From ARC Centres of Excellence for Gravitational Wave Discovery – OzGrav (AU) via phys.org : “Exploring the mysterious origins of the most extreme light flashes in the universe”

From ARC Centres of Excellence for Gravitational Wave Discovery – OzGrav (AU)

via

phys.org

October 18, 2021

Artist’s illustration of a gamma ray burst. Credit: Carl Knox, OzGrav-The Swinburne University of Technology (AU).

Our universe shines bright with light across the electromagnetic spectrum. While most of this light comes from stars like our sun in galaxies like our own, we are often treated with brief and bright flashes that outshine entire galaxies themselves. Some of these brightest flashes are believed to be produced in cataclysmic events, such as the death of massive stars or the collision of two stellar corpses known as neutron stars. Researchers have long studied these bright flashes or “transients” to gain insight into the deaths and afterlives of stars and the evolution of our universe.

Astronomers are sometimes greeted with transients that defy expectations and puzzle theorists who have long predicted how various transients should look. In October 2014, a long-term monitoring program of the southern sky with the Chandra telescope—NASA’s flagship X-Ray telescope—detected one such enigmatic transient called CDF-S XT1: a bright transient lasting a few thousandths of a second.

National Aeronautics and Space Administration Chandra X-ray telescope(US)

The amount of energy CDF-S XT1 released in X-rays was comparable to the amount of energy the sun emits over a billion years. Ever since the original discovery, astrophysicists have come up with many hypotheses to explain this transient; however, none have been conclusive.

In a recent study, a team of astrophysicists led by OzGrav postdoctoral fellow Dr. Nikhil Sarin (Monash University (AU)) found that the observations of CDF-S XT1 match predictions of radiation expected from a a high-speed jet traveling close to the speed of light. Such “outflows” can only be produced in extreme astrophysical conditions, such as the disruption of a star as it gets torn apart by a massive black hole, the collapse of a massive star or the collision of two neutron stars.

Sarin et al’s study found that the outflow from CDF-S XT1 was likely produced by two neutron stars merging together.

Merging neutron stars. Image Credit: Mark Garlick, University of Warwick (UK).

This insight makes CDF-S XT1 similar to the momentous 2017 discovery called GW170817—the first observation of gravitational-waves, cosmic ripples in the fabric of space and time—although CDF-S XT1 is 450 times further away from Earth. This huge distance means that this merger happened very early in the history of the universe; it may also be one of the furthest neutron star mergers ever observed.

Neutron star collisions are the main places in the universe where heavy elements such as gold, silver and plutonium are created. Since CDF-S XT1 occurred early on in the history of the universe, this discovery advances our understanding of Earth’s chemical abundance and elements.

Recent observations of another transient AT2020blt in January 2020—primarily with the Zwicky Transient Facility—have puzzled astronomers.

Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Credit: Caltech Optical Observatories

Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, U.S.A., altitude 1,712 m (5,617 ft). Credit: Caltech.

This transient’s light is like the radiation from high-speed outflows launched during the collapse of a massive star. Such outflows typically produce higher energy gamma-rays; however, they were missing from the data—they were not observed. These gamma rays can only be missing due to one of three reasons: 1) The gamma-rays were not produced, 2) The gamma rays were directed away from Earth, 3) The gamma-rays were too weak to be seen.

In a separate study [The Astrophysical Journal Letters], led again by OzGrav researcher Dr. Sarin, the Monash University astrophysicists teamed up with researchers in Alabama, Louisiana, Portsmouth and Leicester to show that AT2020blt probably did produce gamma-rays pointed toward Earth, they were just really weak and missed by our current instruments.

Dr. Sarin says: “Together with other similar transient observations, this interpretation means that we are now starting to understand the enigmatic problem of how gamma-rays are produced in cataclysmic explosions throughout the Universe.”

The class of bright transients collectively known as gamma-ray bursts, including CDF-S XT1, AT2020blt, and AT2021any, produce enough energy to outshine entire galaxies in just one second.

“Despite this, the precise mechanism that produces the high-energy radiation we detect from the other side of the universe is not known,” explains Dr. Sarin. “These two studies have explored some of the most extreme gamma-ray bursts ever detected. With further research, we’ll finally be able to answer the question we’ve pondered for decades: How do gamma ray bursts work?”

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OzGrav (AU)

ARC Centres of Excellence for Gravitational Wave Discovery OzGrav (AU)
A new window of discovery.
A new age of gravitational wave astronomy.
One hundred years ago, Albert Einstein produced one of the greatest intellectual achievements in physics, the theory of general relativity. In general relativity, spacetime is dynamic. It can be warped into a black hole. Accelerating masses create ripples in spacetime known as gravitational waves (GWs) that carry energy away from the source. Recent advances in detector sensitivity led to the first direct detection of gravitational waves in 2015. This was a landmark achievement in human discovery and heralded the birth of the new field of gravitational wave astronomy. This was followed in 2017 by the first observations of the collision of two neutron-stars. The accompanying explosion was subsequently seen in follow-up observations by telescopes across the globe, and ushered in a new era of multi-messenger astronomy.

The mission of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) is to capitalise on the historic first detections of gravitational waves to understand the extreme physics of black holes and warped spacetime, and to inspire the next generation of Australian scientists and engineers through this new window on the Universe.

OzGrav is funded by the Australian Government through the Australian Research Council Centres of Excellence funding scheme, and is a partnership between Swinburne University of Technology (AU) (host of OzGrav headquarters), the Australian National University (AU), Monash University (AU), University of Adelaide (AU), University of Melbourne (AU), and University of Western Australia (AU), along with other collaborating organisations in Australia and overseas.
________________________________________________________

The objectives for the ARC Centres of Excellence are to:

undertake highly innovative and potentially transformational research that aims to achieve international standing in the fields of research envisaged and leads to a significant advancement of capabilities and knowledge

link existing Australian research strengths and build critical mass with new capacity for interdisciplinary, collaborative approaches to address the most challenging and significant research problems

develope relationships and build new networks with major national and international centres and research programs to help strengthen research, achieve global competitiveness and gain recognition for Australian research

build Australia’s human capacity in a range of research areas by attracting and retaining, from within Australia and abroad, researchers of high international standing as well as the most promising research students

provide high-quality postgraduate and postdoctoral training environments for the next generation of researchers

offer Australian researchers opportunities to work on large-scale problems over long periods of time

establish Centres that have an impact on the wider community through interaction with higher education institutes, governments, industry and the private and non-profit sector.

From ESOblog (EU): Women in STEM-Cyrielle Opitom “Capricious Cosmos”

From ESOblog (EU)

at

European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL)

18 June 2021
Science@ESO

Juan Carlos Muñoz Mateos.
Juan Carlos Muñoz Mateos is Media Officer at ESO in Garching and editor of the ESO blog. He completed his PhD in astrophysics at Complutense University of Madrid[Universidad Complutense Madrid](ES) . Previously he worked for several years at ESO in Chile, combining his research on galaxy evolution with duties at Paranal Observatory.

Astronomical observations are usually planned months in advance, which is not a problem as most celestial objects remain unchanged for millions if not billions of years. But certain astronomical phenomena can occur unexpectedly on timescales of just days –– sometimes even minutes. To learn how we can deal with these sudden events we have talked to three astronomers who study some of the most unpredictable phenomena in the Universe.

“Comets are like cats: they have tails and they do precisely what they want.” This quote by David H. Levy, an amateur astronomer who co-discovered a comet that impacted on Jupiter in 1994, perfectly describes the capricious personality of comets –– large blocks of ice and rock that traverse the solar system.

A NASA Hubble Space Telescope (HST) image of comet Shoemaker-Levy 9, taken on May 17, 1994, with the Wide Field Planetary Camera 2 (WFPC2) in wide field mode.

National Aeronautics and Space Administration(US)/European Space Agency [Agence spatiale européenne] [Europäische Weltraumorganisation] (EU) Hubble Space Telescope

Hubble WFPC2 no longer in service.

But these aren’t the only unpredictable objects out there. Violent supernova explosions, black holes gobbling material from closeby stars, or neutron stars smashing against each other are just a few examples of astronomical phenomena known for not caring about the daily routine of the astronomers who study them. How can we observe these events without even knowing when or where they will happen? Let’s find out.

Celestial wanderers

Cyrielle Opitom, a former ESO Fellow and now a Royal Astronomical Society (UK) Norman Lockyer Research Fellow at the University of Edinburgh (SCT), is very familiar with the changeable nature of comets –– fossils that allow us to study how our own solar system formed and evolved. “Comets are very unpredictable,” she says. “Some suddenly split into different fragments, crash into a planet, or become ten times brighter from one day to the next. And we are still trying to understand why those things are happening. That also makes comets very fun to study. You never know what to expect and it never gets boring.”

When a comet gets close to the Sun, its ices become gaseous. This ejects dust particles as well, creating a huge envelope of dust and gas around the nucleus of the comet, called ‘coma’. Cyrielle uses spectroscopy, a technique that splits light into its constituent colours or wavelengths, “to detect molecules in the coma, and to know what cometary ices are made of.”

But it’s hard to know in advance when a comet may undergo a sudden burst of activity. To address this, ESO and other observatories offer a type of observing programme called “Target of Opportunity”. “This allows us to decide in advance that we want to observe an outburst of activity, have the observations ready to be executed, and when an event is detected we can ask for the observations to be done within just a few days,” says Cyrielle.

A Target of Opportunity still requires astronomers to submit an observing proposal months in advance describing their scientific idea, even if they don’t know when they will trigger the observations. “But there are events that we can’t predict, or new interesting comets that are discovered after the deadline for observing time proposals has passed.” For situations like these, observatories offer the opportunity to obtain observing slots using “Director’s Discretionary Time (DDT)”, which allows astronomers to submit an observing proposal on-the-fly for urgent scientific reasons. For instance, these DDT slots came in handy to observe 2I/Borisov, the first interstellar comet, immediately after its discovery, allowing astronomers to study this alien visitor while it was still close to the Earth.

Comets can still surprise you even when you are already pointing a telescope at them, as Cyrielle knows all too well. In December 2018 she was observing comet 46P/Wirtanen with the ESPRESSO spectrograph at the UT3 telescope [1], part of ESO’s Very Large Telescope.

ESPRESSO — the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations. ESPRESSO will use the light from any one, or all four, of the Unit Telescopes of the Very large Telescope to extend our capability to find planets around other stars and to measure the fundamental constants of physics.

ESO/ESPRESSO on the VLT installed at the incoherent combined Coudé facility of the VLT. It is an ultra-stable fibre-fed échelle high-resolution spectrograph (R~140,000, 190,000, or 70,000) which collects the light from either a single UT or the four UTs simultaneously via the so-called UT Coudé trains.

Espresso Layout

“The comet was bright and very close to the Earth,” she says, “so it was quite big in the sky. However, when we tried to point the instrument at the comet, we could not find it.”

As it turns out, the comet was too far from its predicted position. Luckily, she was observing it simultaneously with the UVES spectrograph on the UT2 telescope. “We managed to find it with UVES, which has a larger field of view.

UVES spectrograph mounted on the VLT at the Nasmyth B focus of UT2.

UVES spectrograph mounted on the VLT at the Nasmyth B focus of UT2.

We computed the offset from the predicted position and finally found the comet with ESPRESSO as well. But our problems were not over: the comet was not moving the way we expected, so we had to constantly adjust the position of the telescope during the observations. Thanks to the great skills of our support astronomer we got amazing data in the end.”

When black holes take a midnight snack

Combining observations done with ESO’s Very Large Telescope and NASA’s Chandra X-ray telescope, astronomers have uncovered the most powerful pair of jets ever seen from a stellar black hole.

National Aeronautics and Space Administration Chandra X-ray telescope(US)

The black hole blows a huge bubble of hot gas, 1000 light-years across or twice as large and tens of times more powerful than the other such microquasars. The stellar black hole belongs to a binary system as pictured in this artist’s impression. Credit:L. Calçada/M.Kornmesser/ESO.

Black holes may not be as evasive as comets, but they are still tricky to observe. Teo Muñoz-Darias, a Ramón y Cajal Fellow at the Institute of Astrophysics of the Canaries[Instituto de Astrofísica de Canarias] (ES), is trying to understand what makes black holes hungry. “I study systems called X-ray binaries,” he says, “where a normal star orbits a black hole at such close distance that the black hole steals material from the star.” But since both are rotating around each other, the material doesn’t fall directly into the black hole; instead, it forms an accretion disc around it. “Gas in the accretion disc gets really hot, up to ten million degrees, thus emitting highly energetic radiation like X-rays.”

This doesn’t happen all the time though. “Most black holes are sleeping and they wake up every now and then,” Teo explains.“If there is little gas in the disc, it will just stay there orbiting the black hole. But when enough gas accumulates, it becomes hotter and friction increases; the gas then loses energy and spirals towards the black hole.” Not all the gas suffers that demise, though; sometimes it can leave the system via powerful winds and jets.

When one of these systems becomes active, dedicated space telescopes will pick up the sudden burst of X-rays. Astronomers worldwide are notified about this and start collecting additional data from ground-based telescopes, quickly sharing their findings via The Astronomer’s Telegram. Teo constantly keeps an eye on this, as interesting targets can show up anytime. “Black holes don’t care about Saturdays, Sundays, or holidays,” he jokes. “In fact they tend to pick holidays!”

Upon finding a suitable target, Teo triggers Target of Opportunity observations with various instruments, like the X-shooter spectrograph at ESO’s VLT.

ESO X-shooter on VLT on UT2 at Cerro Paranal (CL)

“X-shooter is probably the best instrument worldwide for this kind of science,” he says. “With it you are able to get a spectrum all the way from the ultraviolet to the near infrared in one go, and this is fantastic. It’s not only that you get a lot of data, but you get it simultaneously.” This is key with rapidly changing objects, as it allows astronomers to follow how they evolve at different colours without having to coordinate observations with separate instruments.

Thanks to observations like these, Teo and his team could study in great detail the complex balance between gas accretion onto the black hole and gas being expelled outwards due to winds. They found that winds are present even when the system is asleep, and that when they are awake their activity can end prematurely when a lot of gas is removed.

The most energetic explosions in the Universe

When it comes to unpredictability, nothing beats gamma-ray bursts (GRBs) –– sudden flashes of high-energy gamma radiation. “GRBs are the brightest things known to science,” says Nial Tanvir, a professor at the University of Leicester (UK).

Gamma-ray burst. Credit NASA SWIFT/Cruz Dewilde.

National Aeronautics and Space Administration(US) Neil Gehrels Swift Observatory.

“Some are produced when a massive star implodes at the end of its lifetime, leaving behind a neutron star or a black hole. Other GRBs are caused by the merger of two neutron stars. GRBs give us access to the most extreme physics that we know of in the Universe.”

As opposed to comets and black holes feasting off closeby stars, which require astronomers to react within days, GRBs sometimes need to be observed minutes after they occur. “GRBs start out bright and decline in luminosity quickly,” Nial says. “So if you can get there early, there’s just so much more information that you can get with a shorter amount of telescope time. If you can get observations within minutes and then continue to monitor over a few hours, in some cases you see variability, which can tell you important things about the GRB and its environment.”

To allow astronomers to react so quickly, ESO offers them a unique observing mechanism called “Rapid Response Mode”. When this mode is triggered, an alarm instantly goes off in the Paranal control room: the ongoing observations will be aborted –– if it’s safe to do so –– and the telescope will automatically slew towards the sky coordinates of the GRB. Unfortunately, this requires knowing the exact location of the GRB from the get go, which isn’t always the case.

One of the most exciting events that Nial has studied was the first-ever detection of light from two merging neutron stars. On 17 August 2017 the LIGO and Virgo interferometers registered gravitational waves –– ripples in space-time –– passing through Earth. Two seconds later, the Fermi and INTEGRAL space telescopes detected a GRB coming from the same area of the sky.

National Aeronautics and Space Administration(US) Fermi Large Area Telescope

National Aeronautics and Space Administration(US)/Fermi Gamma Ray Space Telescope.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) Integral spacecraft

Both were the smoking-gun evidence of a kilonova: two neutron stars smashing against each other.

Merging neutron stars. Image Credit: Mark Garlick, University of Warwick (UK).

As night fell in Chile, dozens of telescopes started to chase this unique event. “Neither the gravitational waves nor the gamma rays gave us a tremendously accurate localisation,” says Nial. So this was like looking for a needle in a haystack, scanning a large patch of the sky looking for a small dot that wasn’t there before. The Swope telescope at Las Campanas Observatory was the first one to locate the host galaxy: NGC4993, an elliptical galaxy about 140 million lightyears away.

Carnegie Institution 1-meter Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena, near the north end of a 7 km (4.3 mi) long mountain ridge, Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile.

Carnegie Las Campanas Observatory in the southern Atacama Desert of Chile in the Atacama Region approximately 100 kilometres (62 mi) northeast of the city of La Serena,near the southern end and over 2,500 m (8,200 ft) high.

Five other teams found it independently during those hectic first couple of hours, including Nial’s group using ESO’s VISTA telescope [below].

“You just did have that strong sense that you were sort of living through history, perhaps more so than anything else I’ve been involved with.” During the next few weeks, astronomers worldwide monitored the evolution of this object with pretty much every telescope they could, including 14 instruments from 7 ESO-related telescopes. “As the days went by this thing started to become redder and redder, just as predicted. The collision pulled very radioactive material out of the neutron stars, which then decayed to form a whole lot of elements heavier than iron like gold, platinum and uranium, whose origin had previously been quite mysterious.”

It’s all about teamwork

Observing these unpredictable events is only possible thanks to team spirit. In the case of the kilonova, for instance, astronomers barely had a couple of hours after sunset to observe it before it sank under the horizon. As Nial says, “The success of all of these campaigns really came down to the staff at the telescopes, who were doing their very best to squeeze in those observations in difficult circumstances.”

But this is only part of the story, as good planning is also key. “ESO is not just Paranal or La Silla observatories,” explains Cyrielle. “It also has an amazing team at the User Support Department to help us prepare and adjust our observations, so that we make the best possible use of the instruments. When I was preparing observations of the interstellar comet 2I/Borisov, they helped me design unusual observations that spanned several months. Without them we could never have obtained such high-quality data to study an interstellar comet.”
Note

[1] Unlike other instruments, ESPRESSO isn’t physically attached to a Unit Telescope. The light from any UT, even all four of them, can be fed into the instrument.

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European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design,

European Southern Observatory(EU) La Silla HELIOS (HARPS Experiment for Light Integrated Over the Sun)

ESO 3.6m telescope & HARPS atCerro LaSilla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

MPG Institute for Astronomy [Max-Planck-Institut für Astronomie](DE) 2.2 meter telescope at/European Southern Observatory(EU) Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

European Southern Observatory(EU)La Silla Observatory 600 km north of Santiago de Chile at an altitude of 2400 metres.

European Southern Observatory(EU) , Very Large Telescope at Cerro Paranal in the Atacama Desert •ANTU (UT1; The Sun ) •KUEYEN (UT2; The Moon ) •MELIPAL (UT3; The Southern Cross ), and •YEPUN (UT4; Venus – as evening star). Elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo. [/caption]

European Southern Observatory(EU)VLTI Interferometer image, Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening

ESO Very Large Telescope 4 lasers on Yepun (CL)

Glistening against the awesome backdrop of the night sky above ESO’s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT, a major asset of the Adaptive Optics system.

ESO/NTT NTT at Cerro La Silla , Chile, at an altitude of 2400 metres.

Part of ESO’s Paranal Observatory, the VLT Survey Telescope (VISTA) observes the brilliantly clear skies above the Atacama Desert of Chile. It is the largest survey telescope in the world in visible light, with an elevation of 2,635 metres (8,645 ft) above sea level.

European Southern Observatory/National Radio Astronomy Observatory(US)/National Astronomical Observatory of Japan(JP) ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.

European Southern Observatory(EU) ELT 39 meter telescope to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

European Southern Observatory(EU)/MPG Institute for Radio Astronomy [MPG Institut für Radioastronomie](DE) ESO’s Atacama Pathfinder Experiment(CL) high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft).

Leiden MASCARA instrument cabinet at Cerro La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft).

ESO Next Generation Transit Survey telescopes, an array of twelve robotic 20-centimetre telescopes at Cerro Paranal,(CL) 2,635 metres (8,645 ft) above sea level.

TAROT telescope at Cerro LaSilla, 2,635 metres (8,645 ft) above sea level.

European Southern Observatory(EU) ExTrA telescopes at erro LaSilla at an altitude of 2400 metres.

A novel gamma ray telescope under construction on Mount Hopkins, Arizona. A large project known as the Čerenkov Telescope Array composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile at, ESO Cerro Paranal site The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the. University of Wisconsin–Madison and capable of taking pictures at a billion frames per second. Credit: Vladimir Vassiliev.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU), The new Test-Bed Telescope 2is housed inside the shiny white dome shown in this picture, at ESO’s LaSilla Facility in Chile. The telescope has now started operations and will assist its northern-hemisphere twin in protecting us from potentially hazardous, near-Earth objects.The domes of ESO’s 0.5 m and the Danish 0.5 m telescopes are visible in the background of this image.Part of the world-wide effort to scan and identify near-Earth objects, the European Space Agency’s Test-Bed Telescope 2 (TBT2), a technology demonstrator hosted at ESO’s La Silla Observatory in Chile, has now started operating. Working alongside its northern-hemisphere partner telescope, TBT2 will keep a close eye on the sky for asteroids that could pose a risk to Earth, testing hardware and software for a future telescope network.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) The open dome of The black telescope structure of the‘s Test-Bed Telescope 2 peers out of its open dome in front of the rolling desert landscape. The telescope is located at ESO’s La Silla Observatory, which sits at a 2400 metre altitude in the Chilean Atacama desert.

From DESY German Electron Synchrotron [Deütsches Elektronen-Synchrotron] (DE) :Women in STEM-Sylvia Zhu and Edna Ruiz-Velasco “Front-row view reveals exceptional cosmic explosion”

From DESY German Electron Synchrotron [Deütsches Elektronen-Synchrotron] (DE)

Observation challenges established theory of gamma-ray bursts in the universe

Scientists have gained the best view yet of the brightest explosions in the universe: A specialised observatory in Namibia has recorded the most energetic radiation and longest gamma-ray afterglow of a so-called gamma-ray burst (GRB) to date. The observations with the High Energy Stereoscopic System (H.E.S.S.) challenge the established idea of how gamma-rays are produced in these colossal stellar explosions which are the birth cries of black holes, as the international team reports in the journal Science.

Artist’s impression of a relativistic jet of a gamma-ray burst (GRB), breaking out of a collapsing star, and emitting very-high-energy photons. Credit: DESY, Science Communication Lab.

“Gamma-ray bursts are bright X-ray and gamma-ray flashes observed in the sky, emitted by distant extragalactic sources,” explains DESY scientist Sylvia Zhu, one of the authors of the paper. “They are the biggest explosions in the universe and associated with the collapse of a rapidly rotating massive star to a black hole. A fraction of the liberated gravitational energy feeds the production of an ultrarelativistic blast wave. Their emission is divided into two distinct phases: an initial chaotic prompt phase lasting tens of seconds, followed by a long-lasting, smoothly fading afterglow phase.”

On 29 August 2019 the satellites Fermi and Swift detected a gamma-ray burst in the constellation of Eridanus.

National Aeronautics and Space Administration(US)/Fermi Gamma Ray Space Telescope.

National Aeronautics and Space Administration(US) Neil Gehrels Swift Observatory.

The event, catalogued as GRB 190829A according to its date of occurrence, turned out to be one of the nearest gamma-ray bursts observed so far, with a distance of about one billion lightyears. For comparison: The typical gamma-ray burst is about 20 billion lightyears away. “We were really sitting in the front row when this gamma-ray burst happened,” explains co-author Andrew Taylor from DESY. The team caught the explosion’s afterglow immediately when it became visible to the H.E.S.S. telescopes. “We could observe the afterglow for several days and to unprecedented gamma-ray energies,” reports Taylor.

H.E.S.S. Čerenkov Telescope Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg searches for cosmic rays, altitude, 1,800 m (5,900 ft).

X-rays from the GRB were detected by NASA’s Swift satellite in Earth’s orbit. Very-high-energy gamma rays entered the atmosphere and initiated air showers that were detected by H.E.S.S. from the ground (artist’s impression). Credit: DESY, Science Communication Lab.

Cosmic rays produced by high-energy astrophysics sources ASPERA collaboration AStroParticle ERAnet

The comparatively short distance to this gamma-ray burst allowed detailed measurements of the afterglow’s spectrum, which is the distribution of “colours” or photon energies of the radiation, in the very-high energy range. “We could determine GRB 190829A’s spectrum up to an energy of 3.3 tera-electronvolts, that’s about a trillion times as energetic as the photons of visible light,” explains co-author Edna Ruiz-Velasco from the MPG Institute for Nuclear Physics [MPG Institut für Kernphysik] (DE) in Heidelberg. “This is what’s so exceptional about this gamma-ray burst – it happened in our cosmic backyard where the very-high-energy photons were not absorbed in collisions with background light on their way to Earth, as it happens over larger distances in the cosmos.”

The team could follow the afterglow up to three days after the initial explosion. The result came as a surprise: “Our observations revealed curious similarities between the X-ray and very-high energy gamma-ray emission of the burst’s afterglow,” reports Zhu. Established theories assume that the two emission components must be produced by separate mechanisms: the X-ray component originates from ultra-fast electrons that are deflected in the strong magnetic fields of the burst’s surroundings. This “synchrotron” process is quite similar to how particle accelerators on Earth produce bright X-rays for scientific investigations.

However, according to existing theories it seemed very unlikely that even the most powerful explosions in the universe could accelerate electrons enough to directly produce the observed very-high-energy gamma rays via this synchrotron process. This is due to a “burn-off limit”, which is determined by the balance of acceleration and cooling of particles within an accelerator. Producing very-high-energy gamma rays through synchrotron radiation requires electrons with energies well beyond the burn-off limit. Instead, current theories assume that in a gamma-ray burst, fast electrons collide with synchrotron photons and thereby boost them to gamma-ray energies in a process dubbed synchrotron self-Compton.

Artist’s impression of very-high-energy photons from a GRB entering Earths’ atmosphere and initiating air showers that are being recorded by the H.E.S.S. telescopes. Credit: DESY, Science Communication Lab.

But the observations of GRB 190829A’s afterglow now show that both components, X-ray and gamma ray, faded in sync. Also, the gamma-ray spectrum clearly matched an extrapolation of the X-ray spectrum. Together, these results are a strong indication that X-rays and very-high-energy gamma rays in this afterglow were produced by the same mechanism. “It is rather unexpected to observe such remarkably similar spectral and temporal characteristics in the X-ray and very-high energy gamma-ray energy bands, if the emission in these two energy ranges had different origins,” says co-author Dmitry Khangulyan from Rikkyo University [立教大学](JP) in Tokyo. This poses a challenge for the synchrotron self-Compton origin of the very-high energy gamma-ray emission.

The far-reaching implication of this possibility highlights the need for further studies of very-high energy GRB afterglow emission. GRB 190829A is only the fourth gamma-ray burst detected at very high energies from the ground. However, the earlier detected explosions occurred much farther away in the cosmos and their afterglow could only be observed for a few hours each and not to energies above 1 tera-electronvolts (TeV). “Looking to the future, the prospects for the detection of gamma-ray bursts by next-generation instruments like the Čerenkov Telescope Array that is currently being built in the Chilean Andes and on the Canary Island of La Palma look promising,” says H.E.S.S. spokesperson Stefan Wagner from Observatory at Königstuhl[Landessternwarte Königstuhl] at Ruprecht Karl University of Heidelberg [Ruprecht-Karls-Universität Heidelberg] (DE).

Proposed CTA Telescopes, Čerenkov Telescope Array depiction at ESO’s Cerro Paranal Observatory. This image illustrates all three classes of the 99 telescopes planned for the southern hemisphere at ESO’s Paranal Observatory, as viewed from the centre of the array. This rendering is not an accurate representation of the final array layout, but it illustrates the enormous scale of the CTA telescopes and the array itself.

“The general abundance of gamma-ray bursts leads us to expect that regular detections in the very-high energy band will become rather common, helping us to fully understand their physics.”

More than 230 scientists from 41 institutes in 15 countries (Namibia, South Africa, Germany, France, the UK, Ireland, Italy, Austria, the Netherlands, Poland, Sweden, Armenia, Japan, China and Australia), comprising the international H.E.S.S. collaboration, contributed to this research. H.E.S.S. is a system of five Imaging Atmospheric Čerenkov Telescopes that investigates cosmic gamma rays. The name H.E.S.S. stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Franz Hess, who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. Four H.E.S.S. telescopes went into operation in 2002/2003, the much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity. In 2015-2016, the cameras of the first four H.E.S.S. telescopes were fully refurbished using state of the art electronics and in particular the NECTAr readout chip designed for the next big experiment in the field, the Čerenkov Telescope Array (CTA), for which the data science management centre will be hosted by DESY on its Zeuthen site.

See the full article here .

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DESY German Electron Synchrotron [Deütsches Elektronen-Synchrotron] (DE) is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

DESY Petra III interior

DESY Petra III

DESY/FLASH

H1 detector at DESY HERA ring

DESY DORIS III

DESY LUX beamline

[/caption]

From “Physics”: “Gamma Rays Provide New Quantum Gravity Constraint”

About Physics

From “Physics”

July 9, 2020 [Brought to me by “Marcus” at https://whois.arin.net/rest/ip/120.21.107.75%5D
Marric Stephens

An analysis of the speed of the most energetic photons ever observed from a gamma-ray burst sets new constraints on certain theories of quantum gravity.

Cosmic rays produced by high-energy astrophysics sources (ASPERA collaboration – AStroParticle ERAnet).

MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia, La Palma (ES), Altitude 2,396 m (7,861 ft).

To develop a theory of quantum gravity—the holy grail of theoretical physics—researchers might need to sacrifice one of the field’s central principles: the invariance of the speed of light. The predictions of general relativity and quantum theory are expected to coincide somewhere near the Planck energy (about 1.22×10^19GeV) (see Focus: The Period of the Universe’s Clock). But some theories indicate consequences at far lower energies, which would manifest as modifications to the propagation speed of very-high-energy gamma rays (VHEGRs). Now, by analyzing the arrival times of the most energetic photons ever measured from a gamma-ray burst (GRB 190114C), the MAGIC Collaboration shows that if the photons’ speed does deviate from the speed of light, it is by a factor of less than 1.7×10^−15.

The best place to find evidence of photons traveling faster or slower than the speed of light is in signals that have traversed much of the Universe. This colossal distance is needed to make detectable the tiny potential discrepancy in the speeds of photons of different energies. At 4.5 billion light years, GRB 190114C was closer than other GRBs that have been used to constrain quantum gravity theories. But its photons were more energetic, coming in at 2 TeV, compared with less than 100 GeV for previously measured events. This high energy allowed a unique test of a specific light-speed invariance-violation model in which the relationship between photon energy and velocity is quadratic.

The team found that the speed of photons from GRB 190114C matches the speed of light, indicating that under the invariance-violation model, unification must occur at an energy above 5.6×10^10 GeV. They hope that future observations of more distant GRBs will let them tighten the constraint.

This research is published in Physical Review Letters.

See the full article here .

See also https://sciencesprings.wordpress.com/2021/01/31/from-physics-the-period-of-the-universes-clock/ here.

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Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

From NASA Fermi: “NASA Missions Unmask Magnetar Eruptions in Nearby Galaxies”

NASA/Fermi LAT.

From NASA Fermi

Jan. 13, 2021

Francis Reddy
francis.j.reddy@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Media contact:
Claire Andreoli
claire.andreoli@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

On April 15, 2020, a brief burst of high-energy light swept through the solar system, triggering instruments on several NASA and European spacecraft. Now, multiple international science teams conclude that the blast came from a supermagnetized stellar remnant known as a magnetar located in a neighboring galaxy.

This finding confirms long-held suspicions that some gamma-ray bursts (GRBs) – cosmic eruptions detected in the sky almost daily – are in fact powerful flares from magnetars relatively close to home.

NASA Missions Unveil Magnetar Eruptions in Nearby Galaxies
A pulse of X-rays and gamma rays lasting just 140 milliseconds swept across the solar system on April 15, 2020. The event was a giant flare from a magnetar, a type of city-sized stellar remnant that boasts the strongest magnetic fields known. Watch to learn more. Credit: NASA’s Goddard Space Flight Center.

“This has always been regarded as a possibility, and several GRBs observed since 2005 have provided tantalizing evidence,” said Kevin Hurley, a Senior Space Fellow with the Space Sciences Laboratory at the University of California, Berkeley, who joined several scientists to discuss the burst at the virtual 237th meeting of the American Astronomical Society. “The April 15 event is a game changer because we found that the burst almost certainly lies within the disk of the nearby galaxy NGC 253.”

Papers analyzing different aspects of the event and its implications were published on Jan. 13 in the journals Nature and Nature Astronomy.

GRBs, the most powerful explosions in the cosmos, can be detected across billions of light-years. Those lasting less than about two seconds, called short GRBs, occur when a pair of orbiting neutron stars – both the crushed remnants of exploded stars – spiral into each other and merge. Astronomers confirmed this scenario for at least some short GRBs in 2017, when a burst followed the arrival of gravitational waves – ripples in space-time – produced when neutron stars merged 130 million light-years away.

Magnetars are neutron stars with the strongest-known magnetic fields, with up to a thousand times the intensity of typical neutron stars and up to 10 trillion times the strength of a refrigerator magnet. Modest disturbances to the magnetic field can cause magnetars to erupt with sporadic X-ray bursts for weeks or longer.

Rarely, magnetars produce enormous eruptions called giant flares that produce gamma rays, the highest-energy form of light.

Most of the 29 magnetars now cataloged in our Milky Way galaxy exhibit occasional X-ray activity, but only two have produced giant flares. The most recent event, detected on Dec. 27, 2004, produced measurable changes in Earth’s upper atmosphere despite erupting from a magnetar located about 28,000 light-years away.

Shortly before 4:42 a.m. EDT on April 15, 2020, a brief, powerful burst of X-rays and gamma rays swept past Mars, triggering the Russian High Energy Neutron Detector aboard NASA’s Mars Odyssey spacecraft, which has been orbiting the Red Planet since 2001.

NASA/Mars Odyssey Spacecraft

About 6.6 minutes later, the burst triggered the Russian Konus instrument aboard NASA’s Wind satellite, which orbits a point between Earth and the Sun located about 930,000 miles (1.5 million kilometers) away.

NASA Wind Spacecraft

After another 4.5 seconds, the radiation passed Earth, triggering instruments on NASA’s Fermi Gamma-ray Space Telescope [above], as well as on the European Space Agency’s INTEGRAL satellite and Atmosphere-Space Interactions Monitor (ASIM) aboard the International Space Station.

ESA/Integral.

ESA ASIM

The eruption occurred beyond the field of view of the Burst Alert Telescope (BAT) on NASA’s Neil Gehrels Swift Observatory, so its onboard computer did not alert astronomers on the ground.

NASA Neil Gehrels Swift Observatory.

However, thanks to a new capability called the Gamma-ray Urgent Archiver for Novel Opportunities (GUANO), the Swift team can beam back BAT data when other satellites trigger on a burst. Analysis of this data provided additional insight into the event.

The pulse of radiation lasted just 140 milliseconds – as fast as the blink of an eye or a finger snap.

The giant flare, cataloged as GRB 200415A, reached detectors on different NASA spacecraft at different times. Each instrument pair established its possible location in different swaths of the sky, but the bands intersect in the central part of the bright spiral galaxy NGC 253. This is the most precise position yet established for a magnetar located well beyond our galaxy.
Credits: NASA’s Goddard Space Flight Center and Adam Block/Mount Lemmon SkyCenter/University of Arizona.

The Fermi, Swift, Wind, Mars Odyssey and INTEGRAL missions all participate in a GRB-locating system called the InterPlanetary Network (IPN). Now funded by the Fermi project, the IPN has operated since the late 1970s using different spacecraft located throughout the solar system. Because the signal reached each detector at different times, any pair of them can help narrow down a burst’s location in the sky. The greater the distances between spacecraft, the better the technique’s precision.

The IPN placed the April 15 burst, called GRB 200415A, squarely in the central region of NGC 253, a bright spiral galaxy located about 11.4 million light-years away in the constellation Sculptor. This is the most precise sky position yet determined for a magnetar located beyond the Large Magellanic Cloud, a satellite of our galaxy and host to a giant flare in 1979, the first ever detected.

Giant flares from magnetars in the Milky Way and its satellites evolve in a distinct way, with a rapid rise to peak brightness followed by a more gradual tail of fluctuating emission. These variations result from the magnetar’s rotation, which repeatedly brings the flare location in and out of view from Earth, much like a lighthouse.

Observing this fluctuating tail is conclusive evidence of a giant flare. Seen from millions of light-years away, though, this emission is too dim to detect with today’s instruments. Because these signatures are missing, giant flares in our galactic neighborhood may be masquerading as much more distant and powerful merger-type GRBs.

A detailed analysis of data from Fermi’s Gamma-ray Burst Monitor (GBM) and Swift’s BAT provides strong evidence that the April 15 event was unlike any burst associated with mergers, noted Oliver Roberts, an associate scientist at Universities Space Research Association’s Science and Technology Institute in Huntsville, Alabama, who led the study.

In particular, this was the first giant flare known to occur since Fermi’s 2008 launch, and the GBM’s ability to resolve changes at microsecond timescales proved critical. The observations reveal multiple pulses, with the first one appearing in just 77 microseconds – about 13 times the speed of a camera flash and nearly 100 times faster than the rise of the fastest GRBs produced by mergers. The GBM also detected rapid variations in energy over the course of the flare that have never been observed before.

“Giant flares within our galaxy are so brilliant that they overwhelm our instruments, leaving them to hang onto their secrets,” Roberts said. “For the first time, GRB 200415A and distant flares like it allow our instruments to capture every feature and explore these powerful eruptions in unparalleled depth.”

Giant flares are poorly understood, but astronomers think they result from a sudden rearrangement of the magnetic field. One possibility is that the field high above the surface of the magnetar may become too twisted, suddenly releasing energy as it settles into a more stable configuration. Alternatively, a mechanical failure of the magnetar’s crust – a starquake – may trigger the sudden reconfiguration.

Roberts and his colleagues say the data show some evidence of seismic vibrations during the eruption. The highest-energy X-rays recorded by Fermi’s GBM reached 3 million electron volts (MeV), or about a million times the energy of blue light, itself a record for giant flares. The researchers say this emission arose from a cloud of ejected electrons and positrons moving at about 99% the speed of light. The short duration of the emission and its changing brightness and energy reflect the magnetar’s rotation, ramping up and down like the headlights of a car making a turn. Roberts describes it as starting off as an opaque blob – he pictures it as resembling a photon torpedo from the “Star Trek” franchise – that expands and diffuses as it travels.

The torpedo also factors into one of the event’s biggest surprises. Fermi’s main instrument, the Large Area Telescope (LAT), also detected three gamma rays, with energies of 480 MeV, 1.3 billion electron volts (GeV), and 1.7 GeV – the highest-energy light ever detected from a magnetar giant flare. What’s surprising is that all of these gamma rays appeared long after the flare had diminished in other instruments.

Nicola Omodei, a senior research scientist at Stanford University in California, led the LAT team investigating these gamma rays, which arrived between 19 seconds and 4.7 minutes after the main event. The scientists conclude that this signal most likely comes from the magnetar flare. “For the LAT to detect a random short GRB in the same region of the sky and at nearly the same time as the flare, we would have to wait, on average, at least 6 million years,” he explained.

Magnetar Giant Flare Produces Gamma Rays
Astronomers explain the observations of GRB 200415A with the sequence of events illustrated here. A sudden reconfiguration of the magnetar’s magnetic field produced a quick, powerful pulse of X-rays and gamma rays. The event also ejected a blob of matter, which followed the pulse traveling at about 99% the speed of light. After a few days, they both reached the boundary, called a bow shock, where a steady outflow from the magnetar causes a pile-up of interstellar gas. Light from the flare passed through, followed many seconds later by the ejected cloud. The fast-moving matter interacted with gas at the bow shock, creating shock waves that accelerated particles and produced high-energy gamma rays. This accounts for the delay in the arrival of the most energetic gamma rays detected by NASA’s Fermi spacecraft. Credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR).

A magnetar produces a steady outflow of fast-moving particles. As it moves through space, this outflow plows into, slows, and diverts interstellar gas. The gas piles up, becomes heated and compressed, and forms a type of shock wave called a bow shock.

In the model proposed by the LAT team, the flare’s initial pulse of gamma rays travels outward at the speed of light, followed by the cloud of ejected matter, which is moving nearly as fast. After several days, they both reach the bow shock. The gamma rays pass through. Seconds later, the cloud of particles – now expanded into a vast, thin shell – collides with accumulated gas at the bow shock. This interaction creates shock waves that accelerate particles, producing the highest-energy gamma rays after the main burst.

The April 15 flare proves that these events constitute their own class of GRBs. Eric Burns, an assistant professor of physics and astronomy at Louisiana State University in Baton Rouge, led a study investigating additional suspects using data from numerous missions. The findings will appear in The Astrophysical Journal Letters. Bursts near the galaxy M81 in 2005 and the Andromeda galaxy (M31) in 2007 had already been suggested to be giant flares, and the team additionally identified a flare in M83, also seen in 2007 but newly reported. Add to these the giant flare from 1979 and those observed in our Milky Way in 1998 and 2004.

“It’s a small sample, but we now have a better idea of their true energies, and how far we can detect them,” Burns said. “A few percent of short GRBs may really be magnetar giant flares. In fact, they may be the most common high-energy outbursts we’ve detected so far beyond our galaxy – about five times more frequent than supernovae.”

See the full article here .

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The Fermi Gamma-ray Space Telescope, formerly referred to as the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden.

From Symmetry: “A bit of MAGIC”

From Symmetry<

08/25/20
Liz Kruesi

The MAGIC telescope’s first observation of a gamma-ray burst gave astronomers surprising new insight into the phenomenon.

MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia, La Palma, Spain)), Altitude 2,396 m (7,861 ft)

Elena Moretti was filling out paperwork in her room at the astronomer dormitories at the Canary Islands’ Roque de los Muchachos Observatory when, just before 9 p.m., her phone rang. On the line was Moretti’s colleague Cosimo Nigro, who was at the control room of the Major Atmospheric Gamma Imaging Čerenkov telescope, known as MAGIC. He wanted to know whether someone was running a test on their project.

“No, we are not doing a test,” Moretti said. “Why?”

Within minutes, she was running down the road to investigate.

Moretti’s specialty is gamma-ray bursts, incredibly powerful blasts of high-energy radiation marking stellar death. She was in the Canary Islands overseeing construction work on MAGIC’s successor, the Čerenkov Telescope Array.

Čerenkov Telescope Array, http://www.isdc.unige.ch/cta/ at Cerro Paranal, located in the Atacama Desert of northern Chile searches for cosmic rays on Cerro Paranal at 2,635 m (8,645 ft) altitude, 120 km (70 mi) south of Antofagasta; and at at the Instituto de Astrofisica de Canarias (IAC), Roque de los Muchachos Observatory in La Palma, Spain

While scientists have detected GRBs for decades, there seemed to be a limit on the energy level of the gamma rays they captured. Was the limit a feature of GRBs? Some scientists thought that it was not, and they hoped observations could prove it.

If the very-high-energy gamma rays showed up the way some theorists suspected they would, the energy spectrum that observatories measured would show a second higher-energy intensity bump in the data.

Some scientists had given up on the idea of the second bump—after decades’ worth of searching, it had yet to appear.

But that night, January 14, 2019, with its very first detection of a GRB, MAGIC became the observatory that found the second bump—and helped change the scientific world’s understanding of one the great mysteries of the universe.

“It was incredible,” says Moretti, an astrophysicist at the Institute for High Energy Physics in Barcelona, Spain. “The way you look for something for a long time, and then suddenly it materializes and it’s so bright, so undoubtful.”

Bursts born of stellar death

Gamma-ray bursts were serendipitously discovered in the midst of the Cold War, when the US military was scouring the sky for evidence of nuclear weapons. At the time, US satellites spied radiation with energy much higher than visible light that would have come from nuclear blasts. It wasn’t until the next decade, after those detections were declassified, that scientists realized the blasts were coming from cosmic sources. It was later still, in the 1990s, when they realized the signals were coming from outside the galaxy.

Since then, several space-based observatories—including NASA’s Neil Gehrels Swift Observatory and the Gamma-ray bursts were serendipitously discovered in the midst of the Cold War, when the US military was scouring the sky for evidence of nuclear weapons. At the time, US satellites spied radiation with energy much higher than visible light that would have come from nuclear blasts. It wasn’t until the next decade, after those detections were declassified, that scientists realized the blasts were coming from cosmic sources. It was later still, in the 1990s, when they realized the signals were coming from outside the galaxy.

Since then, several space-based observatories—including NASA’s Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope, which is supported by NASA, the US Department of Energy and international partners—have been especially revolutionary to astronomers’ understanding of what leads to such enormously energetic blasts of gamma-ray radiation. Gamma-ray Space Telescope, which is supported by NASA, the US Department of Energy and international partners—have been especially revolutionary to astronomers’ understanding of what leads to such enormously energetic blasts of gamma-ray radiation.

NASA Neil Gehrels Swift Observatory

NASA/Fermi LAT

NASA/Fermi Gamma Ray Space Telescope

GRBs come in two types: short- and long-duration, based on whether their initial gamma-ray blasts last less or more than two seconds. A short-duration GRB happens when two compact objects, such as two neutron stars, slam into each other. A long-duration GRB, on the other hand, results from a specific type of core-collapse supernova, what happens when a star around at least 10 times as massive as our sun exhausts the fuel supplies at its core, causing it to collapse into a black hole.

In both GRB types, the energetic event (whether a collision or collapse) blasts out jets of particles traveling at nearly the speed of light. Interactions inside the jets—between blobs of plasma that are emitted at different times and have different speeds—create a GRB’s initial burst of gamma rays.

As the jets travel through gases surrounding the event site, they create another characteristic feature of GRBs: the afterglow, which shines across the electromagnetic spectrum in radio up to gamma rays.

A gamma-ray puzzle

Astrophysicists have learned a great deal about how such cosmic events generate gamma rays and other electromagnetic radiation. According to astrophysicist David Williams, who has studied GRBs for decades, they produce what’s called synchrotron radiation. “As the burst ejecta plow into the surrounding medium, that burst accelerates electrons,” which spiral around magnetic field lines, shooting out photons like mud off a tractor tire.

But some theorists have thought there could be more to the story—later in the afterglow, a second bump. The second bump would come from the electrons slamming into some of their just-produced photons and bumping those photons’ energies even higher in a process called synchrotron self-Compton.

Twenty years ago, several theorists published papers arguing that, given what scientists knew then about GRBs, synchrotron self-Compton should occur. University of Nevada physicist Bing Zhang, who wrote one of those papers, says he was therefore not surprised by the MAGIC detection. “Synchrotron self-Compton is inevitable, and it should be detected,” he says.

Searching for a second bump

The space-based observatories that have provided so much information about GRBs have never seen the kind of gamma rays that would produce a second bump—because they can’t.

This is for two reasons. First, Swift, which predominantly detects X-rays and ultraviolet rays, would need different technology to detect such high-energy rays. Second, space-based observatories tend to be equal to or smaller than the size of a full-sized refrigerator. Very-high-energy gamma rays are rare, and it takes a much larger detector than that to capture them.

But satellites like Fermi and Swift can still help. When they detect a GRB, they automatically signal additional telescopes to swing around to take a closer look at what they’ve seen.

That day in January 2019, the MAGIC telescope’s computer received the alert and reacted quickly. It analyzed where in the sky the GRB was and whether MAGIC could see it. When it determined it could, it gave a command to quickly move both of MAGIC’s 17-meter-wide telescopes’ thin mirrors, each mounted on a lightweight carbon-fiber frame.

“In this whole process, there is no human,” Moretti says. “We received the alert more or less 20 seconds after the beginning of the GRB. Another 30 seconds—so 50 seconds from the GRB—and we were pointing.”

It was around then that Nigro called. While on the phone, he emailed Moretti the first plot of the GRB signal, which sent Moretti running. “In five minutes, I was on site,” she says.

On the computer screens in the control room, she watched the automatically generated analysis of how the signal’s energy changed over time. Moretti, Nigro and the other observers checked the location for other bright sources, noisy signals that could be confused with a GRB, and found none.

It was MAGIC’s first detection of a GRB. And the source they saw was briefly 100 times as bright as gamma-ray astronomy’s calibration source, the Crab Nebula.

Supernova remnant Crab nebula

MAGIC collected photons over the next few hours with energies from 300 GeV up to 2 TeV, or 2 trillion times the energy of visible light. Once data across multiple wavelengths of radiation from other observatories were combined, the signal became clear: The spectrum in gamma-ray energies peaked not once, but twice.

In that moment, Moretti says she knew “we were looking into the other component, the one that we had been searching for for a long time… I did not know that it was this beautiful, but I knew that it would come. It was just a matter of time.”

More to come

Moretti says she looks forward to the Čerenkov Telescope Array, the next-generation gamma-ray observatory she is also working on in the Canary Islands. Most of the more than 30 institutions that participate in MAGIC—in Germany, Spain, Italy, Japan, Switzerland, Croatia, Finland, Poland, India, Bulgaria, Brazil and Armenia—also participate in the CTA.

Currently in the prototype stage and slated to begin observations in the mid 2020s, CTA will have several times the observing sensitivity as any of the current observatories. It will also be able to monitor a larger area of the sky. CTA will comprise two separate arrays, one in Chile and the other just down the street from MAGIC.

The two-bump January 2019 GRB, along with a one-bump—but also promising—2018 GRB observation by the High Energy Stereoscopic System, or HESS, in Namibia, gives CTA researchers reason to expect many more GRB detections. The current estimates suggest CTA will capture a few each year.

As more observations rack up, it will lead to an even better understanding of these enormously bright blasts.

See the full article here .

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Symmetry is a joint Fermilab/SLAC publication.

From phys.org: “Binary-driven hypernova model gains observational support”

From phys.org

May 19, 2020
by ICRANet

Fig. 1 Taken from 2020ApJ…893..148R. Schematic evolutionary path of a massive binary up to the emission of a BdHN. (a) Binary system composed of two main-sequence stars, say 15 and 12 solar masses, respectively. (b) At a given time, the more massive star undergoes the core-collapse SN and forms a NS (which might have a magnetic field B~1013 G). (c) The system enters the X-ray binary phase. (d) The core of the remaining evolved star, rich in carbon and oxygen, for short CO star, is left exposed since the hydrogen and helium envelope have been striped by binary interactions and possibly multiple common-envelope phases (not shown in this diagram). The system is, at this stage, a CO-NS binary, which is taken as the initial configuration of the BdHN model [2]. (e) The CO star explodes as SN when the binary period is of the order of few minutes, the SN ejecta of a few solar masses start to expand and a fast rotating, newborn NS, for short vNS, is left in the center. (f) The SN ejecta accrete onto the NS companion, forming a massive NS (BdHN II) or a BH (BdHN I; this example), depending on the initial NS mass and the binary separation. Conservation of magnetic flux and possibly additional MHD processes amplify the magnetic field from the NS value to B~1014 G around the newborn BH. At this stage the system is a vNS-BH binary surrounded by ionized matter of the expanding ejecta. (g) The accretion, the formation and the activities of the BH contribute to the GRB prompt gamma-ray emission and GeV emission. Credit: ICRANet

The change of paradigm in gamma-ray burst (GRBs) physics and astrophysics introduced by the binary driven hypernova (BdHN) model, proposed and applied by the ICRA-ICRANet-INAF members in collaboration with the University of Ferrara and the University of Côte d’Azur, has gained further observational support from the X-ray emission in long GRBs. These novel results are presented in the new article, published on April 20, 2020, in The Astrophysical Journal, co-authored by J. A. Rueda, Remo Ruffini, Mile Karlica, Rahim Moradi, and Yu Wang.

The GRB emission is composed by episodes: from the hard X-ray trigger and the gamma-ray prompt emission, to the high-energy emission in GeV, recently observed also in TeV energies in GRB 190114C, to the X-ray afterglow. The traditional model of GRBs attempts to explain the entire GRB emissions from a single-component progenitor, i.e., from the emission of a relativistic jet originating from a rotating black hole (BH). Differently, the BdHN scenario proposes GRBs originate from a cataclysmic event in the last evolutionary stage of a binary system composed of a carbon-oxygen (CO) star and a neutron star (NS) companion in close orbit. The gravitational collapse of the iron core of the CO star produces a supernova (SN) explosion ejecting the outermost layers of the star, and at the same time, a newborn NS (vNS) at its center. The SN ejecta trigger a hypercritical accretion process onto the NS companion and onto the vNS. Depending on the size of the orbit, the NS may reach, in the case of short orbital periods of the order of minutes, the critical mass for gravitational collapse, hence forming a newborn BH. These systems where a BH is formed are called BdHN of type I. For longer periods, the NS gets more massive but it does not form a BH. These systems are BdHNe II. Three-dimensional simulations of all this process showing the feasibility of its occurrence, from the SN explosion to the formation of the BH, has been recently made possible by the collaboration between ICRANet and the group of Los Alamos National Laboratory (LANL) guided by Prof. C. L. Fryer (see Figure 1).

The role of the BH for the formation of the high-energy GeV emission has been recently presented in The Astrophysical Journal. There, the “inner engine” composed of a Kerr BH, with a magnetic field aligned with the BH rotation axis immersed in a low-density ionized plasma, gives origin, by synchrotron radiation, to the beamed emission in the MeV, GeV, and TeV, currently observed only in some BdHN I, by the Fermi-LAT and MAGIC instruments. In the new publication, the ICRA-ICRANet team addresses the interaction of the vNS with the SN due to hypercritical accretion and pulsar-like emission. They show that the fingerprint of the vNS appears in the X-ray afterglow of long GRBs observed by the XRT detector on board the Niels Gehrels Swift observatory. Therefore, the vNS and the BH have well distinct and different roles in the long GRB observed emission.

Fig. 2 :Model evolution of synchrotron spectral luminosity at various times compared with measurements in various spectral bands for GRB 160625B.

Fig. 3 The brown, deep blue, orange, green and bright blue points correspond to the bolometric (about 5 times brighter than the soft X-ray observed by Swift-XRT data) light-curves of GRB 160625B, 160509A, 130427A, 190114C and 180728A, respectively. The solid lines are theoretical light-curves obtained from the rotational energy loss of the vNS powering the late afterglow (t>5000 s, white background), while in the earlier times (300300 s, where data are more available. At earlier times, only GRB 130427A and GRB 190114C in this same have available data. Credit: ICRANet

The emission from the magnetized vNS and the hypercritical accretion of the SN ejecta into it, gives origin to the afterglow observed in all BdHN I and II subclasses. The early (~few hours) X-ray emission during the afterglow phase is explained by the injection of ultra-relativistic electrons from the vNS into the expanding ejecta, producing synchrotron radiation (see Figure 2). The magnetic field inferred from the synchrotron analysis agrees with the expected toroidal/longitudinal magnetic field component of the vNS. Furthermore, from the analysis of the XRT data of these GRBs at times t>10^4 s, it has been shown that the power-law decaying luminosity is powered by the vNS rotational energy loss by the torque acted upon it by its dipole+quadrupole magnetic. From this, it has been inferred that the vNS possesses a magnetic field of strength ~ 10^12 to 10^13 G, and a rotation period of the order of a millisecond (see Figure 3). It is shown that the inferred millisecond rotation period of the vNS agrees with the conservation of angular momentum in the gravitational collapse of the iron core of the CO star which the vNS came from.

The inferred structure of the magnetic field of the “inner engine” agrees with a scenario in which, along the rotational axis of the BH, it is rooted in the magnetosphere left by the NS that collapsed into a BH.

On the equatorial plane, the field is magnified by magnetic flux conservation.

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From Max Planck Institute for Extraterrestrial Physics: “What powers the most powerful explosions in the Universe?”

From Max Planck Institute for Extraterrestrial Physics

October 21, 2019 [Just now in social media]

Dr. J. Michael Burgess
Burgess, J. Michael
Humboldt Research Fellow
+49 (0)89 30000-3842 491736046869
jburgess@mpe.mpg.de

Dr. Jochen Greiner
scientist
+49 (0)89 30000-3847
jcg@mpe.mpg.de

The physical process driving Gamma-Ray Bursts might be synchrotron radiation after all.

A new analysis of Fermi/GBM archival data of gamma-ray bursts (GRB), the most energetic objects in the Universe, has revealed that the process producing this emission might indeed be electrons that are cooled from near-relativistic speeds in a magnetic field. This so-called synchrotron radiation was dismissed in earlier, more indirect analyses. Scientists at the Max Planck Institute for Extraterrestrial Physics were able to fit a high fraction of GRB spectra with an idealized synchrotron model, making a convincing case for this explanation.

NASA/Fermi LAT

NASA/Fermi Gamma Ray Space Telescope

Gamma-ray bursts (GRB) are the most energetic sources in the Universe: in a few seconds, a typical GRB will release more energy than the Sun in its entire lifetime. While there has been some progress identifying the progenitors of various types of GRBs, the physical origin of their emission is still unknown. Synchrotron emission, i.e. radiation emitted by charged particles if their path is bent somehow, was one of the early contenders, but was disregarded as it did not manage to fit some of the properties of the observed GRB spectra. Alternatively, the spectra were fit with other models, e.g. including shocks, but there were always some GRBs that violated certain limits of these models.

The spectral energy distribution of the GRBs analysed in this study. The top graphic shows how synchrotron emission changes with various amounts of cooling, while the inset shows the predictions from previous empirical models from all fitted spectra. These are ranked (top to bottom) according to the median cooling time in the synchrotron model. © MPE

An international team of scientists led by the Max Planck Institute for Extraterrestrial Physics (MPE) revisited the synchrotron idea and has now taken a closer look at archival data of several GRBs observed with the Fermi Gamma-ray Burst Monitor over the past ten years. They selected a subset of GRBs with a known distance (i.e. redshift) and a single continuous, pulse-like structure, which is most likely due to a single physical event. For their sample of nearly 200 observed GRB spectra, the scientists simulated synchrotron emission from cooling electrons and applied the so-called detector response directly. Thus, they could produce mock observations and compare these models directly to the data.

“We wanted to test the simplest synchrotron models that include time-dependent cooling of electrons. The models are idealized, but the best place to start,” explains J. Michael Burgess, first author of the study now published in Nature. “Each spectrum was individually fitted and subjected to rigorous testing leading to a surprisingly high fraction of well-fit spectra using this single spectral model.”

The reason that synchrotron radiation was rejected for a long time is that historically, due to the limited power of computers, researchers used simple tests to see if the observed gamma-ray radiation looked like synchrotron. These tests checked if various shapes similar to a synchrotron (but not the synchrotron radiation itself) resembled the Gamma “rainbow”, i.e. the observed the energy distribution. Many researchers agreed that the observed shapes looked nothing like a synchrotron.

As computers are now faster, and methods for looking at the data from satellites are more advanced, the team was now able to directly simulate how radiation originating from the synchrotron process would be observed and compare all the properties energy distribution to actual data. A critical element of the synchrotron model proved to be a magnetic field, which decelerates the electrons, “cooling” them down from their relativistic energies. The amount of cooling, however, varies across the different GRBs, and in some GRBs the researchers even found evolution of the cooling.

“The ability to model so many GRB spectra at once with a single model is very convincing,” states Jochen Greiner, senior scientist at MPE. “And as we expect the more structured GRB light curves to be a superposition of single pulses, we hope that we can we apply our analysis to all GRBs.” However, as these individual pulses overlap, the scientists will need more advanced predictions about the time-evolution of the emission.

The next step will be to an explanation not only of the shape of the spectra but also of the overall, huge energy output. This means that the dynamics and particle acceleration of moderately magnetized astrophysical outflows will need to be studied in more detail.

In a Gamma-Ray Burst, a massive star collapses to a black hole and sends a jet moving out into space at near the speed of light. In this jet, electrons are accelerated by shock fronts and radiate synchrotron emission as they are cooled by magnetic fields. This radiation can then be observed with telescopes such as the Fermi Gamma-ray Burst Monitor. © MPE

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For their astrophysical research, the MPE scientists measure the radiation of far away objects in different wavelenths areas: from millimetere/sub-millimetre and infared all the way to X-ray and gamma-ray wavelengths. These methods span more than twelve decades of the electromagnetic spectrum.

The research topics pursued at MPE range from the physics of cosmic plasmas and of stars to the physics and chemistry of interstellar matter, from star formation and nucleosynthesis to extragalactic astrophysics and cosmology. The interaction with observers and experimentalists in the institute not only leads to better consolidated efforts but also helps to identify new, promising research areas early on.

The structural development of the institute mainly has been directed by the desire to work on cutting-edge experimental, astrophysical topics using instruments developed in-house. This includes individual detectors, spectrometers and cameras but also telescopes and integrated, complete payloads. Therefore the engineering and workshop areas are especially important for the close interlink between scientific and technical aspects.

The scientific work is done in four major research areas that are supervised by one of the directors:

Center for Astrochemical Studies (CAS)
Director: P. Caselli

High-Energy Astrophysics
Director: P. Nandra

Infrared/Submillimeter Astronomy
Director: R. Genzel

Optical & Interpretative Astronomy
Director: R. Bender

Within these areas scientists lead individual experiments and research projects organised in about 25 project teams.

The Max Planck Society is Germany’s most successful research organization. Since its establishment in 1948, no fewer than 18 Nobel laureates have emerged from the ranks of its scientists, putting it on a par with the best and most prestigious research institutions worldwide. The more than 15,000 publications each year in internationally renowned scientific journals are proof of the outstanding research work conducted at Max Planck Institutes – and many of those articles are among the most-cited publications in the relevant field.

What is the basis of this success? The scientific attractiveness of the Max Planck Society is based on its understanding of research: Max Planck Institutes are built up solely around the world’s leading researchers. They themselves define their research subjects and are given the best working conditions, as well as free reign in selecting their staff. This is the core of the Harnack principle, which dates back to Adolph von Harnack, the first president of the Kaiser Wilhelm Society, which was established in 1911. This principle has been successfully applied for nearly one hundred years. The Max Planck Society continues the tradition of its predecessor institution with this structural principle of the person-centered research organization.

The currently 83 Max Planck Institutes and facilities conduct basic research in the service of the general public in the natural sciences, life sciences, social sciences, and the humanities. Max Planck Institutes focus on research fields that are particularly innovative, or that are especially demanding in terms of funding or time requirements. And their research spectrum is continually evolving: new institutes are established to find answers to seminal, forward-looking scientific questions, while others are closed when, for example, their research field has been widely established at universities. This continuous renewal preserves the scope the Max Planck Society needs to react quickly to pioneering scientific developments.

From NASA/ESA Hubble Telescope: “Hubble Studies Gamma-Ray Burst with the Highest Energy Ever Seen”

NASA/ESA Hubble Telescope

From NASA/ESA Hubble Telescope

November 20, 2019

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu

Andrew Levan
Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, The Netherlands
+44 7714250373
a.levan@astro.ru.nl

NASA, ESA M. Kornmesser

Mega-Blast From The Past Came From Distant Galaxy.

NASA’s Hubble Space Telescope has given astronomers a peek at the location of the most energetic outburst ever seen in the universe—a blast of gamma-rays a trillion times more powerful than visible light. That’s because in a few seconds the gamma-ray burst (GRB) emitted more energy than the Sun will provide over its entire 10-billion year life.

In January 2019, an extremely bright and long-duration GRB was detected by a suite of telescopes, including NASA’s Swift and Fermi telescopes, as well as by the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes on the Canary islands. Follow-up observations were made with Hubble to study the environment around the GRB and find out how this extreme emission is produced.

NASA Neil Gehrels Swift Observatory

NASA/Fermi Gamma Ray Space Telescope

MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia, La Palma, Spain)), Altitude 2,396 m (7,861 ft)

“Hubble’s observations suggest that this particular burst was sitting in a very dense environment, right in the middle of a bright galaxy 5 billion light years away. This is really unusual, and suggests that this concentrated location might be why it produced this exceptionally powerful light,” explained one of the lead authors, Andrew Levan of the Institute for Mathematics, Astrophysics and Particle Physics Department of Astrophysics at Radboud University in the Netherlands.

“Scientists have been trying to observe very high energy emission from gamma-ray bursts for a long time,” explained lead author Antonio de Ugarte Postigo of the Instituto de Astrofísica de Andalucía in Spain. “This new Hubble observation of accompanying lower-energy radiation from the region is a vital step in our understanding of gamma-ray bursts [and] their immediate surroundings.”

The complementary Hubble observations reveal that the GRB occurred within the central region of a massive galaxy. Researchers say that this is a denser environment than typically observed (for GRBs) and could have been crucial for the generation of the very-high-energy radiation that was observed. The host galaxy of the GRB is actually one of a pair of colliding galaxies. The galaxy interactions may have contributed to spawning the outburst.

Known as GRB 190114C, some of the radiation detected from the object had the highest energy ever observed. Scientists have been trying to observe such very high energy emission from GRBs for a long time, so this detection is considered a milestone in high-energy astrophysics, say researchers.

Previous observations revealed that to achieve this energy, material must be emitted from a collapsing star at 99.999% the speed of light. This material is then forced through the gas that surrounds the star, causing a shock that creates the gamma-ray burst itself.

Science paper:
Nature

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The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

From Science Alert: “Faster-Than-Light Speeds Could Be Why Gamma-Ray Bursts Seem to Go Backwards in Time”

From Science Alert

30 SEP 2019
MICHELLE STARR

Artist’s impression of a relativistic jet. (DESY, Science Communication Lab)

Time, as far as we know, moves only in one direction. But last year, researchers found events in some gamma-ray burst pulses that seemed to repeat themselves as though they were going backwards in time.

Now, new research suggests a potential answer for what might be causing this time reversibility effect. If waves within the relativistic jets that produce gamma-ray bursts travel faster than light – at ‘superluminal’ speeds – one of the effects could be time reversibility.

Such speeding waves could actually be possible. We know that when light is travelling through a medium (such as gas or plasma), its phase velocity is slightly slower than c – the speed of light in a vacuum, and, as far as we know, the ultimate speed limit of the Universe.

Therefore, a wave could travel through a gamma-ray burst jet at superluminal speeds without breaking relativity. But to understand this, we need to back up a little to look at the source of those jets.

Gamma-ray bursts are the most energetic explosions in the Universe. They can last from a few milliseconds to several hours [The Astrophysical Journal], they’re extraordinarily bright, and we don’t yet have a comprehensive list of what causes them.

We know from the 2017 observations [Physical Review Letters] of colliding neutron stars that these smash-ups can create gamma-ray bursts. Astronomers also think such bursts are produced when a massive, rapidly spinning star collapses into a black hole, violently ejecting material into the surrounding space in a colossal hypernova.

That black hole is then surrounded by a cloud of accretion material around its equator; if it’s rotating quickly enough, the fallback of the initially exploded material will result in relativistic jets shooting from the polar regions, blasting through the outer envelope of the progenitor star before producing gamma-ray bursts.

Now, back to those waves travelling faster than light.

We know that, when travelling through a medium, particles can move faster than light does. This phenomenon is responsible for the famous Čerenkov radiation, often seen as a distinctive blue glow.

Čerenkov radiation

That glow – a ‘luminal boom’ – is produced when charged particles such as electrons move faster through water than the phase velocity of light.

Astrophysicists Jon Hakkila of the College of Charleston and Robert Nemiroff of the Michigan Technological University believe that this same effect can be observed in gamma-ray burst jets, and have conducted mathematical modelling to demonstrate how.

“In this model an impactor wave in an expanding gamma-ray burst jet accelerates from subluminal to superluminal velocities, or decelerates from superluminal to subluminal velocities,” they write in their paper [The Astrophysical Journal].

“The impactor wave interacts with the surrounding medium to produce Čerenkov and/or other collisional radiation when travelling faster than the speed of light in this medium, and other mechanisms (such as thermalised Compton or synchrotron shock radiation) when travelling slower than the speed of light.

“These transitions create both a time-forward and a time-reversed set of [gamma-ray burst] light curve features through the process of relativistic image doubling.”

Such relativistic image doubling is thought to occur in Čerenkov detectors. When a charged particle travelling at near light-speed enters water, it moves faster than the Čerenkov radiation it produces, and therefore can hypothetically appear to be in two places at once: one image appearing to move forward in time and the other appearing to move backwards.

Mind you, this doubling has not yet been experimentally observed. But if it does occur, it could also be responsible for producing the time-reversibility seen in gamma-ray burst light curves, occurring both when the impactor wave travelling through the jet medium accelerates to speeds faster than light, and decelerates to subluminal speeds.

More work is needed, of course. The researchers assumed that the impactor responsible for creating a gamma-ray burst would be a large-scale wave produced by changes in, say, density, or the magnetic field. That will need further analysis. And if the plasmas involved aren’t transparent to superluminal radiation, all bets are off.

However, the researchers said, their model provides better explanations for the characteristics of gamma-ray burst light curves than models that don’t include time reversibility.

“Standard gamma-ray burst models have neglected time-reversible light curve properties,” Hakkila said. “Superluminal jet motion accounts for these properties while retaining a great many standard model features.”

See the full article here .

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