From Niels Bohr Institute: “Catching a glimpse of the gamma-ray burst engine”

University of Copenhagen

Niels Bohr Institute bloc

From Niels Bohr Institute

16 January 2019

A gamma-ray burst registered in December of 2017 turns out to be “one of the closets GRBs ever observed”. The discovery is featured in Nature [co-authors are: Jonathan Selsing, Johan Fynbo, Jens Hjorth and Daniele Malesani from the Niels Bohr Institute, Giorgos Leloudas from the Technical University of Denmark and Kasper Heintz from University of Iceland] – and it has yielded valuable information about the formation of the most luminous phenomenon in the universe. Scientists from the Niels Bohr Institute at the University of Copenhagen helped carrying out the analysis.

Jonatan Selsing frequently receives text messages from a certain sender regarding events in space. It happens all around the clock, and when his cell phone goes ‘beep’ he knows that yet another gamma-ray burst (GRB) notification has arrived. Which, routinely, raises the question: Does this information – originating from the death of a massive star way back, millions if not billions of years ago – merit further investigation?

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The development in a dying star until the gamma ray burst forms. Attribution: National Science Foundation

Gamma ray bursts – bright signals from space

“GRBs represent the brightest phenomenon known to science – the luminous intensity of a single GRB may in fact exceed that of all stars combined! And at the same time GRBs – which typically last just a couple of seconds – represent one of the best sources available, when it comes to gleaning information about the initial stages of our universe”, explains Jonatan Selsing.

He is astronomer and postdoc at Cosmic Dawn Center at the Niels Bohr Institute in Copenhagen. And he is one of roughly 100 astronomers in a global network set up to ensure that all observational resources needed can be instantaneously mobilized when the GRB-alarm goes off.

Quick action must be taken when a gamma ray burst is registered

The alarm sits on board the international Swift-telescope which was launched in 2004 – and has orbited Earth ever since with the mission of registering GRBs.

NASA Neil Gehrels Swift Observatory

Swift is capable of constantly observing one third of the night sky, and when the telescope registers a GRB – which on average happens a couple of times per week – it will immediately text the 100 astronomers. The message will tell where in space the GRB has been observed – whereupon the astronomer on duty must make a here-and-now decision:

Is there reason to assume that this specific GRB is of such importance that we should ask the VLT-telescope in Chile to immediately take a closer look at it?

ESO VLT 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,

Or should we consider the information from Swift sheer routine, and leave it at that?

On December 5th 2017 – just around 09 o’clock in the morning Copenhagen time – the GRB-alarm went off. Luca Izzo, Italian astronomer, was on duty – and Izzo did not harbor the slightest doubt: He right away alerted VLT – the Very Large Telescope in Chile – which is run by 11 European countries, including Germany, Great Britain, Italy, France, Sweden and Denmark.

At that time it was early in the morning in Chile – 05 o’clock – and dawn was rapidly approaching, tells Jonatan Selsing: “For VLT to take a closer look at the GRB, action had to be taken immediately – since the telescope is only capable of working against a background of the night sky. And fortunately this was exactly what happened, when Izzo contacted VLT”.

This is also why Luca Izzo is listed as first author of the scientific article describing this GRB – an article which has just been published in Nature, one of the world’s most influential scientific journals. The article is based on analyses of the VLT-recordings, and the recordings reveal that this GRB in more than one respect can be described as unusual, says Jonatan Selsing:

“Not least because this is one of the closest GRBs ever observed. GRB171205A – which has since become the official name of this gamma-ray burst – originated a mere 500 million years ago, and has ever since traveled through space at the speed of light, i.e. at 300.000 kilometer per second”. Working closely with a number of his colleagues at the Niels Bohr Institute, Jonatan Selsing contributed to the Nature-article with an analysis which – put simply – represents “a glimpse” of the very engine behind a gamma-ray burst.

Gamma ray bursts are the results of violent events in space

When a massive star – rotating at very high speed – dies, its core may collapse, thus creating a so-called black hole.

This computer-simulated image of a supermassive black hole at the core of a galaxy. Credit NASA, ESA, and D. Coe, J. Anderson

A massive star may weigh up to 300 times more than the Sun, and due to combustion the star is transforming light elements to heavier elements. This process, which takes place in the core, is the source of energy not only in massive stars, but in all stars.

Ashes – the by-product of combustion – may over time become such a heavy load that a massive star can no longer carry its own weight, which is why it finally collapses. When that happens, the outer layers will gradually fall towards the core – towards the black hole – at which point a disc is formed.

Due to the star’s rotation, the disc will function as a dynamo creating a gigantic magnetic field – which will emit two jets, both going away from the black hole at a velocity close to the speed of light. During this process, the dying star is also releasing – spewing – matter, which lightens up with extreme intensity.
This light is the very gamma-ray burst – the GRB itself. And the matter which is released from the center of the star is set free in the form of a so-called jet cocoon.

The gamma ray burst confirms our assumptions about the elements stars produce

“One of the unique features of GRB171205A is that it proved possible to determine which elements this gamma-ray burst released via the jet cocoon 500 million years ago. That was measured here at the Niels Bohr Institute, and that is our contribution to the Nature-article. These measurements were carried out via X-shooter – an extremely sensitive piece of equipment mounted on the VLT-telescope”, says Jonatan Selsing.

X-shooter analyzed the VLT-footage of the gamma-ray burst – and this analysis led to the conclusion that the jet cocoon from GRB171205A contained iron, cobalt and nickel which had formed in the center of the star, explains Jonatan Selsing:

“This corresponds with our theoretical expectations – and therefore also corroborates our model for a star-collapse of this magnitude. Being able to establish that it actually did happen in this way is, however, really special. That’s when you get a glimpse of the very engine behind a gamma-ray burst”.

ESO X-shooter on VLT on UT2 at Cerro Paranal, Chile


ESO X-shooter on VLT on UT2 at Cerro Paranal, Chile

See the full article here .


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Niels Bohr Institute Campus

The Niels Bohr Institute (Danish: Niels Bohr Institutet) is a research institute of the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

The Institute was founded in 1921, as the Institute for Theoretical Physics of the University of Copenhagen, by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institute.[1] Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.[2]

During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institute.

The University of Copenhagen (UCPH) (Danish: Københavns Universitet) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.

The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge, Yale University, The Australian National University, and UC Berkeley, amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient

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From European Space Agency: “Team of telescopes finds X-ray engine inside mysterious supernova”

ESA Space For Europe Banner

From European Space Agency

10 January 2019

Raffaella Margutti
Department of Physics and Astronomy
Northwestern University
Evanston, IL, USA
Email: raffaella.margutti@northwestern.edu

Indrek Vurm
Tartu Observatory
University of Tartu, Estonia
Email: indrek.vurm@ut.ee

Volodymyr Savchenko
Department of Astronomy
University of Geneva, Switzerland
Email: Volodymyr.Savchenko@unige.ch

Carlo Ferrigno
Department of Astronomy
University of Geneva, Switzerland
Email: Carlo.Ferrigno@unige.ch

Giulia Migliori
INAF–Institute of Radioastronomy
University of Bologna, Italy
Email: g.migliori@ira.inaf.it

Erik Kuulkers
ESA Integral Project Scientist
European Space Agency
Email: ekuulker@sciops.esa.int

Norbert Schartel
ESA XMM-Newton Project Scientist
European Space Agency
Email: norbert.schartel@sciops.esa.int

Markus Bauer








ESA Science Communication Officer









Tel: +31 71 565 6799









Mob: +31 61 594 3 954









Email: markus.bauer@esa.int

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An image of supernova explosion AT2018cow and its host galaxy, CGCG 137-068, which is located some 200 million light years away. The image was obtained on 17 August 2018 using the DEep Imaging and Multi-Object Spectrograph (DEIMOS) on the W. M. Keck Observatory in Hawaii.

Keck/DEIMOS on Keck 2


Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level, showing also NASA’s IRTF and NAOJ Subaru

Credit: R. Margutti/W. M. Keck Observatory

The supernova was first spotted on 16 June 2018 with the ATLAS telescope, also in Hawaii. Further observations performed with a large team of telescopes – including ESA’s high-energy space telescopes Integral and XMM-Newton – revealed a source of powerful X-rays at the centre of this unprecedentedly bright and rapidly evolving stellar explosion, suggesting that it could either be a nascent black hole or neutron star with a powerful magnetic field, sucking in the surrounding material. Credit: R. Margutti/W. M. Keck Observatory

ESA’s high-energy space telescopes Integral and XMM-Newton have helped to find a source of powerful X-rays at the centre of an unprecedentedly bright and rapidly evolving stellar explosion that suddenly appeared in the sky earlier this year.

ESA/XMM Newton

ESA/Integral

The ATLAS telescope in Hawaii first spotted the phenomenon, since then named AT2018cow, on 16 June.

ATLAS is an asteroid impact early warning system of two telescopes being developed by the University of Hawaii and funded by NASA


ATLAS telescope, First Asteroid Terrestrial-impact Last Alert system (ATLAS) fully operational 8/15/15 Haleakala , Hawaii, USA, Altitude 4,205 m (13,796 ft)

They soon realised this was something completely new. In only two days the object exceeded the brightness of any previously observed supernova – a powerful explosion of an aging massive star that expels most of its material into the surrounding space, sweeping up the interstellar dust and gases in its vicinity.

A new paper, accepted for publication in The Astrophysical Journal, presents the observations from the first 100 days of the object’s existence, covering the entire electromagnetic spectrum of the explosion from radio waves to gamma rays.

The analysis, which includes observations from ESA’s Integral and XMM-Newton, as well as NASA’s NuSTAR and Swift space telescopes, found a source of high-energy X-rays sitting deep inside the explosion.

NASA NuSTAR X-ray telescope

NASA Neil Gehrels Swift Observatory

The behaviour of this source, or engine, as revealed in the data, suggests that the strange phenomenon could either be a nascent black hole or neutron star with a powerful magnetic field, sucking in the surrounding material.

“The most exciting interpretation is that we might have seen for the first time the birth of a black hole or a neutron star,” says Raffaella Margutti of Northwestern University, USA, lead author of the paper.

“We know that black holes and neutron stars form when stars collapse and explode as a supernova, but never before have we seen one right at the time of birth,” adds co-author Indrek Vurm of Tartu Observatory, Estonia, who worked on modelling the observations.

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An image of supernova explosion AT2018cow and its host galaxy, CGCG 137-068, which is located some 200 million light years away. The image was obtained on 17 August 2018 using the DEep Imaging and Multi-Object Spectrograph (DEIMOS) on the W. M. Keck Observatory in Hawaii. The insert in the top left shows a zoom onto the galaxy, indicating the location of the supernova. The supernova was first spotted on 16 June 2018 with the ATLAS telescope, also in Hawaii. Further observations performed with a large team of telescopes – including ESA’s high-energy space telescopes Integral and XMM-Newton – revealed a source of powerful X-rays at the centre of this unprecedentedly bright and rapidly evolving stellar explosion, suggesting that it could either be a nascent black hole or neutron star with a powerful magnetic field, sucking in the surrounding material. Credit: R. Margutti/W. M. Keck Observatory

The AT2018cow explosion was not only 10 to 100 times brighter than any other supernova previously observed: it also reached peak luminosity much faster than any other previously known event – in only a few days compared to the usual two weeks.

Integral made its first observations of the phenomenon about five days after it had been reported and kept monitoring it for 17 days. Its data proved crucial for the understanding of the strange object.

“Integral covers a wavelength range which is not covered by any other satellite,” says Erik Kuulkers, Integral project scientist at ESA. “We have a certain overlap with NuSTAR in the high-energy X-ray part of the spectrum but we can see higher energies, too.”

So while data from NuSTAR revealed the hard X-ray spectrum in great detail, with Integral the astronomers were able to see the spectrum of the source entirely, including its upper limit at soft gamma-ray energies.

“We saw a kind of a bump with a sharp cut-off in the spectrum at the high-energy end,” says Volodymyr Savchenko, an astronomer at the University of Geneva, Switzerland, who worked on the Integral data. “This bump is an additional component of the radiation released by this explosion, shining through an opaque, or optically thick, medium.”

“This high-energy radiation most likely came from an area of very hot and dense plasma surrounding the source,” adds Carlo Ferrigno, also of the University of Geneva.

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The evolution of supernova explosion AT2018cow as observed at soft X-rays with NASA’s Swift (red circles) and ESA’s XMM-Newton (red triangles) space observatories, and at hard X-rays with NASA’s NuSTAR (orange circles) and ESA’s INTEGRAL (yellow circles) satellites. The supernova was first spotted on 16 June 2018 with the ATLAS telescope in Hawaii. The data shown in this animation were collected between 22 June and 22 July. These observations revealed a source of powerful X-rays at the centre of this unprecedentedly bright and rapidly evolving stellar explosion, suggesting that it could either be a nascent black hole or neutron star with a powerful magnetic field, sucking in the surrounding material. Credit: R. Margutti et al (2019)

Because Integral kept monitoring the AT2018cow explosion over a longer period of time, its data was also able to show that the high-energy X-ray signal was gradually fading.

Raffaella explains that this high-energy X-ray radiation that went away was the so-called reprocessed radiation – radiation from the source interacting with material ejected by the explosion. As the material travels away from the centre of the explosion, the signal gradually wanes and eventually disappears completely.

In this signal, however, the astronomers were able to find patterns typical of an object that draws in matter from its surroundings – either a black hole or a neutron star.

“This is the most unusual thing that we have observed in AT2018cow and it’s definitely something unprecedented in the world of explosive transient astronomical events,” says Raffaella.

Meanwhile, XMM-Newton looked at this unusual explosion twice over the first 100 days of its existence. It detected the lower-energy part of its X-ray emission, which, according to the astronomers, comes directly from the engine at the core of the explosion. Unlike the high-energy X-rays coming from the surrounding plasma, the lower-energy X-rays from the source are still visible.

The astronomers plan to use XMM-Newton to perform a follow-up observation in the future, which will allow them to understand the source’s behaviour over a longer period of time in greater detail.

“We are continuing to analyse the XMM-Newton data to try to understand the nature of the source,” says co-author Giulia Migliori of University of Bologna, Italy, who worked on the X-ray data. “Accreting black holes leave characteristic imprints in X-rays, which we might be able to detect in our data.”

“This event was completely unexpected and it shows that there is a lot of which we don’t completely understand,” says Norbert Schartel, ESA’s XMM-Newton project scientist. “One satellite, one instrument alone, would never be able to understand such a complex object. The detailed insights we were able to gather into the inner workings of the mysterious AT2018cow explosion were only achievable thanks to the broad cooperation and combination of many telescopes.”

See the full article here .


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The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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From The Conversation: “Swift’s telescope reveals birth, deaths and collisions of stars through 1 million snapshots in UV”

Conversation
From The Conversation

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Imagine if the color camera had never been invented and all our images were in black and white. The world would still look beautiful, but incomplete. For thousands of years, that was how humans saw the universe. On Earth, we can only see part of the light that stars emit.

Much of what we can’t see – in the infrared, the ultraviolet, the X-ray and the gamma ray wavelengths – is blocked by the Earth’s atmosphere. For the most part, this is a good thing. The atmosphere traps infrared light keeping the Earth warm at night and blocks high-energy ultraviolet light, X-rays and gamma rays, keeping us safe from deadly cosmic radiation, while letting in visible portions of the spectrum of light. For astronomers, however, this has a drawback: We look at the universe with one eye shut, unable to receive all of the information the universe is sending to us.

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Visible light is just a tiny part of the electromagnetic spectrum. NASA

Launched on November 20, 2004, and orbiting an altitude of 340 miles, NASA’s Neil Gehrels Swift Observatory has three telescopes that monitor the universe using wavelengths of light that are blocked by Earth’s atmosphere.

NASA Neil Gehrels Swift Observatory

These included the X-Ray Telescope, the gamma-ray-sensitive Burst-Alert Telescope and the Ultraviolet Optical Telescope (UVOT). The UVOT recently delivered its 1 millionth image – data that astrophysicists like me use to gain insights into everything from the origins of the universe to the chemical composition of nearby comets.

Watching the birth of black holes

Swift’s primary mission is to study the afterglow of gamma ray bursts (GRBs) – which document the birth of black holes. Black holes are forged in the most violent explosions in the universe – the explosion of a massive star or the merging of two neutron stars (the shriveled husks left over from past stellar explosions). These explosions are so powerful – producing tens to hundreds of billions of times more energy than the sun – that even though they occur billions of light years away from Earth, they can still be detected by instruments like Swift. In fact, the first GRBs were detected by the Vela satellites, which were built to detect the explosions of nuclear weapons.

Over nearly 14 years, Swift has studied over a thousand GRBs. In doing so, it has revealed what powers them and given us glimpses into the furthest reaches of the cosmos, to the time when the first stars were being formed after the Big Bang.

However, one of the things you learn working on a space telescope mission is that if you build it, they will come. The mission provides capabilities to the community of astrophysicists – simultaneous X-ray/UV imaging and a rapid response to requests to observe and photograph specific sections of the sky – which are only available to Swift. We can focus our telescopes on an object of interest within hours of a “Target of Opportunity” request through our website, something no other mission can do. UVOT also fills an important niche by observing larger areas of the sky than can be observed with the more powerful UV instruments aboard the Hubble Space Telescope. These capabilities have proved a boon to the community and enabled study all sorts of objects and phenomenon beyond GRBs.

Swift’s ultraviolet-aided discoveries

Nearby galaxies are full of activity with new stars being formed. Swift is able to capture panoramic ultraviolet images that highlight the youngest, most massive stars in these galaxies. This gives us insight into what the universe has been doing over the last few hundred million years. My research team’s work has focused on nearby galaxies – like Andromeda and the Magellanic Clouds – to reveal what processes drive their past and ongoing star formation.

Andromeda Galaxy Messier 31 with Messier32 -a satellite galaxy copyright Terry Hancock.

Magellanic Clouds ESO S. Brunier

With UVOT, we get a much better view of supernova explosions. These can occur when a white dwarf, the remnant of a star like the sun, explodes, or during the final death throes of a massive star, more than eight times the mass of the sun. These events generate enormous amounts of ultraviolet light, and Swift has a unique ability to observe them within hours of discovery.

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On the left is an ultraviolet composite made from several images of the Whirpool Galaxy (M51) taken between 2005-2007. The image on the right was made in June 2011, shortly after astronomers detected the explosion of a massive star in one of the galaxy’s outer spiral arms. The object is marked by the red circle. NASA/Swift/E. Hoversten, PSU, CC BY-ND

Comets sweep through our solar system, transforming from a frozen solid ball to a vapor as they approach the sun and creating magnificent tails of ionized particles. Swift studies these comets, and analyzes their chemical composition by breaking the light they emit into different wavelengths. Swift also allows scientists to measure a comet’s rotation by seeing how the light changes over time. This has revealed that violent eruptions on the comet surface can dramatically alter a comet’s path.

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This image of Comet Lulin was taken by Swift on January 28, 2009. It shows data obtained by Swift’s Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). The image of the star field (white) was acquired by the Digital Sky Survey. At the time of the observation, comet Lulin was 99.5 million miles from Earth and 115.3 million miles from the sun. The ultraviolet light comes from hydroxyl molecules and shows that, at this time, Lulin was shedding 800 gallons of water every second. D. Bodewits/Swift/NASA, CC BY-ND

One of the most exciting discoveries that Swift made was connected with the recent discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO).


VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation


Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

ESA/eLISA the future of gravitational wave research

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Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

Gravitational waves are distortions in the fabric of spacetime created by the motions of extremely massive objects. In August of 2017, two neutrons stars collided in a distant galaxy, creating gravitational waves powerful enough to be detected on Earth. Swift was one of an army of telescopes that looked for the source of the gravitational waves. The mad scramble over those few days led to one of the most exciting discoveries of the last decade – a luminous afterglow from the source of the gravitational waves. This has opened up new branches of science by connecting a new way of studying the universe – through gravitational waves – to the traditional way – through light.

See also https://sciencesprings.wordpress.com/2017/10/16/from-ucsc-a-uc-santa-cruz-special-report-neutron-stars-gravitational-waves-and-all-the-gold-in-the-universe/

UVOT has been taking snapshots of the universe since 2004 and finally piled up its millionth image. Its success is a testament to the international team of engineers, scientists and staff at the three institutions that support it – the Pennsylvania State University; Mullard Space Science Laboratory in Surrey, England; and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. It has been my privilege to be a part of this team for the last nine years. What does the future hold for UVOT? We hope to find more sources of gravitational waves, survey nearby galaxies, study even more supernovae, and monitor how objects in the universe change over time.

Here’s to the next million images.

See the full article here .

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From Orbiter.ch: “NASA’s Newly Renamed Swift Mission Spies a Comet Slowdown”

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Orbiter.ch

1.10.18

NASA Neil Gehrels Swift Observatory

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NASA – Swift Mission patch.

Observations by NASA’s Swift spacecraft, now renamed the Neil Gehrels Swift Observatory after the mission’s late principal investigator, have captured an unprecedented change in the rotation of a comet. Images taken in May 2017 reveal that comet 41P/Tuttle-Giacobini-Kresák — 41P for short — was spinning three times slower than it was in March, when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.

The abrupt slowdown is the most dramatic change in a comet’s rotation ever seen.


Swift Mission Catches a Comet Slowdown. NASA’s Swift satellite detected an unprecedented slowdown in the rotation of comet 41P/Tuttle-Giacobini-Kresák when it passed nearest to Earth in early 2017. Watch to learn more. Video Credits: NASA’s Goddard Space Flight Center.

“The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days,” said Dennis Bodewits, an associate research scientist at the University of Maryland (UMD) in College Park who presented the findings Wednesday, Jan. 10, at the American Astronomical Society (AAS) meeting in Washington. “By contrast, 41P spun down by more than 10 times as much in just 60 days, so both the extent and the rate of this change is something we’ve never seen before.”

The comet orbits the Sun every 5.4 years, traveling only about as far out as the planet Jupiter, whose gravitational influence is thought to have captured it into its present path. Estimated to be less than 0.9 mile (1.4 kilometers) across, 41P is among the smallest of the family of comets whose orbits are controlled by Jupiter. This small size helps explain how jets on the surface of 41P were able to produce such a dramatic spindown.

As a comet nears the Sun, increased heating causes its surface ice to change directly to a gas, producing jets that launch dust particles and icy grains into space. This material forms an extended atmosphere, called a coma. Water in the coma quickly breaks up into hydrogen atoms and hydroxyl molecules when exposed to ultraviolet sunlight. Because Swift’s Ultraviolet/Optical Telescope (UVOT) is sensitive to UV light emitted by hydroxyl, it is ideally suited for measuring how comet activity levels evolve throughout the orbit.

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Image above: On March 14, 2017, two weeks before its closest approach to Earth, comet 41P/Tuttle-Giacobini-Kresák glides beneath the galaxy NGC 3198. The green glow comes from light emitted by diatomic carbon molecules. Image Credits: Copyright 2017 by Chis Schur, used with permission.

Ground-based observations established the comet’s initial rotational period at about 20 hours in early March 2017 and detected its slowdown later the same month. The comet passed 13.2 million miles (21.2 million km) from Earth on April 1, and eight days later made its closest approach to the Sun. Swift’s UVOT imaged the comet from May 7 to 9, revealing light variations associated with material recently ejected into the coma. These slow changes indicated 41P’s rotation period had more than doubled, to between 46 and 60 hours.

UVOT-based estimates of 41P’s water production, coupled with the body’s small size, suggest that more than half of its surface area contains sunlight-activated jets. That’s a far greater fraction of active real estate than on most comets, which typically support jets over only about 3 percent of their surfaces.

“We suspect that the jets from the active areas are oriented in a favorable way to produce the torques that slowed 41P’s spin,” said Tony Farnham, a principal research scientist at UMD. “If the torques continued acting after the May observations, 41P’s rotation period could have slowed to 100 hours or more by now.”

Such a slow spin could make the comet’s rotation unstable, allowing it to begin tumbling with no fixed rotational axis. This would produce a dramatic change in the comet’s seasonal heating. Bodewits and his colleagues note that extrapolating backward suggests the comet was spinning much faster in the past, possibly fast enough to induce landslides or partial fragmentation and exposing fresh ice. Strong outbursts of activity in 1973 and 2001 may be related to 41P’s rotational changes.

A less extreme relationship between a comet’s shape, activity and spin was previously seen by the European Space Agency’s Rosetta mission, which entered orbit around comet 67P/Churyumov-Gerasimenko in 2014. The comet’s spin sped up by two minutes as it approached the Sun, and then slowed by 20 minutes as it moved farther away. As with 41P, scientists think these changes were produced by the interplay between the comet’s shape and the location and activity of its jets.

A paper detailing these findings will be published in the journal Nature on Jan. 11.

NASA’s Swift spacecraft has conducted a broad array of science investigations for 13 years — monitoring comets, studying stars hosting exoplanets, and catching outbursts from supernovas, neutron stars and black holes — and it continues to be fully operational. NASA announced at the AAS meeting that the mission has now been renamed in honor of Neil Gehrels, who helped develop Swift and served as its principal investigator until his death on Feb. 6, 2017.

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Neil Gehrels

Neil Gehrels Maniac Lecture, September 29, 2015 [1 hour]

Video above: Neil Gehrels talks about his adventures in astrophysics in this talk given at NASA’s Goddard Space Flight Center in 2015. Video Credits: NASA’s Goddard Space Flight Center Library.

Swift’s rapid scheduling capability, plus a trio of telescopes covering optical to gamma-ray wavelengths, continues to deliver important contributions in the study of gamma-ray bursts — the most powerful explosions in the universe — while maintaining a critical role in monitoring how astronomical objects as diverse as comets, stars and galaxies change over time.

“The Neil Gehrels Swift Observatory is a name that reflects Swift’s current status as the go-to facility for rapid-response, multiwavelength follow-up of time-variable sources,” said Paul Hertz, director of NASA’s Astrophysics Division in Headquarters, Washington. “With Swift, Neil helped usher in the era of time-domain astronomy. He would have been very excited about today’s discovery.”

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Image above: NASA’s Swift spacecraft, now renamed the Neil Gehrels Swift Observatory after the mission’s late principal investigator, has become the go-to facility for rapid-response, multiwavelength follow-up of time-variable sources. This illustration highlights the diversity of Swift’s work, which ranges from comets in our solar system to observations of variable sources in our galaxy and beyond. Image Credits: NASA’s Goddard Space Flight Center.

“Swift is still going strong, and we continue to receive four urgent ‘target-of-opportunity’ observing requests from the broader astronomical community each day,” said S. Bradley Cenko, who was recently appointed as the mission’s principal investigator. “Neil’s leadership and vision continue to guide the project, and we can think of no better way to honor this legacy than with the new name.”

Prof. Neil Gehrels (Future Time Dimension)

Video above: The Dan David Prize compiled this video tribute to Goddard’s Neil Gehrels, who was posthumously named a 2017 laureate. Video Credits: Dan David Prize.

Goddard manages the Swift mission in collaboration with Penn State in University Park, the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy.

Swift Mission Overview

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