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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From AAS NOVA: “Compact Objects Charging Toward Merger”

From AAS NOVA

19 April 2019
Susanna Kohler

Artist’s illustration showing two inspiralling neutron stars shortly before they merge. Could electric charge play a role in the radiation we see from compact-binary mergers? [Goddard Media Studios/NASA]

When two compact objects — neutron stars or black holes — merge, will they emit light? A recent study looks at a neglected factor that could affect the answer: electric charge.

Dark or Light?

Gamma-ray burst credit NASA SWIFT Cruz Dewilde

Most theories agree that a compact binary containing a neutron star can emit light when it merges. This is because these systems contain lots of neutron-rich matter that can then radiate in the final stages of merger, in the form of gamma-ray bursts, kilonovae, and afterglows.

But what about compact binaries containing two black holes? Or so-called “plunging” black-hole–neutron-star mergers in which the neutron star plunges directly into the black hole before it can be disrupted? Are these mergers all doomed to darkness?

Possible Charge

Not according to Bing Zhang, a scientist at University of Nevada Las Vegas. Recently, Zhang proposed [The Astrophysical Journal Letters] that black holes might carry electric charge in a surrounding magnetosphere. As charged black holes spiral around and around each other during a merger, they could generate electromagnetic radiation: a characteristic signal that rises sharply just before merger.

Now Zhang is back with a generalized model for the merger of charged compact objects, which also explores possible signatures from electrically charged neutron stars. In a new study, he works out the details and reports on where we might be able to detect these signals.

Searching for a Signal

All compact binaries containing a neutron star should emit radiation from electric charge, since neutron stars are definitely charged — they’re essentially spinning magnets. But for most systems containing a neutron star, Zhang demonstrates, the radiation associated with the object’s charge will be non-detectable, since it’s so much dimmer than other electromagnetic signatures from merger (like a gamma-ray burst).

The Crab pulsar is a highly magnetized, spinning neutron star that powers the Crab nebula seen in this composite image. [X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz]

There’s hope, though, in the scenario of a plunging neutron-star–black-hole merger. If the neutron star is less than 20% the size of the black hole, it can be consumed whole, preventing any of the typical electromagnetic signatures from occurring. In this case, the radiation from the charged, inspiralling neutron star is the only electromagnetic signal present.

If the neutron star in such a system has a magnetic field similar to that of the Crab pulsar — possible in young star clusters — the charge signal can reach detectable levels, according to Zhang’s calculations. In fact, it’s possible that we could observe such a signal as a fast radio burst, the mysterious millisecond radio bursts that we’ve seen originating from beyond our galaxy.

Looking Ahead

Many unknowns are still present in this picture. How is the electric radiation converted into observable emission? How commonly do we expect plunging neutron-star–black-hole mergers to occur as described? Will we be able to link radiation from charged mergers to a gravitational-wave chirp?

One thing is for certain: if we can, indeed, observe the light from charge in a compact-binary merger, this would provide an exciting new opportunity to further probe these distant, exotic systems.

Citation

“Charged Compact Binary Coalescence Signal and Electromagnetic Counterpart of Plunging Black Hole–Neutron Star Mergers,” Bing Zhang 2019 ApJL 873 L9.
https://iopscience.iop.org/article/10.3847/2041-8213/ab0ae8/meta

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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From Harvard-Smithsonian Center for Astrophysics: “What Ionized the Universe?”

From Harvard-Smithsonian Center for Astrophysics

A NASA/ESA Hubble Space Telescope image of the rapidly fading visible-light fireball from a gamma-ray burst (GRB) in a distant galaxy. A new study used the spectra of 140 GRB afterglows to estimate the amount of ionizing radiation from massive stars that escapes from galaxies to ionize the intergalactic medium, and finds the surprising result that it is very small. Andrew Fruchter (STScI) and NASA/ESA

The sparsely distributed hot gas that exists in the space between galaxies, the intergalactic medium, is ionized. The question is, how? Astronomers know that once the early universe expanded and cooled enough, hydrogen (its main constituent) recombined into neutral atoms. Then, once newly formed massive stars began to shine in the so-called “era of reionization,” their extreme ultraviolet radiation presumably ionized the gas in processes that continue today.

Reionization era and first stars, Caltech

One of the key steps, however, is not well understood, namely the extent to which the stellar ionizing radiation escapes from the galaxies into the IGM. Only if the fraction escaping was high enough during the era of reionization could starlight have done the job, otherwise some other significant source of ionizing radiation is required. That might imply the existence of an important population of more exotic objects like faint quasars, X-ray binary stars, or perhaps even decaying/annihilating particles.

Direct studies of extreme ultraviolet light are difficult because the neutral gas absorbs it very strongly. Because the universe is expanding, the spectrum absorbed covers more and more of the optical range with distance until optical observations of cosmologically remote galaxies are essentially impossible. CfA astronomer Edo Berger joined a large team of colleagues to estimate the amount of absorbing gas by looking at the spectra of gamma-ray burst (GRB) afterglows. GRBs are very bright bursts of radiation produced when the core of a massive star collapses. They are bright enough that when their radiation is absorbed in narrow spectral features by gas along the line-of sight, those features can be measured and used to calculate the amount of absorbing atomic hydrogen. That number can then be directly converted into an escape fraction for the ultraviolet light of the associated galaxy. Although a single observation of a GRB in one galaxy does not provide a robust measure, a sample of GRBs is thought to be able to provide a representative measure across all sightlines to massive stars.

The astronomers carefully measured the spectra of 140 GRB afterglows in galaxies ranging as far away as epochs slightly less than one billion years after the big bang. They find a remarkably small escape fraction – less than about 1% of the ionizing photons make it out into the intergalactic medium. The dramatic result finds that stars provide only a small contribution to the ionizing radiation budget in the universe from that early period until today, not even in galaxies actively making new stars. The authors discuss possible reasons why GRBs might not provide an accurate measure of the absorption, although none is particularly convincing. The result needs confirmation and additional measurements, but suggests that a serious reconsideration of the ionizing budget of the intergalactic medium of the universe is needed.

Science paper:
“The Fraction of Ionizing Radiation from Massive Stars That Escapes to the Intergalactic Medium,” N. R. Tanvir et al.
MNRAS

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The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

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

University of Copenhagen

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?

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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Stem Education Coalition

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

From Instituto de Astrofísica de Canarias – IAC: “The observation of a rare hypernova, complete the story of the death of the most massive stars.

From Instituto de Astrofísica de Canarias – IAC

8/1/19
Manu Astrónomus

Contact:
Institute of Astrophysics of Andalusia (IAA-CSIC)
Dissemination and Communication Unit
Silbia Lopez de Lacalle – sll@iaa.es – 958230676
https://www.iaa.csic.es
https://divulgacion.iaa.csic.es

[I have done my best to correct the translation.]

Explosion image obtained by the Gran Telescopio Canarias in the period of maximum brightness of the event.

A study led by the Institute of Astrophysics of Andalusia (IAA-CSIC) and published in Nature, studied in detail to the life of a star, which produces a gamma – ray burst (GRB) and a hypernovae.

The end of the life of stars holds placid scenarios in the case of low-mass stars like the sun. Not so in the case of very massive stars, which undergo explosive events so intense that they can get to outshine all the galaxy that hosts. An international group of astronomers has studied in detail the end of a massive star that has been a gamma-ray burst (GRB) and hypernovae, which has detected a new component in this type of phenomena. The study, published in the journal Nature [above], provides the link to complete the story that links hypernovae with GRBs.

“In 1998 the first hypernovae was detected, a version of the very energy supernovae, which followed a burst of gamma rays and which was the first evidence of the connection between the two phenomena” says Luca Izzo Institute investigator Andalusia Astrophysics (IAA-CSIC) headed the study.

The proposed scenario to explain the phenomenon involved a star of more than twenty solar masses, to exhaust their fuel undergoes a process of core collapse. To collapse on itself, the core generates a black hole or neutron stars, while two polar jets of matter that cross the outer layers of the star and, emerging into the medium, produce gamma ray bursts occur ( GRBs). Hypernovae finally burst, which can be tens of times more intense than a supernova occurs.

Hypernovae artistic representation. The interaction of the jet
with the outer layers of the star forms a sheath around
the jet head and begins to spread laterally with respect
to the jet direction. The jet is able to completely pierce the
shell of the parent star, issue the issuance of a type of high – energy,
responsible for GRB. Source: Anna Serena Esposito.

But, even after twenty years of studying the relationship between GRBs and hypernovae seems clear, it is not met in the opposite direction, as they have detected several hypernovae not have associated gamma-ray bursts. “This work has allowed us to identify the missing link between these two subtypes hypernovae in the form of a new component: a kind of hot envelope is formed around the jet according propagates through the parent star -apunta Izzo (IAA CSIC) -. The jet transfers a significant part of its energy to the shell and, if it goes through the surface of the star will produce gamma ray emission we identify as GRB “.

However, the jet may spoil within the star and not emerge to medium lacking sufficient energy, a circumstance occurs hypernovae but not a GRB. Thus, the casing detected in this investigation represents the link between the two subtypes hypernovae studied so far, and these “jets damped” (English choked-jets) naturally explain the differences.

EVENT HISTORY

On December 5 the GRB171205A outbreak was detected in a galaxy located just five hundred million light years from Earth, making it the fourth GRB nearest known. “Phenomena of this kind occur on average once every ten years, so immediately began an intense campaign observation with the Gran Telescopio Canarias to observe the emerging hypernovae from the early stages -apunta Christina Thöne, researcher at the Institute of Astrophysics of Andalusia ( IAA-CSIC) participating in the hallazgo-. In fact, it is the earliest detection of a hypernovae to date, less than a day after the collapse of the star. ”

And indeed, once the first evidence of the presence of a hypernovae were observed. “This was possible because the luminosity of the jets was much weaker than normal because usually outshine the emission of the supernova He points during the first week Antonio de Ugarte Postigo, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) participating in the hallazgo-. However, a peculiar hypernovae, was already showing very high growth rates and a different chemical abundances to those recorded in similar events “.

This unique chemical composition and velocities associated fit the existence of a jet surrounded by an envelope that cuts on the surface of the star, which had been predicted earlier but had not yet observed. Sheath accompanying the jet during the first days drag material from the interior of the star, and in the case study allowed us to determine its chemical structure. After a few days, this comoponent disappeared and hypernovae evolved similarly to those observed previously.

The total energy emitted by the envelope was higher than the GRB, which implies that the jet deposited much of its energy in it. But also it shows that the energy of GRB depends on the interaction of the jet with stellar material and this new component, the wrapper. And also highlights the need to review the model: “While the standard model supernovae core collapse leads to nearly spherical explosions, evidence of such energy emission produced by a sheath of this type suggests that the jet plays an important role in central collapse supernovae, and we need to take into account the role of the jet explosion models of supernovae, “says Izzo (IAA-CSIC).

This study was coordinated by researchers from the group Phenomena Transients High Energy and Environment (High-Energy Transients and Their Hosts, HETH) of the IAA-CSIC. Christina headed by Thöne, studying the physics of transient astronomical phenomena, the environment in which they occur and the galaxies that host them.

See the full article here.

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Please help promote STEM in your local schools.

Stem Education Coalition

The Instituto de Astrofísica de Canarias(IAC) is an international research centre in Spain which comprises:

The Instituto de Astrofísica, the headquarters, which is in La Laguna (Tenerife).
The Centro de Astrofísica en La Palma (CALP)
The Observatorio del Teide (OT), in Izaña (Tenerife).

These centres, with all the facilities they bring together, make up the European Northern Observatory(ENO).

The IAC is constituted administratively as a Public Consortium, created by statute in 1982, with involvement from the Spanish Government, the Government of the Canary Islands, the University of La Laguna and Spain’s Science Research Council (CSIC).

The International Scientific Committee (CCI) manages participation in the observatories by institutions from other countries. A Time Allocation Committee (CAT) allocates the observing time reserved for Spain at the telescopes in the IAC’s observatories.

The exceptional quality of the sky over the Canaries for astronomical observations is protected by law. The IAC’s Sky Quality Protection Office (OTPC) regulates the application of the law and its Sky Quality Group continuously monitors the parameters that define observing quality at the IAC Observatories.

The IAC’s research programme includes astrophysical research and technological development projects.

The IAC is also involved in researcher training, university teaching and outreachactivities.

The IAC has devoted much energy to developing technology for the design and construction of a large 10.4 metre diameter telescope, the ( Gran Telescopio CANARIAS, GTC), which is sited at the Observatorio del Roque de los Muchachos.

Gran Telescopio Canarias at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, SpainGran Telescopio CANARIAS, GTC

From AAS NOVA: ” Two Explosions with Similar Quirks”

From AAS NOVA

12 October 2018
Susanna Kohler

Artist’s by now iconic illustration of the merger of two neutron stars, producing a short gamma-ray burst. [NSF/LIGO/Sonoma State University/A. Simonnet]

High-energy radiation released during the merger of two neutron stars last year has left astronomers puzzled. Could a burst of gamma rays from 2015 help us to piece together a coherent picture of both explosions?

A Burst Alone?

When two neutron stars collided last August, forming a distinctive gravitational-wave signal and a burst of radiation detected by telescopes around the world, scientists knew that these observations would change our understanding of short gamma-ray bursts (GRBs).Though we’d previously observed thousands of GRBs, GRB 170817A was the first to have such a broad range of complementary observations — both in gravitational waves and across the electromagnetic spectrum — providing insight into its origin.

Total isotropic-equivalent energies for Fermi-detected gamma-ray bursts with known redshifts. GRB 170817A (pink star) is a factor of ~1,000 dimmer than typical short GRBs (orange points). GRB 170817A and GRB 150101B (green star) are two of the closest detected short GRBs. [Adapted from Burns et al. 2018]

But it quickly became evident that GRB 170817A was not your typical GRB. For starters, this burst was unusually weak, appearing 1,000 times less luminous than a typical short GRB. Additionally, the behavior of this burst was unusual: instead of having only a single component, the ~2-second explosion exhibited two distinct components — first a short, hard (higher-energy) spike, and then a longer, soft (lower-energy) tail.

The peculiarities of GRB 170817A prompted a slew of models explaining its unusual appearance. Ultimately, the question is: can our interpretations of GRB 170817A safely be applied to the general population of gamma-ray bursts? Or must we assume that GRB 170817A is a unique event, not representative of the general population?

New analysis of a GRB from 2015 — presented in a recent study led by Eric Burns (NASA Goddard SFC) — may help to answer this question.

A Matter of Angles

What does a burst from 2015 have to do with the curious case of GRB 170817A? Burns and collaborators have demonstrated that this 2015 burst, GRB 150101B, exhibited the same strange behavior as GRB 170817A: its emission can be broken down into two components consisting of a short, hard spike, followed by a long, soft tail. Unlike GRB 170817A, however, GRB 150101B is not underluminous — and it lasted less than a tenth of the time.

Fermi count rates in different energy ranges showing the short hard spike and the longer soft tail in GRB 150101B. The short hard spike is visible above 50 keV (top and middle panels). The soft tail is visible in the 10–50 keV channel (bottom panel). [Burns et al. 2018]

Intriguingly, these similarities and differences can all be explained by a single model. Burns and collaborators propose that GRB 150101B and GRB 170817A exhibit the exact same two-component behavior, and their differences in luminosity and duration can be explained by quirks of special relativity.

High-speed outflows such as these will have different apparent luminosities and durations depending on whether we view them along their axis or slightly from the side. Burns and collaborators demonstrate that these the two bursts could easily have the same profile — but GRB 150101B was viewed nearly on-axis, whereas GRB 170817A was viewed from an angle.

If this is true, then perhaps more GRBs have hard spikes and soft tails similar to these two; the tails may just be difficult to detect in more distant bursts. While more work remains to be done, the recognition that GRB 170817A may not be unique is an important one for understanding both its behavior and that of other short GRBs.

Citation

“Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail,” E. Burns et al 2018 ApJL 863 L34.
http://iopscience.iop.org/article/10.3847/2041-8213/aad813/meta

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

ESA/eLISA the future of gravitational wave research

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)

NASA/Fermi LAT

NASA/Fermi Gamma Ray Space Telescope

See the full article here .

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Please help promote STEM in your local schools.

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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009