Astronomy

How does the Gamma Ray Burst that occurred when 2 black holes merged compare to other GRB's?

How does the Gamma Ray Burst that occurred when 2 black holes merged compare to other GRB's?


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A Gamma Ray Burst was detected 0.4 seconds after the gravitational wave event, GW150914, caused by a black hole merger, and it was in the same part of the sky. It is uncertain whether that Gamma Ray Burst was associated with the black hole merger. The odds of a GRB being coincident (or just background noise) is 0.22%. That implies a 99.78% chance the black hole merger was related to the GRB. Later analysis suggests the GRB was just a background event that occurred in the same place in the sky at just 0.4 seconds after the black hole merger, and therefore not related.

While a GRB may have relativistic jets beaming out from it in opposite directions, research "excludes the possibility that the event is associated with substantial gamma-ray radiation, directed towards the observer." I interpret that to mean this GRB did not have relativistic jets, but was omnidirectional. (Not sure how astronomers came to that conclusion.)

Anyhow, whether it was coincident or not, I am asking about how the energy output of that GRB compares to other GRB's. The Wikipedia article in the link says the "energy emission in gamma-rays and hard X-rays from the event was less than one millionth of the energy emitted as gravitational waves."

How much energy does a typical GRB emit? What was the spectrum spread of a typical GRB? Over what amount of time does a typical GRB emit that energy? (How many seconds?)

How do the energy levels, spectrums, and time durations of other GRB's compare to the GRB associated with the black hole merger?

If it is similar to other GRB's, that would support the hypothesis this GRB was just a coincident event at nearly the same time and location in the sky.

If this GRB has different energy emissions and time duration than other GRB's, that would support the hypothesis it is truly associated with the black hole merger.

When you answer, please provide data, citations, or quotes from original research. I am not looking for unsupported speculation, but real analysis supported by real data.


The interpretation you suggest in the second paragraph is incorrect. It is understandable, since there is a debate in the literature - different papers come to potentially contradicting conclusions.

"Excluding a possibility that the event is associated with substantial gamma-ray radiation, directed towards the observer" simply means that no observable GRB is found, hinting that may be the original detection was caused by instrumental background: each detector has it's own instrumental backgrounds, only real events should be seen by all sufficiently sensitive instruments.

In principle, it is always possible that the merger generated beamed emission, directed somewhere else. No instrument should have seen it directly, and there is no simple way to know if it happened.

This upper limit is derived from the observation of another satellite (INTEGRAL/SPI-ACS, Savchenko et al 2016) than the original detection (Fermi/GBM, Connaughton et al 2016). Also an alternative analysis of the Fermi/GBM data (Greiner et al 2016) suggested that no event can be found in the GBM data - their opinion is that it was a background fluctuation of some kind.

Right now, the teams who reported these conflicting results are working together, trying to come up with a consistent picture, which might be, in principle, a GRB with this or that properties associated with the GW150914, an unrelated GRB with some properties, or no detection whatsoever. This work is centered on cross-calibrating and comparing instruments, and is also useful to avoid these kind of uncertainties in the future.

One could try to characterize the spectral properties of this event, following the approach of the original Fermi/GBM team. But unfortunately, the measurement appeared to be in very unlucky conditions for Fermi/GBM (in bad direction). Which is why the signal was very weak (below what would be usually reported for a real GRB, though recently the attempts were made to decrease these thresholds, see Goldstein et al 2017 ), and spectral characterization is loose. You can look for some details to Veres et al 2016. With these large uncertainties the spectrum is compatible to that of known short GRBs.

The luminosity estimate depends on the spectrum, but is seems to be at the lower end of the short GRB sample (see e.g. Wanderman et al 2015)
But because the uncertainties are large, the event, if real, might be unusual as well.

The INTEGRAL observation, non-detection, would imply much softer (perhaps, unusual for a short GRB) or/and a weaker burst, possibly incompatible even with the highly uncertain Fermi/GBM data.

Duration of this possible GRB possibly associated with GW event is the easy part and is about 1 s long, typical for a short GRB (Kouveliotou et al 1993).


Did Fermi Detect LIGO’s Merging Black Holes?

By: Camille M. Carlisle April 21, 2016 5

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NASA’s Fermi Gamma-ray Space Telescope might have detected a burst from the same merging black holes that emitted the gravitational waves LIGO detected. Or not.

An artist's impression of the Fermi Gamma-Ray Space Telescope.
Credit: NASA/General Dynamics

Back in February, the LIGO team announced it had detected the unmistakable signal of gravitational waves from two black holes as they merged into one. When the news spread, scientists scrambled to see whether they had recorded the event other ways, too — not as spacetime ripples, but as photons. They dug through archived observations taken around the moment on September 14, 2015, when the gravitational wave signal wobbled LIGO’s two sites.

At face value it was a fool’s errand: astronomers didn’t expect the black holes to have set off any kind of light show. That’s because any emission would have to come from gas, and merging black holes of these masses (a few tens of solar masses) should have swept up all surrounding material during their prolonged fatal approach. In other words, merging stellar-mass black holes don’t wear gas tutus.

Yet on the same day of LIGO’s announcement, scientists with NASA’s Fermi Gamma-ray Space Telescope posted a paper to the preprint server arXiv, reporting that Fermi had seen something — a weak, 1-second burst, just 0.4 second after the LIGO event.

The flash’s spectrum looks like that of a short gamma-ray burst (GRB), which is what it sounds like: a burst of gamma rays. The short breed of GRB lasts for less than 2 seconds, probably the result of colliding neutron stars or (more rarely) black holes. Astronomers have seen the afterglow from a neutron star crash before.

But the September 2015 flash was far weaker than the run-of-the-mill short GRB. It was so weak, in fact, that it didn’t trigger the space telescope’s onboard alert system. That’s partly because the burst went off “underneath” the spacecraft Fermi essentially detected it with peripheral vision. It also doesn’t show up in data from the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (Integral) spacecraft. But the latter doesn’t faze Valerie Connaughton (Universities Space Research Association) and her colleagues, who calculate in their paper that Integral only detects half of the weak, short gamma-ray bursts that Fermi does.

The LIGO and Fermi signals come from the same part of the sky — but “same part of the sky” is a big region, because both observatories are bad at pinpointing where a signal comes from. LIGO constrained the merged black hole’s location to a long arc in the heavens, but that arc covers something like 600 square degrees. That’s equivalent to the celestial territory spanned by the constellation Orion (if you leave out the raised club). Fermi's view overlapped about 200 square degrees of that (more like the span of Cassiopeia’s “W”), as shown in the video below.

The team has identified no alternative source for the Fermi flash. The options, as they stand, are basically

  1. the Fermi signal isn’t real (it's an equipment hiccup or a chance background fluctuation)
  2. the Fermi signal is real, but it’s a coincidence that it came from the same part of sky that the LIGO signal did and
  3. the Fermi signal is real and it’s from the same cosmic collision that created the gravitational waves.

The team estimates that there’s only a 0.2% probability Fermi would have detected a signal by chance so soon after LIGO’s. That probability doesn’t take into account whether the Fermi signal is real or its location on the sky, Connaughton says — it's only based on the timing.

Astronomers are nothing if not optimists. Abraham Loeb (Harvard), known for his out-of-the-box thinking, suggested soon after the Fermi team reported their find that both signals could come from the death of a huge star with the mass of more than 100 Suns. This hypothetical star, formed when two smaller stars merged, would have died in a catastrophic collapse. If its core broke into two clumps which then became two black holes, Loeb suggests, those black holes could be the ones that merged — which would explain why there was gas around to feed the flash. (The merged stars’ cores might also never have united in the first place, but he thinks it’s harder to generate a GRB this way.) The team hasn’t advocated this theory or any other yet.

However, astronomers do see GRBs from neutron star collisions, which will also produce gravitational waves that are detectable by LIGO and the near-operational Virgo interferometer in Europe. That’s why astronomers are excited about the possibilities, and why they’re actively discussing how Fermi and LIGO can work together. Even if this pair of signals turns out to be coincidence, others won’t.


New Kind Of Black Hole Explosion Discovered

Scientists using NASA data are studying a newly recognized type of cosmic explosion called a hybrid gamma-ray burst. As with other gamma-ray bursts, this hybrid blast is likely signaling the birth of a new black hole.

It is unclear, however, what kind of object or objects exploded or merged to create the new black hole. The hybrid burst exhibits properties of the two known classes of gamma-ray bursts yet possesses features that remain unexplained.

NASA's Swift first discovered the burst on June 14. Since the Swift finding, more than a dozen telescopes, including the Hubble Space Telescope and large ground-based observatories, have studied the burst.

"We have lots of data on this event, have dedicated lots of observation time, and we just can't figure out what exploded," said Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Md., lead author on one of four reports appearing in this week's edition of the journal Nature. "All the data seem to point to a new but perhaps not so uncommon kind of cosmic explosion."

Gamma-ray bursts represent the most powerful known explosions in the universe. Yet they are random and fleeting, never appearing twice. Scientists have only recently begun to understand their nature.

Such bursts typically fall into one of two categories, long or short. The long bursts last more than two seconds and appear to be from the core collapse of massive stars forming a black hole. Most of these bursts come from the edge of the visible universe. The short bursts, which are under two seconds and often last just a few milliseconds, appear to be the merger of two neutron stars or a neutron star with a black hole, which subsequently creates a new or bigger black hole.

The hybrid burst, called GRB 060614, after the date it was detected, originated from within a galaxy 1.6 billion light years away in the southern constellation Indus. The burst lasted for 102 seconds, placing it soundly in long-burst territory. But the burst lacked the hallmark of a supernova, or star explosion, commonly seen shortly after long bursts. Also, the burst's host galaxy has a low star-formation rate with few massive stars that could produce supernovae and long gamma-ray bursts. "This was close enough to detect a supernova if it existed," said Avishay Gal-Yam of Caltech, Pasadena, Calif., lead author on another Nature report. "Even Hubble didn't see anything."

Certain properties of the burst concerning its brightness and the arrival time of photons of various energies, called the lag-luminosity relationship, suggest that burst behaved more like a short burst (from a merger) than a long burst. Yet no theoretical model of mergers can support a sustained release of gamma-ray energy for 102 seconds. "This is brand new territory we have no theories to guide us," said Gehrels.

The burst is perhaps not unprecedented. Archived data from the Compton Gamma-Ray Observatory in the 1990s possibly reveal other hybrid "long-short" bursts, but no follow-up observations are available to confirm this. Johan Fynbo of the Niels Bohr Institute in Copenhagen, also lead author on a Nature report, suggests that a burst from May of this year was also long, but had no associated supernova.

Scientists remain divided on whether this was a long-short burst from a merger or a long burst from a star explosion with no supernova. Most conclude, however, that some new process must be at play -- either the model of mergers creating second-long bursts needs a major overhaul, or the progenitor star from an explosion is intrinsically different from the kind that make supernovae.

"We siphoned out all the information we could from GRB 060614," said Massimo Della Valle of the Osservatorio Astrofisico di Arcetri in Firenze, Italy, another lead author on a Nature report. "All we can do now is wait for the next nearby hybrid burst."


Two Merging Black Holes May Have Irradiated Earth Around AD 775

According to a new study conducted by German astronomers Dr Valeri Hambaryan and Dr Ralph Neuhauser, an intense blast of high-energy radiation that struck our planet in the 8th century may have been caused by a nearby short gamma-ray burst, emitted by two merging stellar remnants – black holes, neutron stars or white dwarfs.

Black hole merger (NASA / CXC / A. Hobart / Josh Barnes, Univesity of Hawaii / John Hibbard, NRAO)

In 2012, cosmic-ray physicist Prof Fusa Miyake from Nagoya University in Japan announced the detection of high levels of the isotope carbon-14 and beryllium-10 in tree rings formed in 775 CE, suggesting that a burst of radiation struck the Earth in the year 774 or 775.

Carbon-14 and beryllium-10 form when radiation from space collides with nitrogen atoms, which then decay to these heavier forms of carbon and beryllium. The earlier research ruled out the nearby explosion of a massive star as nothing was recorded in observations at the time and no remnant has been found.

Prof Miyake also considered whether a solar flare could have been responsible, but these are not powerful enough to cause the observed excess of carbon-14. Large flares are likely to be accompanied by ejections of material from the Sun’s corona, leading to vivid displays of the northern and southern lights, but again no historical records suggest these took place.

Following this announcement, researchers pointed to an entry in the Anglo-Saxon Chronicle that describes a ‘red crucifix’ seen after sunset and suggested this might be a supernova. But this dates from 776, too late to account for the carbon-14 data and still does not explain why no remnant has been detected.

In a paper, published in the journal Monthly Notices of the Royal Astronomical Society (arXiv.org version), the astronomers provide a new explanation consistent with both the carbon-14 measurements and the absence of any recorded events in the sky. They suggest that two compact stellar remnants – black holes, neutron stars or white dwarfs – collided and merged together. When this happens, some energy is released in the form of gamma rays, the most energetic part of the electromagnetic spectrum that includes visible light.

In these mergers, the burst of gamma rays is intense but short, typically lasting less than two seconds. These events are seen in other galaxies many times each year but, in contrast to long duration bursts, without any corresponding visible light. If this is the explanation for the 774 / 775 radiation burst, then the merging stars could not be closer than about 3,000 light years, or it would have led to the extinction of some terrestrial life. Based on the carbon-14 measurements, the astronomers believe the gamma-ray burst originated in a system between 3,000 and 12,000 light years from the Sun.

If they are right, then this would explain why no records exist of a supernova or auroral display. Other work suggests that some visible light is emitted during short gamma-ray bursts that could be seen in a relatively nearby event. This might only be seen for a few days and be easily missed, but nonetheless it may be worthwhile for historians to look again through contemporary texts.

“If the gamma ray burst had been much closer to the Earth it would have caused significant harm to the biosphere. But even thousands of light years away, a similar event today could cause havoc with the sensitive electronic systems that advanced societies have come to depend on. The challenge now is to establish how rare such carbon-14 spikes are i.e. how often such radiation bursts hit the Earth. In the last 3,000 years, the maximum age of trees alive today, only one such event appears to have taken place,” said Dr Neuhauser of the University of Jena’s Astrophysics Institute.

Bibliographic information: V. V. Hambaryan and R. Neuhäuser. A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5. MNRAS, published online January 20, 2013 doi: 10.1093/mnras/sts378


LIGO's twin black holes might have been born inside a single star

On Sept. 14, 2015, LIGO detected gravitational waves from two merging black holes, shown here in this artist's conception. The Fermi space telescope detected a burst of gamma rays 0.4 seconds later. New research suggests that the burst occurred because the two black holes lived and died inside a single, massive star. Credit: Swinburne Astronomy Productions

On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves from the merger of two black holes 29 and 36 times the mass of the Sun. Such an event is expected to be dark, but the Fermi Space Telescope detected a gamma-ray burst just a fraction of a second after LIGO's signal. New research suggests that the two black holes might have resided inside a single, massive star whose death generated the gamma-ray burst.

"It's the cosmic equivalent of a pregnant woman carrying twins inside her belly," says Harvard astrophysicist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA).

Normally, when a massive star reaches the end of its life, its core collapses into a single black hole. But if the star was spinning very rapidly, its core might stretch into a dumbbell shape and fragment into two clumps, each forming its own black hole.

A very massive star as needed here often forms out of the merger of two smaller stars. And since the stars would have revolved around each other faster and faster as they spiraled together, the resulting merged star would be expected to spin very quickly.

After the black hole pair formed, the star's outer envelope rushed inward toward them. In order to power both the gravitational wave event and the gamma-ray burst, the twin black holes must have been born close together, with an initial separation of order the size of the Earth, and merged within minutes. The newly formed single black hole then fed on the infalling matter, consuming up to a Sun's worth of material every second and powering jets of matter that blasted outward to create the burst.

Fermi detected the burst just 0.4 seconds after LIGO detected gravitational waves, and from the same general area of the sky. However, the European INTEGRAL gamma-ray satellite did not confirm the signal.

"Even if the Fermi detection is a false alarm, future LIGO events should be monitored for accompanying light irrespective of whether they originate from black hole mergers. Nature can always surprise us," says Loeb.

If more gamma-ray bursts are detected from gravitational wave events, they will offer a promising new method of measuring cosmic distances and the expansion of the universe. By spotting the afterglow of a gamma-ray burst and measuring its redshift, then comparing it to the independent distance measurement from LIGO, astronomers can precisely constrain the cosmological parameters. "Astrophysical black holes are much simpler than other distance indicators, such as supernovae, since they are fully defined just by their mass and spin," says Loeb.

"This is an agenda-setting paper that will likely stimulate vigorous follow-up work, in the crucial period after the initial LIGO discovery, where the challenge is to fathom its full implications. If history is any guide, the 'multi-messenger' approach advocated by Loeb, using both gravitational waves and electromagnetic radiation, again promises deeper insight into the physical nature of the remarkable LIGO source," says Volker Bromm of the University of Texas at Austin, commenting independently.

This research has been accepted for publication in The Astrophysical Journal Letters and is available online.


Exoplanet-hunter TESS telescope Spots A Bright Gamma-ray Burst


TESS full-frame image in the cadence just before the BAT trigger (left) and at the peak flux of the burst (center). The emergence of the afterglow is apparent in the center of the image, indicated by the white arrow. The right panel shows the same region of the sky, with a slightly different orientation, in the Digitized Sky Survey (DSS) a small inset of TESS image is provided in the bottom left corner to demonstrate the change in orientation. CREDIT The Astrophysical Journal

NASA has a long tradition of unexpected discoveries, and the space program's TESS mission is no different.

SMU astrophysicist and her team have discovered a particularly bright gamma-ray burst using a NASA telescope designed to find exoplanets - those occurring outside our solar system - particularly those that might be able to support life.

It's the first time a gamma-ray burst has been found this way.

Gamma-ray bursts are the brightest explosions in the universe, typically associated with the collapse of a massive star and the birth of a black hole. They can produce as much radioactive energy as the sun will release during its entire 10-billion-year existence.

Krista Lynne Smith, an assistant professor of physics at Southern Methodist University, and her team confirmed the blast - called GRB 191016A - happened on Oct. 16 and also determined its location and duration. A study on the discovery has been published in The Astrophysical Journal.

"Our findings prove this TESS telescope is useful not just for finding new planets, but also for high-energy astrophysics," said Smith, who specializes in using satellites like TESS (Transiting Exoplanet Survey Satellite) to study supermassive black holes and gas that surrounds them. Such studies shed light on the behavior of matter in the deeply warped spacetime around black holes and the processes by which black holes emit powerful jets into their host galaxies.

Smith calculated that GRB 191016A had a peak magnitude of 15.1, which means it was 10,000 times fainter than the faintest stars we can see with the naked eyes.

That may sound quite dim, but the faintness has to do with how far away the burst occurred. It is estimated that light from GRB 191016A's galaxy had been travelling 11.7 billion years before becoming visible in the TESS telescope.

Most gamma ray bursts are dimmer - closer to 160,000 times fainter than the faintest stars.

The burst reached its peak brightness sometime between 1,000 and 2,600 seconds, then faded gradually until it fell below the ability of TESS to detect it some 7000 seconds after it first went off.

This gamma-ray burst was first detected by a NASA's satellite called Swift-BAT, which was built to find these bursts. But because GRB 191016A occurred too close to the moon, the Swift-BAT couldn't do the necessary follow-up it normally would have to learn more about it until hours later.

NASA's TESS happened to be looking at that same part of the sky. That was sheer luck, as TESS turns its attention to a new strip of the sky every month.

While exoplanet researchers at a ground-base for TESS could tell right away that a gamma-ray burst had happened, it would be months before they got any data from the TESS satellite on it. But since their focus was on new planets, these researchers asked if any other scientists at a TESS conference in Sydney, Australia were interested in doing more digging on the blast.

Smith was one of the few high-energy astrophysics specialists there at that time and quickly volunteered.

"The TESS satellite has a lot of potential for high-energy applications, and this was too good an example to pass up," she said. High-energy astrophysics studies the behavior of matter and energy in extreme environments, including the regions around black holes, powerful relativistic jets, and explosions like gamma-ray bursts.

TESS is an optical telescope that collects light curves on everything in its field of view, every half hour. Light curves are a graph of light intensity of a celestial object or region as a function of time. Smith analyzed three of these light curves to be able to determine how bright the burst was.

She also used data from ground-based observatories and the Swift gamma-ray satellite to determine the burst's distance and other qualities about it.

"Because the burst reached its peak brightness later and had a peak brightness that was higher than most bursts, it allowed the TESS telescope to make multiple observations before the burst faded below the telescope's detection limit," Smith said. "We've provided the only space-based optical follow-up on this exceptional burst."

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Short Gamma-Ray Burst Localized to Extremely Distant Galaxy

An international of astronomers has observed an optical afterglow of a short gamma-ray burst, thought to be from the merger of two neutron stars, and localized it to a particular host galaxy, which is located 10 billion light-years away in the constellation of Coma Berenices. Dubbed GRB 181123B, the event occurred 3.8 billion years after the Big Bang. It is the second most-distant short gamma-ray burst ever detected and the most distant event with an optical afterglow.

The afterglow of GRB 181123B (marked with a circle), captured by the Gemini-North telescope. Image credit: Gemini Observatory / NOIRLab / NSF / AURA / K. Paterson & W. Fong, Northwestern University / Travis Rector, University of Alaska Anchorage / Mahdi Zamani / Davide de Martin.

Short gamma-ray bursts (SGRBs) are short-lived, highly-energetic bursts of gamma-ray light.

Tought to result from the merger of two neutron stars, they are cataclysmic events that are almost unfathomable in terms of their basic properties, emitting huge amounts of energy.

The gamma-ray light lasts for less than two seconds, while the optical light can last for a matter of hours before fading.

Therefore, rapid follow-up of the optical afterglow of these intense flashes of gamma-ray radiation is critical.

Astronomers typically only detect 7-8 SGRBs each year that are well-localized enough for further observations.

GRB 181123B was detected on November 23, 2018 by NASA’s Neil Gehrels Swift Observatory.

Within just a few hours after the detection and a worldwide alert, Northwestern University astronomer Kerry Paterson and colleagues quickly pointed the 8.1-m Gemini-North telescope, the 10-m Keck I telescope and the Multi-Mirror Telescope toward the location of GRB 181123B and were able to measure its very faint afterglow.

“We were able to obtain deep observations of the burst mere hours after its discovery,” Dr. Paterson said.

“The Gemini images were very sharp, allowing us to pinpoint the location to a specific galaxy in the Universe.”

“We certainly did not expect to discover a distant SGRB, as they are extremely rare and very faint,” said Dr. Wen-fai Fong, also from Northwestern University.

“We perform ‘forensics’ with telescopes to understand its local environment, because what its home galaxy looks like can tell us a lot about the underlying physics of these systems.”

An artist’s impression of how GRB 11823B compares to other short gamma-ray bursts. Except when they are detected by gravitational wave observatories, the gamma ray bursts can only be detected from Earth when their jets of energy are pointed towards us. Image credit: Gemini Observatory / NOIRLab / NSF / AURA / J. Pollard / K. Paterson & W. Fong, Northwestern University / Travis Rector, University of Alaska Anchorage / Mahdi Zamani / Davide de Martin.

After identifying the host galaxy of GRB 181123B and calculating the distance, the astronomers were able to determine key properties of the parent stellar populations within the galaxy that produced the event.

Because GRB 181123B appeared when the Universe was only about 30% of its current age — during an epoch known as ‘Cosmic High Noon’ — it offered a rare opportunity to study the neutron star mergers from when the Universe was a ‘teenager.’

When GRB 181123B occurred, the Universe was incredibly busy, with rapidly forming stars and fast-growing galaxies.

Massive binary stars need time to be born, evolve and die — finally turning into a pair of neutron stars that eventually merge.

“It’s long been unknown how long neutron stars — in particular those that produce SGRBs — take to merge,” Dr. Fong said.

“Finding an SGRB at this point in the Universe’s history suggests that, at a time when the Universe was forming lots of stars, the neutron star pair may have merged fairly rapidly.”

K. Paterson et al. 2020. Discovery of the optical afterglow and host galaxy of short GRB181123B at z=1.754: Implications for Delay Time Distributions. ApJL, in press arXiv: 2007.03715


LIGO’s Black Holes Probably Did Not Come From One Star

“Even if the Fermi detection is a false alarm, future LIGO events should be monitored for accompanying light irrespective of whether they originate from black hole mergers. Nature can always surprise us.” -Avi Loeb

When LIGO detected gravitational waves for the first time, we were delighted, but we weren’t surprised. Theorists had calculated exactly the type of LIGO-sensitive signal that should result from the merger of two massive black holes, including the mass-dependent frequency and amplitude of gravitational waves that would result during the inspiral, merger and ringdown phases. As far as the gravitational wave signal went, we couldn’t have asked for a better alignment of theoretical predictions and observational measurements, in both detectors and with the right delay.

But gravitational waves are only one window into the Universe. We can also look for light of all wavelengths, including in the highest energies: the gamma rays. NASA’s Fermi satellite can look for gamma ray bursts across

70% of the sky at once, and was doing exactly this on September 14th, 2015, when those two black holes merged some 1.3 billion light years away. Based on the LIGO data, they were able to correlate one such signal — a gamma ray burst in the same direction of the sky — with that exact black hole merger, having occurred just 0.4 seconds after the LIGO event. This is a puzzle, because short-period gamma ray bursts are supposed to come from neutron star-neutron star mergers, not from black hole-black hole mergers!

The reason this is such a problem is that gamma rays are emitted from high-energy accelerations of massive particles, normally by intense electromagnetic fields. Neutron stars have the strongest known magnetic fields in the Universe, and when they collide, their outer layers (10% of them are made of charged particles) emit catastrophic amount of gamma-ray radiation, capable of frying any living being within trillions of miles. But black holes don’t have that, and hence they shouldn’t emit gamma rays when they merge. So if they did, how did they do so? Avi Loeb at Harvard recently put forth an idea that’s gotten a lot of traction: perhaps these black holes were merging from the interior of a single, solitary star.

This idea is interesting, because it builds off of an underappreciated idea in astrophysics: that black holes might not only be the end state of supermassive stars, but that they might exist inside of dense, massive stars even while they’re still burning. Remember that a black hole is simply a region of space where matter is so dense and gravity is so strong that even if you moved at the speed of light, you wouldn’t be able to escape its gravitational pull. For stars that are hundreds of times as massive as our Sun, or even for ultra-dense neutron stars of a sufficient mass, perhaps the very central region — where mass concentrations, densities and pressures are the highest — are already black holes, even as the outer layers remain uncollapsed. So maybe, Loeb’s thinking goes, there can be two black holes inside of a star if it rotates rapidly enough. When these black holes spiral in and merge together, perhaps they create the gamma ray burst that the Fermi satellite saw.

The best thing I can say about this idea is that it falls into the category of “not automatically impossible.” The tough thing about it is that even for the most rapid rotators, the stars themselves are still highly non-relativistic, meaning they spin at velocities well below (significantly less than 10%) the speed of light, while the inspiraling black holes were moving at speeds very close to (about 60%) the speed of light. While two merging-and-colliding black holes inside a single, supermassive star could have produced a gamma ray burst, there are other explanations that are highly regarded as more likely:

  1. The two black holes that merged could have had accretion disks, and when the disks collided, they heated up, emitting gamma rays during the merger.
  2. The two separate progenitor stars that led to the black holes had expelled most of their matter into the space around them, but some of that matter remained closely gravitationally bound. When those black holes merged, the matter that was close enough heated up and caused some gamma ray emission.
  3. The interstellar medium near the black hole contained matter, and the changes in magnetic fields during the merger (perhaps they do have strong magnetic fields!) caused a rapid acceleration of those charged particles, leading to gamma ray emissions.

If I had to bet, I’d go with option 1, since that scenario consists of things that are supposed to exist, and provides a nice, simple mechanism not only for how this would have happened, but even explains how there’s a small (less than 1 second) delay from when the gravitational waves arrived to when the photons arrived. Nothing is certain and it’s important to explore all the possibilities, but the single star explanation is probably the least likely of all. As more LIGO events come in, and as we not only use Fermi but other gamma ray observatories (like the ESA’s INTEGRAL satellite), perhaps we’ll come to understand these mergers — and the physics taking place in their environments — even better.


An Explosive Merger … Maybe

On August 16, 2019, both the Fermi Gamma-ray Burst Monitor (GBM) and the Laser Interferometer Gravitational-wave Observatory (LIGO) detected faint blips that didn’t quite register as events. But could these ghost signals actually correspond to the first collision we’ve detected of a black hole with a neutron star?

Neutron Stars and Black Holes: Mix and Match

LIGO’s first detection of gravitational waves was GW150914, the collision of a pair of black holes. In the years since, LIGO has partnered with its European counterpart, Virgo, to make another dozen confirmed detections of binary black holes merging. The collaboration also spotted two instances of binary neutron stars colliding — one of which, GW170817, was accompanied by a short gamma-ray burst and emission spanning the electromagnetic spectrum.

A recent version of the “stellar graveyard”, a plot that shows the masses of the different components of confirmed compact binary mergers. No definite NS–BH mergers have been detected yet. Click to enlarge. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

Could it be that such an event is actually hidden in the reject data from LIGO/Virgo and Fermi?

A Pair of Intriguing (Non-?)Events

The results from LIGO-Virgo’s third observing run, cut short by the pandemic in March 2020, are still being carefully analyzed by the collaboration. The O3 alert data, however, is publicly available — and a team of scientists have taken advantage of this to do some independent analysis, recently detailed in a publication led by Yi-Si Yang (Nanjing University, China).

Yang and collaborators take note of two faint signals that occurred on August 16, 2020:

The accumulated light curve for GBM-190816 shows the duration of the gamma-ray burst, roughly 0.1 seconds. [Adapted from Yang et al. 2020]

  1. A subthreshold gravitational-wave event in the LIGO/Virgo data — i.e., an event with a signal-to-noise ratio below 12, the threshold to qualify as a significant candidate.
  2. A subthreshold gamma-ray burst, GBM-190816, that was picked up by Fermi/GBM just 1.57 seconds after the gravitational-wave event.

If these two signals are both real and related, then GBM-190816 represents a short gamma-ray burst emitted from the merger of two compact objects — and Yang and collaborators’ analysis shows that, with a mass ratio of q

2.26, the system is most likely a neutron-star–black-hole binary. In the simplest explanation, the neutron star was torn apart before the bodies ultimately merged, producing the pair of signals.

Identifying What’s Real

So are these subthreshold events real? We can’t say, yet! The public alerts from LIGO/Virgo only contain a portion of the signal information, so Yang and collaborators had to make a number of assumptions to analyze the event.

The parameters of GBM-190816 (marked by a red star), like the peak-to-background flux ratio vs. duration shown here, are consistent with typical short gamma-ray bursts (blue triangles). [Adapted from Yang et al. 2020]

0.1 seconds, compared to the

2 second duration of GW170817 — which is what caused it to register below the Fermi/GBM trigger threshold.

If confirmed, this event could provide interesting insight into how the light emitted by such a merger escapes and travels to us. Now we just have to wait for the official joint analysis from the LIGO/Virgo/Fermi team!

Citation

“Physical Implications of the Subthreshold GRB GBM-190816 and Its Associated Subthreshold Gravitational-wave Event,” Yi-Si Yang et al 2020 ApJ 899 60. doi:10.3847/1538-4357/ab9ff5


New Kind of Gamma Ray Burst is Ultra Long-Lasting

According to astronomer Andrew Levan, there’s an old adage in studying gamma ray bursts: “When you’ve seen one gamma ray burst, you’ve seen … only one gamma ray burst. They aren’t all the same,” he said during a press briefing on April 16 discussing the discovery of a very different kind of GRB – a type that comes in a new long-lasting flavor.

Three of these unusual long-lasting stellar explosions have recently been discovered using the Swift satellite and other international telescopes, and one, named GRB 111209A, is the longest GRB ever observed, with a duration of at least 25,000 seconds, or about 7 hours.

“We have observed the longest gamma ray burst in modern history, and think this event is caused by the death of a blue supergiant,” said Bruce Gendre, a researcher now associated with the French National Center for Scientific Research who led this study while at the Italian Space Agency’s Science Data Center in Frascati, Italy. “It caused the most powerful stellar explosion in recent history, and likely since the Big Bang occurred.”

The astronomers said these three GRBs represent a previously unrecognized class of these stellar explosions, which arise from the catastrophic deaths of supergiant stars hundreds of times larger than our Sun. GRBs are the most luminous and mysterious explosions in the Universe. The blasts emit surges of gamma rays — the most powerful form of light — as well as X-rays, and they produce afterglows that can be observed at optical and radio energies.

Swift, the Fermi telescope and other spacecraft detect an average of about one GRB each day. As to why this type of GRB hasn’t been detected before, Levan explained this new type appears to be difficult to find because of how long they last.

“Gamma ray telescopes usually detect a quick spike, and you look for a burst — at how many gamma rays come from the sky,” Levan told Universe Today. “But these new GRBs put out energy over a long period of time, over 10,000 seconds instead of the usual 100 seconds. Because it is spread out, it is harder to spot, and only since Swift launched do we have the ability to build up images of GBSs across the sky. To detect this new kind, you have to add up all the light over a long period of time.”

Levan is an astronomer at the University of Warwick in Coventry, England.

He added that these long-lasting GRBs were likely more common in the Universe’s past.

The number, duration and burst class for GRBs observed by Swift are shown in this plot. Colors link each GRB class to illustrations above the plot, which show the estimated sizes of the source stars. For comparison, the width of the yellow circle represents a star about 20 percent larger than the sun. Credit: Andrew Levan, Univ. of Warwick.

Traditionally, astronomers have recognized two types of GRBs: short and long, based on the duration of the gamma-ray signal. Short bursts last two seconds or less and are thought to represent a merger of compact objects in a binary system, with the most likely suspects being neutron stars and black holes. Long GRBs may last anywhere from several seconds to several minutes, with typical durations falling between 20 and 50 seconds. These events are thought to be associated with the collapse of a star many times the Sun’s mass and the resulting birth of a new black hole.

“It’s a very random process and every GRB looks very different,” said Levan during the briefing. “They all have a range of durations and a range of energies. It will take much bigger sample to see if this new type have more complexities than regular gamma rays bursts.”

All GRBs give rise to powerful jets that propel matter at nearly the speed of light in opposite directions. As they interact with matter in and around the star, the jets produce a spike of high-energy light.

Gendre and his colleagues made a detailed study of GRB 111209A, which erupted on Dec. 9, 2011, using gamma-ray data from the Konus instrument on NASA’s Wind spacecraft, X-ray observations from Swift and the European Space Agency’s XMM-Newton satellite, and optical data from the TAROT robotic observatory in La Silla, Chile. The 7-hour burst is by far the longest-duration GRB ever recorded.

Another event, GRB 101225A, exploded on December 25, 2010 and produced high-energy emission for at least two hours. Subsequently nicknamed the “Christmas burst,” the event’s distance was unknown, which led two teams to arrive at radically different physical interpretations. One group concluded the blast was caused by an asteroid or comet falling onto a neutron star within our own galaxy. Another team determined that the burst was the outcome of a merger event in an exotic binary system located some 3.5 billion light-years away.

“We now know that the Christmas burst occurred much farther off, more than halfway across the observable universe, and was consequently far more powerful than these researchers imagined,” said Levan.

Using the Gemini North Telescope in Hawaii, Levan and his team obtained a spectrum of the faint galaxy that hosted the Christmas burst. This enabled the scientists to identify emission lines of oxygen and hydrogen and determine how much these lines were displaced to lower energies compared to their appearance in a laboratory. This difference, known to astronomers as a redshift, places the burst some 7 billion light-years away.

Levan’s team also examined 111209A and the more recent burst 121027A, which exploded on Oct. 27, 2012. All show similar X-ray, ultraviolet and optical emission and all arose from the central regions of compact galaxies that were actively forming stars. The astronomers have concluded that all three GRBs constitute a new kind of GRB, which they are calling “ultra-long” bursts.

Astronomers suggest that blue supergiant stars may be the most likely sources of ultra-long GRBs. These stars hold about 20 times the sun’s mass and may reach sizes 1,000 times larger than the sun, making them nearly wide enough to span Jupiter’s orbit. Credit: NASA’s Goddard Space Flight Center/S. Wiessinger.

“Ultra-long GRBs arise from very large stars,” said Levan, “perhaps as big as the orbit of Jupiter. Because the material falling onto the black hole from the edge of the star has further to fall it takes longer to get there. Because it takes longer to get there, it powers the jet for a longer time, giving it time to break out of the star.”

Levan said that Wolf-Rayet stars best fit the description. “They are born with more than 25 times the Sun’s mass, but they burn so hot that they drive away their deep, outermost layer of hydrogen as an outflow we call a stellar wind,” he said. Stripping away the star’s atmosphere leaves an object massive enough to form a black hole but small enough for the particle jets to drill all the way through in times typical of long GRBs

John Graham and Andrew Fruchter, both astronomers at the Space Telescope Science Institute in Baltimore, provided details that these blue supergiant contain relatively modest amounts of elements heavier than helium, which astronomers call metals. This fits an apparent puzzle piece, that these ultra-long GRBs seem to have a strong intrinsic preference for low metallicity environments that contain just trace amounts of elements other than hydrogen and helium.

“High metalicity long duration GRBs do exist but are rare,” said Graham. “They occur at about 1/25th the rate (per unit of star formation) of the low metallicity events. This is good news for us here on Earth, as the likelihood of this type of GRB going off in our own galaxy is far less than previously thought.”

The astronomers discussed their findings Tuesday at the 2013 Huntsville Gamma-ray Burst Symposium in Nashville, Tenn., a meeting sponsored in part by the University of Alabama at Huntsville and NASA’s Swift and Fermi Gamma-ray Space Telescope missions. Gendre’s findings appear in the March 20 edition of The Astrophysical Journal.


Watch the video: Gamma Ray Camera Assembly (May 2022).


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