New observations from the NASA/ESA Hubble Space Telescope have investigated the nature of the powerful gamma-ray burst GRB 190114C by studying its environment. Gamma-ray bursts are the most powerful explosions in the Universe. They emit most of their energy in gamma rays, light which is much more energetic than the visible light we can see with our eyes. Hubble’s observations suggest that this particular burst displayed such powerful emission because the collapsing star was sitting in a very dense environment, right in the middle of a bright galaxy 5 billion light years away.
Gamma-ray bursts (GRBs) are one of the most energetic phenomena in the Universe, and also one of the least researched. These explosions of energy occur when a massive star goes supernova and emits twin beams of gamma rays that can be seen billions of light-years away. Because they are closely tied with the formation of black holes, scientists have been eager to study this rare occurrence in greater detail.
Unfortunately, few opportunities for this have occurred since GRBs are very short-lived (lasting for just seconds) and most have happened in distant galaxies. But thanks to the efforts using a suite of telescopes, astronomers were able to spot a GRB (designated GRB 190114C) back in January of 2019. Some of the radiation from this GRB was the highest energy ever observed, making this a milestone in the history of astronomy.
An artistic rendering of two neutron stars merging. Credit: NSF/LIGO/Sonoma State/A. Simonnet
In 2017, LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo detected gravitational waves coming from the merger of two neutron stars. They named that signal GW170817. Two seconds after detecting it, NASA’s Fermi satellite detected a gamma ray burst (GRB) that was named GRB170817A. Within minutes, telescopes and observatories around the world honed in on the event.
The Hubble Space Telescope played a role in this historic detection of two neutron stars merging. Starting in December 2017, Hubble detected the visible light from this merger, and in the next year and a half it turned its powerful mirror on the same location over 10 times. The result?
The deepest image of the afterglow of this event, and one chock-full of scientific detail.
The VISIR instrument on ESO’s VLT captured this stunning image of a newly-discovered massive binary star system. Nicknamed Apep after an ancient Egyptian deity, it could be the first gamma-ray burst progenitor to be found in our galaxy. Apep’s stellar winds have created the dust cloud surrounding the system, which consists of a binary star with a fainter companion. With 2 Wolf-Rayet stars orbiting each other in the binary, the serpentine swirls surrounding Apep are formed by the collision of two sets of powerful stellar winds, which create the spectacular dust plumes seen in the image. The reddish pinwheel in this image is data from the VISIR instrument on ESO’s Very Large Telescope (VLT), and shows the spectacular plumes of dust surrounding Apep. The blue sources at the centre of the image are a triple star system — which consists of a binary star system and a companion single star bound together by gravity. Though only two star-like objects are visible in the image, the lower source is in fact an unresolved binary Wolf-Rayet star. The triple star system was captured by the NACO adaptive optics instrument on the VLT. Credit: ESO/Callingham et al.
When stars reach the end of their lifespan, many undergo gravitational collapse and explode into a supernova, In some cases, they collapse to become black holes and release a tremendous amount of energy in a short amount of time. These are what is known as gamma-ray bursts (GRBs), and they are one of the most powerful events in the known Universe.
Recently, an international team of astronomers was able to capture an image of a newly-discovered triple star system surrounded by a “pinwheel” of dust. This system, nicknamed “Apep”, is located roughly 8,000 light years from Earth and destined to become a long-duration GRB. In addition, it is the first of its kind to be discovered in our galaxy.
An artist's impression of a Gamma Ray Burst. Credit: Stanford.edu
Last week’s announcement that Gravitational Waves (GW) have been detected for the first time—as a result of the merger of two black holes—is huge news. But now a Gamma Ray Burst (GRB) originating from the same place, and that arrived at Earth 0.4 seconds after the GW, is making news. Isolated black holes aren’t supposed to create GRB’s; they need to be near a large amount of matter to do that.
NASA’s Fermi telescope detected the GRB, coming from the same point as the GW, a mere 0.4 seconds after the waves arrived. Though we can’t be absolutely certain that the two phenomena are from the same black hole merger, the Fermi team calculates the odds of that being a coincidence at only 0.0022%. That’s a pretty solid correlation.
So what’s going on here? To back up a little, let’s look at what we thought was happening when LIGO detected gravitational waves.
Our understanding was that the two black holes orbited each other for a long time. As they did so, their massive gravity would have cleared the area around them of matter. By they time they finished circling each other and merged, they would have been isolated in space. But now that a GRB has been detected, we need some way to account for it. We need more matter to be present.
According to Abraham Loeb, of Harvard University, the missing piece of this puzzle is a massive star—itself the result of a binary star system combining into one—a few hundred times larger than the Sun, that spawned two black holes. A star this size would form a black hole when it exhausted its fuel and collapsed. But why would there be two black holes?
Again, according to Loeb, if the star was rotating at a high enough rate—just below its break up frequency—the star could actually form two collapsing cores in a dumbbell configuration, and hence two black holes. But now these two black holes would not be isolated in space, they would actually be inside a massive star. Or what was left of one. The remnants of the massive star is the missing matter.
When the black holes joined together, an outflow would be generated, which would produce the GRB. Or else the GRB came “from a jet originating out of the accretion disk of residual debris around the BH remnant,” according to Loeb’s paper. So why the 0.4 s delay? This is the time it took the GRB to cross the star, relative to the gravitational waves.
It sounds like a nice tidy explanation. But, as Loeb notes, there are some problems with it. The main question is, why was the GRB so weak, or dim? Loeb’s paper says that “observed GRB may be just one spike in a longer and weaker transient below the GBM detection threshold.”
But was the GRB really weak? Or was it even real? The European Space Agency has their own gamma ray detecting spacecraft, called Integral. Integral was not able to confirm the GRB signal, and according to this paper, the gamma ray signal was not real after all.
36 years ago today, a strange event was detected over the Southern Indian Ocean that remains controversial. On September 22nd, 1979, an American Vela Hotel satellite detected an atmospheric explosion over the southern Indian Ocean near the Prince Edward Islands. The event occurred at 00:53 Universal Time on the pre-dawn nighttime side of the Earth. Vela’s gamma-ray and x-ray detectors rang out in surprise, along with its two radiometers (known as Bhangmeters) which also captured the event.
The approximate location of the flash seen by the Vela-5b satellite Image credit: Wikimedia Commons/public domain
What was it?
Even today, the source of the Vela Incident remains a mystery. Designed to detect nuclear detonations worldwide and enforce the Partial Nuclear Test Ban Treaty, the Vela satellites operated for about ten years and were also famous for discovering evidence for extra-galactic gamma-ray bursts.
A Vela payload in the lab. Image credit: The U.S. Department of Defense
Vela-5B was the spacecraft from the series that detected the mysterious flash. A Titan-3C rocket launched Vela 5B (NORAD ID 1969-046E) on May 23rd, 1969 from Vandenberg Air Force Base in California.
One of the first things scientists realized early on in the Cold War is that the Universe is a noisy place, and that this extends across the electromagnetic spectrum. Meteors, lightning, cosmic rays and even distant astrophysical sources can seem to mimic certain signature aspects of nuclear detonations. The ability to discern the difference between human-made and natural events became of paramount importance and remains so to this day: the hypothetical scenario of a Chelyabinsk-style event over two nuclear armed states already on a political hair-trigger edge is a case in point.
Over the years, the prime suspect for the Vela Incident has been a joint South African-Israeli nuclear test. The chief piece of evidence is the characteristic ‘double-flash’ recorded by Vela, characteristic of a nuclear detonation. Said event would’ve been an approximately 3 kiloton explosion; for context, the bomb dropped on Hiroshima had a 15 kiloton yield, and the Chelyabinsk event had an estimated equivalent explosive force of 500 kilotons. As a matter of fact, the Vela Incident became a topic of discussion on the day Chelyabinsk occurred, as we sought to verify the assertion of whether Chelyabinsk was ‘the biggest thing’ since the 1908 Tunguska event.
A bolide event captured over Pennsylvania in early 2015. Image credit: Bill Ingalls/NASA
The Carter administration played down the Vela Incident at the time, though U.S. Air Force dispatched several WC-135B surveillance aircraft to the area, which turned up naught. Though detectors worldwide reported no increase of radioactive fallout, the ionospheric observatory at Arecibo did detect an atmospheric wave on the same morning as the event.
Israel ratified the Limited Test Ban Treaty in 1964. To date, Israel has never acknowledged that the test took place or the possession of nuclear weapons. Over the years, other suspect states have included Pakistan, France and India. Today, probably the only true final confirmation would come from someone stepping forward who was directly involved with the test, as it must have required the silence of a large number of personnel.
A comparison of the Vela event with a known nuclear test and a typical “zoo event’. Image credit: Vela Event Alert 747, Los Alamos Nat’l Laboratory
Was it a reentry or a bolide? Again, the signature double flash seen by the Vela satellite makes it unlikely. A micrometeoroid striking the spacecraft could have caused an anomalous detection known as a ‘zoo event,’ mimicking a nuclear test. Los Alamos researchers who have analyzed the event over the years remain convinced in the assertion that the 1979 Vela Incident had all the hallmark signatures of a nuclear test.
A U.S. nuclear detonation during Operation Upshot-Knothole in 1953. Image credit: National Nuclear Security Administration/Public Domain
Shortly after the Cold War, the U.S. Department of Defense made much of its atmospheric monitoring data public, revealing that small meteorites strike us much more often than realized. Sadly, this type of continual monitoring accompanied by public data release has declined in recent years mostly due to budgetary concerns, though monitoring of the worldwide environment for nuclear testing via acoustic microphone on land, sea and eyes overhead in space continues.
And it’s frightening to think how close we came to a nuclear exchange during the Cold War on several occasions. For example: in 1960, an Distant Early Warning System based in Thule, Greenland mistook the rising Moon for a Soviet missile launch (!) The United States also conducted nuclear tests in space shortly before the Test Ban Treaty went into effect, including Starfish Prime:
The Vela Incident remains a fascinating chapter of the Cold War, one where space and the geopolitical intrigue overlap. Even today, parsing out the difference between human-made explosions and the cataclysmic events that pepper the cosmos remains a primary concern for the continued preservation of our civilization.
Tactical nuclear weapons from around the world seen on display at the Nuclear Science Museum in Albuquerque, New Mexico. Image credit: Dave Dickinson
-Listen to an interesting discussion on monitoring nuclear plants worldwide via neutrino emissions.
-For a fascinating in-depth discussion on the continued relevance of the Vela Incident, check out this recent article by The Bulletin of Atomic Scientists.
An artistic image of the explosion of a star leading to a gamma-ray burst. (Source: FUW/Tentaris/Maciej Fro?ow)
Gamma ray bursts (GRBs) are some of the brightest, most dramatic events in the Universe. These cosmic tempests are characterized by a spectacular explosion of photons with energies 1,000,000 times greater than the most energetic light our eyes can detect. Due to their explosive power, long-lasting GRBs are predicted to have catastrophic consequences for life on any nearby planet. But could this type of event occur in our own stellar neighborhood? In a new paper published in Physical Review Letters, two astrophysicists examine the probability of a deadly GRB occurring in galaxies like the Milky Way, potentially shedding light on the risk for organisms on Earth, both now and in our distant past and future.
There are two main kinds of GRBs: short, and long. Short GRBs last less than two seconds and are thought to result from the merger of two compact stars, such as neutron stars or black holes. Conversely, long GRBs last more than two seconds and seem to occur in conjunction with certain kinds of Type I supernovae, specifically those that result when a massive star throws off all of its hydrogen and helium during collapse.
Perhaps unsurprisingly, long GRBs are much more threatening to planetary systems than short GRBs. Since dangerous long GRBs appear to be relatively rare in large, metal-rich galaxies like our own, it has long been thought that planets in the Milky Way would be immune to their fallout. But take into account the inconceivably old age of the Universe, and “relatively rare” no longer seems to cut it.
In fact, according to the authors of the new paper, there is a 90% chance that a GRB powerful enough to destroy Earth’s ozone layer occurred in our stellar neighborhood some time in the last 5 billion years, and a 50% chance that such an event occurred within the last half billion years. These odds indicate a possible trigger for the second worst mass extinction in Earth’s history: the Ordovician Extinction. This great decimation occurred 440-450 million years ago and led to the death of more than 80% of all species.
Today, however, Earth appears to be relatively safe. Galaxies that produce GRBs at a far higher rate than our own, such as the Large Magellanic Cloud, are currently too far from Earth to be any cause for alarm. Additionally, our Solar System’s home address in the sleepy outskirts of the Milky Way places us far away from our own galaxy’s more active, star-forming regions, areas that would be more likely to produce GRBs. Interestingly, the fact that such quiet outer regions exist within spiral galaxies like our own is entirely due to the precise value of the cosmological constant – the factor that describes our Universe’s expansion rate – that we observe. If the Universe had expanded any faster, such galaxies would not exist; any slower, and spirals would be far more compact and thus, far more energetically active.
In a future paper, the authors promise to look into the role long GRBs may play in Fermi’s paradox, the open question of why advanced lifeforms appear to be so rare in our Universe. A preprint of their current work can be accessed on the ArXiv.
A new research paper reveals more details of the effect gamma ray bursts (GRB) have had on the development of complex life throughout the cosmos. Illustration depicts a beam from a GRB as might have been directed toward early life on Earth during the Cambrian or Ordovician periods, ~500 million years ago. (Illustration Credit: T. Reyes)
If too close to an environment harboring complex life, a gamma ray burst could spell doom for that life. But could GRBs be the reason we haven’t yet found evidence of other civilizations in the cosmos? To help answer the big question of “where is everybody?” physicists from Spain and Israel have narrowed the time period and the regions of space in which complex life could persist with a low risk of extinction by a GRB.
GRBs are some of the most cataclysmic events in the Universe. Astrophysicists are astounded by their intensity, some of which can outshine the whole Universe for brief moments. So far, they have remained incredible far-off events. But in a new paper, physicists have weighed how GRBs could limit where and when life could persist and evolve, potentially into intelligent life.
In their paper, “On the role of GRBs on life extinctions in the Universe”, published in the journal Science, Dr. Piran from Hebrew University and Dr. Jimenez from University of Barcelona consider first what is known about gamma ray bursts. The metallicity of stars and galaxies as a whole are directly related to the frequency of GRBs. Metallicity is the abundance of elements beyond hydrogen and helium in the content of stars or whole galaxies. More metals reduce the frequency of GRBs. Galaxies that have a low metal content are prone to a higher frequency of GRBs. The researchers, referencing their previous work, state that observational data has shown that GRBs are not generally related to a galaxy’s star formation rate; forming stars, including massive ones is not the most significant factor for increased frequency of GRBs.
As fate would have it, we live in a high metal content galaxy – the Milky Way. Piran and Jimenez show that the frequency of GRBs in the Milky Way is lower based on the latest data available. That is the good news. More significant is the placement of a solar system within the Milky Way or any galaxy.
The brightest gamma-ray burst ever seen in X-rays temporarily blinded Swift’s X-ray Telescope on 21 June 2010. This image merges the X-rays (red to yellow) with the same view from Swift’s Ultraviolet/Optical Telescope, which showed nothing extraordinary. Credit: NASA/Swift/Stefan Immler
The paper states that there is a 50% chance of a lethal GRB’s having occurred near Earth within the last 500 million years. If a stellar system is within 13,000 light years (4 kilo-parsecs) of the galactic center, the odds rise to 95%. Effectively, this makes the densest regions of all galaxies too prone to GRBs to permit complex life to persist.
The Earth lies at 8.3 kilo-parsecs (27,000 light years) from the galactic center and the astrophysicists’ work also concludes that the chances of a lethal GRB in a 500 million year span does not drop below 50% until beyond 10 kilo-parsecs (32,000 light years). So Earth’s odds have not been most favorable, but obviously adequate. Star systems further out from the center are safer places for life to progress and evolve. Only the outlying low star density regions of large galaxies keep life out of harm’s way of gamma ray bursts.
The paper continues by describing their assessment of the effect of GRBs throughout the Universe. They state that only approximately 10% of galaxies have environments conducive to life when GRB events are a concern. Based on previous work and new data, galaxies (their stars) had to reach a metallicity content of 30% of the Sun’s, and the galaxies needed to be at least 4 kilo-parsecs (13,000 light years) in diameter to lower the risk of lethal GRBs. Simple life could survive repeated GRBs. Evolving to higher life forms would be repeatedly set back by mass extinctions.
Piran’s and Jimenez’s work also reveals a relation to a cosmological constant. Further back in time, metallicity within stars was lower. Only after generations of star formation – billions of years – have heavier elements built up within galaxies. They conclude that complex life such as on Earth – from jelly fish to humans – could not have developed in the early Universe before Z > 0.5, a cosmological red-shift equal to ~5 billion years ago or longer ago. Analysis also shows that there is a 95% chance that Earth experienced a lethal GRB within the last 5 billion years.
The question of what effect a nearby GRB could have on life has been raised for decades. In 1974, Dr. Malvin Ruderman of Columbia University considered the consequences of a nearby supernova on the ozone layer of the Earth and on terrestrial life. His and subsequent work has determined that cosmic rays would lead to the depletion of the ozone layer, a doubling of the solar ultraviolet radiation reaching the surface, cooling of the Earth’s climate, and an increase in NOx and rainout that effects biological systems. Not a pretty picture. The loss of the ozone layer would lead to a domino effect of atmospheric changes and radiation exposure leading to the collapse of ecosystems. A GRB is considered the most likely cause of the mass extinction at the end of the Ordovician period, 450 million years ago; there remains considerable debate on the causes of this and several other mass extinction events in Earth’s history.
The paper focuses on what are deemed long GRBs – lGRBs – lasting several seconds in contrast to short GRBs which last only a second or less. Long GRBs are believed to be due to the collapse of massive stars such as seen in supernovas, while sGRBs are from the collision of neutron stars or black holes. There remains uncertainty as to the causes, but the longer GRBs release far greater amounts of energy and are most dangerous to ecosystems harboring complex life.
The paper narrows the time and space available for complex life to develop within our Universe. Over the age of the Universe, approximately 14 billion years, only the last 5 billion years have been conducive to the creation of complex life. Furthermore, only 10% of the galaxies within the last 5 billion years provided such environments. And within only larger galaxies, only the outlying areas provided the safe distances needed to evade lethal exposure to a gamma ray burst.
This work reveals how well our Solar System fits within the ideal conditions for permitting complex life to develop. We stand at a fairly good distance from the Milky Way’s galactic center. The age of our Solar System, at approximately 4.6 billion years, lies within the 5 billion year safe zone in time. However, for many other stellar systems, despite how many are now considered to exist throughout the Universe – 100s of billions in the Milky Way, trillions throughout the Universe – simple is probably a way of life due to GRBs. This work indicates that complex life, including intelligent life, is likely less common when just taking the effect of gamma ray bursts into consideration.
Observations of the IC 310 radio galaxy have revealed the first lightning flashes observed from a black hole, astronomers say. Credit: Valencian Universities Network for the Promotion of Research, Development and Innovation (RUVID)
We know black holes are dangerous to people and galactic objects alike due to their immense gravity. But it turns out the galaxies that host supermassive black holes also have stormy interiors, at least according to one new study.
Scientists have found gamma-ray euptions emerging from the center of the IC 310 radio galaxy in Perseus — the strongest such variations in brightness ever found, they say — which they are comparing to a lightning storm.
It’s common for changes in brightness to happen in these galaxies as falling matter plunges into the black hole. The radio galaxies also produce jets that shoot matter away from the center at close to the speed of light.
Artist rendering of a supermassive black hole. Credit: NASA / JPL-Caltech.
What baffles researchers for IC 310 is how quickly they saw brightness shifts– on the order of five minutes, which is odd considering that the black hole’s event horizon (the point where there’s no way you’ll get out of there) requires 25 minutes to go across. This means the lightning is likely coming from a region that is smaller than the event horizon itself.
“We believe that in the black hole’s polar regions there are huge electric fields, which are able to accelerate fundamental particles at relativist speeds,” stated study leader Eduardo Ros, a researcher from the Max Planck Institute for Radio Astronomy and the Universitat de València.
“When they interact with others of lower energy, [they] are able to produce highly energized gamma rays,” he added. “We can imagine this process as a fierce electrical thunderstorm.”
Results of the study were published in the journal Science. Observatories participating included the Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes (MAGIC) at La Palma in the Canary Islands, and the European Very Large Baseline Interferometer Network.
Source: Valencian Universities Network for the Promotion of Research, Development and Innovation (RUVID)
The blazar 3C 454.3 photographed by the Sloan Digital Sky Survey. It's currently in bright outburst and nearly as bright as the star next to it. Both are about magnitude +13.6. Credit: SDSS
Have an 8-inch or larger telescope? Don’t mind staying up late? Excellent. Here’s a chance to stare deeper into the known fabric of the universe than perhaps you’ve ever done before. The violent blazer 3C 454.3 is throwing a fit again, undergoing its most intense outburst seen since 2010. Normally it sleeps away the months around 17th magnitude but every few years, it can brighten up to 5 magnitudes and show in amateur telescopes. While magnitude +13 doesn’t sound impressive at first blush, consider that 3C 454.3 lies 7 billion light years from Earth. When light left the quasar, the sun and planets wouldn’t have skin in the game for another two billion years.
If we could see the blazar 3C 354.3 up close it would look something like this. A bright accretion disk surrounds a black hole. Twin jets of radiation beam from the center. Credit: Cosmovision
Blazars form in the the cores of active galaxies where supermassive black holes reside. Matter falling into the black hole spreads into a spinning accretion disk before spiraling down the hole like water down a bathtub drain.
Superheated to millions of degrees by gravitational compression the disk glows brilliantly across the electromagnetic spectrum. Powerful spun-up magnetic fields focus twin beams of light and energetic particles called jets that blast into space perpendicular to the disk.
Blazars and quasars are thought to be one and the same, differing only by the angle at which we see them. Quasars – far more common – are actively- munching supermassive black holes seen from the side, while in blazars – far more rare – we stare directly or nearly so into the jet like looking into the beam of a flashlight.
An all-sky view in gamma ray light made with the Fermi Gamma-ray Space Telescope shows bright gamma-ray emission in the plane of the Milky Way (center), bright pulsars and super-massive black holes including the active blazar 3C 454.3 at lower left. Credit: NASA/DOE/International LAT Team
3C 454.3 is one of the top ten brightest gamma ray sources in the sky seen by the Fermi Gamma-ray Space Telescope. During its last major flare in 2005, the blazar blazed with the light of 550 billion suns. That’s more stars than the entire Milky Way galaxy! It’s still not known exactly what sets off these periodic outbursts but possible causes include radiation bursts from shocked particles within the jet or precession (twisting) of the jet bringing it close to our line of sight.
3c 454.3 is near the star Alpha Pegasi just to the west of the Great Square. Use this chart to star hop from Alpha to IM Peg (mag. ~ 5.7). Once there, the detailed map below will guide you to the blazar. Stellarium
The current outburst began in late May when the Italian Space Agency’s AGILE satellite detected an increase in gamma rays from the blazar. Now it’s bright visually at around magnitude +13.6 and fortunately not difficult to find, located in the constellation Pegasus near the bright star Alpha Pegasi (Markab) in the lower right corner of the Great Square asterism.
Using the wide view map, find your way to IM Peg via Markab and then make a copy of the detailed map below to use at the telescope to star hop to 3C 454.3. The blazar lies immediately south of a star of similar magnitude. If you see what looks like a ‘double star’ at the location, you’ve nailed it. Incredible isn’t it to look so far into space back to when the universe was just a teenager? Blows my mind every time.
Detailed map showing the location of the blazar 3C 454.3. I’ve drawn a small asterism using a group of brighter stars with their magnitudes marked. A scale showing 30 arc minutes (1/2 degree) is at right. Click to enlarge. Created with Chris Marriott’s SkyMap software
To further explore 3C 454.3 and blazars vs. quasars I encourage you to visit check out Stefan Karge’s excellent Frankfurt Quasar Monitoring site. It’s packed with great information and maps for finding the best and brightest of this rarified group of observing targets. Karge suggests that flickering of the blazar may cause it to appear somewhat brighter or fainter than the current magnitude. You’re watching a violent event subject to rapid and erratic changes. For an in-depth study of 3C 454.3, check out the scientific paper that appeared in the 2010 Astrophysical Journal.
Learn more about quasars and blazers with a bit of great humor
Finally, I came across a wonderful video while doing research for this article I thought you’d enjoy as well.
An artist’s conception of the environment around GRB 020819B based on ALMA observations. Image Credit: NAOJ
Gamma-ray bursts (GRBs) represent the most powerful explosions in the cosmos, sending out as much energy in a matter of seconds as our Sun will give off during its entire 10-billion-year lifespan.
These powerful explosions are thought to be triggered when dying stars collapse into jet-spewing black holes. Yet no one has ever witnessed a GRB directly. Instead astronomers are left to study their fading light.
But some GRBs mysteriously seem to have no afterglow. Now, observations from the Atacama Large Millimeter/submillimeter Array (ALMA) are shedding light on these so-called dark bursts.
One possible explanation is that dark bursts explode so far away their visible light is extinguished due to the expansion of the Universe. Another possible explanation is that dark bursts explode in galaxies with unusually thick amounts of interstellar dust, which absorb a burst’s light.
Neither explanation, however, seems likely as astronomers anticipate that GRB progenitors — massive stars — are found in active star-forming regions surrounded by large amounts of molecular gas. But unfortunately there has never been an observational result to back up this theory either.
So astronomers have been working hard to better understand GRBs by studying their host galaxies. Now, a Japanese team of astronomers led by Bunyo Hatsukade from the National Astronomical Observatory in Japan, has used ALMA to report the first-ever map of molecular gas and dust in two galaxies that were previously rocked by GRBs.
Hatsukade and colleagues detected the radio emission from molecular gas and dust in two dark host galaxies — GRB 020819B and GRB 051022 — at about 4.3 billion and 6.9 billion light-years away, respectively.
“We have been searching for molecular gas in GRB host galaxies for over 10 years using various telescopes around the world,” said Kotaro Kohno from the University of Tokyo in a press release. “As a result of our hard work, we finally achieved a remarkable breakthrough using the power of ALMA. We are very excited with what we have achieved.”
Watch the video below for an artist concept animation of the environment around GRB 020819B based on ALMA observations:
The telescope’s high sensitivity enabled the team of astronomers to detect the emission from molecular gas, as opposed to most telescopes, which can only probe absorption along the line of sight. This combined with its high spatial resolution provided the first detailed map of the molecular gas and dust throughout a GRB host galaxy.
Surprisingly, less gas was observed than expected, and correspondingly much more dust. The ratio of dust to molecular gas at the GRB site is 10 times higher than in normal environments.
Observations of the host galaxy for GRB 020819B. Radio measurements of molecular gas (left) and dust (middle), both of which are observed with ALMA. An image in visible-light captured by the Frederick C. Gillett Gemini North Telescope (right). The cross indicates the location of the GRB site. Image Credit: Bunyo Hatsukade(NAOJ), ALMA (ESO/NAOJ/NRAO)
“We didn’t expect that GRBs would occur in such a dusty environment with a low ratio of molecular gas to dust,” said Hatsukade. “This indicates that the GRB occurred in an environment quite different from a typical star-forming region.”
The research team thinks the high proportion of dust compared to molecular gas is likely due to the intense ultraviolet radiation from the young, massive stars, which will break up any molecular gas while leaving the dust relatively undisturbed.
It’s becoming clear that dust absorbs the afterglow radiation, causing these dark gamma-ray bursts. The team plans to carry out further observations and is excited to use ALMA’s incredible sensitivity to probe other host galaxies.
