What would a gamma-ray burst sound like? No one really knows, but members of the team that work with the Fermi Large Area Telescope (LAT) have translated gamma-ray measurements into musical notes and have created a “song” from the photons from one of the most energetic of these powerful explosions, GRB 080916C which occurred in September of 2008.
“In translating the gamma-ray measurements into musical notes we assigned the photons to be “played” by different instruments (harp, cello, or piano) based on the probabilities that they came from the burst,” the team wrote in the Fermi blog. “By converting gamma rays into musical notes, we have a new way of representing the data and listening to the universe.” Continue reading “A Gamma-Ray Burst as Music”
When it comes to high-energy sources, no one knows them better than NASA’s Fermi Gamma-ray Space Telescope. Taking a portrait of the entire sky every 240 minutes, the program is continually renewing and updating its sources and once a year the scientists harvest the data. These annual gatherings are then re-worked with new tools to produce an ever-deeper look into the Universe around us.
Fermi is famous for its analysis of steady gamma-ray sources, numerous transient events, the dreaded GRB and even flares from the Sun. Its all-sky map absolutely bristles with the energy that’s out there and earlier this year a second catalog of objects was released to eager public eyes. An astounding 1,873 objects were detected by the satellite’s Large Area Telescope (LAT) and this high energy form of light is turning some heads.
“More than half of these sources are active galaxies, whose massive black holes are responsible for the gamma-ray emissions that the LAT detects,” said Gino Tosti, an astrophysicist at the University of Perugia in Italy and currently a visiting scientist at SLAC National Accelerator Laboratory in Menlo Park, California.
One of the scientists who led the new compilation, Tosti presented a paper on the catalog at a meeting of the American Astronomical Society’s High Energy Astrophysics Division in Newport, R.I. “What is perhaps the most intriguing aspect of our new catalog is the large number of sources not associated with objects detected at any other wavelength,” he noted.
If we were to look at Fermi’s gathering experience as a harvest, we’d see two major components – crops and mystery. Add to that a bushel of pulsars, a basket of supernova remnants and a handful of other things, like galaxies and globular clusters. For Fermi farmers, harvesting new types of gamma-ray-emitting objects that are from “unassociated sources” would account for about 31% of the cash crop. However, the brave little Fermi LAT is producing results from some highly unusual sources. Mystery growth? Think this way… If it’s a light source, then it has a spectrum. When it comes to gamma rays, they’re seen at different energies. “At some energy, the spectra of many objects display what astronomers call a spectral break, that is, a greater-than-expected drop-off in the number of gamma rays seen at increasing energies.” Let’s take a look at two…
Within our galaxy is 2FGL J0359.5+5410. Right now, scientists just don’t understand what it is… only that it’s located in the constellation Camelopardalis. Since it appears about midplane, we’re just assuming it belongs to the Milky Way. From its spectrum, it might be a pulsar – but one without a pulse. Or how about 2FGL J1305.0+1152? It also resides along the midplane and smack dab in the middle of galaxy country – Virgo. Even after two years, Fermi can’t tease out any more details. It doesn’t even have a spectral break!
NASA’s Swift, Hubble Space Telescope and Chandra X-ray Observatory have teamed up to study one of the most puzzling cosmic blasts yet observed. More than a week later, high-energy radiation continues to brighten and fade from its location.
Astronomers say they have never seen anything this bright, long-lasting and variable before. Usually, gamma-ray bursts mark the destruction of a massive star, but flaring emission from these events never lasts more than a few hours.
Although research is ongoing, astronomers say that the unusual blast likely arose when a star wandered too close to its galaxy’s central black hole. Intense tidal forces tore the star apart, and the infalling gas continues to stream toward the hole. According to this model, the spinning black hole formed an outflowing jet along its rotational axis. A powerful blast of X- and gamma rays is seen if this jet is pointed in our direction.
On March 28, Swift’s Burst Alert Telescope discovered the source in the constellation Draco when it erupted with the first in a series of powerful X-ray blasts. The satellite determined a position for the explosion, now cataloged as gamma-ray burst (GRB) 110328A, and informed astronomers worldwide.
As dozens of telescopes turned to study the spot, astronomers quickly noticed that a small, distant galaxy appeared very near the Swift position. A deep image taken by Hubble on April 4 pinpoints the source of the explosion at the center of this galaxy, which lies 3.8 billion light-years away.
That same day, astronomers used NASA’s Chandra X-ray Observatory to make a four-hour-long exposure of the puzzling source. The image, which locates the object 10 times more precisely than Swift can, shows that it lies at the center of the galaxy Hubble imaged.
“We know of objects in our own galaxy that can produce repeated bursts, but they are thousands to millions of times less powerful than the bursts we are seeing now. This is truly extraordinary,” said Andrew Fruchter at the Space Telescope Science Institute in Baltimore.
“We have been eagerly awaiting the Hubble observation,” said Neil Gehrels, the lead scientist for Swift at NASA’s Goddard Space Flight Center in Greenbelt, Md. “The fact that the explosion occurred in the center of a galaxy tells us it is most likely associated with a massive black hole. This solves a key question about the mysterious event.”
Most galaxies, including our own, contain central black holes with millions of times the sun’s mass; those in the largest galaxies can be a thousand times larger. The disrupted star probably succumbed to a black hole less massive than the Milky Way’s, which has a mass four million times that of our sun
Astronomers previously have detected stars disrupted by supermassive black holes, but none have shown the X-ray brightness and variability seen in GRB 110328A. The source has repeatedly flared. Since April 3, for example, it has brightened by more than five times.
Scientists think that the X-rays may be coming from matter moving near the speed of light in a particle jet that forms as the star’s gas falls toward the black hole.
“The best explanation at the moment is that we happen to be looking down the barrel of this jet,” said Andrew Levan at the University of Warwick in the United Kingdom, who led the Chandra observations. “When we look straight down these jets, a brightness boost lets us view details we might otherwise miss.”
This brightness increase, which is called relativistic beaming, occurs when matter moving close to the speed of light is viewed nearly head on.
Astronomers plan additional Hubble observations to see if the galaxy’s core changes brightness.
Gamma Ray Bursts (GRBs) are among the most energetic phenomena astronomers regularly observe. These events are triggered by massive explosions and a large amount of the energy if focused into narrow beams that sweep across the universe. These beams are so tightly concentrated that they can be seen across the visible universe and allow astronomers to probe the universe’s history. If such an event happened in our galaxy and we stood in the path of the beam, the effects would be pronounced and may lead to large extinctions. Yet one of the most energetic GRBs on record (GRB 080607) was shrouded in cloud of gas and dust dimming the blast by a factor of 20 – 200, depending on the wavelength. Despite this strong veil, the GRB was still bright enough to be detected by small optical telescopes for over an hour. So what can this hidden monster tell astronomers about ancient galaxies and GRBs in general?
GRB 080607 was discovered on June 6, 2008 by the Swift satellite. Since GRBs are short lived events, searches for them are automated and upon detection, the Swift satellite immediately oriented itself towards the source. Other GRB hunting satellites quickly joined in and ground based observatories, including ROTSE-III and Keck made observations as well. This large collection of instruments allowed astronomers, led by D. A. Perley of UC Berkley, to develop a strong understanding of not just the GRB, but also the obscuring gas. Given that the host galaxy lies at a distance of over 12 billion light years, this has provided a unique probe into the nature of the environment of such distant galaxies.
One of the most surprising features was unusually strong absorption near 2175 °A. Although such absorption has been noticed in other galaxies, it has been rare in galaxies at such large cosmological distances. In the local universe, this feature seems to be most common in dynamically stable galaxies but tends to be “absent in more disturbed locations such as the SMC, nearby starburst galaxies” as well as some regions of the Milky Way which more turbulence is present. The team uses this feature to imply that the host galaxy was stable as well. Although this feature is familiar in nearby galaxies, observing it in this case makes it the furthest known example of this phenomenon. The precise cause of this feature is not yet known, although other studies have indicated “polycyclic aromatic hydrocarbons and graphite” are possible suspects.
Earlier studies of this event have shown other novel spectral features. A paper by Sheffer et al. notes that the spectrum also revealed molecular hydrogen. Again, such a feature is common in the local universe and many other galaxies, but never before has such an observation been made linked to a galaxy in which a GRB has occurred. Molecular hydrogen (as well as other molecular compounds) become disassociated at high temperatures like the ones in galaxies containing large amounts of star formation that would produce regions with large stars capable of triggering GRBs. With observations of one molecule in hand, this lead Sheffer’s team to suspect that there might be large amounts of other molecules, such as carbon monoxide (CO). This too was detected making yet another first for the odd environment of a GRB host.
This unusual environment may help to explain a class of GRBs known as “subluminous optical bursts” or “dark bursts” in which the optical component of the burst (especially the afterglow) is less bright than would be predicted by comparison to more traditional GRBs.
A record-breaking gamma ray burst from beyond the Milky Way temporarily blinded the X-ray eye on NASA’s Swift space observatory on June 21, 2010. The X-rays traveled through space for 5-billion years before slamming into and overwhelming the space-based telescope. “This gamma-ray burst is by far the brightest light source ever seen in X-ray wavelengths at cosmological distances,” said David Burrows, senior scientist and professor of astronomy and astrophysics at Penn State University and the lead scientist for Swift’s X-ray Telescope (XRT).
A gamma-ray burst is a violent eruption of energy from the explosion of a massive star morphing into a new black hole. This mega burst, named GRB 100621A, is the brightest X-ray source that Swift has detected since the observatory began X-ray observation in early 2005.
Although Swift satellite was designed specifically to study gamma-ray bursts, the instrument was not designed to handle an X-ray blast this bright. “The intensity of these X-rays was unexpected and unprecedented” said Neil Gehrels, Swift’s principal investigator at NASA’s Goddard Space Flight Center. “Just when we were beginning to think that we had seen everything that gamma-ray bursts could throw at us, this burst came along to challenge our assumptions about how powerful their X-ray emissions can be.”.
What is a supernova? Well, “nova” means “new star”, and “super” means “really big”, like supermarket, so a supernova is a really bright new star. That’s where the word comes from, but today it has a rather more precise meaning, namely a once-off variable star which has a peak brightness similar to, or greater than, that of a typical galaxy.
Supernovae aren’t new stars in the sense that they were not stars before they became supernovae; the progenitor – what the star was before it went supernova – of a supernova is just a star (or a pair of stars), albeit an unusual one.
From what we see – the rise of the intensity of light (and electromagnetic radiation in general) to a peak, its decline; the lines which show up in the spectra (and the ones which don’t), etc – we can classify supernovae into several different types. There are two main types, called Type I and Type II. The difference between them is that Type I supernovae have no lines of hydrogen in their spectra, while Type II ones do.
Centuries of work by astronomers and physicists have given us just two kinds of progenitors: white dwarfs and massive (>8 sols) stars; and just two key physical mechanisms: nuclear detonation and core collapse.
Core collapse supernovae happen when a massive star tries to fuse iron in its core … bad move, because fusing iron requires energy (rather than liberates it), and the core suddenly collapses due to its gravity. A lot of interesting physics happens when such a core collapses, but it either results in a neutron star or a black hole, and a vast amount of energy is produced (most of it in the form of neutrinos!). These supernovae can be of any type, except a sub-type of Type I (called Ia). They also produce the long gamma-ray bursts (GRB).
Detonation is when a white dwarf star undergoes almost simultaneous fusion of carbon or oxygen throughout its entire body (it can do this because a white dwarf has the same temperature throughout, unlike an ordinary star, because its electrons are degenerate). There are at least two ways such a detonation can be triggered: steady accumulation of hydrogen transferred from a close binary companion, or a collision or merger with a neutron star or another white dwarf. These supernovae are all Type Ia.
One other kind of supernova: when two neutron stars merge, or a ~solar mass black hole and a neutron star merge – as a result of loss of orbital energy due to gravitational wave radiation – an intense burst of gamma-rays results, along with a fireball and an afterglow (as the fireball cools). We see such an event as a short GRB, but if we were unlikely enough to be close to such a stellar death, we’d certainly see it as a spectacular supernova!
Are the relativistic jets of long gamma ray bursts (GRBs) produced by brand new black holes? Do some core-collapse supernovae result in black holes and relativistic jets?
The answer to both questions is ‘very likely, yes’! And what recent research points to those answers? Study of an Ic supernova (SN 2007gr), and an Ibc one (SN 2009bb), by two different teams, using archived Gamma-Ray Burst Coordination Network data, and trans-continental Very Long Baseline Interferometry (VLBI) radio observations.
“In every respect, these objects look like gamma-ray bursts – except that they produced no gamma rays,” said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.
The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away (it’s one of the closest Ic supernovae detected in the radio waveband). The team which studied this supernova using VLBI was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, and included Chryssa Kouveliotou, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.
The researchers searched for gamma-rays associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA’s Goddard Space Flight Center in Greenbelt, Md. This project distributes and archives observations of gamma-ray bursts by NASA’s SWIFT spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.
“The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light,” explained Kouveliotou, co-author of one of the new studies. “Yet, in these new objects, we’re tracking gas moving some 20 times faster than this.”
Unlike typical core-collapse supernovae, the stars that produce long gamma-ray bursts possess a “central engine” – likely a nascent black hole – that drives particle jets clocked at more than 99 percent the speed of light (short GRBs are likely produced by the collision/merger of two neutron stars, or a neutron star and a stellar mass black hole).
By contrast, the fastest outflows detected from SN 2009bb reached 85 percent of the speed of light and SN 2007gr reached more than 60 percent of light speed; this is “mildly relativistic”.
“These observations are the first to show some supernovae are powered by a central engine,” Soderberg said. “These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites.”
The VLBI radio observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur. The team led by Paragi included 14 members from 12 institutions spread over seven countries, the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.
“Using the electronic VLBI technique eliminates some of the major issues,” said Huib Jan van Langevelde, the director of JIVE “Moreover it allows us to produce immediate results necessary for the planning of additional measurements.”
Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a long gamma-ray burst. In some cases, the star’s jets may not be angled in a way to produce a detectable burst; in others, the energy of the jets may not be enough to allow them to blast through the overlying bulk of the dying star.
“We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” Kouveliotou said. “These likely represent most of the population.”
Nova, “new star”; supernova, a “super” nova; hypernova, a super-duper, or super super, nova!
This word appeared in the astronomical literature at least as early as 1982, and refers to a kind of core-collapse supernova far brighter (>100 times) than usual; its meaning has changed somewhat, and today generally refers to the core collapse of particularly massive stars (>100 sols), whether or not they are spectacularly brighter than other core-collapse supernovae (though they are that too).
Most times you’ll come across hypernovae in material on gamma ray bursts (GRBs), many of which seem to involve emission of electromagnetic radiation with total energy many times that from ordinary supernovae (whether core collapse or Type Ia). Long-duration GRBs have jets, presumably from the poles of the temporary accretion disk which forms around the new black hole at the heart of the collapsed core of the progenitor (short-duration GRBs, which also produce jets, are thought to be the merger of two neutron stars, or a neutron star and a stellar-mass black hole), but even when viewed side-on (i.e. not looking into one of the jets), these GRBs are intrinsically much brighter than other core collapse supernovae.
If a supernova were to occur a few hundred light-years from us, we’d certainly notice it, and there might be some impact on our atmosphere; if there was a hypernova the same distance away, we’d suffer (not only from the increased incidence of cancer due to the far greater intensity of cosmic rays, but also from changes in weather and climate, and damage to ecosystems); if the jet were aimed directly at us, we’d be toast (while those on the other side of the world would survive the few seconds-long blast, they’d die from the consequences).
Fortunately, it seems there are no stars likely to go hypernova on us … at least not within a few tens of thousands of light-years. Whew!
For the first time, astronomers have found a supernova explosion with properties similar to a gamma-ray burst, but without seeing any gamma rays from it. Radio observations with the Very Large Array (VLA) showed material expelled from supernova explosion SN2009bb at speeds approaching the speed of light. The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. But astronomers don’t think this blast is one-of-a-kind, and say that more radio observations will point the way toward locating many more examples of these mysterious explosions.
“We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites,” said Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics.
Usually supernova explosions blasts the star’s material outward in a roughly-spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.
When the nuclear fusion reactions at the cores of very massive stars no longer can provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star’s material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a “core-collapse supernova” as the cause of one kind of gamma-ray burst.
The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.
“This is the only way we know that a supernova explosion could accelerate material to such speeds,” Soderberg said.
Until now, no such “engine-driven” supernova had been found any way other than by detecting gamma rays emitted by it.
“Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough. With the new capabilities of the Expanded VLA coming soon, we believe we’ll find more in the future through radio observations than with gamma-ray satellites,” Soderberg said.
Why didn’t anyone see gamma rays from this explosion? “We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen,” Soderberg said. In that case, finding such blasts through radio observations will allow scientists to discover a much larger percentage of them in the future.
“Another possibility,” Soderberg adds, “is that the gamma rays were ‘smothered’ as they tried to escape the star. This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites.”
One important question the scientists hope to answer is just what causes the difference between the “ordinary” and the “engine-driven” core-collapse supernovae. “There must be some rare physical property that separates the stars that produce the ‘engine-driven’ blasts from their more-normal cousins,” Soderberg said. “We’d like to find out what that property is.”
One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen. However, Soderberg points out, that does not seem to be the case for this supernova.
This research will be published in January 28 issue of the journal Nature.
Fermi’s Large Area Telescope has detected bursts of gamma-rays in the binary system Cygnus X-3, which astronomers say are coming from a microquasar. While microquasars have strong emissions across is a broad range of wavelengths, this is the first time this type of object has been detected in gamma rays. “Cygnus X-3 is a genuine microquasar and it’s the first for which we can prove high-energy gamma-ray emission,” said Stéphane Corbel at Paris Diderot University in France.
Microquasars are stellar mass object that displays in miniature some of the properties of quasars: a normal star begins shedding its matter onto either a neutron star or a black hole. This phenomenon produces large amounts of radiation and “jets” of material moving at relativistic speeds—more than 10% the speed of light—away from the star. These “relativistic jets” are a great mystery that astronomers are still trying to understand, but this new gamma-ray microquasar could provide new ways to study them.
At the center of Cygnus X-3 lies a massive Wolf-Rayet star. With a surface temperature of 100,255.372 Kelvin (180,000 degrees F,) or about 17 times hotter than the sun, the star is so hot that its mass bleeds into space in the form of a powerful outflow called a stellar wind. “In just 100,000 years, this fast, dense wind removes as much mass from the Wolf-Rayet star as our sun contains,” said Robin Corbet at the University of Maryland, Baltimore County.
The researchers matched the gamma-rays to the known orbital period of the Cygnus X-3 microquasar in order to confirm that the strong pulses of radiation were, in fact, originating from the object. They also matched the gamma-rays with radio emission from the relativistic jets of Cygnus X-3.
Every 4.8 hours, a compact companion embedded in a disk of hot gas wheels around the star. “This object is most likely a black hole, but we can’t yet rule out a neutron star,” Corbet said.
Between Oct. 11 and Dec. 20, 2008, and again between June 8 and Aug. 2, 2009, Cygnus X-3 was unusually active. The team found that outbursts in the system’s gamma-ray emission preceded flaring in the radio jet by roughly five days, strongly suggesting a relationship between the two.
These new findings should provide more information about the formation of such mysterious and fast-moving relativistic jets. This research appears in the 26 November issue of Science Express.
Read the team’s abstract