Dying supergiant stars implicated in hours-long gamma-ray bursts

GRB 111209A exploded on December 9, 2011. The blast produced high-energy emission for an astonishing seven hours, earning a record as the longest-duration GRB ever observed. This false-color image shows the event as captured by the X-ray Telescope aboard NASA's Swift satellite. //NASA/Swift/B. Gendre (ASDC/INAF-OAR/ARTEMIS)

By NASA's Goddard Space Flight Center, Greenbelt, Maryland

Published: April 17, 2013

Three unusually long-lasting stellar explosions discovered by NASA’s Swift satellite represent a previously unrecognized class of gamma-ray bursts (GRBs). Two international teams of astronomers studying these events conclude that they likely arose from the catastrophic death of supergiant stars hundreds of times larger than the 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 are visible at optical and radio energies. Swift, Fermi, and other spacecraft detect an average of about one GRB each day.

“We have seen thousands of gamma-ray bursts over the past four decades, but only now are we seeing a clear picture of just how extreme these extraordinary events can be,” 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.

Prior to Swift’s launch in 2004, satellite instruments were much less sensitive to GRBs that unfolded over comparatively long timescales.

Traditionally, astronomers have recognized two GRB types, 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.

Both scenarios 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 December 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 burst continued to produce high-energy emission for an astonishing seven hours, making it by far the longest-duration GRB ever recorded.

Another event, GRB 101225A, exploded on Christmas Day in 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 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 Andrew Levan from the University of Warwick in Coventry, England.

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.

As a part of this study, Levan’s team also examined 111209A and the more recent burst 121027A, which exploded October 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 conclude that all three GRBs constitute a hitherto unrecognized group of “ultralong” bursts.

To account for the normal class of long GRBs, astronomers envision a star similar to the Sun’s size but with many times its mass. The mass must be high enough for the star to undergo an energy crisis, with its core ultimately running out of fuel and collapsing under its own weight to form a black hole. Some of the matter falling onto the nascent black hole becomes redirected into powerful jets that drill through the star, creating the gamma-ray spike, but because this burst is short-lived, the star must be comparatively small.

“Wolf-Rayet stars fit these requirements,” said Levan. “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.” 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.

Because ultralong GRBs persist for periods up to 100 times greater than long GRBs, they require a stellar source of correspondingly greater physical size. Both groups suggest that the likely candidate is a supergiant, a star with about 20 times the Sun’s mass that still retains its deep hydrogen atmosphere, making it hundreds of times the Sun’s diameter.

Gendre’s team goes further, suggesting that GRB 111209A marked the death of a blue supergiant containing relatively modest amounts of elements heavier than helium, which astronomers call metals.

“The metal content of a massive star controls the strength of its stellar wind, which determines how much of the hydrogen atmosphere it retains as it grows older,” Gendre said. The star’s deep hydrogen envelope would take hours to complete its fall into the black hole, which would provide a long-lived fuel source to power an ultralong GRB jet.

Metal content also plays a strong role in the development of long GRBs, according to a detailed study presented by John Graham and Andrew Fruchter, both from the Space Telescope Science Institute in Baltimore, Maryland.

Stars make heavy elements throughout their energy-producing lives and during supernova explosions, and each generation of stars enriches interstellar gas with a greater proportion of them. While astronomers have noted that long GRBs occur much more frequently in metal-poor galaxies, a few of them have suggested that this pattern is not intrinsic to the stars and their environments.

To examine this possibility, Graham and Fruchter developed a novel method that allowed them to compare galaxies by their underlying rates of star formation. They then examined galaxies that served as hosts for long GRBs and various types of supernovae as well as a control sample of 20,000 typical galaxies in the Sloan Digital Sky Survey.

The astronomers found that 75 percent of long GRBs occurred among the 10 percent of star formation with the lowest metal content. While the study found a few long GRBs in environments with high-metal content, like our galaxy, these occur at only about 4 percent the rate seen in low-metal environments per unit of underlying star formation.

“Most stars form in metal-rich environments, and this has a side effect of decreasing the prevalence of long GRBs as the universe grows older,” Graham said. “And while a nearby long GRB would be catastrophic to life on Earth, our study shows that galaxies like our own are much less likely to produce them.”

The astronomers suspect this pattern reflects a difference in how well a massive star manages to retain its rotation speed. Rising metal content means stronger stellar winds. As these winds push material off the star’s surface, the star’s rotation gradually decreases in much the same way as a spinning ice skater slows when she extends her arms. Stars with more rapid rotation may be more likely to produce a long GRB.

Graham and Fruchter hypothesize that the few long GRBs found in high-metal environments received an assist from the presence of a nearby companion star. By feeding mass — and with it, rotational energy — onto the star that explodes, a companion serves as the physical equivalent of someone pushing a slowly spinning ice skater back up to a higher rotational speed.

GRB 101225A, better known as the "Christmas burst," was an unusually long-lasting gamma-ray burst. Because its distance was not measured, astronomers came up with two radically different interpretations. In the first, a solitary neutron star in our  galaxy shredded and accreted an approaching comet-like body. In the second, a neutron star is engulfed by, spirals into, and merges with an evolved giant star in a distant galaxy. Now, thanks to a measurement of the Christmas burst’s host galaxy, astronomers have determined that it represented the collapse and explosion of a supergiant star hundreds of times larger than the Sun

Swift Satellite records early phase of gamma ray burst

Illustration of GRB
Credit:NASA
Monday, March 02, 2009

UK astronomers, using a telescope aboard the NASA Swift Satellite, have captured information from the early stages of a gamma ray burst - the most violent and luminous explosions occurring in the Universe since the Big Bang. The work was published on Friday 27th February in the Monthly Notices of the Royal Astronomical society.

Swift is able to both locate and point at gamma ray bursts (GRBs) far quicker than any other telescope, so by using its Ultraviolet/Optical Telescope (UVOT) the astronomers were able to obtain an ultraviolet spectrum of a GRB just 251 seconds after its onset - the earliest ever captured. Further use of the instrument in this way will allow them to calculate the distance and brightness of GRBs within a few hundred seconds of their initial outburst, and gather new information about the causes of bursts and the galaxies they originate from.

“The UVOT’s wavelength range, coupled with the fact that Swift is a space observatory with a speedy response rate, unconstrained by time of day or weather, has allowed us to collect this early ultraviolet spectrum,” said Martin Still from the Mullard Space Science Laboratory (MSSL) at UCL.

Paul Kuin, also from MSSL, who works on the calibration of the UVOT instrument explained: “By looking at these earlier moments of gamma ray bursts, we will not only be able to better calculate things such as the luminosity and distance of a burst, but to find out more about the galaxies that play host to them and the impact these explosions have on their environments. Once this new technique is applied to much brighter bursts, we’ll have a wealth of new data.”

Massimiliano De Pasquale, a GRB scientist of the UVOT team from MSSL, added, “The UVOT instrument is particularly suited to study bursts with an average to high redshift – a part of the ultraviolet spectrum that is difficult for even the very big ground-based telescopes to study. Using UVOT with Swift, we can now find redshifts for bursts that were difficult to capture in the past and find out more about their distant host galaxies, about ten billion light years away.

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, said, “This is an amazing first for the UVOT instrument and an exciting new development in the study of these most violent and energetic explosions. Thanks to the hard work of our UK scientists at MSSL, and their partners, we can now gather far more information about gamma ray bursts and the early Universe.

Since its launch in 2004, the Swift satellite has provided the most comprehensive study so far of GRBs and their afterglows. Using the UVOT to obtain ultraviolet spectrums, the Swift team will be able to build on this study and even determine more about the host galaxies’ chemistry.

Paul Kuin said, “The new spectrum has not only allowed us to determine the distance of the gamma ray burst’s host galaxy but has revealed the density of its hydrogen clouds. Learning more about these far-away galaxies helps us to understand how they formed during the early universe. The gamma ray burst observed on this occasion originated in a galaxy 8 billion light years from Earth.

Swift is a NASA mission in collaboration with the STFC in the UK and the Italian Space Agency (ASI).

NASA and Gemini probe mysterious explosion

Nobody knows how the short gamma-ray burst GRB 070714B was triggered, but a leading possibility is the inspiral and merger of two neutron stars, depicted in this artist rendition. NASA/Dana Berry

January 9, 2008

Using the powerful one-two combo of NASA's Swift satellite and the Gemini Observatory, astronomers have detected a mysterious type of cosmic explosion farther back in time than ever before. The explosion, known as a short gamma-ray burst (GRB), took place 7.4 billion years ago, more than halfway back to the Big Bang.

"This discovery dramatically moves back the time at which we know short GRBs were exploding. The short burst is almost twice as far as the previous confirmed record holder," says John Graham of the Johns Hopkins University, in Baltimore, Maryland. Graham presented his group's discovery on Tuesday in a poster at the American Astronomical Society's 2008 winter meeting in Austin, Texas.

GRBs are among the most powerful explosions in the universe, releasing enormous amounts of energy in the form of X-rays and gamma rays. Most bursts fall in one of two categories: long bursts and short bursts, depending on whether they last longer or shorter than 3 seconds. Astronomers think that long GRBs are triggered by the collapse and explosion of massive stars. In contrast, a variety of mechanisms have been proposed for short bursts. The most popular model says that most short GRBs occur when two neutron stars smash into each other and collapse into a black hole, ejecting energy in two counterflowing beams.



These illustrations show the view from afar. In the third image, we see two jets shooting away from the collision site. These jets are where the gamma rays are emitted. NASA/Dana Berry


The record-setting short burst is known as GRB 070714B, since it was the second GRB detected on July 14, 2007. Swift discovered the GRB in the constellation Taurus. The burst's high energy and 3-second duration firmly place it in the short GRB category. Rapid follow-up observations with the 2-meter Liverpool Telescope and the 4-meter William Herschel Telescope found an optical afterglow in the same location as the burst, which allowed astronomers to identify the GRB's host galaxy.Next, Graham and his colleagues, Andrew Fruchter of the Space Telescope Science Institute, in Baltimore, and Andrew Levan of the University of Warwick, U.K., trained the 8-meter Gemini North Telescope in Hawaii on the galaxy. It revealed that the host galaxy has a spectral line from ionized oxygen. The amount that line was shifted toward the red end of the spectrum yields a redshift of 0.92. A redshift of 0.92 translates to a distance of 7.4 billion light-years, meaning the explosion occurred 7.4 billion years ago.

"The fact that this short burst is so far away means this subclass has a broad range of distances, although they still tend to be closer on average than long GRBs," says Swift lead scientist Neil Gehrels of NASA's Goddard Space Flight Center.

Gehrels adds that GRB 070714B's energy is about 100 times higher than average for short bursts, and is more similar to the typical energy of a long GRB. "It is unclear whether another mechanism is needed to explain this explosion, such as a neutron star-black hole merger. Or it could be that there are a wide range of energies for neutron star-neutron star mergers, but that seems unlikely."

Another possibility is that GRB 070714B concentrated its energy in two very narrow beams, and one of the beams happened to be aimed directly at Earth. This would make the burst seem more powerful than it really was. Perhaps most short GRBs eject their energy in wider and less-concentrated beams.

"We now have a good idea of the type of star that produces the brighter long bursts. But how short bursts are formed remains a mystery," says Fruchter.

Brightest Galactic Flash Ever Detected Hits Earth

photo: Artist impression of the eruption striking Earth's magnetic field and atmosphere. Credit: NASA

18 February, 2005

A huge explosion halfway across the galaxy packed so much power it briefly altered Earth's upper atmosphere in December, astronomers said Friday.

No known eruption beyond our solar system has ever appeared as bright upon arrival.

But you could not have seen it, unless you can top the X-ray vision of Superman: In gamma rays, the event equaled the brightness of the full Moon's reflected visible light.

The blast originated about 50,000 light-years away and was detected Dec. 27. A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion kilometers).

The commotion was caused by a special variety of neutron star known as a magnetar. These fast-spinning, compact stellar corpses -- no larger than a big city -- create intense magnetic fields that trigger explosions. The blast was 100 times more powerful than any other similar eruption witnessed, said David Palmer of Los Alamos National Laboratory, one of several researchers around the world who monitored the event with various telescopes."Had this happened within 10 light-years of us, it would have severely damaged our atmosphere and possibly have triggered a mass extinction," said Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics (CfA).

There are no magnetars close enough to worry about, however, Gaensler and two other astronomers told . But the strength of the tempest has them marveling over the dying star's capabilities while also wondering if major species die-offs in the past might have been triggered by stellar explosions.

'Once-in-a-lifetime'

The Sun is a middle-aged star about 8 light-minutes from us. It's tantrums, though cosmically pitiful compared to the magnetar explosion, routinely squish Earth's protective magnetic field and alter our atmosphere, lighting up the night sky with colorful lights called aurora.



photo: The burst from SGR1806-20 as seen in radio wavelength. Credit: University of Hawaii

Solar storms also alter the shape of Earth's ionosphere, a region of the atmosphere 50 miles (80 kilometers) up where gas is so thin that electrons can be stripped from atoms and molecules -- they are ionized -- and roam free for short periods. Fluctuations in solar radiation cause the ionosphere to expand and contract.

"The gamma rays hit the ionosphere and created more ionization, briefly expanding the ionosphere," said Neil Gehrels, lead scientist for NASA's gamma-ray watching Swift observatory.

Gehrels said in an email interview that the effect was similar to a solar-induced disruption but that the effect was "much smaller than a big solar flare."

Still, scientists were surprised that a magnetar so far away could alter the ionosphere.

"That it can reach out and tap us on the shoulder like this, reminds us that we really are linked to the cosmos," said Phil Wilkinson of IPS Australia, that country's space weather service.

"This is a once-in-a-lifetime event," said Rob Fender of Southampton University in the UK. "We have observed an object only 20 kilometers across [12 miles], on the other side of our galaxy, releasing more energy in a tenth of a second than the Sun emits in 100,000 years."

Some researchers have speculated that one or more known mass extinctions hundreds of millions of years ago might have been the result of a similar blast altering Earth's atmosphere. There is no firm data to support the idea, however. But astronomers say the Sun might have been closer to other stars in the past.

A similar blast within 10 light-years of Earth "would destroy the ozone layer," according to a CfA statement, "causing abrupt climate change and mass extinctions due to increased radiation."

The all-clear has been sounded, however.

"None of the known sample [of magnetars] are closer than about 4,000-5,000 light years from us," Gaensler said. "This is a very safe distance."

Cause a mystery

Researchers don't know exactly why the burst was so incredible. The star, named SGR 1806-20, spins once on its axis every 7.5 seconds, and it is surrounded by a magnetic field more powerful than any other object in the universe.

"We may be seeing a massive release of magnetic energy during a 'starquake' on the surface of the object," said Maura McLaughlin of the University of Manchester in the UK.

Another possibility is that the magnetic field more or less snapped in a process scientists call magnetic reconnection.

Gamma rays are the highest form of radiation on the electromagnetic spectrum, which includes X-rays, visible light and radio waves too.

The eruption was also recorded by the National Science Foundation's Very Large Array of radio telescopes, along with other European satellites and telescopes in Australia.

Explosive details

A neutron star is the remnant of a star that was once several times more massive than the Sun. When their nuclear fuel is depleted, they explode as a supernova. The remaining dense core is slightly more massive than the Sun but has a diameter typically no more than 12 miles (20 kilometers).

Millions of neutron stars fill the Milky Way galaxy. A dozen or so are ultra-magnetic neutron stars -- magnetars. The magnetic field around one is about 1,000 trillion gauss, strong enough to strip information from a credit card at a distance halfway to the Moon, scientists say.

Of the known magnetars, four are called soft gamma repeaters, or SGRs, because they flare up randomly and release gamma rays. The flare on SGR 1806-20 unleashed about 10,000 trillion trillion trillion watts of power.

"The next biggest flare ever seen from any soft gamma repeater was peanuts compared to this incredible Dec. 27 event," said Gaensler of the CfA.

Tsunami Connection?

Several readers wondered if the magnetar blast could be related to the December tsunami. Scientists have made no such connection. The blast affected Earth's ionosphere, which is routinely affected to a greater extent by changes in solar activity.

Gamma ray burst progenitors

photo: A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. Eta Carinae was observed by Hubble in September 1995 with the Wide Field Planetary Camera 2 (WFPC2). Images taken through red and near-ultraviolet filters were subsequently combined to produce the color image shown. A sequence of eight exposures was necessary to cover the object's huge dynamic range: the outer ejecta blobs are 100,000 times fainter than the brilliant central star. Eta Carinae suffered a giant outburst about 160 years ago, when it became one of the brightest stars in the southern sky. Though the star released as much visible light as a supernova explosion, it survived the outburst. The explosion produced two lobes and a large, thin equatorial disk, all moving outward at about 1 million kilometers per hour.

Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.

There are at least two different types of progenitors (sources) of GRBs:

one responsible for the long-duration, soft-spectrum bursts and one (or possibly more) responsible for short-duration, hard-spectrum bursts. The progenitors of long GRBs are believed to be massive, low-metallicity stars exploding due to the collapse of their cores. The progenitors of short GRBs are still unknown but mergers of neutron stars is probably the most popular model as of 2007.


Long GRBs: massive stars

Collapsar model

As of 2007, there is almost universal agreement in the astrophysics community that the long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar or hypernova.Very massive stars are able to fuse material in their centers all the way to iron, at which point a star cannot continue to generate energy by fusion and collapses, in this case, immediately forming a black hole. Matter from the star around the core rains down towards the center and (for rapidly rotating stars) swirls into a high-density accretion disk. The infall of this material into the black hole drives a pair of jets out along the rotational axis, where the matter density is much lower than in the accretion disk, towards the poles of the star at velocities approaching the speed of light, creating a relativistic shock wave[4] at the front. If the star is not surrounded by a thick, diffuse hydrogen envelope, the jets' material can pummel all the way to the stellar surface. The leading shock actually accelerates as the density of the stellar matter it travels through decreases, and by the time it reaches the surface of the star it may be traveling with a Lorentz factor of 100 or higher (that is, a velocity of 0.9999 times the speed of light). Once it reaches the surface, the shock wave breaks out into space, with much of its energy released in the form of gamma-rays.

Three very special conditions are required for a star to evolve all the way to a gamma-ray burst under this theory: the star must be very massive (probably at least 40 Solar masses on the main sequence) to form a central black hole in the first place, the star must be rapidly rotating to develop an accretion torus capable of launching jets, and the star must have low metallicity in order to strip off its hydrogen envelope so the jets can reach the surface. As a result, gamma-ray bursts are far rarer than ordinary core-collapse supernovae, which only require that the star be massive enough to fuse all the way to iron.

Evidence for the collapsar view

This consensus is based largely on two lines of evidence. First, long gamma-ray bursts are found without exception in systems with abundant recent star formation, such as in irregular galaxies and in the arms of spiral galaxies. This is strong evidence of a link to massive stars, which evolve and die within a few hundred million years and are never found in regions where star formation has long ceased. This does not necessarily prove the collapsar model (other models also predict an association with star formation) but does provide significant support.

Second, there are now several observed cases where a supernova has immediately followed a gamma-ray burst. While most GRBs occur too far away for current instruments to have any chance of detecting the relatively faint emission from a supernova at that distance, for lower-redshift systems there are several well-documented cases where a GRB was followed within a few days by the appearance of a supernova. These supernovae that have been successfully classified are type Ib/c, a rare class of supernovae caused by core collapse. Type Ib/c supernovae lack hydrogen absorption lines, consistent with the theoretical prediction of stars that have lost their hydrogen envelope. The GRBs with the most obvious supernova signatures include GRB 060218 (SN 2006aj),GRB 030329 (SN 2003dh),and GRB 980425 (SN 1998bw),and a handful of more distant GRBs show supernova "bumps" in their afterglow light curves at late times.

Possible exceptions to this theory were recently discovered when two nearby long gamma-ray bursts lacked a signature of any type of supernova: both GRB060614 and GRB 060505 defied predictions that a supernova would emerge despite intense scrutiny from ground-based telescopes. Both events were, however, associated with actively star-forming stellar populations. One possible implication is that it now appears that a supernova can fail utterly during the core collapse of a massive star, perhaps when the black hole swallows the entire star before the supernova blast can reach the surface.

Short GRBs: degenerate binary systems?

Short gamma-ray bursts appear to be an exception. Until 2007, only a handful of these events have been localized to a definite galactic host. However, those that have been localized appear to show significant differences from the long-burst population. While at least one short burst has been found in the star-forming central region of a galaxy, several others have been associated with the outer regions and even the outer halo of large elliptical galaxies in which star formation has nearly ceased. All the hosts identified so far have also been at low redshift. Furthermore, despite the relatively nearby distances and detailed follow-up study for these events, no supernova has been associated with any short GRB.

Neutron star and Neutron star/Black hole mergers

While the astrophysical community has yet to settle on a single, universally favored model for the progenitors of short GRBs, the generally preferred model is the merger of two compact objects as a result of gravitational inspiral: two neutron stars,or a neutron star and a black hole. While thought to be rare in the Universe, a small number of cases of close neutron star - neutron star binaries are known in our Galaxy, and neutron star - black hole binaries are believed to exist as well. According to Einstein's theory of general relativity, systems of this nature will slowly lose energy due to gravitational radiation and the two degenerate objects will spiral closer and closer together, until in the last few moments, tidal forces rip the neutron star (or stars) apart and an immense amount of energy is liberated before the matter plunges into a single black hole. The whole process is believed to occur extremely quickly and be completely over within a few seconds, accounting for the short nature of these bursts. Unlike long-duration bursts, there is no conventional star to explode and therefore no supernova.This model has been well-supported so far by the distribution of short GRB host galaxies, which have been observed in old galaxies with no star formation (for example, GRB050509B, the first short burst to be localized to a probable host) as well as in galaxies with star formation still occurring (such as GRB050709, the second), as even younger-looking galaxies can have significant populations of old stars. However, the picture is clouded somewhat by the observation of X-ray flaring in short GRBs out to very late times (up to many days), long after the merger should have been completed, and the failure to find nearby hosts of any sort for some short GRBs.

Magnetar giant flares

One final possible model that may describe a small subset of short GRBs are the so-called magnetar giant flares (also called megaflares or hyperflares). Members of a rare class of powerfully magnetized neutron stars known as "magnetars" (only five such objects are known in our Galaxy) are capable of producing brief but enormous outbursts of high-energy photons. Indeed, for a long time outbursts of this nature were a favorite model for producing all gamma-ray bursts. However, none of these events were observed to be luminous enough for bursts from similar events outside our Galaxy and its satellites to be detectable until 27 December 2004, when a blast of radiation from the magnetar SGR 1806-20 saturated the detectors of every gamma-ray satellite in orbit and significantly disrupted Earth's ionosphere.Such an event would easily be detectable from beyond our Galaxy, and it has been speculated that a handful of known GRBs may be associated with these events. As of 2007, a definitive link with any specific GRB is lacking, though there is suggestive evidence of association in the case of GRB051103. Furthermore, only a small fraction of known GRBs have spectral properties with any resemblance to the properties of giant flares.

Hubble Pinpoints Record-Breaking Explosion

April 10, 2008

ABOUT THIS IMAGE:

Peering across 7.5 billion light-years and halfway back to the Big Bang, NASA's Hubble Space Telescope has photographed the fading optical counterpart of a powerful gamma ray burst that holds the record for being the intrinsically brightest naked-eye object ever seen from Earth. For nearly a minute this single star was as bright as 10 million galaxies. Hubble Wide Field and Planetary Camera 2 (WFPC2) images of GRB 080319B, taken on Monday, April 7, show the fading optical counterpart of the titanic blast. The object erupted in a brilliant flash of gamma rays and other electromagnetic radiation at 2:12 a.m. EDT on March 19, and was detected by Swift, NASA's gamma ray burst watchdog satellite. Immediately after the explosion, the gamma ray burst glowed as a dim 5th magnitude "star" in the spring constellation Bootes. Designated GRB 080319B, the intergalactic firework has been fading away ever since then. Hubble astronomers had hoped to see the host galaxy where the burst presumably originated, but were taken aback that the light from the GRB is still drowning out the galaxy's light even three weeks after the explosion. This is particularly surprising because it was such a bright GRB initially. Previously, bright bursts have tended to fade more rapidly, which fits in to the theory that brighter GRBs emit their energy in a more tightly confined beam. The slow fading leaves astronomers puzzling about just where the energy came from to power this GRB, and makes Hubble's next observations of this object in May all the more crucial. Called a long-duration gamma ray burst, such events are theorized to be caused by the death of a very massive star, perhaps weighing as much as 50 times our Sun. Such explosions, sometimes dubbed "hypernovae," are more powerful than ordinary supernova explosions and are far more luminous, in part because their energy seems to be concentrated into a blowtorch-like beam that, in this case, was aimed directly at Earth. The Hubble exposure also shows field galaxies around the fading optical component of the gamma ray burst, which are probably unrelated to the burst itself.

NASA Satellite Sees Oldest-Ever Gamma-Ray Burst, Long Before Milky Way Existed

photo: GRB 080913 exploded Sept. 13 at a whopping distance of 12.8 billion light-years away in the constellation Eridanus. The box indicates the sky area shown in the Swift image.
Credit: DSS/STScI/AURA



photo: This image merges the view through Swift's UltraViolet and Optical Telescope, which shows bright stars, and its X-ray Telescope, which captures the burst (orange and yellow).
Credit: NASA/Swift/Stefan Immler

Whenever satellites like NASA's Swift sees a gamma-ray burst, it's really a look back in time. Now, scientists have seen one that happened farther back in time than any other seen before.

Gamma-ray bursts or "GRBs" are the most powerful and brightest explosions of energy in our universe. They last only a few milliseconds to several minutes and they outshine all other sources of gamma rays combined. Astronomers now think that most GRBs, those lasting 2 seconds or longer, are associated with the explosive deaths of massive stars. These stars collapse and explode when they run out of nuclear fuel.

Now, NASA's Swift satellite has found the most distant gamma-ray burst ever detected! The blast was named "GRB 080913." The GRB number is actually the date YYMMDD of the burst, with letters used for the first, second, etc. burst of the day. This burst came from an exploding star 12.8 billion light-years away. A light-year is the distance light travels, at a speed 186,000 miles per second, in one year.

"This is the most amazing burst Swift has seen," says the mission’s lead scientist Neil Gehrels at Goddard Space Flight Center in Greenbelt, Md. "It's coming to us from near the edge of the visible universe."

Because light moves at a set speed, looking farther into the universe means looking back in time. GRB 080913's "lookback time" reveals that the burst occurred less than 825 million years after the universe began. Scientists think that the universe is 13.7 billion years old. That means this gamma-ray burst happened 12.8 billion years ago! That's long before our galaxy, called the Milky Way, even existed. Scientists believe the galaxy formed about 10 billion years ago.

The star that created this gamma-ray burst died when the universe was less than one-seventh its present age. "This burst accompanies the death of a star from one of the universe’s early generations," says Patricia Schady of the Mullard Space Science Laboratory at University College London, who is organizing Swift observations of the event.

The star's gamma-rays were registered on NASA's Swift satellite at 1:47 a.m. EDT on Sept. 13. The spacecraft established the burst's location in the constellation Eridanus. The previous record holder was a burst from about 12.1 billion years ago.