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A Slow Explosion

Why would a gamma ray burst fade so slowly? This behavior (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v422/n6929/abs/nature01504_fs), recorded last October 2002, is considered a new clue into the cause of gamma-ray bursts, the most powerful explosions known in the universe. The burst, first detected by the orbiting HETE satellite and later tracked by numerous ground-based telescopes, showed an unusually slow and tumultuous decay in visible light. Speculations on the cause of the unusual light curve include a blast wave striking a windy circumburst medium, a blast wave energetically refreshed by a faster outgoing shock, and non-uniformity in a fast moving jet. Pictured above is the massive Wolf-Rayet star WR124, a star itself undergoing a slow explosion by producing a very powerful but tumultuous wind. Popular candidate progenitor sources for GRBs include supernova or hypernova explosions from massive stars, possibly ones with similarities to Wolf-Rayet stars. (Larger Version)

In astronomy, Gamma-ray bursters (GRBs) are flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow. They occur at random positions in the sky several times each day. As of summer 2003, they are believed to be associated with the creation of a black hole from a dying star. The black hole, surrounded by a rotating disk of matter falling into it somehow emits energetic beam parallel to the axis of rotation.

This document provides a history of the GRB search and its current status.

Table of contents

The Discovery of GRBs

Cosmic gamma-ray bursts were discovered in the late 1960s by the US "Vela" nuclear test detection satellites. The Velas were launched to detect radiation emitted by weapons tests, but they picked up occasional bursts of gamma rays from unknown sources. While the sensors on the Vela satellites had low angular resolution, in 1973 researchers at the US Los Alamos National Laboratory in New Mexico were able to use the data from the satellites to determine that the bursts came from deep space.

Gamma ray bursts can only be observed directly from space, as the atmosphere blocks gamma rays. Astronomers believed that once better gamma-ray detectors were put in orbit, they would be able to quickly pin down the locations of the GRBs. After all, that is what happened with X-ray sources. However, when such improved detectors were sent into space in the 1970s, optical searches of the regions where the bursts originated showed nothing of interest. The sensors were not accurate enough to pinpoint the location of the bursts for detailed study.

Further information on the burst sources proved hard to obtain, and led to more questions than answers. The first question posed by the GRBs was: are they local to our own Galaxy, or do they occur in the distant reaches of the Universe? The second question was: what mechanism causes the bursts? If they do occur in the distant Universe, the mechanism must be producing enormous amounts of energy.

Little progress was made on the matter through the 1980s, but in April 1991, the US National Aeronautics and Space Administration (NASA) launched the "Compton Gamma Ray Observatory[?]" on board the space shuttle. One of the instruments on board Compton was the "Burst & Transient Source Experiment (BATSE[?])", which could detect gamma-ray bursts and locate their positions in the sky with reasonable accuracy.

Within a year, BATSE determined that GRBs occur twice or three times a day, and are randomly distributed over the entire sky. If they were events occurring in our own Galaxy, they would be preferentially distributed in the plane of the Milky Way. Even if they were associated with the galactic halo[?], they would still be preferentially distributed towards the galactic center, 30,000 light years away, unless the halo were truly enormous. Besides, if that were the case, nearby galaxies would be expected to have similar haloes, but they did not show up as "hot spots" of faint gamma-ray bursts.

To many astronomers, this implied that the GRBs originated in the distant Universe, but that led to the problem of finding a mechanism that could generate so much energy. Other theorists were also still able to come up with "local" models for the GRBs, and BATSE couldn't resolve the issue.

Pinpointing a Burster: GRB 970228

By the late 1990s, the local hypothesis for GRBs had been ruled out. The first clue came from the Italian-Dutch BeppoSAX satellite, which was launched in 1996.

BeppoSAX carries a gamma-ray detector that works in conjunction with a pair of wide-field X-ray cameras. While the satellite's gamma-ray detector has poor resolution, a gamma-ray burst will generally have an X-ray component, which should allow the X-ray cameras to quickly pinpoint the source for observation by optical and other telescopes.

On February 28, 1997, BeppoSAX managed to pin down the location of an optical counterpart to a gamma-ray burst, which was designated "GRB 970228" in accordance with the date of the event. Some observations seemed to show the object was moving rapidly across the sky. That meant it couldn't be too far away, implying the bursters are a local phenomenon.

Then, on May 8, 1997, BeppoSAX recorded another burst in the constellation Cameleopardis[?], and the spacecraft's science team sent out an alert over the Internet. Seven hours later, an optical source was detected by astronomer Howard Bond[?], using a 90-centimeter telescope at Kitt Peak National Observatory in Arizona.

On May 11, astronomers used the 10 meter Keck II telescope on the island of Mauna Kea[?], Hawaii, to obtain a spectrum of the object. The spectrum showed "absorption lines", or frequencies where the light was absorbed by gases between the object and Earth. The patterns of absorption lines are specific to different atoms and molecules.

The motion of an object causes a "Doppler shift" in the wavelengths of these lines, with the expansion of the Universe causing a "redshift" towards longer wavelengths. The amount of redshift is proportional to the distance of the object, and in this case the spectrum showed a redshift of 0.835, indicating the object was billions of light-years away.

Astronomers were baffled. One observation indicated a local origin, the other a distant origin. Astronomers suspected that at least one of the correlated optical sources may have had nothing to do with a gamma-ray burst, and simply happened to be in the right place at the right time. Some astronomers were also unable to detect any proper motion in the object linked to GRB 970228.

Following these observations, astronomers were able to track down more faint visible-light and radio "afterglows" of GRBs, hours or days after the occurrence of the burst. A few more redshifts were obtained, and confirmed that the bursts occurred in the distant cosmos. The high proper motion reported for GRB 970228 was clearly erroneous, and in fact observations made by the Hubble Space Telescope in September 1997 showed no proper motion in the faint afterglow that remained from GRB 970228.

Visible light observations of several of these GRB locations in 1997 and 1998 identified possible links between the bursts and supernovas. The observations were not conclusive, but they were encouraging to astrophysicists who believed that the GRBs were associate with supernovas, and gave astronomers hunting visible components of GRBs something to investigate in more detail.

Caught in the Act: GRB 990123

Astronomers finally managed to obtain a visible-light image of a GRB as it occurred on January 23, 1999, using an ingenious contraption named the "Robotic Optical Transient Search Experiment 1 (ROTSE-1)", sited in Los Alamos, New Mexico. ROTSE-1 consists of an array of four commercial 200 millimeter telephoto lenses, focused on CCD electronic imaging arrays and mounted on an automated platform.

While 200 millimeter lenses are modest even by the standards of amateur astronomy, ROTSE-1 has a wide field of view and can be quickly repositioned to scan any part of the visible sky. ROTSE-1 is operated by a team under Dr. Carl Akerlof[?] of the University of Michigan.

In the dark hours of the morning of January 23, the Compton satellite recorded a gamma-ray burst that lasted for about a minute and a half. There was a peak of gamma and X-ray emission 25 seconds after the event was first detected, followed by a somewhat smaller peak 40 seconds after the beginning of the event. The emission then fizzled out in a series of small peaks over the next 50 seconds, and eight minutes after the event had faded to a hundredth of its maximum brightness. The burst was so strong that it ranked in the top 2% of all bursts detected.

Compton reported the burst to its ground control facility at NASA Goddard Space Flight Center[?] in Maryland the moment it began, and Goddard immediately sent the data out over the "Gamma Ray Burst Coordinates Network (GCN)". While Compton, as mentioned, cannot provide precise locations of bursts, the location was good enough for the wide-field ROTSE-1. The camera array automatically focused on the region of the sky and obtained an image of the burst 22 seconds after it was detected by Compton, with subsequent images obtained every 25 seconds after that.

ROTSE-1 can image cosmic objects as faint as magnitude 16, and GRB hunters had expected the visible component of a GRB to be very faint. Instead, the visible component reached magnitude 9. It was so bright that it could have been seen by an amateur astronomer with a good pair of binoculars. The object that produced it increased in brightness by a factor of 4,000 in less than a minute.

The news of ROTSE-1's accomplishment didn't make it out on the networks until later in the day, and in the meantime other observatories were focusing on the event, by then designated "GRB 990123".

The BeppoSAX satellite had also seen the burst, and pinned down its location to within a few arc-minutes. This data was sent out, and four hours after the burst the area was imaged with the 1.52 meter (60 inch) Schmitt camera at Palomar Mountain[?] in California. The image revealed a magnitude 18 optical transient that wasn't on archive images of the same area.

The next night, the fading object, by now down to magnitude 20, was imaged by the Keck telescope, and the 2.6 meter Nordic Optical Telescope[?] in the Canary Islands. The observations revealed absorption lines with a redshift of 1.6, implying a distance of 9 billion light years.

The Hubble Space Telescope performed observations on the location of GRB 990123, sixteen days after the event. It had faded by more than a factor of three million in that time. The Hubble was able to pick up the traces of a faint galaxy, whose blue color suggested it was forming new stars at a rapid rate.

What is a GRB?

The combination of obvious brightness and implied distance of GRB 990123 lead to two possiblities. The first was that the radiation of the gamma ray burster was spread evenly. This implied that 9 billion light years, the gamma-ray energy released by the burster was equivalent of converting the entire mass of a star 1.3 times the mass of our Sun completely into gamma radiation. At visual wavelengths, if the burster had gone off in our own Galaxy 2,000 light years away, it would have shined twice as bright in our night sky as the Sun does during the day.

Another possibility was that the gamma ray was not evenly distributed but was tightly beamed in a narrow region. While this would still imply a massive emission of energy, the energies would be on the order of supernova and thus would require less exotic physics.

Astrophysicists have have been challenged to come up with convincing mechanisms to explain the sheer power of these bursts. One line of thought proposed that collisions between neutron stars, or between a neutron star and a black hole, could do the job. Another proposed that the bursts were caused by supernova explosions of very large stars, sometimes called "hypernovas", with the explosive collapse of the star creating a black hole, rather than a neutron star as would be the case for a smaller star.

The Hubble observations that showed GRB 990123 to be associated with a young galaxy tended to discourage theorists who believed that the bursts were due to collisions between neutron stars or the like, since that implied a fairly high density of dead stars and that was inconsistent with a young galaxy. Supernovas, on the other hand, occur frequently in star-forming galaxies, since the big stars that die in supernovas have short lifetimes.

Even the supernova model had trouble accounting for the energy output. One way around the problem was to assume that the burst energy was only sent out in specific directions, rather than in all directions, much as some stars and galaxies emit directional high-energy "cosmic jets[?]" from violent events. Another explanation for the the great brightness of the burst was that its light had been focused by a "gravitational lens", caused by the distortion of space by a large galaxy between Earth and the GRB.

The lensing theory was supported by observations that seemed to indicate there was in fact a galaxy between the Earth and the GRB. However, the "galaxy" turned out to be a photographic flaw. This didn't rule out gravitational lensing, but interest in the idea faded when Bradley E. Schafer of Yale pointed out that at a redshift of 1.6, the density of galaxies made the probability of lensing only about one in a thousand.

In any case, if limits needed to be imposed, the "beaming" hypothesis was much more plausible than lensing. Astrophysicists Bohdan Paczynski of Princeton and Stan Woosely of the University of California, Santa Cruz, independently suggested that a supernova might emit a narrow beam of gamma ray energy during its explosive collapse into a black hole, with the tightly focused beam giving the impression of a much more energetic event.

Exactly how the collapse would generate such a beam remains a puzzle. However, an analysis of the afterglows of 17 GRBs that was published in the fall of 2001 did place limits on the width of the beam, stating that it was probably only a few degrees across. With such a narrow beam, the energy of a GRB could be provided by a supernova only slightly more powerful than average. Incidentally, theorists say that if the Earth were hit by a gamma-ray beam from a burster only a few hundred light years away, it would incinerate the surface of our planet.

The narrowness of the beam also suggested that only one in 500 GRBs are seen from Earth, meaning they are a fairly common phenomenon in the Universe, probably occurring about once every minute. This means that astronomers might be able to observe "orphan afterglows", exactly like those following a GRB, but not associated with a gamma-ray burst.

The brightness of GRBs varies rapidly, implying that their source objects are quite small: whatever causes the brightness variation cannot travel faster than the speed of light across the object. Very densely packed photons prevent each other from escaping, and astronomers therefore theorize that the energy initially leaves the object as a jet of matter, with gamma rays being created at a certain distance by internal shock waves.

There is some direct evidence of an association of a GRB with a supernova. A supernova synthesizes a wide range of heavy elements during its collapse, and many of these, particularly isotopes of nickel, are highly unstable and break down very quickly, releasing radiation. This means that a supernova actually gets brighter for a few days or weeks after its occurrence.

BeppoSAX targeted a GRB on 21 November 2001, and following the burst the Hubble Space Telescope tracked the evolution of "GRB 011121" for an extended period of time. The light curve obtained matched that expected of a supernova. However, no valid spectrum was obtained of GRB 011121 that would conclusively prove a link to a supernova.

Closing in on the Answer

Data on GRBs is still sketchy and they remain mysterious. Spectra have proven difficult to obtain, as are accurate estimates of their distance.

Distance can be estimated from the redshift of the GRB. However, a redshift can't be obtained from gamma ray measurements as they don't have a distinctive line structure, and so must be obtained by visible observations of afterglows, which are hard to spot. Some astrophysicists believe that the rate at which a GRB flickers may provide a useful index of its distance, and might even be a useful "standard candle" for determining distances to the far reaches of the cosmos.

There is also the puzzle that the burst durations fall into distinct "long" and "short" categories. The long bursts are generally agreed to be associated with supernovas, but the short bursts may be associated with an entirely different mechanism.

Despite the fuzzy data and many questions, astronomers now feel they are closing in on a solution to the mystery, and remain very excited. They are making good use of the tools available for the job. Although BeppoSAX was only supposed to be a five-year mission, to end in 2001, at last report the satellite is still in good shape, and the program is now being extended another year.

The astronomers expect to obtain more information from the "US High Energy Transient Explorer 2 (HETE-2[?])" satellite, launched on 9 October 2000. The first HETE[?] satellite had been launched on 4 November 1996, but it had been trapped in orbit in its payload shroud. Burst hunters were bitterly disappointed, but they were able to obtain a replacement. HETE-2 is specifically designed to quickly and precisely locate gamma-ray bursts, permitting other observatories, such as the NASA Chandra X-Ray Observatory[?], to obtain more details of the bursts.

A new mission to investigate GRBs is now in development. The "Swift Gamma Ray Burst Explorer[?]" satellite is now under construction by Spectrum Astro[?], and is slated to be launched in September 2003 from Cape Canaveral. Swift will be put into a 600 kilometer (373 mile) high, low-inclination orbit to observe gamma-ray bursts. Swift will include a "burst alert telescope" to alert the spacecraft to a gamma-ray burst. The satellite will then quickly realign itself to focus more sensitive instruments on the burst. Swift can shift 50 degrees in less than 50 seconds to come to a stable focus on a precise sky coordinate.

As the number of detailed observations of GRBs and the instruments and theoretical tools are refined, it seems only a matter of time before the nature of the bursters is understood.

See also


  • Neil Gehrels et.al. The Brightest Explosions in the Universe, Scientific American, Vol 287, No. 6, December 2002

Originally based on the document [v1.1.0 / 01 jul 02 / gvgoebel@earthlink.net / public domain]

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