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ScienceWeek
ASTRONOMY: ON SHORT GAMMA-RAY BURSTS
The following points are made by Stephan Rosswog (Science 2004 303:46):
1) More than 35 years after their initial discovery, gamma-ray bursts (GRBs) remain one the most exciting and mysterious events in our Universe. GRBs are flashes of energetic photons that outshine, for a short moment, the whole rest of the gamma-ray sky. They are classified as "short" or "long" depending on whether their duration is shorter or longer than 2 s. A wealth of information about long GRBs has been gathered, including afterglows at different wavelengths, redshifts, host galaxies, and indications that the bursts occur close to star-forming regions (1). In contrast, information about short GRBs is so far restricted to gamma rays alone.
2) In principle, the two classes of GRBs could be caused by two different types of progenitors, or they could have the same kind of progenitor, which acts in two different ways depending on its initial conditions (such as its initial rotation). It is more likely that each class of GRB has a different kind of progenitor, because there are substantial differences apart from the duration of the burst. For example, the number of subpulses that constitute the light curve of a GRB is different for short and for long bursts (2). The spectra of short GRBs are harder (3); that is, they have a larger fraction of high-energy photons, their peak energy is larger (4), and they evolve differently in time (2). The peak energies of the spectra are influenced by the cosmological expansion -- that is, for more remote bursts, one would expect lower peak energies -- and by the speed with which the radiation-producing matter is ejected from the central engine. Therefore, a higher peak energy of the short bursts could mean either that their central engines produce higher outflow velocities or that they occur on average closer to Earth. Statistical arguments (5) also point to a relatively local origin of the short GRB class.
3) Compact binary mergers -- the coalescence of either two neutron stars or of a neutron star and a low-mass black hole --have long been the standard model for the central engine of GRBs. Such mergers provide huge energy reservoirs of several 10^(53) ergs and would yield a natural explanation for the shortest time scales observed in GRBs. More recently, the association of long GRBs with supernovae has been substantiated. Compact binary mergers are now thought to power only short GRBs.
4) If the gamma-ray emission were beamed into a narrow jet, we could detect a short GRB only if it is by chance directed toward Earth, and we would miss most bursts that occur in the Universe. This would mean that the "true" event rate is much higher than our observed one. The estimated compact binary merger rates are high enough to explain all short GRBs, even if the gamma-ray emission is strongly beamed. Because of the long time from the birth of the binary system to its death in the final coalescence, GRBs resulting from compact binary mergers are expected to occur relatively late in the life of the Universe. It is estimated that they occur at 0.5 to 0.8 times the cosmological redshift of long GRBs, providing further support for a relatively local origin of short GRBs.
References (abridged):
1. A. MacFadyen, Science 303, 45 (2004)
2. J. P. Norris, G. F. Marani, J. T. Bonnel, Astrophys. J. 534, 248 (2000)
3. C. Kouveliotou et al., Astrophys. J. 413, L101 (1993)
4. W. S. Paciesas, R. D. Preece, R. S. Mallozzi, in Gamma-Ray Bursts in the Afterglow Era, E. Costa, F. Frontera, J. Hjorth, Eds. (Springer, Berlin/Heidelberg, 2001), pp. 248-251
5. S. Mao, R. Narayan, T. Piran, Astrophys. J. 420, 171 (1994)
Science http://www.sciencemag.org
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ON GAMMA RAY BURSTS
The following points are made by Peter Meszaros (Nature 2003 423:809):
1) The fog surrounding the identity of the progenitors of gamma-ray bursts (GRBs) is beginning to lift, at least for the class of GRBs known as "long" bursts. This is thanks to a series of observations of a burst that began on 29 March 2003, very close to our Galaxy. Analysis reveals the evolution of this burst in unprecedented detail -- and demonstrates that behind this GRB is the unmistakable signature of a supernova.
2) The GRB population divides neatly into long ones and short ones, depending on whether the burst of gamma-rays lasts more or less than a few seconds. About two-thirds of all observed bursts are long, and these are the only ones for which longer-lasting "afterglows" at x-ray, optical, and radio wavelengths have also been found. These afterglows may last up to several months, and from them the distance to the GRB and the identity of its host galaxy can be determined. There is good evidence that long bursts are largely associated with active, star-forming regions in small blue galaxies. And, in at least three cases, there has been tantalizing evidence that GRBs are associated with a particular type of supernova -- although that interpretation has so far been fraught with uncertainty.
3) A "usual" supernova arises when the core of a massive star collapses, ejecting the stellar outer envelope. The majority of such supernovae result from parent stars that are less than about 30 times heavier than the Sun, and the core collapse produces a neutron star. These supernovae are normally detected weeks after the collapse, because the ejected envelope only brightens sufficiently to be detected at optical wavelengths some weeks later. The only signals of the collapse that are expected to reach the Earth promptly are a flux of neutrinos (which was picked up for the supernova SN1987a by the Japanese neutrino detector Kamiokande) and gravitational waves (which have yet to be detected).
4) For heavier stars, however, the core is thought to collapse into a black hole, and the resulting brief episode of mass accretion has been proposed as the central engine driving GRBs. This kind of collapse was initially referred to as a "failed" supernova, as it was thought that the stellar envelope would not be ejected. A GRB would instead result from a relativistic jet of gas fed by the black hole; it would break through the stellar envelope, leading to radiative shocks in the rarefied environment outside the star.
5) In 1998, observations of GRB980425 showed an anomalous brightening of its optical afterglow a few weeks after the burst, possibly linking it to a roughly contemporaneous supernova, known as SN1998bw, whose ejected envelope would have brightened at about that time. Suspicions grew that long GRBs might, after all, be associated with "successful" supernovae. In fact, the few supernovae tentatively linked to GRBs appeared even more energetic than usual, and were dubbed "hypernovae", or "collapsars". There is also a more elaborate offshoot of the supernova idea -- the "supra-nova". Here, the core collapse is assumed to be a two-step affair: the first step produces a temporary neutron star and a supernova; in the second step, a few weeks or months later, the neutron star collapses into a black hole, producing a GRB.
Nature http://www.nature.com/nature
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ON DARK GAMMA-RAY BURSTS.
The following points are made by Gerald J. Fishman (Nature 2002 419:259):
1) Gamma-ray bursts (GRBs) are intense flashes of radiation that originate in the furthest reaches of the Universe. The bursts are expected to be followed by an afterglow of radiation at optical wavelengths. But for many GRBs (approximately 60% of the total sample of less than 40 bursts that have now been followed up), this afterglow seems to be missing. Hence, they have become known as "dark" bursts. Berger et al (2002) have reported data from an armada of telescopes that reveal a faint optical afterglow from a GRB that would otherwise have been classed as "dark", and their observations suggest that the seeming lack of optical emission from dark bursts is in fact a result of insufficiently prompt and sensitive observations.
2) Astronomers once thought that GRBs occurred within our Galaxy, but in the early 1990s it was realized that these explosive events actually occur at cosmological (extra-galactic) distances, of the order of 15 billion light years away. The distance issue was settled by an observational breakthrough -- the accurate and rapid location of GRBs by the Italian Dutch spacecraft Beppo SAX. Following the initial detection and accurate location of gamma-rays by Beppo SAX, the optical-radiation counterpart could be determined by other telescopes. This also makes it possible to measure the distance of the burst from the Earth. More importantly, detailed follow-up observations could then be made of the burst afterglow and of its host galaxy at other wavelengths by some of the most powerful telescopes in the world, both ground-based and space-based (in particular, the Hubble Space Telescope and the Chandra X-ray Observatory).
3) The field of prompt GRB follow-up observations has burgeoned in the past four years, in both the observational and the theoretical areas. Afterglow observations are being made at X-ray, optical, microwave, infrared and radio wavelengths. In each observational band, and by combining data from different bands, astronomers can make clever use of the data, such as providing a measure of the angular size of the emitting region by observing radio variability(.
Nature http://www.nature.com/nature
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