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Gamma-Ray Burst Physics

Gamma-ray bursts (GRB) are sudden, intense flashes of gamma-rays which, for a few blinding seconds, light up in an otherwise fairly dark gamma-ray sky. They are detected at the rate of about once a day, and while they are on, they outshine every other gamma-ray source in the sky, including the sun. Major advances have been made in the last three or four years, including the discovery of slowly fading x-ray, optical and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and finding evidence for many of them being associated with star forming regions and possibly supernovae. Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock model. This is described in a recent non-specialist GRB review or a more detailed review on GRB and afterglows. A summary of some of the specific research activities on GRB at Penn State is given in the previous link. The rest of this page gives a general overview of GRB.

Until a few years ago, GRB were thought to be just that, bursts of gamma-rays which were largely devoid of any observable traces at any other wavelengths. GRBs were first reported in 1973, based on 1969-71 observations by the Vela military satellites monitoring for nuclear explosions in verification of the Nuclear Test Ban Treaty. When these mysterious gamma-ray flashes were first detected, which did not come from Earth's direction, the first suspicion (quickly abandoned) was that they might be the product of an advanced extraterrestrial civilization. Soon, however, it was realized that this was a new and extremely puzzling cosmic phenomenon. A major advance occurred in 1991 with the launch of the Compton Gamma-Ray Observatory (CGRO), whose results have been summarized Fishman & Meegan 1995. The all-sky survey from the Burst and Transient Experiment (BATSE) onboard CGRO, which measured about 3000 bursts, showed that they were isotropically distributed, suggesting a cosmological distribution, with no dipole and quadrupole components. Some of the related work at Penn State on the cosmological GRB distribution is in the previous link. This isotropic distribution and the brightness distribution (log N- log P) provided strong support for a cosmological origin, and the detailed gamma-ray spectra and time histories imposed significant constraints on viable models, which led to the development of the fireball shock model.

A dramatic development in the last several years has been the measurement and localization of fading x-ray signals a number of GRBs by the Beppo-SAX satellite . These afterglows, lasting typically for weeks, made possible the optical and radio detection of afterglows, which, as fading beacons, mark the location of the fiery and brief GRB event. These afterglows in turn enabled the measurement of redshift distances, the identification of host galaxies, and the confirmation that GRB were, as suspected, at cosmological distances of the order of billions of light-years, similar to those of the most distant galaxies and quasars. Even at those distances they appear so bright that their energy output during its brief peak period has to be larger than that of any other type of source, of the order of a solar rest-mass if isotropic, or some percent of that if collimated. This energy output rate is comparable to burning up the entire mass-energy of the sun in a few tens of seconds, or to emit over that same period of time as much energy as our entire Milky Way does in a hundred years.

The energy density in a GRB event is so large that an optically thick pair/photon fireball is expected to form, which will expand carrying with itself some fraction of baryons. The main challenge in the early 90's was not so much the ultimate energy source, but how to turn this energy into predominantly gamma rays with the right nonthermal broken power law spectrum with the right temporal behavior. To explain the observations, the relativistic fireball shock model was proposed by Rees and Meszaros (1992, 1994), following pioneering earlier earlier work by Cavallo & Rees, Paczynski, Goodman and Shemi & Piran. This model has been quite succesful in explaining the various features of GRB, and a general discussion of it is given, e.g. here.

Much of the recent work has concentrated on GRB afterglows, a highlight of which was the successful prediction (Meszaros & Rees, ApJ 476, 232, Feb 10, 1997) of the general X-ray and optical behavior of GRB 970228. Since then more than 40 afterglows have been studied in detail, and a number of interestinf developments have occured. A prompt optcial flash (also predicted by theory) was found in one burst; many afterglows were found to be collimated, easing the energy constraints; X-ray lines believed to be from Iron and other metals have been reported from a number of bursts; and a new variety of softer bursts dubbed "X-ray flashes" has been identified, which are very similar to classical GRB but have a softer spectrum. Other work has concentrated on identifying the progenitors of GRB. Many of the afterglows identified by Beppo-SAX (all belonging to the class of "long" bursts, >10 s duration) have been shown to be associated with massive young stars, and in some cases a peculiar supernova "(hypernova") may be associated, as suggested by Woosley and Paczynski. This has led to work by Meszaros, Rees, Lazzati and others using X-ray lines as a diagnostic for distinguishing a massive progenitor. Other work has concentrated on modeling the central engine resposible for the energy release. The main ideas invoke the formation of a several solar mass black hole with a disrupted debris torus which is rapidly accrreted, which feeds an MHD or electron-positron-baryon jet. This can result from either the merger of a compact binary, such as a double neutron star (which is expected to lead to short bursts (< 10 s), observed in gamma-rays but so far without identified long-wavelenght afterglows) or by the collapse of the fast-totating core of a massive star, in some cases dubbed a collapsar, which leads to long bursts (>10 s) and could be associated with a suupernova-like phenomenon. More details and references are given in my recent review on GRB and afterglows.