Gamma-ray bursts: recent developments

Peter Meszaros, Penn State

What are GRBs? Gamma-ray bursts (GRBs) are brief gamma-ray flashes detected with space-based detectors in the range 0.1-100 MeV, with typical photon fluxes of photons/cm/s and durations 0.1-1000 seconds. Their origin is clearly outside the solar system, and more than 2000 events have been recorded so far. Before there was any firm evidence on the isotropy of classical gamma-ray bursts, the most plausible interpretations involved magnetospheric events on neutron stars (NS) within our Galaxy. However, the remarkable isotropy of these events discovered within the last two years by the BATSE experiment on the NASA Compton Gamma Ray Observatory (together with the `flatter than Newtonian' counts) clearly shifts the odds substantially in favor of a cosmological interpretation. Irrespective of the distance (i.e., even in the galactic halo, but more so in cosmological models), 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 (e.g. Cavallo and Rees, 1978, Paczynski, 1986, Shemi and Piran, 1990). The main challenge in these models is not so much the ultimate energy source (which may involve stellar collapse or binary compact star merger) but rather how to turn the energy of a fraction of a stellar rest mass into predominantly gamma rays with the right non-thermal broken power law spectrum with the right temporal behavior. The dissipative relativistic fireball model proposed by Rees and Meszaros (1992, 1993, 1994; see also Narayan, Paczynski and Piran, 1992; Meszaros, Laguna and Rees, 1993, Meszaros, Rees and Papathanassiou 1994, Katz, 1994; Sari, Narayan and Piran, 1996) is largely successful in solving these problems, and is discussed in several reviews, e.g. Meszaros (1995, 1997).

The Significance of GRB After-glows and Counterparts The recent discovery (1997) of X-ray, optical and radio after-glows of gamma-ray bursts (GRB) amounts to a major qualitative leap in the type of independent observational hand-holds on these objects. Together with existing gamma-ray signatures, these provide significantly more severe constraints on possible models, and may indeed represent the light at the end of the tunnel for understanding this long-standing puzzle of astrophysics. The report of long wavelength observations of GRB 970228 over time scales of days to weeks at X-ray (X), and months at optical (O) wavelengths (Costa etal, 1997) was the most dramatic recent development in the field. In this and subsequent IAU circulars, it was pointed out that the overall behavior of the long term radiation agreed with theoretical expectations from the simplest relativistic fireball afterglow models published in advance of the observations (Meszaros & Rees, 1997a). A number of theoretical papers were stimulated by this and subsequent observations (e.g. Tavani, 1997; Waxman, 1997a; Reichart, 1997; Wijers, etal, 1997, among others), and interest has continued to grow as new observations provided apparently controversial evidence for the distance scale, possible variability and the candidate host (Sahu etal, 1997). New evidence was added when the optical counterpart to the second discovered afterglow (GRB 970508) yielded a redshift lower limit placing it at a clearly cosmological distance (Metzger etal, 1997), and this was strengthened by the detection of a radio counterpart (Frail etal, 1997; Taylor etal, 1997) as well as evidence for the constancy of the associated diffuse source and continued power law decay of the point source (Fruchter, etal, 1997). A third GRB afterglow (GRB971214) has also been detected in X-rays and optical, and appears to follow the canonical power law time decay (Heise, et al, 1997, and follwing IAU circulars). This new evidence reinforces the conclusions from previous work on the isotropy of the burst distribution which suggested a cosmological origin (e.g Fishman & Meegan, 1995). Observational material on this is provided chiefly by a superb data base (currently of over 1800 bursts in the 4B catalog) which continues being accumulated by the BATSE instrument, complemented by data from the OSSE and Comptel instruments on CGRO, as well as Ulysses, KONUS and other experiments. At gamma-ray energies, much new information has been collected and analyzed, relevant to the spatial distribution, the time histories, possible repeatability, spectra, and various types of classifications and correlations have been investigated. At the same time, investigations of the physics of fireball models of GRB have continued to probe the gamma-ray behavior of these objects, as well as the after-glows. Much of the recent theoretical work has concentrated on modeling the time structure expected from internal and external shock models, multi-wavelength spectra, the time evolution and the spectral-temporal correlations (e.g. Papathanasiou & Meszaros, 1996; Kobayashi, Sari and Piran 1997; Waxman, 1997b; Katz & Piran, 1997; Panaitescu & Meszaros 1997a, 1997b; Sari, Piran & Narayan, 1997, etc.).

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Jorge Pullin
Sun Feb 8 20:46:09 EST 1998