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Gamma ray bursts

(section 2.4 from "Detection strategies for a multi-interferometer triggered search", M.S. thesis)
by Wm. Robert Johnston
last modified 31 May 2004

Astronomical gammas ray bursts (GRBs) were first reported in 1973, but only recently have observations begun to identify their sources. The first GRB was detected in 1967 by Vela satellites, military satellites tasked with detection of terrestrial nuclear explosions. Besides being of short overall duration (of order seconds or less), GRBs were observed to vary in intensity over periods of order ms. Since causality arguments lead to the conclusion that the timescale of such variability must set an upper limit on the light-travel time across the source region, this constrained GRB source regions to sizes of at most a few hundred km. Subsequently other satellites made additional GRBs detections, and in a few cases X-ray emissions were also observed. However, little else was known until the beginning of observations by the Compton Gamma-Ray Observatory (CGRO), in orbit from 1991 to 2000. The Burst and Transient Experiment, or BATSE sensor, aboard Compton identified a total of about 2700 GRBs. This data indicated a uniform sky distribution, eliminating any association with the Milky Way Galaxy. BATSE data further indicated that GRBs included two distinct classes: "short" GRBs with durations less than 2 s (some less than 0.1 s) and "long" GRBs lasting more than 2 s up to a few minutes (Meszaros 2002).

Follow-up observations in other electromagnetic energies eventually provided additional information on GRBs. The Gamma Ray Burst Coordinates Network (GCN) was developed to automatically notify astronomers of GRB detections to aid in rapid follow-up (Barthelmy 2003). X-ray flashes (XRFs) have been observed and linked to GRBs. With BeppoSAX satellite and other satellite observations in X-rays, improved source locations for GRBs were obtained. Such information allowed observations of optical afterglows of GRBs beginning in 1997. Automated systems involving the GCN have improved the response time, as illustrated by the ROTSE-1 team which obtained automatic observations of GRB 990123 in 1999 within seconds of the gamma ray burst itself. In turn, afterglow observations served to identify presumed host galaxies for some GRBs, with redshift measurements giving typical distances of several Gpc. With GRBs established as occurring at cosmological distances, energy releases could be calculated as up to 1054 ergs, or the energy equivalent of one solar mass, if the energy emission was anisotropic. Another key development was development of evidence linking GRBs and supernovae. This connection was proposed in the case of GRB 980425 and SN 1998bw in 1998, but established in the case of GRB 030329 in 2003, based on observations of the afterglow (Willingale et al. 2004). As of 2002 over 40 GRB afterglows had been observed, with host galaxies identified in over 30 cases (see review by Meszaros 2002), and through 2004 associated redshift measurements have been obtained for over 30 GRBs. Spacecraft currently providing GRB observations include HETE, INTEGRAL, RXTE, and Ulysses. NASA's Swift satellite to be orbited in late 2004 is expected to significantly extend detection and observation capabilities. During its two year mission, Swift's gamma-ray detectors will provide arc minute accurate positions within about 10 s of a GRB, then direct its onboard X-ray and optical/UV telescopes onto the GRB within 20-70 s for follow-up observations (NASA 2004).

Together, these observations have led to collapsar models for long GRBs which explain them as a special type of supernova. In these models, the core of a massive collapsing star forms a black hole, and rapidly infalling material forms an accretion disk. Either due to the star's rapid rotation or strong magnetic fields or both, particles and radiation preferentially escape in jets perpendicular to the accretion disk. As these jets collide with stellar material or surrounding gas at Lorenz factors up to 1000, they produce beamed gamma-ray and X-ray bursts. While the degree of beaming remains unclear, beaming is demonstrated by the intensity decay of GRB afterglows. The rate of this decay abruptly increases at a particular point in time, corresponding to the end of spreading of the relativistic beaming effect. It is also suggested that observed X-ray flashes are slightly off-axis GRBs. In attempting to relate observed energy fluxes to energy available in a stellar collapse, beaming is a key factor. If beaming occurs, this would reduce the required source energies by a factor of 100 or more, to levels that correspond with such collapse models.

Short GRBs, however, remain poorly modeled because of the lack of follow-up observations that might provide observational constraints, this lack due to their short duration. One possibility is that they result from the final merger of a binary neutron star into a black hole, either with or without gamma rays preferentially emitted in jets perpendicular to a short-lived accretion disk. Alternately, the source could be an unrecognized type.

In the cases of both types of GRBs, they appear to be promising sources of gravitational waves, given the indications of relativistic motion of compact masses. They are currently the most studied event for triggered burst searches of gravitational waves due to their frequency, the current ability to identify source location, and the relatively short time delay between the GRB trigger and the gravitational wave signal. Observable GRBs are typically at distances of several Gpc, much more distant that some other potential burst sources.

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© 2004 by Wm. Robert Johnston.
Last modified 31 May 2004.
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