5.4. Gamma-ray bursts
Gamma-ray bursts were first detected by the military Vela satellites in
1967. These are short flashes of gamma-rays (typically with a peak
energy around 100 keV), lasting between a few milliseconds and tens
of minutes. Already in the early 1990s, the BATSE experiment on board
the Compton Gamma-Ray Observatory demonstrated that the rate of
gamma-ray bursts is about 1-2 per day, and that they are distributed
isotropically in the sky. The data from BATSE also showed that gamma-ray
bursts (GRBs) come in two distinct classes: (i) short-duration
( 2 sec) with hard
spectra, and (ii) long duration with softer spectra
(Kouveliotou
et al. 1993,
Fishman 2001).
Due to the relatively poor localization capabilities of gamma-ray
detectors (which did not allow identification of the sources in other
wavebands), two thousand bursts have been detected before it became
possible to determine the distance to the bursts. The isotropic sky
distribution argued for either a cosmological origin or an extremely
local one (e.g., the Galactic halo). This situation changed dramatically
since the operation of the Italian-Dutch BeppoSAX satellite in
1997. BeppoSAX was able to determine the positions of bursts to within
arcminutes. The discovery of rapidly declining "afterglows" in the
X-ray, optical and radio bands
(Costa et
al. 1997,
van Paradijs
et al. 1997,
Metzger et
al. 1997,
Frail et al.
1999)
allowed for redshift determinations that immediately placed GRBs at
cosmoligical distances. I should note that to date, only afterglows of
the long-duration bursts have been observed. Thus, it is not even clear
if the short bursts produce afterglows (the best limit to date, for the
burst GRB 020531, failed to detect an afterglow candidate [down to
V ~ 25] about 20 hours after the burst;
Salamanca et
al. 2002).
Typically, the afterglows decay with time as
t-, with
in the ranges 1-2. This
behavior was predicted by a model in which a fireball is
expanding into a homogeneous external medium
(Mészáros
and Rees 1997).
At the time of this writing redshifts have been determined to more than
two dozen GRBs, and they usually lie in the z = 0.5-1.5 range
(although redshifts as high as 4.5 have been recorded). The observed
fluxes imply energies of up to 1054 ergs for isotropic
emission. However, there is increasing evidence, in the form of kinks in
the afterglow light curve, and in polarization detected in a few bursts,
that gamma-ray bursts are in fact collimated into narrow jets. If the
observer's line of sight is within the jet solid angle,
j, then
as long as the Lorentz factor
satisfies
j-1/2, the light-cone is within the
jet boundary. However, as the jet decelerates,
eventually
drops below
j-1/2. Consequently, the (transverse)
emitting area starts to grow more slowly (as r||
j-1/2 instead of (r|| /
)2), resulting in a break (faster decay) in the
light curve
(Rhoads 1997,
Mészáros
and Rees 1999,
Livio and
Waxman 2000),
consistent with observations in GRB 990123
(Kulkarni et
al. 1999,
Fruchter et
al. 1999).
When collimation is taken into account, the average total energy of GRBs
is estimated to be of the order of 2 × 1051 ergs
(Frail et
al. 2001,
Panaitescu and
Kumar 2001).
Based on the energetics, and the fact that the production of astrophysical jets typically relies on the collimation and acceleration provided by an accretion disk around a compact object (Livio 2000), the most popular models for GRBs involve the formation of stellar-mass black holes, surrounded by a debris torus from which mass accretion onto the central object occurs (Mészáros 2002). The two most likely progenitors to produce such a configuration are the core collapse of massive stars (Woosley 1993, Paczynski 1998), or the ongoing merger of neutron star-neutron star or black hole-neutron star binaries (e.g., Paczynski 1986, Goodman 1986, Eichler et al. 1989). In both cases the main source of energy is gravitational, even though tapping into the (large) spin energy of the black hole is also possible in principle (Blandford and Znajek 1977, Krolik 1999b). A schematic describing how a GRB and its afterglow may be generated by internal and external shocks (respectively) in a collimated jet arising from a stellar collapse, is shown in Figure 22.
![]() |
Figure 22. Schematic gamma-ray burst from
internal shocks and afterglow from external shock, arising from a
relativistic jet produced by the collapse of a massive star. Internal
shocks produce
|
There are several pieces of evidence suggesting that the long duration GRBs may indeed be associated with stellar collapses. First, in merging neutron stars (or a black hole and a neutron star) the associated timescales (dictated at that point by the emission of gravitational wave radiation) are probably too short to produce multi-second-long bursts. On the other hand, core collapses that lead to a black hole and an accretion disk are naturally associated with longer timescales. Second, there exists quite strong evidence that at least some GRBs are associated with supernova explosions.
In particular, the supernova SN 1998bw (of the relatively rare type Ic, caused by the explosion of a massive star that had rid itself of its hydrogen envelope prior to exploding; Filippenko and Sargent 1986) was found to be approximately coincident in both position and time with the relatively weak GRB 980425, at redshift z = 0.0085 (Galama et al. 1998). Furthermore, the supernova light curve was found to be consistent with the formation of a black hole [Iwamoto et al. 1998). In a few other bursts (e.g., GRB 011121) the optical and near infrared afterglow light curve exhibits a "bump" with a time delay and amplitude that are consistent with resulting from a supernova explosion occuring simultaneously with the GRB (e.g., Greiner et al. 2001).
The contributions of HST to this exciting field have been in a few areas.
First, by resolving the host galaxies and being able to pinpoint the GRB on the host, HST has shown unambiguously that at least some GRBs are not associated with galactic nuclei. In fact, they sometimes occur far from the nucleus or in spiral arms (Sahu et al. 1997, Andersen et al. 2002). Second, the Hubble Space Telescope has shown (Fruchter et al. 1999, Fruchter 2002) that the colors of the hosts of GRBs corresond to the bluest colors observed for galaxies in the Hubble Deep Fields (see Section VI). This means that GRBs with afterglows occur preferentially in galaxies with high star formation rates - a finding that is consistent with GRBs being associated with collapses of massive stars. Furthermore, the fact that in almost all cases the GRB's position was found to be within the extent of the rest-frame ultraviolet image (where star formation is intense) of the hosts, also argues in favor of the "collapsar" model for the long-duration bursts. Neutron star binaries, on the other hand, are born with "kicks" of as much as a few hundred km/sec, which could drive them out of the star-forming regions.
The Hubble Space Telescope also helped to firm the association of at
least some GRBs with supernova explosions. In GRB 011121, for
example, HST detected an intermediate-time flux excess ("red bump") that
was redder in color relative to the GRB afterglow. This "bump" could be
well described by a redshifted Type Ic supernova that exploded
approximately at the same time as the GRB
(Bloom et
al. 2002,
Garnavich et
al. 2003).
Near-infrared and radio observations of the afterglow further provided
evidence for extensive mass loss
( ~ 2 ×
10-7
M
yr-1) from the massive stellar progenitor of GRB 011121
(Price et
al. 2002).
To conclude this topic, it is very likely that every time a GRB goes
off, a black hole is born. Gamma Ray Bursts therefore offer, in
principle at least, a direct measure of the massive-star formation rate
in the early universe. Future observations with the HETE-2 spacecraft
and with the Swift multi-wavelength GRB afterglow mission (equipped with
-ray, X-ray
and optical detectors), as well as follow-ups with HST and other
observatories from the ground and space, will hopefully reveal the true
nature of these most dramatic explosions (including the short-duration
bursts).