Gamma-Ray Bursts (GRBs) are short and intense pulses of soft
-rays.
The bursts last from a fraction of a second to several hundred
seconds. GRBs arrive from cosmological distances from random
directions in the sky. The overall observed fluences range from
10-4ergs/cm2 to 10-7ergs/cm2
(the lower limit
depends, of course, on the characteristic of the detectors and not
on the bursts themselves). This corresponds to isotropic
luminosity of
1051 - 1052ergs/sec, making GRBs the most
luminous objects in the sky. However, we know today that most GRBs
are narrowly beamed and the corresponding energies are "only"
around 1051ergs
[105,
291,
310],
making them comparable to Supernovae in the total energy release.
The GRBs are followed by afterglow - lower energy, long lasting emission in the X-ray, optical and radio. The radio afterglow was observed in some cases several years after the bursts. The accurate afterglow positions enabled the identification of host galaxies in almost all cases when afterglow was detected and this in turn enabled the determination of the corresponding redshifts that range from 0.16 (or possibly even down to 0.0085) to 4.5. Within the host galaxies there is evidence that (long duration) GRBs arise within star forming regions and there is evidence that they follow the star formation rate.
While not all observed features are understood there is an overall agreement between the observations and the fireball model. According to the fireball model GRBs are produced when the kinetic energy of an ultra-relativistic flow is dissipated. The GRB itself is produced by internal dissipation within the flow while the afterglow is produced via external shocks with the circum-burst medium. I will focus in this review on this model.
The numerous observations of the GRB and the observations of the afterglow constrain the fireball model that describes the emitting regions. The evidence on nature of the inner engine that powers the GRB and produces the ultra-relativistic flow is however, indirect. The energetic requirements and the time scales suggest that GRB involve the formation of the black hole via a catastrophic stellar collapse event or possibly a neutron star merger. Additional indirect evidence arises from the requirement of the fireball model of long (several dozen seconds) activity of the inner engine. This hints towards an inner engine built on an accreting black hole. On the other hand, the evidence of association of GRBs with star forming regions indicates that GRBs progenitors are massive stars. Finally, the appearance of Supernova bumps in the afterglow light curve (most notably in GRB 030329) suggest association with Supernovae and stellar collapse.
I review here the theory of GRB, focusing as mentioned earlier on
the fireball internal-external shocks model. I begin in
Section II with a brief discussion of the
observations. I turn
in Section III to some generally accepted
properties of
GRB models - such as the essential ultra-relativistic nature of
this phenomenon. Before turning to a specific discussion of the
fireball model I review in Section IV several
relativistic
effects and in Section V the physical
process, such as synchrotron emission or particle acceleration in
relativistic shocks that are essential ingredients of this model.
In Section VI I turn to a discussion of the prompt
emission and the GRB. In Section VII I discuss
modelling the afterglow emission. I consider other related
phenomenon - such as TeV
-rays
emission, high energy neutrinos,
Ultra High energy cosmic rays and gravitational radiation in
Section VIII. Finally, I turn in
Section IX to
examine different `inner engines' and various aspects related to
their activity. I conclude with a discussion of open questions and
observational prospects.
While writing this review I realized how large is the scope of this field and how difficult it is to cover all aspects of this interesting phenomenon. Some important aspects had to be left out. I also did not attempt to give a complete historical coverage of the field. I am sure that inadvertently I have missed many important references. I refer the reader to several other recent review papers [98, 119, 177, 231, 232, 308, 309, 416] that discuss these and other aspects of GRB theory and observations from different points of view.