Astron. Astrophys. 351, 759-765 (1999)

2. -ray bursts (GRBs) as a heat source

Since their discovery thirty years ago the properties of GRBs have been determined by an outstanding series of experiments that were deployed on more than twenty spacecraft at distances up to several astronomical units (AU) from the Earth and during one period 11 spacecraft were used to study the same GRBs. The properties of GRBs have recently been reviewed (Fishman & Meegan, 1995; Piran, 1999) and are now the subject of intense research. GRBs are extragalactic in origin and release collossal amounts of energy, to ergs assuming isotropic emission, for the GRBs with known redshift. The BeppoSAX satellite discovered x-ray afterglow that decayed with time t in the range to (Costa et al., 1997; Piro et al., 1998; Nicastro et al., 1998). Simultaneous optical emission was detected from the spectacular GRB 990123 at the level of about of the energy in -rays (Akerlof et al., 1999), but this emission is too weak to influence chondrule formation.

The progenitors of GRBs are not known but merging neutron stars have been suggested. One GRB is known to have occurred in a luminous infrared galaxy (Hanlon et al. 1999, in preparation) and some GRBs are close to star forming regions suggesting a connection with massive stars (Bloom et al., 1998). Models of `failed supernova' and `hypernova' have been proposed (Woosley, 1993; Paczynski, 1998) in which the inner core of a massive rotating star collapses to a black hole while the outer core forms a massive disk or torus that somehow generates a relativistic fireball and GRB. In these models GRBs represent the violent end to massive stars.

GRBs have a lognormal bimodal distribution of durations that peak at about 0.3 s and 30 s respectively. The wide range of pulse shapes with complex time profiles have been described using lognormal distributions and GRBs have been called cosmic lightning because of their statistical similiarities with terrestrial lightning (McBreen et al., 1994; Hurley et al., 1998; Stern & Svensson, 1996; Li & Fenimore, 1996). The photon spectra are well described by a power-law with a low energy slope , a break energy E_o and a high energy power law with slope . The functional form, for low energies, is given by (Band et al., 1993):

The value of ranges from 2 keV to over 1 MeV and the indices and are typically -1 and -2 respectively. There is an excess of 10 keV x-rays above this functional form in about 15% of GRBs (Preece et al., 1996).

The spectral energy distributions of a sample of GRBs have been extrapolated and integrated from 0.1 keV to 10 MeV using a sample of values for compatible with BATSE (Band et al., 1993) and Ginga results (Strohmayer et al., 1998). The cumulative fraction of the total energy in GRBs is given in Fig. 1 where a redshift correction of z = 0.8 has been applied to all the spectra. There is sufficient energy to melt solar nebula grains provided absorption of the x-rays and -rays is reasonably efficient.

Fig. 1. The cumulative fraction of GRB energy as a function of photon energy for assumed spectral parameters = -1, = -2, = 5, 10, 15, 25, 50 and 100 keV and redshift of 0.8.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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