Spectral Properties

Although GRBs release their energy mostly in the energy range 50-1000 keV, photons down to a few keV and up to 18 GeV have been detected as well. The non-thermal spectra are well described by the Band law ([Band et al., 1993]), that is a smoothed broken power law, defined by the following functional form:

$$N_E(E) = \left\{\begin{array}{lccr} A\left(\frac{E}{100keV}\right)^{\alpha... ...\frac{E}{100keV})^{\beta}, & ~ & ~ & (\alpha-\beta)E_0 \le E \end{array}\right.$$ (3)

The function  has been constructed so that it and its derivative are continuous. The common description of the spectrum continuum is reproduced with eq.  with $\alpha$ peaking in the range [-1,$0.5$] and $\beta \sim -2$. This functional form fits the burst spectra satisfactorily in the energy range from 10 keV to 100 MeV; the break energy $E_0$ typically ranges from 100 keV up to some MeV. The usual spectral evolution with time is hard-to-soft, but different trends were found as well ([Norris et al., 1986], [Ford et al., 1995]). The BATSE catalog did not found any of the spectral features claimed by some previous experiments: [Band et al., 1996], found neither absorption lines in the 20-40 keV band (claimed by [Murakami et al., 1988]), nor emission features around 400 keV (claimed by [Mazets et al., 1980]).