3 Spectra and spectral evolution

The hard X-ray and gamma-ray spectra of GRBs have been studied in great detail (cf. papers in the 4th Huntsville GRB Symposium Proceedings, Meegan et al. 1998, and references therein). The general spectral form is well-described by what has become known as the Band function (Band et al. 1993), both in the time-integrated spectra and in shorter intervals within bursts. The low energy characteristics (below 20 keV) of GRBs have been examined in detail in several recent papers (Preece et al. 1996; Strohmayer et al. 1998). This is a crucial region to test emission models, such as the synchrotron shock model and the thick Comptonization model. Joint fits of spectral data from BeppoSAX and BATSE should become available in the near future.

The high-energy spectra of GRBs that are seen to extend above 10 MeV are best fit by a power-law with a spectral index of $\sim -2$, but the number of GRBs for which these data are available are limited (cf. Schaefer et al. 1998). However, many GRB spectra are seen to cut-off much more steeply and do not have detectable emission above 300 keV (Pendleton et al. 1997). Spectra within the  BATSE range from 20 keV to $\sim$2 MeV have been fit to numerous GRBs (Mallozzi et al. 1995). In these spectra, both the low and high-energy ends of the broad spectral distribution are fit by smoothly joined power-laws within the BATSE bandpass. The peaks of these energy spectra ($\nu F_{\nu}$) are referred to as the $E_{\rm p}$ energy. The $E_{\rm p}$ distribution for a large number of GRBs is shown in Fig. 4. A comparison of $E_{\rm p}$ with GRB intensity shows a correlation. This correlation is consistent with a redshift of the continuum spectrum with distance (Mallozzi et al. 1995).

  

Figure 4: The BATSE GRB $E_{\rm p}$ distribution (Mallozzi et al. 1995)

Spectral softening is usually  (but not always)  seen throughout a GRB and also in sub-pulses within a burst. This is illustrated in Fig. 5, where a sub-peak is seen to have soft emission (20-50 keV), for an extended duration, $\sim$8 s, following the hard peak, which only lasts for $\sim$2 s. In this particular case, the peaks of both the hard and soft emission occur simultaneously. However, in many cases the peaks are offset, with the softer peak delayed by up to a few seconds. The evolution of spectral properties is an active area of research. A member of the BATSE group, Giblin, is working on spectral evolution by characterizing changes and noting patterns in time-dependent color-color diagrams for ensembles of BATSE GRBs. This work is an extension of earlier color-color diagrams of BATSE GRBs (Kouveliotou et al. 1993b). Liang & Kargatis (1996) have noted a correlation between the spectral evolution of sub-peaks within a given GRB and the intensity of these sub-peaks. Spectral evolution within GRBs is another property that must be considered in burst emission models but is usually not, in contrast to the well-modeled spectral evolution of the GRB afterglow emission.