A Variety of Decays of Gamma-Ray Burst Pulses

The main target of this study is the GRB light curve during the decay phase of long, bright pulses. As shown by Ryde & Svensson (2000; hereafter RS00) approximately half of these decays can be described by a power law ∝ 1/(time). This happens for cases when the hardness-fluence correlation (HFC) is an exponential function, Epk(Φ) ∝ e −Φ/Φ0 , and the hardness-intensity correlation (HIC) is a power law, Epk(N) ∝ (N/N0) . Here, N(t) is the instantaneous photon flux, Epk(t) is the corresponding photon energy, at which the E NE-spectrum peaks and is used as a measure of the spectral hardness, and the photon fluence is defined by Φ(t) = ∫ t N(t) dt. These most commonly assumed correlations were found by Liang & Kargatis (1996; HFC) and Golenetskii et al. (1983; HIC). There obviously exists a large group of GRB pulses which decay in a different way. In this paper, we search for alternative descriptions of the spectral/temporal evolution. We use the complete sample of long pulses in strong bursts presented in Ryde & Svensson (2001) consisting of 25 pulses within 23 bursts observed by BATSE on the CGRO during its entire mission (1991 – 2000). The spectral analysis of the LAD/HERB data (∼ 25 − 1900 keV) was performed with the WINGSPAN/MFIT package (Preece et al. 1996). For each time bin the photon spectrum with the background subtracted was determined using the Band et al. (1993) function with both its power law indices left free to vary. The instantaneous, integrated photon flux, N(t), was found by integrating the modeled photon spectrum over the available energy band.