Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts

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Abstract
We report evidence that the asymptotic low-energy power-law slope α (below the spectral break) of BATSE gamma-ray burst (GRB) photon spectra evolves with time rather than remaining constant. We find that a high degree of positive correlation exists between the time-resolved spectral break energy Epk and α. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of α was found to correlate with that of the spectral break energy Epk at the 99.7% and 98% confidence levels, respectively. We also find that in the flux rise phase of hard-to-soft pulses, the mean value of α is often positive, and in some bursts the maximum value of α is consistent with a value > +1. BATSE burst 3B 910927, for example, has an αmax equal to 1.6 ± 0.3. These findings challenge GRB spectral models in which α must be negative or remain constant.
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