Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts
Crider, A., Liang, E. P., Smith, I. A., Preece, R. D., Briggs, M. S.,
Pendleton, G. N., Paciesas, W. S., Band, D. L., and Matteson, J. L.
We report evidence that the asymptotic low-energy power law slope
alpha (below the spectral break) of BATSE gamma-ray burst 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 alpha.
In samples of 18 ``hard-to-soft'' and 12 ``tracking'' pulses,
evolution of alpha 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 alpha is often positive and in some bursts
the maximum value of alpha is consistent with a value > +1.
BATSE burst 3B 910927,
for example, has an alpha_max equal to 1.6 +/- 0.3.
These findings challenge GRB spectral models
in which alpha must be negative or remain constant.
Status:
April 10, 1997, ApJ, 479, L39
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