Time-Integrated Gamma-Ray Burst Synchrotron Spectra from Blast Wave/Cloud Interactions

Abstract

We show that the spectral shape of the low energy tails found for the time-integrated spectra of gamma-ray bursts, even in the absence of strong synchrotron cooling, can be significantly softer than the $\nu F_{\nu }\propto {\nu }^{4/3}$ asymptote predicted by synchrotron shock models. As we have noted in a previous work, blast wave deceleration via interaction with ambient material causes the characteristic electron injection energy to decrease in proportion to the bulk Lorentz factor of the blast wave, and under certain conditions, this effect will at least partially account for the observed increase in pulse widths with decreasing energy. This spectral softening can also be reflected in the time-integrated pulse spectrum. Using a simple model for the blast wave interaction with a dense cloud of material, we show that just below the $\nu F_{\nu }$ spectral peak the integrated spectrum behaves as $\nu F_{\nu }\sim {\nu }^{1/2}$ and rolls over to a ${\nu }^{4/3}$ dependence at lower energies, thus a spectral shape arises which is similar to that predicted for the spectrum of a strongly synchrotron-cooled electron population. We discuss the implications of this work in the context of models of burst light curve variability which are based on blast wave/cloud interactions.

5 pages, 2 eps figures, LaTeX, uses emulateapj.sty and apjfonts.sty, submitted to ApJ

References in corpus (10)

• Spectra and Light Curves of Gamma-Ray Burst Afterglows
• The Synchrotron Shock Model Confronts a `Line of Death' in the BATSE Gamma-Ray Burst Data
• Implications of the radio afterglow from the gamma-ray burst of May 8, 1997
• Synchrotron and SSC Emission and the Blast-Wave Model of Gamma-Ray Bursts
• Fireballs Loading and the Blast Wave Model of Gamma Ray Bursts
• The afterglow of gamma ray bursts II: the cases of GRB 970228 and GRB 970508
• Radiative Regimes in Gamma-Ray Bursts and Afterglows
• On Spectral and Temporal Variability in Blazars and Gamma Ray Bursts
• The Duration -- Photon Energy Relation of Gamma Ray Bursts and Its Interpretations
• Energy-Dependent Gamma-Ray Burst Peak Durations and Blast-Wave Deceleration

Cited by in corpus (1)

• Constraints on the emission mechanisms of gamma-ray bursts