Beaming, Baryon Loading, and the Synchrotron Self‐Compton Component in Gamma‐Ray Bursts

Abstract

We present detailed calculations of nonthermal synchrotron and synchrotron\nself-Compton (SSC) spectra radiated by blast waves that are energized by\ninteractions with a uniform surrounding medium. Radio, optical, X-ray and\ngamma-ray light curves and spectral indices are calculated for a standard\nparameter set that yields hard GRB spectra during the prompt emission phase.\nBecause no lateral spreading of the blast-wave is assumed, the calculated\ntemporal breaks represent the sharpest breaks possible from collimated outflows\nin a uniform surrounding medium. Absence of SSC hardenings in observed GRB\nX-ray afterglows indicates magnetic field generation toward equipartition as\nthe blast wave evolves. EGRET detections of 100 MeV-GeV photons observed\npromptly and 90 minutes after GRB 940217 are attributed to nonthermal\nsynchrotron radiation and SSC emission from a decelerating blast wave,\nrespectively. The SSC process will produce prompt TeV emission that could be\nobserved from GRBs with redshifts $z \\lesssim 0.1$, provided $\\gamma$-$\\gamma$\nopacity in the source is small. Measurements of the time dependence of the 100\nMeV-GeV spectral indices with the planned {\\it GLAST} mission will chart the\nevolution of the SSC component and test the external shock scenario. Transient\noptical and X-ray emissions from misaligned GRBs are generally much weaker than\non-axis emissions produced by dirty and clean fireballs that would themselves\nnot trigger a GRB detector; thus detection of long wavelength transients not\nassociated with GRBs will not unambiguously demonstrate GRB beaming.\n