On the synchrotron self-compton emission from relativistic shocks and its implications for gamma-ray burst afterglows

We consider the effects of inverse Compton scattering of synchrotron photon s from relativistic electrons in gamma -ray burst (GRB) afterglows. We comp ute the spectrum of the inverse Compton emission and find that it can domin ate the total cooling rate of the afterglow for several months or even year s after the initial explosion. We demonstrate that the presence of strong i nverse Compton cooling can be deduced from the effect it has on the time ev olution of the cooling break in the synchrotron spectral component, and the refore on the optical and X-ray afterglow light curves. We then show how th e physical interpretation of the observed characteristics of the synchrotro n spectrum must be modified to take into consideration this extra source of cooling and give a revised prescription for computing physical parameters characterizing the expanding shock wave from the observed quantities. We fi nd that for a given set of observables (synchrotron break frequencies and f luxes) there is either no consistent physical interpretation or two of them . Finally we discuss the prospects of directly detecting the inverse Compto n emission with Chandra. We argue that such a detection is possible for GRB s exploding in a reasonably dense (n greater than or similar to 1 cm(-3)) m edium.