DETECTION OF A THERMAL SPECTRAL COMPONENT IN THE PROMPT EMISSION OF GRB 100724B

Abstract

Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor (GBM) find\nthat the spectrum is dominated by the typical Band functional form, which is\nusually taken to represent a non-thermal emission component, but also includes\na statistically highly significant thermal spectral contribution. The\nsimultaneous observation of the thermal and non-thermal components allows us to\nconfidently identify the two emission components. The fact that these seem to\nvary independently favors the idea that the thermal component is of\nphotospheric origin while the dominant non-thermal emission occurs at larger\nradii. Our results imply either a very high efficiency for the non-thermal\nprocess, or a very small size of the region at the base of the flow, both quite\nchallenging for the standard fireball model. These problems are resolved if the\njet is initially highly magnetized and has a substantial Poynting flux.\n