Accounting for the XRT early steep decay in models of the prompt GRB emission

TL;DR

This paper evaluates different models of GRB prompt emission to explain the early steep decay observed in X-ray afterglows, finding internal shocks most naturally fit the observed decay timescale.

Contribution

It compares internal shocks, photospheric emission, and magnetic reconnection models to determine which best explains the early steep decay in GRB X-ray afterglows.

Findings

Internal shocks predict decay timescales matching observations.

Photospheric models predict too short decay timescales.

Reconnection models require ad hoc assumptions to fit data.

Abstract

The Swift-XRT observations of the early X-ray afterglow of GRBs show that it usually begins with a steep decay phase. A possible origin of this early steep decay is the high latitude emission that subsists when the on-axis emission of the last dissipating regions in the relativistic outflow has been switched-off. We wish to establish which of various models of the prompt emission are compatible with this interpretation. We successively consider internal shocks, photospheric emission, and magnetic reconnection and obtain the typical decay timescale at the end of the prompt phase in each case. Only internal shocks naturally predict a decay timescale comparable to the burst duration, as required to explain XRT observations in terms of high latitude emission. The decay timescale of the high latitude emission is much too short in photospheric models and the observed decay must then correspond to an effective and generic behavior of the central engine. Reconnection models require some ad hoc assumptions to agree with the data, which will have to be validated when a better description of the reconnection process becomes available.