Thermal emission in the early X-ray afterglows of GRBs: following the prompt phase to the late times

TL;DR

This paper investigates thermal emission in early X-ray afterglows of GRBs, identifying a thermal component in several cases, suggesting it originates from late photospheric emission rather than shock break-out, and linking it to prompt emission.

Contribution

It presents the first systematic search for thermal emission in early X-ray afterglows of GRBs and proposes a new interpretation linking it to prompt phase photospheric emission.

Findings

Thermal components identified in 8 GRBs with tracked evolution.

Evidence of super-luminal expansion indicating relativistic origin.

High luminosities and temperatures suggest a jet photosphere origin.

Abstract

Thermal radiation, peaking in soft X-rays, has now been detected in a handful of GRB afterglows and has to date been interpreted as shock break-out of the GRB's progenitor star. We present a search for thermal emission in the early X-ray afterglows of a sample of Swift bursts selected by their brightness in X-rays at early times. We identify a clear thermal component in eight GRBs and track the evolution. We show that at least some of the emission must come from highly relativistic material since two show an apparent super-luminal expansion of the thermal component. Furthermore we determine very large luminosities and high temperatures for many of the components-too high to originate in a SN shock break-out. Instead we suggest that the component may be modelled as late photospheric emission from the jet, linking it to the apparently thermal component observed in the prompt emission of some GRBs at gamma-ray and hard X-ray energies. By comparing the parameters from the prompt emission and the early afterglow emission we find that the results are compatible with the interpretation that we are observing the prompt quasi-thermal emission component in soft X-rays at a later point in its evolution.