Are GRB Blackbodies an Artifact of Spectral Evolution?

Abstract

The analysis of gamma-ray burst (GRB) spectra with multi-component emission models has become an important part of the field. In particular, multi-component analysis where one component is a blackbody representing emission from a photosphere has enabled both a more detailed understanding of the energy content of the jet as well as the ability to examine the dynamic structure of the outflow. While the existence of a blackbody-like component has been shown to be significant and not a byproduct of background fluctuations, it is very possible that it can be an artifact of spectral evolution of a single component that is being poorly resolved in time. Herein, this possibility is tested by simulating a single component evolving in time and then folding the spectra through the $Fermi$ detector response to generate time-tagged event Gamma-ray Burst Monitor (GBM) data. We then fit both the time integrated and resolved generated spectral data with a multi-component model using standard tools. It is found that in {\it time-integrated} spectra, a blackbody can be falsely identified due to the spectral curvature introduced by the spectral evolution. However, in time-resolved analysis defined by time bins that can resolve the evolution of the spectra, the significance of the falsely identified blackbody is very low. Additionally, the evolution of the artificial blackbody parameters does not match the recurring behavior that has been identified in the actual observations. These results reinforce the existence of the blackbody found in {\it time-resolved} analysis of GRBs and stress the point that caution should be taken when using time-integrated spectral analysis for identifying physical properties of GRBs.