Taking the Band Function Too Far: A Tale of Two $α$'s

The long standing problem of identifying the emission mechanism operating in gamma-ray bursts (GRBs) has produced a myriad of possible models that have the potential of explaining the observations. Generally, the empirical Band function is fit to the observed gamma-ray data and the fit parameters are used to infer which radiative mechanisms are at work in GRB outflows. In particular, the distribution of the Band function's low-energy power law index, $α$, has led to the so-called synchrotron "line-of-death" (LOD) which is a statement that the distribution cannot be explained by the simplest of synchrotron models alone. As an alternatively fitting model, a combination of a blackbody in addition to the Band function is used, which in many cases provide a better or equally good fit. It has been suggested that such fits would be able to alleviate the LOD problem for synchrotron emission in GRBs. However, these conclusions rely on the Band function's ability to fit a synchrotron spectrum within the observed energy band. In order to investigate if this is the case, we simulate synchrotron and synchrotron+blackbody spectra and fold them through the instrumental response of the $Fermi$ Gamma-ray Burst Monitor (GBM). We then perform a standard data analysis by fitting the simulated data with both Band and Band+blackbody models. We find two important results: the synchrotron LOD is actually more severe than the original predictions: $α_{\bf LOD} \sim -0.8$. Moreover, we find that intrinsic synchrotron+blackbody emission is insufficient to account for the entire observed $α$ distribution. This implies that some other emission mechanism(s) are required to explain a large fraction of observed GRBs.