The spectral analysis of GRB 150902A and the nature of its outflow

Abstract

ABSTRACT The degree of magnetization of the jets remains an important question in Gamma-Ray Burst (GRB) physics. The magnetization at a large distance from the initial radius also determines whether the process for non-thermal prompt emission is magnetic reconnection or internal shocks. In this article, we aim to address these questions for GRB 150902A. We carry out the spectral analysis of GRB 150902A between 8 keV and 100 GeV with the data from the Fermi Gamma-ray SpaceTelescope. In both the time-integrated and time-resolved analyses, we find that the fit of the spectrum is significantly improved by the addition of a sub-dominant black-body component to the Band function. These results allow us to study and constrain the properties of the outflow. In particular, we consider different values of the efficiency of the non-thermal emission process fNT and evaluate possible ranges of the initial fraction of power in thermal form ϵTh for different initial radii. We find that GRB 150902A has a strong magnetic component in the initial outflow, such that ϵTh ≤ 0.5. Small values of the initial radius tend to favour internal shocks; high efficiency magnetic reconnection is completely excluded and relatively low efficiency magnetic reconnection is only possible if the outflow is almost entirely accelerated thermally. For larger initial radii, ϵTh ≪ 0.5 such that the outflow is dominated by the magnetic component initially. Such initial radii allow for the non-thermal emission process to be either magnetic reconnection or internal shocks, depending on the efficiency of the acceleration by the magnetic component.