Fermi Gamma-ray Space Telescope

GRB 131014A: a Natural Laboratory to Study the Prompt-Emission Thermal-Like Component and its Connection to the Non-Thermal Contribution

Sylvain Guiriec
Julie McEnery, Neil Gehrels, Judith Racusin, Chryssa Kouveliotou

Abstract:

Over the past years we have been accumulating evidences for the existence of a thermal-like component during the prompt phase of Gamma Ray Bursts (GRBs). However, this component is usually subdominant compared to a much stronger non-thermal one. The prompt emission of GRB 131014A -- detected by Fermi -- provides an unique opportunity (i) to trace the history of this thermal-like component and (ii) to identify its true spectral shape. Indeed, the thermal-like emission in GRB 131014A is much more intense than in other GRBs and after a nearly purely thermal episode exhibiting a clear monotonic cooling, the non-thermal emission kicks off and its intensity progressively increases until being energetically dominant at late time similarly to what is usually observed. This is the perfect scenario to disentangle the thermal-like component from the non-thermal one. The broader spectral shape of the thermal-like component compared to a pure Planck function is also in perfect agreement with the jet photosphere models that predict a broadening because of sub-photospheric dissipation and geometrical jet curvature effects. Finally, a strong correlation is obtained between the time-resolved energy flux of the non-thermal component and its nuFnu peak energy with a slope similar to the one we reported previously in other GRBs. Assuming that this new luminosity-hardness relation is universal -- as suggest by our previous works -- we derived a redshift of ~1.55+/-0.03 for GRB 131014A that is a typical value for long GRBs.