Fermi Gamma-ray Space Telescope

Towards a Unied Model for the Gamma-Ray Burst Prompt Emission & a New Luminosity-Hardness Relation for Cosmology

S. Guiriec


We suggest here to replace the historical spectral model (Band function) for the Gamma-Ray Burst (GRB) prompt emission (keV-MeV energy regime) with a new one. We show that the complex GRB spectral shapes are well described with a combination of three separate components: (i) a thermal-like component interpreted as emission from a non-dissipative jet photosphere, (ii) a non-thermal component interpreted either as synchrotron radiation or as a dissipative photosphere, and (iii) a second non-thermal component--not always detected--most likely of inverse Compton origin. The smooth evolution of all three components during the burst reinforces the validity of this new model. Detailed studies of these components provide insights on the nature and composition of GRB jets as well as on their magnetic fields. Moreover, this new model enables a new luminosity-hardness relation based on the first non-thermal component showing that GRBs may be standard candles. If statistically confirmed, this relation will be used to (i) constrain the mechanisms powering GRB jets, (ii) estimate GRB distances, (iii) probe the early Universe, and (iv) constrain the cosmological parameters. I will present this new model using analysis of GRBs detected with various observatories and instruments such as Fermi and CGRO/BATSE. I will discuss here the striking similarities of GRB spectral shapes as well as the possible universality of the proposed luminosity-hardness relation in the context of the new model.