Fermi Gamma-ray Space Telescope

A Unified Model for GRB Prompt Emission from Optical to Gamma Rays: Exploring GRBs as Standard Candles

sylvain Guiriec
(Sylvain Guiriec, Frederic Daigne, Chryssa Kouveliotou, Dieter Hartmann, Tsvi Piran, Jonathan Granot, Robert Mochkovitch, Julie McEnery, Judith Racusin & Katsuaki Asano)


The Band function traditionally used for Gamma Ray Bursts (GRB) often fails to fit their prompt emission spectra. Our new model composed of three separate components provides an excellent description of the time-resolved prompt emission: a thermal-like and two non-thermal components. This multi-component model has only one extra free-parameter with respect to the Band function. For the first time, analysis of GRBs with correlated optical and gamma-ray prompt emission show that our new model describes very accurately the whole broadband spectrum from the optical regime to higher energy gamma rays. In addition, this new model enables a new luminosity/hardness relation intrinsic to one of the non-thermal components 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 (a crucial tool in the gravitational wave astrophysics Era), (iii) probe the early Universe, and (iv) constrain the cosmological parameters. I will present this new unified model using analysis of GRBs detected with various observatories and instruments such as Fermi, CGRO/BATSE and the combination of the three instruments onboard Swift and Suzaku/WAM. I will discuss here the striking similarities of GRB spectral shapes, whose components inform on the nature of the prompt emission, as well as the possible universality of the proposed luminosity/hardness relation in the context of our new model.