Toward a Better Understanding of the GRB Phenomenon: A New Model For GRB Prompt Emission and its Effects on the New Lint– Epeak,irest,NT Relation

S. Guiriec
C. Kouveliotou


Gamma-ray burst (GRB) prompt emission spectra in the keV–MeV energy range are usually considered to be adequately fitted with the empirical Band function. Recent observations with the Fermi Gamma-ray Space Telescope (Fermi) revealed deviations from the Band function, sometimes in the form of an additional blackbody (BB) component, while on other occasions in the form of an additional power law (PL) component extending to high energies. In this article we investigate the possibility that the three components may be present simultaneously in the prompt emission spectra of two very bright GRBs (080916C and 090926A) observed with Fermi, and how the three components may affect the overall shape of the spectra. While the two GRBs are very different when fitted to a single Band function, they look like "twins" in the three-component scenario. Through fine-time spectroscopy down to the 100 ms timescale, we follow the evolution of the various components. We succeed in reducing the number of free parameters in the three-component model, which results in a new semi-empirical model—but with physical motivations—to be competitive with the Band function in terms of number of degrees of freedom. From this analysis using multiple components, the Band function is globally the most intense component, although the additional PL can overpower the others in sharp time structures. The Band function and the BB component are the most intense at early times and globally fade across the burst duration. The additional PL is the most intense component at late time and may be correlated with the extended high-energy emission observed thousands of seconds after the burst with Fermi/Large Area Telescope. Unexpectedly, this analysis also shows that the additional PL may be present from the very beginning of the burst, where it may even overpower the other components at low energy. We investigate the effect of the three components on the new time-resolved luminosity–hardness relation in both the observer and rest frames and show that a strong correlation exists between the flux of the non-thermal Band function and its Epeak only when the three components are fitted simultaneously to the data (i.e., ${F}_{i}^{\mathrm{NT}}$–${E}_{\mathrm{peak},i}^{\mathrm{NT}}$ relation). In addition, this result points toward a universal relation between those two quantities when transposed to the central engine rest frame for all GRBs (i.e., ${L}_{i}^{\mathrm{NT}}$–${E}_{\mathrm{peak},i}^{\mathrm{rest},\mathrm{NT}}$ relation). We discuss a possible theoretical interpretation of the three spectral components within this new empirical model. We suggest that (i) the BB component may be interpreted as the photosphere emission of a magnetized relativistic outflow, (ii) the Band component has synchrotron radiation in an optically thin region above the photosphere, either from internal shocks or magnetic field dissipation, and (iii) the extra PL component extending to high energies likely has an inverse Compton origin of some sort, even though its extension to a much lower energy remains a mystery.