Extremely hard GRB spectra prune down the forest of emission models

By G. Ghirlanda, A. Celotti, G. Ghisellini. 2002

140 citations recorded by the source corpus.

We consider the evidence for very hard low energy spectra during the prompt phase of Gamma-Ray Bursts (GRB). In particular we examine the spectral evolution of GRB 980306 together with the detailed analysis of some other bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927 and GRB 970111), and check for the significance of their hardness (i.e. extremely steep spectral slopes below the EFE peak) by applying dierent tests. These bursts, detected by the Burst And Transient Source Experiment (BATSE) in the30 keV-2 MeV energy range, are suciently bright to allow time resolved spectral studies on time intervals of the order of tenths of a second. We discuss the hard spectra of these bursts and their evolution in the context of several non-thermal emission models, which all appear inadequate to account for these cases. The extremely hard spectra, which are detected in the early part of the BATSE light curve, are also compared with a black body spectral model: the resulting fits are remarkably good, except for an excess at high energies (in several cases) which could be simply accounted for by the presence of a supra-thermal component. The findings on the possible thermal character of the evolving spectrum and the implications on the GRB physical scenario are considered in the frameworks of photospheric models for a fireball which is becoming optically thin, and of Compton drag models, in which the fireball boosts "ambient" seed photons by its own bulk motion. Both models, according to simple estimates, appear to be qualitatively and quantitatively consistent with the found spectral characteristics, although their possible caveats are discussed.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/08711a21eab978b7df84175b18b10bffbc036f5c

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Citations in the corpus

Is Thermal Emission in Gamma-Ray Bursts Ubiquitous? (2005, background): “Some bursts are found to have quasi-thermal spectra during the initial phases, before they become non-thermal (Ghirlanda et al. 2003; Kaneko et al. 2003; Ryde 2004).”
A New Method of Determining the Initial Size and Lorentz Factor of Gamma-Ray Burst Fireballs Using a Thermal Emission Component (2007, background): “…increasing evidence has appeared that, during the first stages of the prompt emission of long duration gamma-ray bursts (GRBs), a thermal component accompanies the underlying non-thermal emission [Ryde (2004, 2005); Campana et al. (2006); see also Ghirlanda et al. (2003); Kaneko et al. (2003)].”
Prompt GRB emission from gradual energy dissipation (2007, background): “Despite its attractive features, the synchrotron model has theoretical and observational difficulties (as discussed, for example, in Ghirlanda et al. 2003).”
Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation? (2010, background): “The scenario proposed in this paper can never produce low-energy slopesα steeper than−2/3, whereas such slopes are measured in a non negligible fraction of GRB spectra (Ryde 2004; Ghirlanda et al. 2003).”
QUASI-BLACKBODY COMPONENT AND RADIATIVE EFFICIENCY OF THE PROMPT EMISSION OF GAMMA-RAY BURSTS (2008, background): “Analysis of time-resolved spectra done by Crider et al. (1998) and Ghirlanda et al. (2003), showed that neither the synchrotron nor the SSC models can explain the instantaneous GRB spectra and their evolution.”
The Cooling Behavior of Thermal Pulses in Gamma-Ray Bursts (2004, methodology): “A temporal variation in kT will be followed by a similar variation in the flux.1 However, an increase of the temperature during the rise phase of the pulses studied above is not observed (see also Ghirlanda et al. 2003).”
General relativistic magnetohydrodynamic simulations of the jet formation and large-scale propagation from black hole accretion systems (2006, methodology): “However, by the radius simulated, the Lorentz factor is not yet large enough for the standard GRB internal shock model to be efficient (Kobayashi et al. 1997; Mészáros 2002; Ghirlanda et al. 2003; Piran 2005).”
DETECTION OF A THERMAL SPECTRAL COMPONENT IN THE PROMPT EMISSION OF GRB 100724B (2010, background): “Observationally, Ghirlanda et al. (2003), Ryde (2004, 2005), and Ryde et al. (2010) argued that a photospheric component is present in CGRO BATSE data.”