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The short GRB 120323A had the highest flux ever detected with the Gamma-Ray Burst Monitor on board the Fermi Gamma-Ray Space Telescope. Here we study its remarkable spectral properties and their evolution using two spectral models: (1) a single emission component scenario, where the spectrum is modeled by the empirical Band function (a broken power law), and (2) a two-component scenario, where thermal (a Planck-like function) emission is observed simultaneously with a non-thermal component (a Band function).We find that the latter model fits the integrated burst spectrum significantly better than the former, and that their respective spectral parameters are dramatically different: when fit with a Band function only, the Epeak of the event is unusually soft for a short gamma-ray burst (GRB; 70 keV compared to an average of 300 keV), while adding a thermal component leads to more typical short GRB values (Epeak ~ 300 keV). Our time-resolved spectral analysis produces similar results.We argue here that the two-component model is the preferred interpretation for GRB 120323A based on (1) the values and evolution of the Band function parameters of the two component scenario, which are more typical for a short GRB, and (2) the appearance in the data of a significant hardness–intensity correlation, commonly found in GRBs, when we employee two-component model fits; the correlation is non-existent in the Band-only fits.GRB 110721A, a long burst with an intense photospheric emission, exhibits the exact same behavior.We conclude that GRB 120323A has a strong photospheric emission contribution, observed for the first time in a short GRB. Magnetic dissipation models are difficult to reconcile with these results, which instead favor photospheric thermal emission and fast cooling synchrotron radiation from internal shocks. Finally, we derive a possibly universal hardness–luminosity relation in the source frame using a larger set of GRBs (LiBand = (1.59 ± 0.84) × 1050(Epeak,irest)1.33±0.07 erg s-1), which could be used as a possible redshift estimator for cosmology. [ABSTRACT FROM AUTHOR]

Context. Gamma-ray burst (GRB) spectra globally appear non-thermal, but recent observations of a few bursts with Fermi GBM have confirmed previous indications from BATSE of the presence of an underlying thermal component. Photospheric emission is indeed expected when the relativistic outflow emerging from the central engine becomes transparent to its own radiation, with a quasiblackbody spectrum in absence of additional sub-photospheric dissipation. However, its intensity strongly depends on the acceleration mechanism - thermal or magnetic - of the flow. Aims. We aim to compute the thermal and non-thermal emissions (light curves and spectra) produced by an outflow with a variable Lorentz factor, where the power Ėiso injected at the origin is partially thermal (fraction ϵth ⩽ 1) and partially magnetic (fraction 1-ϵth). The thermal emission is produced at the photosphere, and the non-thermal emission in the optically thin regime. Apart from the value of ϵth, we want to test how the other model parameters affect the observed ratio of the thermal to non-thermal emission. Methods. We followed the adiabatic cooling of the flow from the origin to the photosphere and computed the emitted radiation, which is a sum of modified black bodies at different temperatures (as the temperature strongly depends on the Lorentz factor of each shell at transparency). If the non-thermal emission comes from internal shocks, it is obtained from a multi-shell model where a fraction of the energy dissipated in shell collision is transferred to electrons and radiated via the synchrotron mechanism. If, conversely, the non-thermal emission originates in magnetic reconnection, the lack of any detailed theory for this process forced us to use a very simple parametrisation to estimate the emitted spectrum. Results. If the non-thermal emission is made by internal shocks, we self-consistently obtained the light curves and spectra of the thermal and non-thermal components for any distribution of the Lorentz factor in the flow. If the non-thermal emission results from magnetic reconnection we were unable to produce a light curve and could only compare the respective non-thermal and thermal spectra. In the different considered cases, we varied the model parameters to see when the thermal component in the light curve and/or spectrum is likely to show up or, on the contrary, to be hidden. We finally compared our results to the proposed evidence for the presence of a thermal component in GRB spectra. Focussing on GRB 090902B and GRB10072B, we showed how these observations can be used to constrain the nature and acceleration mechanism of GRBoutflows. [ABSTRACT FROM AUTHOR]

Context. The Swift-XRT observations of the early X-ray afterglow of GRBs show that it usually begins with a steep decay phase. Aims. A possible origin of this early steep decay is the high latitude emission that subsists when the on-axis emission of the last dissipating regions in the relativistic outflow has been switched-off. We wish to establish which of serious models of the prompt emission are compatible with this interpretation. Methods. We successively consider internal shocks, photospheric emission, and magnetic reconnection and obtain the typical decay timescale at the end of the prompt phase in each case. Results. Only internal shocks naturally predict a decay timescale comparable to the burst duration, as required to explain XRT observations in terms of high latitude emission. The decay timescale of the high latitude emission is much too short in photospheric models and the observed decay must then correspond to an effective and generic behavior of the central engine. Reconnection models require some ad hoc assumptions to agree with the data, which will have to be validated when s better description of the reconnection process becomes available. [ABSTRACT FROM AUTHOR]