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The early X-ray afterglow of gamma-ray bursts revealed by Swift carried many surprises. Following an initial steep decay the light curve often exhibits a plateau phase that can last up to several 104 s, with in addition the presence of flares in 50 per cent of the cases. We focus in this paper on the plateau phase whose origin remains highly debated. We confront several newly discovered correlations between prompt and afterglow quantities (isotropic emitted energy in gamma-rays, luminosity and duration of the plateau) to several models proposed for the origin of plateaus in order to check if they can account for these observed correlations. We first show that the scenario of plateau formation by energy injection into the forward shock leads to an efficiency crisis for the prompt phase and therefore study two possible alternatives: the first one still takes place within the framework of the standard forward shock model but allows for a variation of the microphysics parameters to reduce the radiative efficiency at early times; in the second scenario the early afterglow results from a long-lived reverse shock. Its shape then depends on the distribution of energy as a function of Lorentz factor in the ejecta. In both cases, we first present simple analytical estimates of the plateau luminosity and duration and then compute detailed light curves. In the two considered scenarios we find that plateaus following the observed correlations can be obtained under the condition that specific additional ingredients are included. In the forward shock scenario, the preferred model supposes a wind external medium and a microphysics parameter ϵe that first varies as n−ξ (n being the external density), with ξ ∼ 1 to get a flat plateau, before staying constant below a critical density n0. To produce a plateau in the reverse shock scenario the ejecta must contain a tail of low Lorentz factor with a peak of energy deposition at Γ ≳ 10. [ABSTRACT FROM AUTHOR]

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]

ABSTRACT Recent detections of GeV photons in a few gamma-ray bursts (GRBs) by Fermi Large Area Telescope (LAT) imply huge bulk Lorentz factors to avoid a large γγ optical depth at high energy. Estimates can be as high as Γ≃ 103 in the most extreme cases. This puts severe constraints on models of the central engine and the jet acceleration in GRBs. These estimates are, however, obtained from a simplified single-zone model. We present here a more realistic calculation of the γγ opacity which takes into account the time, space and direction dependent photon field existing in an outflow with several relativistically moving emitting zones. The formalism is very general and can be applied to many models of the prompt GRB emission. We present results obtained for a numerical implementation in the framework of the internal shock model. We show the following results in this paper. (i) The minimum Lorentz factor Γmin in bright Fermi-LAT GRBs is reduced by a factor of ≃2-3 compared to previous estimates if the GeV and MeV emission are produced in the same region, and by an additional factor of ≃2-8 if the GeV emission is produced at larger radii. We provide an improved approximate formula for Γmin which is in good agreement with our numerical results and which can be directly applied to Fermi-LAT GRB data. (ii) A delayed onset of the GeV emission can be due to the time evolution of the opacity in a GRB outflow. As an illustration of these first two results, we present a synthetic GRB that reproduces most features of GRB 080916C with a mean Lorentz factor of ≃340, an optically thin regime for γγ opacity at 3 GeV in time bin 'b', a variability time-scale of ≃0.5 s in the MeV light curve and a delayed onset of ≃5 s of the GeV emission. (iii) The γγ opacity can smooth the short time-scale variability in the GeV light curve. This last result implies that the observed variability at high energy is not necessarily a good test to distinguish between an internal and an external origin for the GeV emission in GRBs. [ABSTRACT FROM AUTHOR]

GRB 041219A is one of the longest and brightest gamma-ray bursts (GRBs) ever observed. It was discovered by the INTEGRAL satellite, and thanks to a precursor happening about 300 s before the bulk of the burst, ground-based telescopes were able to catch the rarely observed prompt emission in the optical and in the near-infrared bands. Here we present the detailed analysis of its prompt gamma-ray emission, as observed with IBIS onboard INTEGRAL, and of the available X-ray afterglow data collected by X-Ray Telescope onboard Swift. We then present the late-time multiband near-infrared imaging data, collected at the Telescopio Nazionale Galileo (TNG) and the Canada-France-Hawaii Telescope (CFHT), that allowed us to identify the host galaxy of the GRB as an underluminous, irregular galaxy of at best-fitting redshift of . We model the broad-band prompt optical to gamma-ray emission of GRB 041219A within the internal shock model. We were able to reproduce the spectra and light curve invoking the synchrotron emission of relativistic electrons accelerated by a series of propagating shock waves inside a relativistic outflow. On the other hand, it is less easy to simultaneously reproduce the temporal and spectral properties of the infrared data. [ABSTRACT FROM AUTHOR]