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Register a new userThe origin of the prompt GRB spectrum
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F. Daigne
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Using a detailed model of the internal shock phase, we discuss the origin of the prompt emission in gamma-ray bursts. We focus on the identification of the dominant radiative process (Fermi-GBM range) and propose an explanation for some features observed by Fermi-LAT at high energy in some GRB lightcurves.
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GRB 080503, detected by Swift, belongs to the class of bursts whose prompt phase consists of an initial short spike followed by a longer soft tail. It did not show any transition to a regular afterglow at the end of the prompt emission but exhibited a surprising rebrightening after one day. We aim to explain this rebrightening with two different scenarios - refreshed shocks or a density clump in the circumburst medium - and two models for the origin of the afterglow, the standard one where it comes from the forward shock, and an alternative one where it results from a long-lived reverse shock. We computed afterglow light curves either using a single-zone approximation for the shocked region or a detailed multizone method that more accurately accounts for the compression of the material. We find that in several of the considered cases the detailed model must be used to obtain a reliable description of the shock dynamics. The density clump scenario is not favored. We confirm previous results that the presence of the clump has little effect on the forward shock emission, except if the microphysics parameters evolve when the shock enters the clump. Moreover, we find that the rebrightening from the reverse shock is also too weak when it is calculated with the multi-zone method. On the other hand, in the refreshed-shock scenario both the forward and reverse shock models provide satisfactory fits of the data under some additional conditions on the distribution of the Lorentz factor in the ejecta and the beaming angle of the relativistic outflow.
The high-fluence GRB131108A at redshift z=2.4, was detected by the Mini-Calorimeter (MCAL, 0.35-100 MeV) and the Gamma- Ray Imaging Detector (GRID, 30 MeV - 30 GeV) onboard the AGILE satellite. The burst emission consisted of a very bright initial peak,lasting 0.1 s, followed by a fainter emission detected for ~25 s with the MCAL and ~80 s with the GRID. The AGILE spectra, when compared with those reported at lower energies, indicate the presence of a prominent high-energy component with peak energy in the 10-20 MeV region. Contrary to other GRBs, this high-energy component is present also during the initial peak, with power law photon index of about -1.6 below 10 MeV and -2.35+-0.2 above 30 MeV.
There are the results of gamma-ray bursts observations obtained using the MASTER robotic telescope in 2007 - 2009. We observed 20 error-boxes of gamma-ray bursts this period.The limits on their optical brightnesses have been derived. There are 5 prompt observations among them, obtained at our very wide field cameras. Also we present the results of the earliest observations of the optical emission of the gamma-ray bursts GRB 050824 and GRB 060926.
The Swift-XRT observations of the early X-ray afterglow of GRBs show that it usually begins with a steep decay phase. 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 various models of the prompt emission are compatible with this interpretation. 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. 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 a better description of the reconnection process becomes available.
Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor (GBM) find that the spectrum is dominated by the typical Band functional form, which is usually taken to represent a non-thermal emission component, but also includes a statistically highly significant thermal spectral contribution. The simultaneous observation of the thermal and non-thermal components allows us to confidently identify the two emission components. The fact that these seem to vary independently favors the idea that the thermal component is of photospheric origin while the dominant non-thermal emission occurs at larger radii. Our results imply either a very high efficiency for the non-thermal process, or a very small size of the region at the base of the flow, both quite challenging for the standard fireball model. These problems are resolved if the jet is initially highly magnetized and has a substantial Poynting flux.
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