Extinction properties of the X-ray bright/optically faint afterglow of GRB 020405

We present an optical-to-X-ray spectral analysis of the afterglow of GRB 020405. The optical spectral energy distribution not corrected for the extragalactic extinction is significantly below the X-ray extrapolation of the single powerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain the observed spectral ``mismatch'' by testing several types of extinction curves. For the first time we test extinction curves computed with time-dependent numerical simulations of dust grains destruction by the burst radiation. We find that an extinction law weakly depen dent on wavelength can reconcile the unabsorbed optical and X-ray data with the expected synchrotron spectrum. A gray extinction law can be provided by a dust grain size distribution biased toward large grains.Comment: 6 pages, 5 figures, accepted for publication on A&

Detection of a very bright optical flare from a gamma-ray burst at redshift 6.29

In this letter we discuss the flux and the behavior of the bright optical flare emission detected by the 25 cm TAROT robotic telescope during the prompt high-energy emission and the early afterglow. We combine our data with simultaneous observations performed in X-rays and we analyze the broad-band spectrum. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude. While GRB 990123 was, until now, considered as a unique event, this observation suggests the existence of a population of GRBs which have very large isotropic equivalent energies and extremely bright optical counterparts. The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift Universe and might be used to probe the first generation of stars.Comment: 9 pages, 3 figures. Accepted in ApJ

Fall back accretion and energy injections in gamma-ray bursts

Intense flares that occur at late times relative to the prompt phase have been observed by the $Swift$ satellite in the X-ray afterglows of gamma-ray bursts (GRBs). Here, we present a detailed analysis on the fall back accretion process to explain the intense flare phase in the very early X-ray afterglow light curves. To reproduce the afterglow at late times, we resort to the external shock by engaging energy injections. By applying our model to GRBs 080810, 081028 and 091029, we show that their X-ray afterglow light curves can be reproduced well. We then apply our model to the ultra-long $Swift$ GRB 111209A, which is the longest burst ever observed. The very early X-ray afterglow of GRB 111209A showed many interesting features, such as a significant bump observed at around 2000 s after the $Swift$/BAT trigger. We assume two constant energy injection processes in our model. These can explain the observed plateau at X-ray wavelength in the relatively early stage ($8.0\times10^{3}$ s) and a second X-ray plateau and optical rebrightening at about $10^{5}$ s. Our analysis supports the scenario that a significant amount of material may fall back toward the central engine after the prompt phase, causing an enhanced and long lived mass accretion rate powering a Poynting-flux-dominated outflow.Comment: 2 tables, 3 figures, accepted by MNRA

Continuous optical monitoring during the prompt emission of GRB 060111B

We present the time-resolved optical emission of GRB 060111B during its prompt phase, measured with the TAROT robotic observatory. This is the first time that the optical emission from a gamma-ray burst has been continuously monitored with a temporal resolution of a few seconds during the prompt gamma-ray phase. The temporal evolution of the prompt optical emission at the level of several seconds is used to provide a clue to the origin of this emission. The optical emission was found to decay steadily from our first measure, 28s after the trigger, in contrast to the gamma-ray emission, which exhibits strong variability at the same time. This behaviour strongly suggests that the optical emission is due to the reverse shock