Aims: We characterize a sample of Gamma-Ray Bursts with low luminosity X-ray afterglows (LLA GRBs), and study their properties. Method: We select a sample consisting of the 12% faintest X-ray afterglows from the total population of long GRBs (lGRBs)
with known redshift. We study their intrinsic properties (spectral index, decay index, distance, luminosity, isotropic radiated energy and peak energy) to assess whether they belong to the same population than the brighter afterglow events. Results: We present strong evidences that these events belong to a population of nearby events, different from that of the general population of lGRBs. These events are faint during their prompt phase, and include the few possible outliers of the Amati relation. Out of 14 GRB-SN associations, 9 are in LLA GRB sample, prompting for caution when using SN templates in observational and theoretical models for the general lGRBs population.
During the pre-Swift era, a clustering of light curves was observed in the X-ray, optical and infrared afterglow of gamma-ray bursts. We used a sample of 254 GRB X-ray afterglows to check this fact in the Swift era. We corrected fluxes for distance,
time dilation and losses of energy due to cosmological effects. With all our data in hand, we faced with a problem: our data were scattered. We investigated 3 possibilities to explain this, namely: the clustering does not exist, there are problems during calibration of data, and there are instrumental problems. We finally confirm that our sample is consistent with Dainotti correlation.
The Swift burst GRB 110205A was a very bright burst visible in the Northern hemisphere. GRB 110205A was intrinsically long and very energetic and it occurred in a low-density interstellar medium environment, leading to delayed afterglow emission and
a clear temporal separation of the main emitting components: prompt emission, reverse shock, and forward shock. Our observations show several remarkable features of GRB 110205A : the detection of prompt optical emission strongly correlated with the BAT light curve, with no temporal lag between the two ; the absence of correlation of the X-ray emission compared to the optical and high energy gamma-ray ones during the prompt phase ; and a large optical re-brightening after the end of the prompt phase, that we interpret as a signature of the reverse shock. Beyond the pedagogical value offered by the excellent multi-wavelength coverage of a GRB with temporally separated radiating components, we discuss several questions raised by our observations: the nature of the prompt optical emission and the spectral evolution of the prompt emission at high-energies (from 0.5 keV to 150 keV) ; the origin of an X-ray flare at the beginning of the forward shock; and the modeling of the afterglow, including the reverse shock, in the framework of the classical fireball model.
The TAROT telescopes (Telescopes a Action Rapide pour les Objets Transitoires) are two robotic observatories designed to observe the prompt optical emission counterpart and the early afterglow of gamma ray bursts (GRBs). We present data acquired betw
een 2001 and 2008 and discuss the properties of the optical emission of GRBs, noting various interesting results. The optical emission observed during the prompt GRB phase is rarely very bright: we estimate that 5% to 20% of GRBs exhibit a bright optical flash (R<14) during the prompt gamma-ray emission, and that more than 50% of the GRBs have an optical emission fainter than R=15.5 when the gamma-ray emission is active. We study the apparent optical brightness distribution of GRBs at 1000 s showing that our observations confirm the distribution derived by other groups. The combination of these results with those obtained by other rapid slewing telescopes allows us to better characterize the early optical emission of GRBs and to emphasize the importance of very early multi-wavelength GRB studies for the understanding of the physics of the ejecta.
Clustering in the luminosity of the afterglows of gamma-ray burst has been reported in the optical and X-ray. We investigate the possibility that a clustering in the luminosity of the afterglows of gamma-ray burst exists in near infrared (J, H, K ban
ds). We use observations of events occurring from 1997 to the end of 2007. We correct the gamma-ray burst afterglow light curve for distance effect and time dilation, and replace all light curves to a common distance of z=1. We used only observations of signal emitted in the near infrared (in the burst frame). We observe a clustering identical to the one observed in optical and similar to the one observed in X-ray. We thus confirm the previous works made in optical. We set a constraint on the total energy of the fireball.
