The luminosity of GRB afterglows as distance estimator

We investigate the clustering of afterglow light curves observed at X-ray and optical wavelengths. We have constructed a sample of 61 bursts with known distance and X-ray afterglow. GRB sources can be divided in three classes, namely optical and X-ray bright afterglows, optical and X-ray dim one s, and optically bright -X-ray dim ones. We argue that this clustering is related to the fireball total energy, the external medium density, the fraction of fireball energy going in relativistic electrons and magnetic fields. We propose a method for the estimation of the GRB source redshift based on the observe d X-ray flux one day after the burst and optical properties. We tested this method on three recently detected SWIFT GRBs with known redshift, and found it i n good agreement with the reported distance from optical spectroscopy.Comment: 6 pages, proceeding of the PCHE session at the Journees de la SF2

The puzzling temporally variable optical and X-ray afterglow of GRB 101024A

Aim: To present the optical observations of the afterglow of GRB 101024A and to try to reconcile these observations with the X-ray afterglow data of GRB 101024A using current afterglow models Method: We employ early optical observations using the Zadko Telescope combined with X-ray data and compare with the reverse shock/forward shock model. Results: The early optical light curve reveals a very unusual steep decay index of alpha~5. This is followed by a flattening and possibly a plateau phase coincident with a similar feature in the X-ray. We discuss these observations in the framework of the standard reverse shock/forward shock model and energy injection.We note that the plateau phase might also be the signature of the formation of a new magnetar.Comment: 5 pages, 2 figures. Accepted for publication in Astronomy and Astrophysic

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

Optical afterglow luminosities in the Swift epoch: confirming clustering and bimodality

We show that Gamma Ray Bursts (GRBs) of known redshift and rest frame optical extinction detected by the Swift satellite fully confirm earlier results concerning the distribution of the optical afterglow luminosity at 12 hours after trigger (rest frame time). This distribution is bimodal and relatively narrow, especially for the high luminosity branch. This is intriguing, given that Swift GRBs have, on average, a redshift larger than pre-Swift ones, and is unexpected in the common scenario explaining the GRB afterglow. We investigate if the observed distribution can be the result of selection effects affecting a unimodal parent luminosity distribution, and find that either the distribution is intrinsically bimodal, or most (60 per cent) of the bursts are absorbed by a substantial amount of grey dust. In both cases we suggest that most dark bursts should belong to the underluminous optical family.Comment: 5 pages 3 figures, minor revision, added reference, accepted for publication in MNRAS Letter

The ultra-long GRB 111209A - II. Prompt to afterglow and afterglow properties

The "ultra-long" Gamma Ray Burst GRB 111209A at redshift z=0.677, is so far the longest GRB ever observed, with rest frame prompt emission duration of ~4 hours. In order to explain the bursts exceptional longevity, a low metallicity blue supergiant progenitor has been invoked. In this work, we further investigate this peculiar burst by performing a multi-band temporal and spectral analysis of both the prompt and the afterglow emission. We use proprietary and publicly available data from Swift, Konus Wind, XMM-Newton, TAROT as well as from other ground based optical and radio telescopes. We find some peculiar properties that are possibly connected to the exceptional nature of this burst, namely: i) an unprecedented large optical delay of 410+/-50 s is measured between the peak epochs of a marked flare observed also in gamma-rays after about 2 ks from the first Swift/BAT trigger; ii) if the optical and X-ray/gamma-ray photons during the prompt emission share a common origin, as suggested by their similar temporal behavior, a certain amount of dust in the circumburst environment should be introduced, with rest frame visual dust extinction of AV=0.3-1.5 mag; iii) at the end of the X-ray "steep decay phase" and before the start of the X-ray afterglow, we detect the presence of a hard spectral extra power law component never revealed so far. On the contrary, the optical afterglow since the end of the prompt emission shows more common properties, with a flux power law decay with index alpha=1.6+/-0.1 and a late re-brightening feature at 1.1 day. We discuss our findings in the context of several possible interpretations given so far to the complex multi-band GRB phenomenology. We also attempt to exploit our results to further constrain the progenitor nature properties of this exceptionally long GRB, suggesting a binary channel formation for the proposed blue supergiant progenitor.Comment: ApJ accepted. Revised version with substantial adjustments, the main results remain unchange

X-ray afterglow light curves : toward standard candle ?

We investigate the clustering of afterglow light curves observed at X-ray and optical wavelengths. We have constructed a sample of 61 bursts with known dis tance and X-ray afterglow. This sample includes bursts observed by BeppoSAX, XMM-Newton, Chandra, and SWIFT. We correct the light curves for cosmological ef fects and compare the observed X-ray fluxes one day after the burst. We check for correlations between the observed flux and the burst spectral and temporal properties. We confirm the previous result of Boer and Gendre (2000) that X-ray afterglow light curves cluster in luminosity, even when we consider the l ast SWIFT data. We observe this clustering only for the afterglow light curves; the inclusion of prompt-related data broaden the distribution. A similar clu stering is observed for the optical light curves; GRB sources can be divided in three classes, namely optical and X-ray bright afterglows, optical and X-ray dim ones, and optically bright -X-ray dim ones. We argue that this clustering is related to the fireball total energy, the external medium density, the fraction of fireball energy going in relativistic electrons and magnetic fields. These parameters can be either fixed to a standard va lue, or correlated. We finally propose a method for the estimation of the GRB source redshift based on the observed X-ray flux one day after the burst and optical properties. Using this method, we compute a redshift of 1.4 +/- 0.2 for GRB 980519 and of 1.9 \+/- 0.3 for GRB 040827. We tested this method on three recently detected SWIFT GRBs with known redshift, and found it in good agreement with the reported distance from optical spectroscopy .Comment: 17 pages, 10 color figures, 1 b&w figure. Accepted for publication by Ap

Early optical observations of GRBs by the TAROT telescopes: period 2001-2008

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 between 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.Comment: 13 pages, 2 color figures, 5 b&w figures. Accepted for publication in Astronomical Journa