CFHT Real Time Analysis System
Optically Selected GRB's Afterglows

The main goal of this Real Time Analysis System (RTAS) is to search optically selected GRB afterglows in near real time in the very-wide survey of the CFHTLS.

Orphan Afterglow Candidate Discovered!!! Click here

Gamma-Ray Bursts (hereafter GRBs) are bright flashes of gamma-rays due to powerful stellar explosions occurring in distant galaxies (from z~0.1 to z ~5). They are usually followed by an afterglow visible during a few days at X-ray, optical and/or radio wavelengths (see figures below). Observation and study of GRB afterglows has become a major field of research in the recent years. These observations provide insight into critical issues like the distance and the luminosity distribution of the sources, the physics of the ejecta, the nature of GRB hosts, the density and the composition of the gas surrounding the sources.

Fig. 1: Afterglow of GRB 021004 (http://www.astro.columbia.edu/~jules/021004.ps)	Fig. 2: Afterglow of GRB 021211 (Della Valle et al. 2003, A&A, 406, L37)

All the afterglows observed to date have been found from the localization of the gamma-ray burst at high energies. We have no example yet of an optically selected GRB. This situation implies that we are seeing a biased sample of the GRB population. The detection of optically selected afterglows would open a new window on GRBs, and represent a unique opportunity to address crucial issues like the GRB energy content, the local GRB rate or the GRB-supernova connection.

The rationale to perform this search in near real time is to allow fast follow-up observations while the afterglow is still bright. Once a candidate afterglow is detected in the CFHTLS, follow-up observations are required to determine the redshift of the host, the total energy content of the burst and the shape of the light curve (providing clues on the existence of an underlying supernova). Since the afterglow luminosity declines as a power law of the time, it is important to perform follow-up observations as soon as possible after the detection, justifying a real time analysis system. Even if afterglow identification takes several hours at the beginning of the survey, having the RTAS implies that nothing will prevent us to go to automatic alert generation.

The CFHTLS provides one of the first credible opportunities to detect GRB afterglows independently of the gamma-ray signal. Calculations based on our current knowledge show that a few detections per year can reasonably be expected in the very-wide survey. Lower rates are expected in the deep survey, which could also be searched for afterglows. GRB afterglows will be identified by comparing two images of the same field taken few days apart. They will appear as sources visible in one image only or with a large difference in magnitude between the two images. The processing is taking place within the few hours following the acquisition of the second image.

In the first months, afterglow candidates will be checked by a member of our collaboration before an alert is generated. The alerts will be distributed via the Gamma-ray burst Coordinate Network (GCN, see http://gcn.gsfc.nasa.gov). A template alert message will be prepared in collaboration with the CFHTLS Steering Group. Our collaboration will ensure that afterglows discovered in the CFHTLS will undergo detailed follow-up observations.

The GRB-RTAS collaboration includes:

J-L. Atteia, T. Contini, J-P. Kneib, F. Malacrino, G. Soucail Laboratoire d'Astrophysique de Toulouse-Tarbes, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France
J.J. Kavelaars Herzberg Institute for Astrophysics, CADC, 5071, Chemin West Saanich, Victoria (C.-B.) V9E 2E7, Canada
M. Boer Observatoire de Haute-Provence, 04870 Saint-Michel l'Observatoire, France
A. Klotz, J-F. Olive C.E.S.R., Observatoire Midi-Pyrénées, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
F. Mirabel Service d'Astrophysique, CEA, L'Orme des Merisiers, Bâtiment 709, 91191 Gif-sur-Yvette cedex, France
F. Daigne, R. Mochkovitch Institut d'Astrophysique de Paris, 98, bis boulevard Arago, 75014 Paris, France