The CFHTLS real time analysis system: “optically selected GRB afterglows”

Abstract

Aims. We describe a deep and wide search for optical GRB afterglows in images taken with MegaCAM at the Canada France Hawaii Telescope, within the framework of the CFHT Legacy Survey. Methods. This search is performed in near real-time thanks to a Real Time Analysis System (RTAS) called "Optically Selected GRB Afterglows", which has been installed on a dedicated computer in Hawaii. This pipeline automatically and quickly analyzes Megacam images to construct catalogs of astronomical objects, and compares catalogs made from images taken at different epochs to produce a list of astrometrically and photometrically variable objects. These objects are then displayed on a web page to be characterized by a member of the collaboration. Results. In this paper, we comprehensively describe the RTAS process from image acquisition to the final characterization of variable sources. We present statistical results based on one full year of operation, showing the quality of the images and the performance of the RTAS. The limiting magnitude of our search is r = 22.5 on average and the observed area amounts to 1178 square degrees. We have detected about 13 × 10 6 astronomical sources of which about 0.01% are found to vary by more than one tenth of a magnitude. We discuss the performance of our instrumental setup with a sample of simulated afterglows. This sample allows us to compare the efficiency of our search with previous works, to propose an optimal observational strategy, and to discuss general considerations on the searches for GRB optical afterglows. We postpone to a forthcoming paper the discussion of the characterization of variable objects we have found, and a more detailled analysis of the nature of those resembling GRB afterglows. Conclusions. The RTAS has been continuously operating since November 2004. Each month 15-30 square degrees are observed many times over a period of 2-3 nights. The real-time analysis of the data has revealed no convincing afterglow candidate so far.

‘Orphan’ afterglows in the Universal structured jet model for γ-ray bursts

“…They searched 490 deg 2 down to R = 22.5. They observed 25–30 deg 2 every month over a period of two to three nights (Malacrino et al 2006). Since T th ≃ 17 d, we can consider their survey a snapshot observation and we predict N oa = 0.1.…”

“…Since T th ≃ 17 days, we can consider their survey a snapshot observation and we predict Noa = 0.1. Recently, Malacrino et al (2007b) have rejected the only candidate they had identified in their first work.…”

“…Totani & Panaitescu 2002;Nakar, Piran & Granot 2002), since the emission is likely to be still relativistically beamed when OAs are detected in those bands. Malacrino et al (2007Malacrino et al ( , 2007b put the tightest optical constraints so far (see their fig. 3).…”

Estimation of the detectability of optical orphan afterglows

“…Other than constraining the beaming factor (Rhoads 1997;Dalal, Griest & Pruet 2002;Totani & Panaitescu 2002), some authors have focused on investigations of the detectability of orphan optical afterglows both theoretically (Nakar, Piran & Granot 2002;Totani & Panaitescu 2002) and experimentally (Hudec 2004;Becker et al 2004;Rykoff et al 2005;Malacrino & Atteia 2006;Rau, Greiner & Schwarz 2006). Becker et al (2004) give the results of a 5-year (1999)(2000)(2001)(2002)(2003)(2004)) survey of optical transients, but none was identified as an orphan.…”

Constraining the rate of GRB visible afterglows with the CFHTLS very wide survey

“…All the objects of figure 1 have been carefully examined in order to determine their nature. Most of them are variable stars which are identified as such because they are present in archived astronomical images 1 , or in the USNO-B1.0 catalog (Monet et al 2003), or in images of the same field acquired during other observational periods of the CFHTLS-VWS. This last method is particularly useful for variable objects fainter than the limit of the USNO-B1.0 catalog (about 21 st mag).…”