Letter to the Editor
Constraining the rate of GRB visible afterglows with the CFHTLS very wide survey*
1
Laboratoire d'Astrophysique de Toulouse-Tarbes, Observatoire Midi-Pyrénées (CNRS-UMR5572/Université Paul Sabatier Toulouse III), 14 avenue Édouard Belin, 31400 Toulouse, France e-mail: fmalacri@ast.obs-mip.fr
2
Observatoire de Haute-Provence, 04870 Saint-Michel l'Observatoire, France
3
Centre d'Étude Spatiale des Rayonnements, Observatoire Midi-Pyrénées (CNRS/UPS), BP 4346, 31028 Toulouse Cedex 04, France
4
Canada-France-Hawaii Telescope Corp., Kamuela, HI 96743, USA
Received:
11
December
2006
Accepted:
19
January
2007
Aims.We analyze images of the CFHTLS Very Wide Survey to search for visible orphan afterglows from gamma-ray bursts (GRBs).
Methods.We have searched 490 square degrees down to magnitude 
for
visible transients similar to GRB afterglows.
We translate our observations into
constraints on the number of GRB visible afterglows in the sky,
by measuring the detection efficiency of our search with a simulation
reproducing the characteristics of our observational
strategy and the properties of on-axis GRB afterglows.
Results.We have found only three potential candidates, of which two are most probably variable
stars, and one presents similarities to an orphan afterglow.
We constrain the number of visible afterglows to be less than 220 down to
in the whole sky at any time. Our observations are marginally
consistent with the most optimistic model, which predicts orphan
afterglows to be about 10 times more frequent than GRBs.
Conclusions.This search has led to the detection of one possible GRB afterglow, and provides the strongest constraints on the rate of GRB visible afterglows as well as an estimation of the observing time required to detect a significant number of GRB afterglows.
Key words: gamma rays: bursts / methods: data analysis
Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.
© ESO, 2007