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

We have analyzed 2231 images of the CFHTLS Very Wide survey to search visible orphan afterglows from GRBs. Our search covers 490 square degrees down to magnitude r[ = 22.5, and to date it provides the strongest constraints on the rate of GRB visible afterglows. We have found zero GRB optical afterglow and one remarkable event, which could be a micro-lensing event. We measure the detection efficiency of our search with a MC simulation reproducing the characteristics of our observational strategy and the properties of on-axis GRB afterglows. This simulation allows us to translate our observations into constraints on the rate of GRB visible afterglows, which is found to be less than 220 afterglows brighter than m = 22.5 in all sky at any time (or 10000 optical afterglows/year/all-sky). Our observations are marginally consistent with the most optimistic models, which predict orphan afterglows to be about 10 times more frequent than GRBs at m = 22.5.