Constraining the rate of GRB visible afterglows with the CFHTLS very wide survey
F. Malacrino, J-L. Atteia, M. Boer, A. Klotz, C. Veillet, J-C. Cuillandre

Abstract: We analyzed images of the CFHTLS Very Wide survey to search visible orphan afterglows from GRBs. To date our search provides the strongest constraints on the rate of GRB visible afterglows. , We have searched 490 square degrees down to magnitude r'=22.5 for visible transients similar to GRB afterglows. We have found only three potential candidates, of which two are most probably variable stars, and one presents similarities with a true GRB afterglow. , 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 number of GRB visible afterglows in the sky. We constrain this rate to be less than 220 afterglows brighter than m=22.5 in all sky at any time. 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.