A Serendipitous Search for GRB Afterglows by Subaru/Suprime-Cam: A Test of GRB Beaming.

Gamma-ray bursts (GRBs) are the most mysterious object in astronomy, and one of the key issues is whether they are isotropic explosions or strongly collimated jet-like ones. A direct consequence of the collimation (or beaming) is that we should be able to find many afterglows without observed prompt gamma-ray emission, since the collimation gets wider and wider at later stages. We describe a proposed serendipitous search for optical GRB afterglows as a test of GRB beaming, by the Suprime-Cam on the 8.2m Subaru telescope, which is a very unique facility for this test by its wide field of view and high sensitivity for faint objects. We will show that the detection probability of such afterglows is of order unity by this observation, if GRBs are beamed by a factor of 4n/ΔΩ > 100, which is a typical beaming factor currently discussed. Therefore, positive detection is possible and it would have a strong impact on our understanding of GRBs. Even if undetected, it would set the strongest constraint on the GRB beaming ever obtained. We also show that discrimination of GRB afterglows from other transient objects is possible. The data taken by this proposal can also be used in other scientific purposes of the high-z supernova search. [ABSTRACT FROM AUTHOR]