Searching gamma‐ray bursts for gravitational lensing echoes

We have searched for gravitational‐lens induced echoes between gamma‐ray bursts (GRBs) in BATSE data. The search was conducted in two phases. In the first phase we compared all GRBs in a brightness complete sample of the first 260 GRBs with recorded angular positions having at least 5% chance of being coincident from their combined positional error. In the second phase, we compared all GRB light curves of the first 611 GRBs with recorded angular positions having at least 55% chance of being coincident from their combined positional error. No unambiguous gravitational lens candidate pairs were found in either phase, although a ‘‘library of close calls’’ was accumulated for future reference. In comparison with the QSO gravitational lens detection rate induced by the known galaxy‐lens field, a limit on the distance to the brightest GRBs was derived. It is found that in a smoothly distributed Ω=1 universe where the density of galaxy‐size mass clumps is approximately constant, BATSE‐detected GRBs existed at an...