[Other -- Oral ]

Chandra Searches for Late-Time Jet Breaks in GRB X-ray Afterglows

David Burrows, Penn State University
G. Garmire (PSU), G. Ricker (MIT), M. Bautz (MIT), J. Nousek (PSU), D. Grupe (PSU), J. Racusin (PSU)

The Swift X-ray Telescope has now studied hundreds of X-ray afterglows of Gamma-Ray Bursts. One surprising finding is the apparent absence of jet breaks in the XRT light curves, suggesting that the Swift GRBs may have larger jet opening angles than those studied before the launch of Swift.We have undertaken a program of late-time observations of GRB afterglows by the Chandra X-ray Observatory to searchfor possible jet breaks at very late times, after the afterglow becomes too faint for Swift to monitor. These Chandra observations push the flux limits down by an order of magnitude, from ~2E-14 cgs to ~2E-15 cgs. I will report on the results of this on-going program, which has found very late jet breaks in some cases, and failed to find evidence of jet breaks to very late times in other afterglows. The results are consistent with the recent findings of Racusin et al., that suggest that some jet breaks occur at very late times but that the opening angles are typically still of order 5-10 degrees.