GRB Energetics in the Swift Era

We examine the rest frame energetics of 76 gamma-ray bursts (GRBs) with known redshift that were detected by the Swift spacecraft and monitored by the satellite’s X-ray Telescope (XRT). Using the bolometric fluence values estimated in Butler et al. (2007) and the last XRT observation for each event, we set a lower limit the their collimation corrected energy Eγ and find that a 68% of our sample are at high enough redshift and/or low enough fluence to accommodate a jet break occurring beyond the last XRT observation and still be consistent with the pre-Swift Eγ distribution for long GRBs. We find that relatively few of the X-ray light curves for the remaining events show evidence for late-time decay slopes that are consistent with that expected from post jet break emission. The breaks in the X-ray light curves that do exist tend to be shallower and occur earlier than the breaks previously observed in optical light curves, yielding a Eγ distribution that is far lower than the pre-Swift distribution. If these early Xray breaks are not due to jet effects, then a small but significant fraction of our sample have lower limits to their collimation corrected energy that place them well above the pre-Swift Eγ distribution. Either scenario would necessitate a much wider post-Swift Eγ distribution for long cosmological GRBs compared to the narrow standard energy deduced from pre-Swift observations. We note that almost all of the pre-Swift Eγ estimates come from jet breaks detected in the optical whereas our sample is limited entirely to X-ray wavelengths, furthering the suggestion that the assumed achromaticity of jet breaks may not extend to high energies. We review several mechanisms that could create dissimilar X-ray and optical light curves but still be within the beamed jet model paradigm. Subject headings: gamma-rays bursts— Astronomy Department, University of California, 601 Campbell Hall, Berkeley, CA 94720