Gamma-ray burst energetics in the Swift ERA

Daniel Kocevski, Nathaniel Butler

Research output: Contribution to journalArticlepeer-review

61 Scopus citations


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 by Butler and coworkers and the last XRT observation for each event, we set a lower limit to their collimation-corrected energy Eγ and find that 68% of our sample is 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 preSwift 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 X-ray breaks are not due to jet effects, then a small but significant fraction of our sample have lower limits to their collimationcorrected 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.

Original languageEnglish (US)
Pages (from-to)531-538
Number of pages8
JournalAstrophysical Journal
Issue number1
StatePublished - Jun 10 2008
Externally publishedYes


  • Gamma rays: bursts

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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