The Collimation and Energetics of the Brightest Swift Gamma-ray Bursts

Creators: Cenko, S. B.; Frail, D. A.; Harrison, F. A.; Kulkarni, S. R.; Nakar, E.; Chandra, P. C.; Butler, N. R.; Fox, D. B.; Gal-Yam, A.; Kasliwal, M. M.; Kelemen, J.; Moon, D.-S.; Ofek, E. O.; Price, P. A.; Rau, A.; Soderberg, A. M.; Teplitz, H. I.; Werner, M. W.; Bock, D. C.-J.; Bloom, J. S.; Starr, D. A.; Filippenko, A. V.; Chevalier, R. A.; Gehrels, N.; Nousek, J. N.; Piran, T.

Abstract

Long-duration gamma-ray bursts (GRBs) are widely believed to be highly collimated explosions (bipolar conical outflows with half-opening angle θ ≈ 1°-10°). As a result of this beaming factor, the true energy release from a GRB is usually several orders of magnitude smaller than the observed isotropic value. Measuring this opening angle, typically inferred from an achromatic steepening in the afterglow light curve (a "jet" break), has proven exceedingly difficult in the Swift era. Here, we undertake a study of five of the brightest (in terms of the isotropic prompt γ-ray energy release, E_(γ,iso)) GRBs in the Swift era to search for jet breaks and hence constrain the collimation-corrected energy release. We present multi-wavelength (radio through X-ray) observations of GRBs 050820A, 060418, and 080319B, and construct afterglow models to extract the opening angle and beaming-corrected energy release for all three events. Together with results from previous analyses of GRBs 050904 and 070125, we find evidence for an achromatic jet break in all five events, strongly supporting the canonical picture of GRBs as collimated explosions. The most natural explanation for the lack of observed jet breaks from most Swift GRBs is therefore selection effects. However, the opening angles for the events in our sample are larger than would be expected if all GRBs had a canonical energy release of ~10^(51) erg. The total energy release we measure for the "hyper-energetic" (E_(tot) ≳ 10^(52) erg) events in our sample is large enough to start challenging models with a magnetar as the compact central remnant.

Additional Information

© 2010 American Astronomical Society. Issue 2 (2010 March 10); received 2009 May 5; accepted for publication 2010 January 19; published 2010 February 17. S.B.C. and A.V.F. acknowledge generous support from Gary and Cynthia Bengier, the Richard and Rhoda Goldman Fund, NASA/Swift grants NNG06GI86G and NNX09AL08G, and NSF grants AST–0607485 and AST–0908886. A.G. acknowledges support by the Israeli Science Foundation, an EU Seventh Framework Programme Marie Curie IRG fellowship and the Benoziyo Center for Astrophysics, a research grant from the Peter and Patricia Gruber Awards, and the William Z. and Eda Bess Novick New Scientists Fund at the Weizmann Institute. J.N.N. is supported by NASA contract NAS5-00136. T.P. acknowledges support from an ERC advanced research grant. P60 operations are funded in part by NASA through the Swift Guest Investigator Program (grant number NNG06GH61G). Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These data are associated with program GO-10551. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank the SSC Director for an award of discretionary time and the Spitzer Operations team for their quick response to our request. This publication has made use of data obtained from the Swift interface of the High-Energy Astrophysics Archive (HEASARC), provided by NASA's Goddard Space Flight Center. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the Associates of the California Institute of Technology, the states of California, Illinois, and Maryland, and the NSF. Ongoing CARMA development and operations are supported by the NSF under a cooperative agreement, and by the CARMA partner universities. PAIRITEL is operated by the Smithsonian Astrophysical Observatory (SAO) and was made possible by a grant from the Harvard University Milton Fund, a camera loan from the University of Virginia, and continued support of the SAO and UC Berkeley. The PAIRITEL project is further supported by NASA/Swift Guest Investigator grants NNG06GH50G and NNX08AN84G. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: VLA, HST (ACS), Swift (XRT), Keck:I (LRIS), PO:1.5m, Hale (LFC), FLWO:2MASS (PAIRITEL), Spitzer (IRS), CARMA

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