Cenko, S. Bradley and Fox, Derek B. and Penprase, Brian E. and Cucchiara, Antonio and Price, Paul A. and Berger, Edo and Kulkarni, Shri R. and Harrison, Fiona A. and Gal-Yam, Avishay and Ofek, Eran O. and Rau, Arne and Chandra, Poonam and Frail, Dale A. and Kasliwal, Mansi M. and Schmidt, Brian P. and Soderberg, Alicia M. and Cameron, P. Brian and Roth, Kathy C. (2008) GRB 070125: The first long-duration gamma-ray burst in a halo environment. Astrophysical Journal, 677 (1). pp. 441-447. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20090408-112338820
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We present the discovery and high signal-to-noise ratio spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB 070125. Unlike all previously observed long-duration afterglows in the redshift range 0.5 ≲ z ≲ 2.0, we find no strong (rest-frame equivalent width Wr ≳ 1.0 Å) absorption features in the wavelength range 4000-10000 Å. The sole significant feature is a weak doublet that we identify as Mg II λλ2796 (Wr = 0.18 ± 0.02 Å), 2803 (Wr = 0.08 ± 0.01 Å) at z = 1.5477 ± 0.0001. The low observed Mg II and inferred H I column densities are typically observed in galactic halos, far away from the bulk of massive star formation. Deep ground-based imaging reveals no host directly underneath the afterglow to a limit of R > 25.4 mag. Either of the two nearest blue galaxies could host GRB 070125; the large offset (d ≥ 27 kpc) would naturally explain the low column densities. To remain consistent with the large local (i.e., parsec scale) circumburst density inferred from broadband afterglow observations, we speculate that GRB 070125 may have occurred far away from the disk of its host in a compact star-forming cluster. Such distant stellar clusters, typically formed by dynamical galaxy interactions, have been observed in the nearby universe and should be more prevalent at z > 1, where galaxy mergers occur more frequently.
|Additional Information:||© 2008 The American Astronomical Society. Received 2007 May 24; accepted 2007 November 13. We wish to thank R. Ellis for obtaining the Keck ToO imaging data. Some of the data were obtained with the Gemini Observatory under Program IDs GN-2006B-Q-21 and GN-2007A-Q-3. 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 National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. S. B. C. and A. M. S. are supported by the NASA Graduate Student Research Program. E.B. is supported by NASA through Hubble Fellowship grant HST-HF-01171.01 awarded by STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. A. G. acknowledges support by NASA through Hubble Fellowship grant HSTHF- 01158.01 awarded by STScI. P. C. is supported by a Jansky fellowship. M. M.K. is supported by the Gordon and Betty Moore Foundation. GRB research at Caltech is supported by NASA and The NSF. The authors wish to 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.|
|Group:||Space Radiation Laboratory|
|Subject Keywords:||gamma rays: bursts|
|Official Citation:||GRB 070125: The First Long-Duration Gamma-Ray Burst in a Halo Environment S. Bradley Cenko, Derek B. Fox, Brian E. Penprase, Antonio Cucchiara, Paul A. Price, Edo Berger, Shri R. Kulkarni, Fiona A. Harrison, Avishay Gal-Yam, Eran O. Ofek, Arne Rau, Poonam Chandra, Dale A. Frail, Mansi M. Kasliwal, Brian P. Schmidt, Alicia M. Soderberg, P. Brian Cameron, and Kathy C. Roth 2008 ApJ 677 441-447 doi: 10.1086/526491|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||26 Aug 2009 22:10|
|Last Modified:||04 Mar 2013 18:55|
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