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Published April 20, 2013 | Published
Journal Article Open

Illuminating the Darkest Gamma-Ray Bursts with Radio Observations


We present X-ray, optical, near-infrared (IR), and radio observations of gamma-ray bursts (GRBs) 110709B and 111215A, as well as optical and near-IR observations of their host galaxies. The combination of X-ray detections and deep optical/near-IR limits establish both bursts as "dark." Sub-arcsecond positions enabled by radio detections lead to robust host galaxy associations, with optical detections that indicate z ≾ 4 (110709B) and z ≈ 1.8-2.9 (111215A). We therefore conclude that both bursts are dark due to substantial rest-frame extinction. Using the radio and X-ray data for each burst we find that GRB 110709B requires A_V^(host) ≳ 5.3 mag and GRB 111215A requires A_V^(host) ≳ 8.5 mag (assuming z = 2). These are among the largest extinction values inferred for dark bursts to date. The two bursts also exhibit large neutral hydrogen column densities of N H, int ≳ 10^(22) cm^(–2) (z = 2) as inferred from their X-ray spectra, in agreement with the trend for dark GRBs. Moreover, the inferred values are in agreement with the Galactic A_V -N_H relation, unlike the bulk of the GRB population. Finally, we find that for both bursts the afterglow emission is best explained by a collimated outflow with a total beaming-corrected energy of E_γ + E_K ≈ (7-9) × 10^(51) erg (z = 2) expanding into a wind medium with a high density, Ṁ ≈ (6-20) x 10^(-5) M_☉ yr^(–1) (n ≈ 100-350 cm^(–3) at ≈ 10^(17) cm). While the energy release is typical of long GRBs, the inferred density may be indicative of larger mass-loss rates for GRB progenitors in dusty (and hence metal rich) environments. This study establishes the critical role of radio observations in demonstrating the origin and properties of dark GRBs. Observations with the JVLA and ALMA will provide a sample with sub-arcsecond positions and robust host associations that will help to shed light on obscured star formation and the role of metallicity in GRB progenitors.

Additional Information

© 2013 American Astronomical Society. Received 2012 September 19; accepted 2013 March 10; published 2013 April 8. We thank R. Chary for helpful discussions regarding obscured star formation in distant galaxies and Y. Cao for his assistance in attaining the optical spectrum of GRB111215A while observing at Keck. The Berger GRB group at Harvard is supported by the National Science Foundation under grant AST-1107973, and by NASA/Swift AO7 grant NNX12AD69G. B.A.Z., E.B., R.M., W.F., and A.S. acknowledge partial support of this research while in residence at the Kavli Institute for Theoretical Physics under National Science Foundation Grant PHY11-25915. E.N. acknowledges partial support by an ERC starting grant. F.O.E. acknowledges funding of his PhD through the Deutscher Akademischer Austausch-Dienst (DAAD). A.J.C.T. acknowledges support from the MINECO Spanish Ministry projects AYA 2009-14000-C01 and AYA 2012-39737-C03-01. We thank the referee for useful comments and suggestions. Funding for GROND was generously granted from the Leibniz Prize to G. Hasinger (DFG grant HA 1850/28-1). D.P. is supported by grant HST-HF-51296.01-A, provided by NASA through a Hubble Fellowship grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support for this work was provided by the National Aeronautics and Space Administration (NASA) through Chandra Award Number 09900712 issued by the Chandra X-ray Observatory Center, which is operated by the SAO for and on behalf of NASA under contract NAS8-03060. The JVLA is operated by the National Radio Astronomy Observatory, a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. JVLA observations were undertaken as part of project numbers 10C-145 and 11B-242. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the James S. McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, 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. CARMA observations were undertaken as part of projects c0773 and cx334. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. SMA observations were undertaken as part of project 2011B-S003. The IRAM Plateau de Bure Interferometer is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Some observations were obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovaci όn Productiva (Argentina). Gemini observations were undertaken as part of programs GS-2011A-Q-25 and GN-2011BQ- 10. Some observations were made with the NASA/ESA Hubble Space Telescope, obtained 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. HST observations were undertaken as part of program 12378. This work utilized observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofsica de Canarias, in the island of La Palma. This research has made use of Swift data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center. This research has made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. Facilities: Swift (XRT), CXO (ACIS-S), Gemini:South (GMOS), Keck:I (LRIS), HST (ACS, WFC3), GTC, OSN, CARMA, SMA, VLA, IRAM:Interferometer, ESO 2.2m/GROND

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