The Swift/Fermi GRB 080928 from 1 eV to 150 keV^⋆

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: rossi@tls-tautenburg.de
² Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ CRESST, Universities Space Research Association and NASA GSFC, Greenbelt, MD 20771, USA
⁵ Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA
⁶ ISR-1, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
⁷ Physics Department, University of Michigan, Ann Arbor, MI 48109, USA
⁸ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
¹⁰ Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
¹¹ ARIES, Manora Peak, Nainital, 263129 Uttaranchal, India
¹² INAF/IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
¹³ University of Rochester, Department of Physics and Astronomy, Rochester, NY 14627-0171, USA
¹⁴ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
¹⁵ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK
¹⁶ School of Physics, University College Dublin, Dublin 4, Republic of Ireland
¹⁷ University of Alabama in Huntsville, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
¹⁸ Physics Department, University of California at Santa Barbara, 2233B Broida Hall, Santa Barbara, CA 93106, USA
¹⁹ Institute of Physics, Eötvös University, Pázmány P. s. 1/A, 1117 Budapest, Hungary
²⁰ CRESST and the Observational Cosmology Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA
²¹ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 1 July 2010
Accepted: 5 February 2011

Abstract

We present the results of a comprehensive study of the gamma-ray burst 080928 and of its afterglow. GRB 080928 was a long burst detected by Swift/BAT and Fermi/GBM. It is one of the exceptional cases where optical emission had already been detected when the GRB itself was still radiating in the gamma-ray band. For nearly 100 s simultaneous optical, X-ray and gamma-ray data provide a coverage of the spectral energy distribution of the transient source from about 1 eV to 150 keV. In particular, we show that the SED during the main prompt emission phase agrees with synchrotron radiation. We constructed the optical/near-infrared light curve and the spectral energy distribution based on Swift/UVOT, ROTSE-IIIa (Australia), and GROND (La Silla) data and compared it to the X-ray light curve retrieved from the Swift/XRT repository. We show that its bumpy shape can be modeled by multiple energy-injections into the forward shock. Furthermore, weinvestigate whether the temporal and spectral evolution of the tail emission of the first strong flare seen in the early X-ray light curve can be explained by large-angle emission (LAE). We find that a nonstandard LAE model is required to explain the observations. Finally, we report on the results of our search for the GRB host galaxy, for which only a deep upper limit can be provided.