The multiwavelength afterglow of GRB 050721: a puzzling rebrightening seen in the optical but not in the X-ray

¹ INAF - Astronomical Observatory of Rome, via Frascati, 33, 00040 Monte Porzio Catone, Rome, Italy e-mail: a.antonelli@oa-roma.inaf.it
² ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Rome, Italy
³ INAF - Astronomical Observatory of Brera, via E. Bianchi 46, Merate (LC) 23807, Italy
⁴ Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
⁵ International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
⁶ Università degli Studi di Milano “Bicocca”, Piazza delle Scienze 3, 20126 Milano, Italy
⁷ INAF - Astrophysical Observatory of Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁸ Kavli Institute for Theoretical Physics University of California Santa Barbara, CA 93106, USA
⁹ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Department of Astronomy & Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park, PA 16802, USA
¹¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
¹² JILA, University of Colorado, Boulder, CO 80309, USA
¹³ Department of Physics and Astronomy, University of Leicester, Leicester LE 1 7RH, UK

Received: 27 March 2006
Accepted: 12 June 2006

Abstract

Context.GRB 050721 was detected by Swift and promptly followed-up, in the X-ray by Swift itself and, in the optical band, by the VLT operated, for the first time, in rapid response mode starting observations about 25 m after the burst. A multiwavelength monitoring campaign was performed in order to study its afterglow's behavior.

Aims.We present the analysis of the early and late afterglow emission in both the X-ray and optical bands, as observed by , a robotic telescope, and the VLT. We compare early observations with late afterglow observations obtained with Swift  and the VLT in different bands in order to constrain the density of the medium in which the fireball is expanding.

Methods.We have analyzed both the X-ray and the optical light curves and compared the spectral energy distribution of the afterglow at two different epochs.

Results.We observed an intense rebrightening in the optical band at about one day after the burst which was not seen in the X-ray band. This is the first observation of a GRB afterglow in which a rebrightening is observed in the optical but not in the X-ray band. The lack of detection in X-ray of such a strong rebrightening at lower energies may be described with a variable external density profile. In such a scenario, the combined X-ray and optical observations allow us to derive that matter located at  cm from the burst is about a factor of 10 higher than in the inner region.