title: The puzzling afterglow of GRB 050721: a rebrightening seen in the optical but not in the X-ray.
abstract: We present here the analysis of the early and late multiwavelength afterglow emission, as observed by Swift a small robotic telescope and very large telescope (VLT). We compare early observations with late afterglow observations obtained with Swift and the VLT and we observe an intense rebrightening in the optical band at about 1 day after the burst, which is not present 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 the matter density located at approximately 1017 cm from the burst is approximately a factor of 10 higher than in the inner region. This is the first time in which a rebrightening has been observed in the optical afterglow of a gamma-ray burst that is clearly absent in the X-ray afterglow.
authors.affiliation: INAF-Astronomical Observatory of Rome, via Frascati, 33, 00040 Monteporzio, Rome, Italy. antonelli@mporzio.astro.it
authors.forename: L Angelo
authors.initials: LA
authors.lastname: Antonelli
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Testa
authors.forename: Patrizia
authors.initials: P
authors.lastname: Romano
authors.forename: Dafne
authors.initials: D
authors.lastname: Guetta
authors.forename: Ken'ichi
authors.initials: K
authors.lastname: Torii
authors.forename: Valerio
authors.initials: V
authors.lastname: D'Elia
authors.forename: Daniele
authors.initials: D
authors.lastname: Malesani