Swift UVOT Detection of GRB 050318
Still, M. ; Roming, P. W. A. ; Mason, K. O. ; ...Giommi, P. ; et al.
Dec - 2005
DOI: 10.1086/497627
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 2
type: Article Journal

Abstract
We present observations of GRB 050318 by the Ultraviolet/Optical Telescope (UVOT) on board the Swift observatory. The data are the first detections of a gamma-ray burst (GRB) afterglow decay by the UVOT instrument, launched specifically to open a new window on these transient sources. We showcase UVOTs ability to provide multicolor photometry and the advantages of combining UVOT data with simultaneous and contemporaneous observations from the high-energy detectors on the Swift spacecraft. Multiple filters covering 1800-6000 \AA reveal a red source with a spectral slope steeper than the simultaneous X-ray continuum. Spectral fits indicate that the UVOT colors are consistent with dust extinction by systems at z = 1.2037 and 1.4436, redshifts where absorption systems have been preidentified. However, the data can be most easily reproduced with models containing a foreground system of neutral gas redshifted by z = 2.8 ± 0.3. For both of the above scenarios, spectral and decay slopes are, for the most part, consistent with fireball expansion into a uniform medium, provided a cooling break occurs between the energy ranges of the UVOT and Swifts X-ray instrumentation.

keywords : P. Giommi: ASI author.,SWIFT I/R/039/04