GRB 050117: Simultaneous Gamma‐Ray and X‐Ray Observations with theSwiftSatellite

The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-rayfollow-up to a newly detected GRB on 2005 January 17, within 193 seconds of theburst trigger by the Swift Burst Alert Telescope. While the burst was still inprogress, the X-ray Telescope obtained a position and an image for anun-catalogued X-ray source; simultaneous with the gamma-ray observation. TheXRT observed flux during the prompt emission was 1.1 x 10^{-8} ergs cm^{-2}s^{-1} in the 0.5-10 keV energy band. The emission in the X-ray band decreasedby three orders of magnitude within 700 seconds, following the prompt emission.This is found to be consistent with the gamma-ray decay when extrapolated intothe XRT energy band. During the following 6.3 hours, the XRT observed theafterglow in an automated sequence for an additional 947 seconds, until theburst became fully obscured by the Earth limb. A faint, extremely slowlydecaying afterglow, alpha=-0.21$, was detected. Finally, a break in thelightcurve occurred and the flux decayed with alpha<-1.2$. The X-ray positiontriggered many follow-up observations: no optical afterglow could be confirmed,although a candidate was identified 3 arcsecs from the XRT position.Comment: 27 pages, 6 figures. Accepted for publication in Ap