GRB 050223: A faint gamma-ray burst discovered by Swift

GRB 050223 was discovered by the Swift Gamma-Ray Burst Explorer on 2005 February 23 and was the first gamma-ray burst (GRB) to be observed by both Swift and XMM-Newton. At the time of writing (2005 May), it has one of the faintest GRB afterglows ever observed. The spacecraft could not slew immediately to the burst, so the first X-ray and optical observations occurred approximately 45 min after the trigger. Although no optical emission was found by any instrument, both Swift and XMM-Newton detected the fading X-ray afterglow. Combined data from both of these observatories show the afterglow to be fading monotonically as 0.99^+0.15_-0.12 over a time-frame between 45 min and 27 h post-burst. Spectral analysis, allowed largely by the higher throughput of XMM-Newton, implies a power law with a slope of Γ = 1.75^+0.19_-0.18and shows no evidence for absorption above the Galactic column of 7 × 10²⁰ cm^-2. From the X-ray decay and spectral slopes, a low electron power-law index of p = 1.3-1.9 is derived; the slopes also imply that a jet-break has not occurred up to 27 h after the burst. The faintness of GRB 050223 may be due to a large jet opening or viewing angle or a high redshift.