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[superscript +0.15, subscript −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[superscript +0.19, subscript −0.18] and shows no evidence for absorption above the Galactic column of 7 × 10[superscript 20] cm[superscript −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.