Page, K. L.K. L.PageRol, E.E.RolLevan, A. J.A. J.LevanGiommi, PaoloPaoloGiommi2020-09-172020-09-172005-10-01https://hdl.handle.net/20.500.13025/376Monthly Notices of the Royal Astronomical Society Letters, Volume 363, Issue 1, pp. L76-L80.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<SUP>+0.15</SUP><SUB>-0.12</SUB> 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 Gamma= 1.75<SUP>+0.19</SUP><SUB>-0.18</SUB> and shows no evidence for absorption above the Galactic column of 7 10<SUP>20</SUP> cm<SUP>-2</SUP>. 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.gamma-rays bursts10.1111/j.1745-3933.2005.00086.xhttp://adsabs.harvard.edu/abs/2005MNRAS.363L..76P2005MNRAS.363L..76P