The First Swift X-Ray Flash: The Faint Afterglow of XRF 050215B

We present the discovery of XRF 050215B and its afterglow. The burst was detected by the Swift BAT during the check-out phase, and observations with the X-Ray Telescope began approximately 30 minutes after the burst. These observations found a faint, slowly fading X-ray afterglow near the center of the error box as reported by the BAT. Infrared data obtained at UKIRT after 10 hr also revealed a very faint K-band afterglow. The afterglow appears unusual since it is very faint, especially in the infrared, with K > 20 only 9 hr postburst. The X-ray and infrared light curves exhibit a slow, monotonic decay with α ~ 0.8 and no evidence for a steepening associated with the jet break to 10 days postburst. We discuss possible explanations for the faintness and slow decay in the context of present models for the production of X-ray flashes.