We present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM-Newton in 2008 January during a survey of M31. The X-ray spectrum is well fitted by either a steep power law plus a blackbody model or a double blackbody model. Prior observations with XMM-Newton, Chandra, Swift, and ROSAT spanning 1991-2007, as well as an additional Swift observation in 2011, all failed to detect this source. No counterpart was detected in deep optical imaging with the Canada-France-Hawaii Telescope down to a 3σ lower limit of g = 26.5 mag. This source has previously been identified as a black hole X-ray binary in M31. While this remains a possibility, the transient behavior, X-ray spectrum, and lack of an optical counterpart are equally consistent with a magnetar interpretation for 2XMMi J003833.3+402133. The derived luminosity and blackbody emitting radius at the distance of M31 argue against an extragalactic location, implying that if it is indeed a magnetar it is located within the Milky Way but 22° out of the plane. The high Galactic latitude could be explained if 2XMMi J003833.3+402133 were an old magnetar, or if its progenitor was a runaway star that traveled away from the plane prior to going supernova.
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