2008ApJ...686L..71B

Swift XRT observations of the H I line source VIRGOHI 21 were performed on 2008 April 22 and 26 for a total exposure time of 9.2 ks. This is the first pointed X-ray observation of VIRGOHI 21, a putative dark galaxy in the Virgo Cluster, and no photons were detected from this source. The nondetection of extended X-ray emission within the angular extent of the H I source corresponds to a 99% confidence upper limit of 2.1x10^–14 ergs/cm2/s in the 0.3-2.0 keV band. The equivalent upper limit to the amount of diffuse hot gas associated with VIRGOHI 21 is in the range 4x10⁷-2 x 10⁸ M_☉ for a hot gas temperature between 0.1 and 1 keV. The nondetection also corresponds to a 99% confidence upper limit on the flux from a pointlike source of 8x10^–15 ergs/cm2/s in the 0.3-2.0 keV band. We discuss the constraints on the nature of VIRGOHI 21 imposed by these observations and the theoretical implications of these results.