\swift \xrt observations of the \hi line source \VV were performed on April 22 and April 26, 2008 for a total exposure time of 9.2ks. This is the first pointed X-ray observation of \VV, a putative dark galaxy in the Virgo cluster, and no photons were detected from this source. The non-detection of extended X-ray emission within the angular extent of the \hi source corresponds to a 99% confidence upper limit of 2.1×10−14\ergcms in the 0.3--2.0 keV band. The equivalent upper limit to the amount of diffuse hot gas associated with \VV is in the range 4×107−2×108 \msun for a hot gas temperature between 0.1 and 1 keV. The non-detection also corresponds to a 99%-confidence upper limit on the flux from a point-like source of 8×10−15\ergcms in the 0.3--2.0 keV band. We discuss the constraints on the nature of \VV imposed by these observations and the theoretical implications of these results.
@article{arxiv.0805.1665,
title = {Swift XRT Observations of the Possible Dark Galaxy VIRGOHI 21},
author = {Massimiliano Bonamente and Douglas Swartz and Martin Weisskopf and Steven Murray},
journal= {arXiv preprint arXiv:0805.1665},
year = {2009}
}