2013ApJ...766...29L

We have analyzed three XMM-Newton observations of the central part of the unidentified TeV γ-ray source HESS J1804-216. We focus on two X-ray sources, 2XMMi J180442.0-214221 (Src 1) and 2XMMi J180432.5-214009 (Src 2), which were suggested to be the possible X-ray counterparts to the TeV source. We discover a 2.93 hr X-ray periodicity from Src 1, with the pulse profile explained with a self-eclipsing pole in an eclipsing polar. Src 2 exhibits a strong Fe emission line (FWHM ∼ 0.3 keV and equivalent width ∼0.8 keV) and large X-ray variability on timescales of hours and is probably an intermediate polar. Thus Src 1 and Src 2 are probably two field sources not responsible for the TeV emission. The observations were contaminated by strong stray light from a nearby bright source, and we see no clear extended X-ray emission that can be attributed to the supernova remnant G8.7-0.1, a popular possible association with the TeV source. The other possible association, the pulsar wind nebula candidate PSR J1803-2137, shows little long-term variability compared with a previous Chandra observation. Many point sources were serendipitously detected, but most of them are probably normal stars. Three new candidate compact object systems (other than Src 1, Src 2, and PSR J1803-2137) are also found. They are far away from the TeV source and are probably also magnetic cataclysmic variables, thus unlikely to be responsible for the TeV emission.