The Field of the TeV Source HESS J1804–216 in X-Rays and Other Wavelengths

The field of the extended TeV source HESS J1804-216 was serendipitously observed with the Chandra ACIS detector on 2005 May 4. The data reveal several X-ray sources within the bright part of HESS J1804-216. The brightest of these objects, CXOU J180432.4-214009, which has been also detected with Swift (2005 November 3) and Suzaku (2006 April 6), is consistent with being a pointlike source, with the 0.3-7 keV flux FX = (1.7 ± 0.2) × 10-13 ergs s-1 cm-2. Its hard and strongly absorbed spectrum can be fitted by the absorbed power-law model with the best-fit photon index Γ ≈ 0.45 and hydrogen column density nH ≈ 4 × 1022 cm-2, both with large uncertainties due to the strong correlation between these parameters. A search for pulsations resulted in a 106 s period candidate, which has a low significance of 97.9%, however. We found no infrared-optical counterparts for this source. The second brightest source, CXOU J180441.9-214224, which has been detected with Suzaku, is either extended or multiple, with the flux FX ~ 1 × 10-13 ergs cm-2 s-1. We found a nearby M dwarf within the X-ray source extension, which could contribute a fraction of the observed X-ray flux. The remaining sources are very faint (FX < 3 × 10-14 ergs cm-2 s-1), and at least some of them are likely associated with nearby stars. Although one or both of the two brighter X-ray sources could be faint accreting binaries or remote pulsars with pulsar wind nebulae (hence possible TeV sources), their relation to HESS J1804-216 remains elusive. The possibility that HESS J1804-216 is powered by the relativistic wind from the young pulsar B1800-21, located at a distance of ~10 pc from the TeV source, still remains a more plausible option.