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 point-like 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 however has a low significance of 97.9%. We found no infraredoptical 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 Xray 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. Subject headings: X-rays: individual (CXOU J180432.4–214009, CXOU J180441.9–214224, HESS J1804– 216) — pulsars: individual (PSR B1800–21=J1803–2137)