TeV source HESS J1804-216 in X-rays and other wavelengths

Abstract

The field of the extended TeV source HESS J1804-216 was serendipitously observed with the Chandra ACIS detector. 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 and Suzaku, is consistent with being a point-like source, with the 0.3-7 keV flux of (1.7\pm0.2)\times10^{-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 of 0.45 and hydrogen column density of 4\times10^{22} 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 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 of about 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 (<3\times 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 about 10 pc from the TeV source, still remains a more plausible option.