Discovery of a possible X-ray counterpart to HESS J1804-216

Abstract

Suzaku deep observations have discovered two highly significant nonthermal X-ray sources, Suzaku J1804-2142 (Src 1) and Suzaku J1804-2140 (Src 2), positionally coincident with the unidentified TeV γ-ray source, HESS J1804-216. The X-ray sources are not time variable and show no counterpart in other wavebands, except for the TeV source. Src 1 is unresolved at Suzaku spatial resolution, whereas Src 2 is extended or composed of multiple sources. The X-ray spectra are highly absorbed, hard, and featureless, and are well fitted by absorbed power-law models with best-fit photon indices and absorption columns of -0.3-0.5+0.5 and 0.2-0.2+2.0 × 1022 cm-2 for Src 1, and 1.7 -1.0+1.4 and 1.1-0.6+1.0 × 1023cm-2 for Src 2. The measured X-ray absorption to the latter source is significantly larger than the total Galactic neutral hydrogen column in that direction. The unabsorbed 2-10keV band luminosities are 7.5 × 1032(d/5kpc)2 erg s-1 (Src 1) and 1.3 × 1033(d/5kpc)2 erg s-1 (Src 2), where d is the source distance. Among the handful of TeV sources with known X-ray counterparts, HESS J1804-216 has the largest ratio of TeV γ-ray to hard X-ray fluxes. We discuss the nature of the emission and propose the Suzaku sources as plausible counterparts to the TeV source, although further observations are necessary to confirm this. © 2007. Astronomical Society of Japan.