* Astronomy

Title: Discovery of a possible X-ray counterpart to HESS J1804-216
Authors: Aya Bamba (1), Katsuji Koyama (2), Junko S. Hiraga (1), John P. Hughes (3), Takayoshi Kohmura (4), Motohide Kokubun (5), Yo****omo Maeda (6), Hironori Matsumoto (2), Atsushi Senda (1), Tadayuki Takahashi (6), Yohko Tsuboi (7), Shigeo Yamauchi (8), Takayuki Yuasa (5) ((1) RIKEN, (2) Kyoto Univ., (3) Rutgers Univ., (4) Kougakuin Univ., (5) Univ. of Tokyo, (6) ISAS/JAXA, (7) Chuo Univ., (8) Iwate Univ.)

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 gamma-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 x 10^22 cm^-2 for Src 1, and 1.7_-1.0^1.4 and 1.1_-0.6^+1.0 x 10^23 cm^-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--10 keV band luminosities are 7.5 x 10^32 (d/5 kpc)^2 ergs s^-1 (Src 1) and 1.3 x 10^33 (d/5 kpc)^2 ergs 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 gamma-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.


HESS image (grey scale; courtesy of W. Homan and S. Funk) overlaid XIS 2.0–7.0 keV image (white contour), with J2000 coordinates. The black contour and circle represents ROSAT image of G8.7-0.1 (Finley & Oegelman 1994) and PSR 1803-2137. All images are in the linear scale. The black boxes show the field of views of XIS.


XIS images of HESS J1804-216 region in the 0.5–2.0 keV band (left) and 2.0–7.0 keV band (right), with J2000 coordinates. The scales are linear. Each picture is smoothed with 20 arcsec scale. The thick crosses in both panels represents the best-fit position of HESS J1804-216. The source regions used for the timing and spectral analysis are shown by thin solid lines in the right panel.

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-- Edited by Blobrana at 12:33, 2006-08-16