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F. E. Bauer (Penn State), R. M. Sambruna (George Mason), W. N. Brandt, G. Chartas (Penn State), H. Netzer (Tel-Aviv), S. Kaspi, G. P. Garmire, J. A. Nousek (Penn State), K. A. Weaver (NASA GSFC)
We present results for the nearby Seyfert 2 galaxy Circinus from the zeroth-order imaging of a Chandra HETGS observation. In particular, we report on the off-nuclear sources detected in the ACIS-S image of the galaxy. We find that many of these serendipitous X-ray sources lie along the disk of the galaxy, but that none have optical counterparts on an archival HST WFPC2 F606W image to within 4\arcsec of their X-ray positions, down to a limiting magnitude of V~5. Their implied X-ray-to-optical flux ratios, fx/fV \ge 15.8, are larger than those typically found for stars, normal galaxies, or AGN. At the distance of Circinus (4 Mpc), their intrinsic 0.5-8 keV luminosities range from 1037 erg s-1 to several times 1039 erg s-1. Nearly half of the off-nuclear X-ray sources exhibit short term flux variability on the order of 1-3 hours. One source in particular demonstrates flux variability that is both periodic and extreme (P \approx 7.5 hours, L\rm 0.5-8\ keV \approx 0 - 6\times1039 erg s-1), making it one of the most luminous eclipsing X-ray binaries known. These properties suggest that the likely origins for the off-nuclear X-ray sources are X-ray binaries and/or ultra-luminous SNRs.