The Nature of the X - Ray Point Source in the Bar of NGC 6822 Paul

We have analysed archival ROSAT PSPC and Einstein HRI data for the x-ray point source in the bar of NGC 6822. The source decreased in x-ray flux by at least half an order of magnitude in the 13 years between the HRI and PSPC observations. It has a PSPC flux of fX ≈ 8× 10 −14 erg cm s, giving LX ≈ 2.4 × 10 36 erg s for a distance of 500 kpc. The source appears to be related to the optical emission-line object Ho 12. It is unclear if Ho 12 is an H II region or a supernova remnant, although the latter interpretation is better supported by the available optical data. The x-rays are unlikely to be direct emission from a supernova remnant, due to the observed strong x-ray variability. The x-ray spectrum of the source is very soft. Acceptable fits are found for either a Raymond-Smith thermal model with kT≈0.56 keV, or a blackbody model with kT≈0.06 keV. The latter model would place the source amongst the class of Super-Soft x-ray sources first found in the Magellanic Clouds. If the thermal model is correct, the source appears similar to Galactic and Magellanic Cloud stellar-mass black hole binary candidates. We have identified a list of potential optical counterparts from published photometry. However, for either model discussed above, we expect the apparent magnitude of the optical counterpart to be in the range 21.5 ∼< mV ∼< 24.0. Thus the optical counterpart may be below the limit of published photometry. [email protected] [email protected]