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Session 2 - Spiral, Irregular and Elliptical Galaxies.
Display session, Monday, June 09
South Main Hall,
We have analysed archival ROSAT PSPC and Einstein HRI data for the x-ray point source in the bar of the Local Group dwarf irregular galaxy NGC 6822. The source decreased in x-ray flux by roughly an order of magnitude in the 13 years between the HRI and PSPC observations. It has a PSPC flux of f_X \approx 8 \times 10^-14 erg cm^-2 s^-1 in the 0.2--2 keV band, giving L_X \approx 2.4 \times 10^36 erg s^-1 for a distance of 500 kpc. The x-ray position is within the isophotes of the optical emission-line object Ho 12. It is unclear if Ho 12 is an \ion H2 region or a supernova remnant, although the SNR interpretation is better supported by the available optical data. The x-rays are unlikely to be direct emission from an SNR, 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\approx0.56 keV, or a blackbody model with kT\approx0.06 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 \la m_V \la 24.0. Thus the optical counterpart may be below the limit of published photometry.
Program listing for Monday