The nature of the X-ray point source in the bar of NGC 6822.
Abstract
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 f_X ~ 8 x 10(-14) erg cm(-2) s(-1) , giving L_X ~ 2.4 x 10(36) erg s(-1) 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 \hii\ 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 la m_V la 24.0. Thus the optical counterpart may be below the limit of published photometry.
- Publication:
-
The Astronomical Journal
- Pub Date:
- September 1997
- DOI:
- 10.1086/118529
- arXiv:
- arXiv:astro-ph/9706183
- Bibcode:
- 1997AJ....114..988E
- Keywords:
-
- X-RAYS: GALAXIES;
- GALAXIES: INDIVIDUAL: NGC 6822;
- Astrophysics
- E-Print:
- 17 pages, 4 PostScript figures, uses AASTeX style files, Accepted for publication in The Astronomical Journal