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Optical counterparts of two candidate ultraluminous X-ray sources in NGC 4536

2023    Avdan, H.; Sonbas, E.; Dhuga, K. S.; Vinokurov, A.; Goegues, E.; Avdan, S.; Solovyeva, Y. N.; Kostenkov, A. E.; Shablovinskaya, E. S.; Goktas, D.

Archival XMM-Newton, Chandra, and Hubble Space Telescope(HST) data have been used to study the X-ray and optical properties of two candidate ultraluminous X-ray sources in NGC 4536. In order to search for potential optical counterparts, relative astrometry between Chandra and HST was improved, and as a result, optical counterparts were detected for both X-ray sources. To complement our findings (based on the archival data), ground-based optical spectra of the counterparts were obtained with the 6-m BTA (Big Telescope Alt-azimuth) Telescope located at the Special Astrophysical Observatory. The calculated redshift (z = 0.4391 +/- 0.0010) for one of the sources (X-3) indicates that the source is, in fact, a background active galactic nucleus. Two possible optical counterparts (s1 and s2) were found for X-2. Whether s1 is point-like or an extended source is unclear: If it is point-like and the emission is dominated by the donor, its spectral type indicates O-B star. The second source (s2) is point-like and is consistent with the colours and absolute magnitudes of a red supergiant.