Optical Counterparts of ULXs and Their Host Environments in NGC 4490/4485

Avdan, Senay
Vinokurov, Alexander
Avdan, Hasan
Fabrika, Sergei
Valeev, Azamat
Balman, Şölen
We report the identification of the possible optical counterparts of five out of seven ultraluminous X-ray sources (ULXs) in the galaxy pair NGC 4490/4485. Using archival Hubble Space Telescope (HST) imaging data, we identified a single optical candidate for two ULXs (X-4 and X-7) and multiple optical candidates for the other three (X-2, X-3, and X-6) within similar to 0.'' 2 error radius at the 90% confidence level. Of the two remaining ULXs, X-1 has no HST imaging data, and photometry could not be performed due to the position of X-5 in NGC 4490. Absolute magnitudes (M-V) of the optical candidates lie between -5.7 and -3.8. Color-magnitude diagrams have been used to investigate the properties of the counterparts and their environments. The locations of the counterparts of X-2, X-4, and X-6 suggest possible association with a nearby group of stars, while the others have no association with a star cluster or group of stars. For comparison purposes, we analyzed three previously unused archival XMM-Newton observations. The long-term X-ray light curves of the sources (except transient X-7) show variability by a factor of three on a timescale of more than a decade. The use of a disk blackbody model for the mass of the compact objects indicates that these objects most likely have masses in the range 10-15 M-circle dot.

Citation Formats
S. Avdan et al., “Optical Counterparts of ULXs and Their Host Environments in NGC 4490/4485,” ASTROPHYSICAL JOURNAL, vol. 875, no. 1, pp. 0–0, 2019, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/57144.