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

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2019-04-10
Avdan, Senay
AKYÜZ, AYSUN
Vinokurov, Alexander
AKSAKER, NAZIM
Avdan, Hasan
Fabrika, Sergei
Valeev, Azamat
AKKAYA ORALHAN, Inci
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.