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

Abstract

We report the identification of the possible optical counterparts of five out of seven Ultraluminous X-ray Sources (ULXs) in NGC 4490/4485 galaxy pair. Using archival Hubble Space Telescope ({\it 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 ULXs (X-2, X-3 and X-6) within $\sim$ $0\farcs2$ error radius at the 90\% confidence level. Of the two remaining ULXs, X-1 has no {\it HST} imaging data and photometry could not be performed due to the position of X-5 in NGC4490. Absolute magnitudes ($M_V$) of the optical candidates lie between $-5.7$ and $-3.8$. Color-Magnitude Diagrams (CMDs) have been used to investigate the properties of counterparts and their environments. The locations of the counterparts of X-2, X-4, and X-6 suggest possible association with nearby group of stars while others have no association with a star cluster or group of stars. For comparison purposes, we analyzed previously unused three 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 in a time scale more than a decade. The use of disk blackbody model for the mass of the compact objects indicates that these objects might have masses most likely in the range 10$-$15 $M_{\sun}$.

20 pages, 13 figures, submitted to ApJ

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