Investigation of the Connection Between X-ray Binaries and Compact Star Clusters in NGC 628

Abstract

We investigated for a possible connection between the types of X-ray binaries (XRBs) and the properties of compact star clusters in the nearby galaxy NGC 628. Using {\it Chandra} archival data covering the years 2001-2018, 75 X-ray sources were detected within the field of view of observations. A total of 69 XRBs, one of which is an ultraluminous X-ray source (ULX), were found to be in the luminosity range of 3$\times 10^{36}\le L_X$ $\le$ 2$\times 10^{39}$ erg s${}^{-1}$. We identified the optical counterpart(s) of 15 of the 42 XRBs that coincide with the {\it HST} field of view via improved astrometry. We classified 15 of them as HMXBs based on the presence of the optical counterparts. The remaining sources with no optical counterparts were classified as LMXBs. We also search compact star clusters in this galaxy using the multi-band optical images drawn from {\it HST} archives. 864 compact star clusters were identified and their ages and masses were determined by applying the best-fit SSP (Simple Stellar Population) model to their color-color diagram. We found that in NGC 628, HMXBs are associated with younger star clusters and LMXBs with older ones. Our findings support a connection between different types of XRBs and cluster ages, already known to exist for other galaxies.

19 pages, 13 figures, accepted for publication in MNRAS

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• The First Glimpse of ULXs Through the Near-Infrared Images Captured by the James Webb Space Telescope