The X-Ray Binary-star Cluster Connection in Late-type Galaxies

Abstract

We conduct one of the largest systematic investigations of bright X-ray binaries (XRBs) in both young star clusters and ancient globular clusters (GCs) using a sample of six nearby star-forming galaxies. Combining complete CXO X-ray source catalogs with optical Physics at High Angular Resolution in Nearby Galaxies-Hubble Space Telescope cluster catalogs, we identify a population of 33 XRBs within or near their parent clusters. We find that GCs that host XRBs in spiral galaxies appear to be brighter, more compact, denser, and more massive than the general GC population. However, these XRB hosts do not appear to be preferentially redder or more metal-rich, pointing to a possible absence of the metallicity-boosted formation of low-mass X-ray binaries (LMXBs) that is observed in the GCs of older galaxies. We also find that a smaller fraction of LMXBs is found in spiral GC systems when compared with those in early-type galaxies: between 8% and 50%, or an average of 20% across galaxies in our sample. Although there is a non-negligible probability of a chance superposition between an XRB and an unrelated young cluster, we find that among clusters younger than 10 Myr, which most likely host high-mass XRBs, the fraction of clusters associated with an XRB increases at higher cluster masses and densities. The X-ray luminosity of XRBs appears to increase with the mass of the cluster host for clusters younger than ∼400 Myr, while the inverse relation is found for XRBs in GCs.

Uncovering an Excess of X-Ray Point Sources in the Halos of Virgo Late-type Galaxies

“…Assuming that those GCs follow the spatial distribution of the known GCs in the Milky Way (a total of 157), we expect 16% (25 of 157) to fall in the range of 4.4 kpc < |z| < 12.0 kpc (Harris 1996), i.e., the Virgo LTG off-disk range where excess sources are found. Again from Hunt et al (2023), 1.2% of GCs contain LMXBs and 24% (6 of 25) of these are brighter than our median detection limit of…”

“…For the Fornax cluster, Jin et al (2019) estimated that 20 out of 74 excess sources around the central giant elliptical galaxy NGC 1399 could be associated with faint, undetected GCs. In addition, Hunt et al (2023) noticed that LTGs have on average fewer GC-LMXBs than ETGs. In particular, 1.2% of the GCs in LTGs contain GC-LMXBs, while for ETGs, this rate rises to 4%-10%.…”

Monitoring the X-Ray Variability of Bright X-Ray Sources in M33

“…If these detections are true transient sources, they could belong to the faint and very faint X-ray transient classes which are both phenomenological classes of likely LMXB systems (Bahramian et al 2021). Many previous studies of extragalactic LMXBs focus on sources in globular clusters (e.g., Bildsten & Deloye 2004;Hunt et al 2023). These sources are not, however, expected to be observed as transient X-ray sources as they are mostly in compact orbits that enable persistent accretion (Armas Padilla et al 2023).…”