Properties of intra-cluster low-mass X-ray binaries in Fornax globular clusters

Abstract

Aims. We present a study of the intra-cluster population of low-mass X-ray binaries (LMXB) residing in globular clusters (GC) in the central 1 deg2 of the Fornax galaxy cluster. Differently from previous studies, which were restricted to the innermost regions of individual galaxies, this work is aimed at comparing the properties of the intra-cluster population of GC-LMXBs with those of the host galaxy. Methods. The data used in this work are a combination of the VLT Survey Telescope (VST) and Chandra observations. We performed a cross-match between the optical and the X-ray catalogue in order to identify the LMXBs residing in GCs. We divided the GC-LMXBs into host-galaxy and intra-cluster objects based on their distance from the nearest galaxy in terms of effective radius (Reff). We found 82 intra-cluster GC-LMXBs and 86 objects that are hosted in galaxies. As the formation of LMXBs also depends on the host GC colour, we performed a Gaussian mixture model to divide the population into red and blue GCs. Results. As has been found for the innermost regions of galaxies, LMXBs tend to form in red and bright GCs in intra-cluster space as well. We find, however, that the likelihood of a red GC to host an LMXB decreases with galactocentric distance, but it remains approximately constant for the blue GC population. Investigating the X-ray properties of the LMXBs residing in GCs, we find a difference in the X-ray luminosity function between the intra-cluster and host-galaxy sample: both follow a power-law down to ∼8.5 × 1037 erg s−1, which is consistent with field LMXBs for the intra-cluster sample, while the latter agree with previous estimates for LMXBs in GCs. We observe a deficiency of bright LMXBs in blue intra-cluster GCs, however. This might indicate a lack of black hole binaries in metal-poor systems. We further investigated the spectral properties of the GC-LMXBs through their hardness-ratio. We detect a tentative difference in the hardness ratio of two populations, where the intra-cluster GC-LMXBs appear to have harder spectra than the host-galaxy objects. We find the same trend when we compare red and blue GC-LMXBs: the spectra of the blue sample are harder spectra than those of the red sample. This result could suggest a relation between the spectral properties of LMXBs and the host GC colour and therefore its metallicity. We discuss the possibilities of spatial biases due to uncertainties in the X-ray spectral response correction and due to contamination by background active galactic nuclei.

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

“…We find that across all galaxies, the mean and median number of chance superpositions between an XRB and cluster per simulation run is approximately zero, except in the case of NGC 3351, which averaged a single superposition per simulation run (equivalent to 5% of the simulated XRB population). Thus, we conclude that the probability of a randomly distributed X-ray source intersecting with a cluster is negligible, consistent with previous estimates (Rangelov et al 2012;Riccio et al 2022;Hunt et al 2023).…”

“…Because LMXBs and GCs tend to follow the underlying, radial stellar mass distributions, and are generally more isolated and more uniformly distributed than higher-mass XRBs and younger clusters, we consider any X-ray source that has a GC within 2σ to be truly associated with a cluster, while we approach the population of XRBs associated with clusters younger that 400 Myr with more caution. This approach to the GC population is independently supported by prior works (e.g., see Riccio et al 2022).…”

“…Analyses of these XRB hosts suggest they are more massive, more luminous, radially smaller, denser, and redder than the general GC population (e.g., Kundu et al 2002;Sivakoff et al 2007;Peacock et al 2009;Brassington et al 2010;Paolillo et al 2011;Riccio et al 2019). Further investigations suggest that metallicity-more so than cluster age-plays a role in the production of LMXBs (Kundu et al 2003(Kundu et al , 2008Fabbiano 2006;Sivakoff et al 2007;Peacock et al 2010;D′ Ago, G. et al 2014;Luan et al 2018;Riccio et al 2022).…”

“…The majority of studies regarding XRBs within star clusters focus on the GCs of elliptical galaxies (e.g., Angelini et al 2001;Jordán et al 2004;Smits et al 2006;Peacock & Zepf 2016;Lehmer et al 2020;Ferrell et al 2021;Riccio et al 2022), a few more recent papers report on XRB associations with GCs in late-type galaxies (e.g., Pfahl et al 2003;Peacock et al 2009;Generozov et al 2018;Hailey et al 2018;Hixenbaugh et al 2022), and fewer still include XRBs in younger clusters (e.g., Rangelov et al 2012;Johns Mulia et al 2019;Avdan et al 2022;Hunt et al 2023). In this paper, we take the first step toward (i) assessing the fraction of star clusters-both old and young-that host XRBs in late-type galaxies, and (ii) comparing the fraction of field versus cluster XRBs in late-type versus early-type galaxies.…”

“…Studies have also shown that in ellipticals, LMXBs are preferentially found in GCs that are brighter (i.e., more massive; e.g., Sivakoff et al 2007;Brassington et al 2010;Fabbiano et al 2010;Riccio et al 2022), more compact (i.e., smaller effective radii and higher densities; e.g., Kundu et al 2002;Pooley et al 2003;Jordán et al 2007;Sivakoff et al 2007;Paolillo et al 2011;Riccio et al 2022), and redder (i.e., metal-rich; e.g., Bellazzini et al 1995;Kundu et al 2002Kundu et al , 2003Kundu et al , 2008Jordán et al 2007;Sivakoff et al 2007;Luan et al 2018;Riccio et al 2022). These surveys have gone a long way toward helping us to understand the mechanism by which XRBs form within dense stellar environments.…”