Deep Chandra observations of NGC 7457, the X-ray point source populations of a low-mass early-type galaxy

Abstract

We present the X-ray point source population of NGC 7457 based on 124 ks of Chandra observations. Previous deep Chandra observations of low-mass X-ray binaries (LMXBs) in early-type galaxies have typically targeted the large populations of massive galaxies. NGC 7457 is a nearby, early-type galaxy with a stellar luminosity of 1.7 × 1010 LK☉, allowing us to investigate the populations in a relatively low-mass galaxy. We classify the detected X-ray sources into field LMXBs, globular cluster LMXBs and background active galactic nuclei based on identifying optical counterparts in new HST/ACS images. We detect 10 field LMXBs within the rext ellipse of NGC 7457 (with semimajor axis ∼9.1 kpc, ellipticity = 0.55). The corresponding number of LMXBs with X-ray luminosities (Lx) > 2 × 1037erg s−1 per stellar luminosity is consistent with that observed in more massive galaxies, ∼7 per 1010 LK☉. We detect a small globular cluster population in these HSTdata and show that its colour distribution is likely bimodal and that its specific frequency is similar to that of other early-type galaxies. However, no X-ray emission is detected from any of these clusters. Using published data for other galaxies, we show that this non-detection is consistent with the small stellar mass of these clusters. We estimate that 0.11 (and 0.03) LMXBs are expected per 106 M☉ in metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and 330) LMXBs per 1010 LK☉, highlighting the enhanced formation efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected, with Lx varying from 2.8 × 1038 to 6.8 × 1038 erg s−1. Combining this Lx with a published dynamical mass estimate for the central supermassive black hole in NGC 7457, we find that Lx/LEdd varies from 0.5 × 10−6 to 1.3 × 10−6