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

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 1.7×10¹⁰L_K⊙, 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 AGN based on identifying optical counterparts in new HST/ACS images. We detect 10 field LMXBs within the rext ellipse of NGC 7457 (with semi-major axis ∼ 9.1 kpc, ellipticity = 0.55). The corresponding number of LMXBs with (L_x) > 2 × 10³⁷erg/s ^-1per stellar luminosity is consistent with that observed in more massive galaxies, ∼7 per 10¹⁰L_K⊙. We detect a small globular cluster population in these HST data 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 10⁶M_⊙ in metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and 330) LMXBs per 10¹⁰L_K⊙, highlighting the enhanced formation efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected with L_x varying from 2.8 ×10³⁸ to 6.8 x 10³⁸ erg/s ^-1. Combining this L_x with a published dynamical mass estimate for the central SMBH in NGC 7457, we find that L_x/L_Edd varies from 0.5 × 10⁻⁶ to 1.3 x 10^-6.