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 $1.7\times10^{10} L_{K\odot}$, 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 $r_{ext}$ ellipse of NGC 7457 (with semi-major axis $\sim$ 9.1
kpc, ellipticity = 0.55). The corresponding number of LMXBs with
$L_{x}>2\times10^{37}erg/s$ per stellar luminosity is consistent with that
observed in more massive galaxies, $\sim 7$ per $10^{10} L_{K\odot}$. 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 $10^{6}M_{\odot}$ in
metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and
330) LMXBs per $10^{10} L_{K\odot}$, highlighting the enhanced formation
efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected
with $L_{x}$ varying from $2.8-6.8\times10^{38}erg/s$. 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 $0.5-1.3\times10^{-6}$.
