Mon. Not. R. Astron. Soc., 466, 4021-4028 (2017/April-3)
Deep Chandra observations of NGC 7457, the X-ray point source populations of a low-mass early-type galaxy.
PEACOCK M.B., ZEPF S.E., KUNDU A., MACCARONE T.J., LEHMER B.D., GONZALEZ A.H. and MARASTON C.
Abstract (from CDS):
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)>2×1037 erg 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 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 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 x 1038 to 6.8 x 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/L_ Edd_ varies from 0.5 x 10–6 to 1.3 x 10–6.
© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
galaxies: individual: NGC 7457 - galaxies: star clusters: general - X-rays: binaries - X-rays: galaxies - X-rays: galaxies
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