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摘要
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 x 10(10) L-K circle dot, 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 r(ext) ellipse of NGC 7457 (with semimajor axis similar to 9.1 kpc, ellipticity = 0.55). The corresponding number of LMXBs with X-ray luminosities (L-x) > 2 x 10(37) erg s(-1) per stellar luminosity is consistent with that observed in more massive galaxies, similar to 7 per 10(10) L-K circle dot. 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-circle dot in metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and 330) LMXBs per 10(10) L-K circle dot, highlighting the enhanced formation efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected, with L-x varying from 2.8 x 10(38) to 6.8 x 10(38) erg s(-1). Combining this L-x with a published dynamical mass estimate for the central supermassive black hole in NGC 7457, we find that L-x/L-Edd varies from 0.5 x 10(-6) to 1.3 x 10(-6).
- 出版日期2017-5
