Stacking star clusters in M51: Searching for faint X-ray binaries

Abstract

The population of low-luminosity (<1035 erg s-1) X-ray binaries (XRBs) has been investigated in our Galaxy and M31 but not further. To address this problem, we have used data from the Chandra X-Ray Observatory and the Hubble Space Telescope to investigate the faint population of XRBs in the grand-design spiral galaxy M51. A matching analysis found 25 star clusters coincident with 20 X-ray point sources within 1.″5 (60 pc). From X-ray and optical color-color plots we determine that this population is dominated by high-mass XRBs. A stacking analysis of the X-ray data at the positions of optically identified star clusters was completed to probe low-luminosity X-ray sources. No cluster type had a significant detection in any X-ray energy band. An average globular cluster had the largest upper limit, 9.23 × 10 34 erg s-1, in the full band (0.3-8 keV) while on average the complete sample of clusters had the lowest upper limit, 6.46 × 10 33 erg s-1 in the hard band (2-8 keV). We determined average luminosities of the young and old star cluster populations and compared the results to those from the Milky Way. We conclude that deeper X-ray data are required to identify faint sources with a stacking analysis. © 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.