A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where Are the X-Ray Binaries?

We present results from a 34 ks Chandra ACIS-S observation of the low-mass X-ray binary (LMXB) population and the hot interstellar medium (ISM) in the nearby (d = 3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually low X-ray luminosity. We detect 11 X-ray point sources within the D25 optical boundary of the galaxy (93% of the light), one-third to one-half of which are likely to be background active galactic nuclei (AGNs). One of the X-ray sources is coincident with the optical nucleus and may be a low-luminosity AGN. Only two sources with an X-ray luminosity greater than 1037 ergs s-1 in the 0.5-5.0 keV band were detected, one of which is statistically likely to be a background AGN. We expected to detect seven or five such luminous sources if the X-ray binary (XRB) population scales linearly with the B-band or J-band magnitudes, respectively, of the host galaxy. By this measure, NGC 5102 has an unusually low number of XRBs. The deficit of LMXBs is even more striking, because some of these sources may in fact be high-mass X-ray binaries (HMXBs). NGC 5102 is unusually blue for its morphological type and has undergone at least two recent bursts of star formation only ~1.5 × 107 and ~3 × 108 yr ago. We present the results of optical/UV spectral synthesis analysis and demonstrate that a significant fraction (>50%) of the stars in this galaxy are comparatively young (<3 × 109 yr old). We discuss the relationship between the XRB population, the globular cluster (GC) population, and the relative youth of the majority of stars in this galaxy. If the lack of X-ray binaries is related to the relative youth of most of the stars, this would support models of LMXB formation and evolution that require wide binaries to shed angular momentum on a timescale of Gyr. We have also analyzed archival Hubble Space Telescope (HST) images of NGC 5102 and find that it has an unusually low specific frequency of GCs (SN ~ 0.4). The lack of LMXBs could also be explained by the small number of GCs. We have also detected diffuse X-ray emission in the central ~1 kpc of the galaxy with an X-ray luminosity of 4.1 × 1037 ergs s-1 in the 0.1-2.0 keV band. This hot gas is most likely a superbubble created by multiple supernovae of massive stars born during the most recent star burst and is driving the shock into the ISM, which was inferred from previous [O III] λ5007 and Hα observations.