ChandraStudy of X‐Ray Point Sources in the Early‐Type Galaxy NGC 4552 (M89)

We present a Chandra ACIS study of the early-type galaxy NGC 4552. We detect47 X-ray point sources, most of which are likely LMXBs, within 4 R_e. Thebrightest X-ray source coincides with the optical/UV/radio center of thegalaxy, and shows variability on >1 hr timescales, indicating the possibleexistence of a LLAGN. The 46 off-center sources and the unresolved pointsources contribute about 29% and 20% to the total luminosity of the galaxy,respectively. The corrected cumulative XLF of the off-center sources is bestfit by a broken power-law model with a break at L_b=4.4+2.0-1.4 e38 ergs/s. Weidentified 210 GCs candidates in a HST WFPC2 optical image of the galaxy'scentral region. Of the 25 off-center LMXBs that fall within the WFPC2 FOV, 10sources are coincident with a GC. Thus the fraction of the GCs hosting brightLMXBs and the fraction of the LMXBs associated with GCs are 4.8% and 40%,respectively. In the V and I bands, the GCs hosting bright LMXBs are typically1-2 magnitudes brighter than the GCs with no detected LMXBs. There are about1.9+-0.4 times as many LMXBs in the red, metal-rich GCs as there are in theblue, metal-poor ones. We find no obvious difference between the luminositydistributions of LMXBs in GCs and in field, but the cumulative spectrum of theLMXBs in GCs tends to be softer than that of the LMXBs in field. We detected 3X-ray sources that have isotropic luminosities larger than 1e39 erg/s. The onelocated in the joint Chandra-HST field is found to be associated with a GC. Bystudying the ACIS spectra we infer that the this may be a candidate black holesystem with a mass of 15-135 M_sun. One of the other sources with a luminositybrighter than 1e39 ergs/s reveals temporal variations in brightness ontimescales greater than an hour.Comment: Accepted for publication in Ap