X-ray Binaries in the Ultrahigh Encounter Rate Globular Cluster NGC 6388

Abstract

We report the results of a joint \chandra-\hst study of the X-ray binary population in the massive, high-density globular cluster NGC 6388. NGC 6388 has one of the highest predicted X-ray binary production rate of any Galactic cluster. We detected a large population of 61 \chandra sources within the half-mass radius with L$_X > 5 \times 10^{30}$ \ergs. From the X-ray colors, luminosities, (lack of) variability, and spectral fitting, we identify five as likely quiescent low-mass X-ray binaries. Due to the extremely crowded nature of the core of NGC 6388, finding optical identifications to \chandra sources is challenging. We have identified four blue, optically variable counterparts to spectrally hard X-ray sources, evidence that these are bright cataclysmic variables (CVs). One showed variability of 2 magnitudes in V, indicative of a dwarf nova eruption. One other likely CV is identified by its X-ray spectrum (partial covering with high $N_H$) and strong variability, making five likely CVs identified in this cluster. The relatively bright optical magnitudes of these sources put them in the same class as CV1 in M15 and the brightest CVs in 47 Tuc.