AN EXAMINATION OF THE X-RAY SOURCES IN THE GLOBULAR CLUSTER NGC 6652

Abstract

We observed the globular cluster NGC 6652 with Chandra for 47.5 ks, detecting six known X-ray sources, as well as five previously undetected X-ray sources. Source A (XB 1832-330) is a well-known bright low-mass X-ray binary (LXMB). The second brightest source, B, has a spectrum that fits well to either a power-law model (Γ ∼ 1.3) or an absorbed hot gas emission model (kT ∼ 34 keV). Its unabsorbed 0.5–10 keV luminosity (LX = 1.6+0.1−0.1 × 1034 erg s−1) is suggestive of a neutron star primary; however, Source B exhibits unusual variability for an LMXB, varying by over an order of magnitude on timescales of ∼100 s. Source C's spectrum contains a strong low-energy component below ∼1 keV. Its spectrum is well fit to a simplified magnetic cataclysmic variable (CV) model, thus the soft component may be explained by a hot polar cap of a magnetic CV. Source D has an average LX (0.5–10 keV) ∼9 × 1032 erg s−1 and its spectrum is well fit to a neutron star atmosphere model. This is indicative of a quiescent neutron star LXMB, suggesting Source D may be the third known LMXB in NGC 6652. Source E has LX (0.5–10 keV) ∼3 × 1032 erg s−1, while Source F has LX (0.5–10 keV) ∼1 × 1032 erg s−1. Their relatively hard X-ray spectra are well-fit by power-law or plasma emission models. Five newly detected fainter sources have luminosities between 1 and 5 × 1031 erg s−1. NGC 6652 has an unusually flat X-ray luminosity function compared to other globular clusters, which may be connected to its extremely high central density.