An Examination of the X-ray Sources in the Globular Cluster NGC 6652

TL;DR

This study used Chandra observations to identify and analyze X-ray sources in NGC 6652, revealing known and new sources with diverse spectral properties, and suggesting a high-density environment influences its X-ray luminosity distribution.

Contribution

The paper provides the first detailed X-ray source catalog for NGC 6652, including the discovery of five new sources and insights into their nature and the cluster's unique luminosity function.

Findings

Detected six known X-ray sources and five new ones.

Identified a potential quiescent neutron star LMXB in the cluster.

Found an unusually flat X-ray luminosity function linked to high central density.

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 (Gamma ~ 1.3) or an absorbed hot gas emission model (kT ~ 34 keV). Its unabsorbed 0.5-10 keV luminosity (L_X = 1.6+-0.1*10^34 erg/s) is suggestive of a neutron star primary; however, Source B exhibits unusual variability for a 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 L_X (0.5-10 keV) ~ 9*10^32 erg/s, 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 L_X (0.5-10 keV) ~ 3*10^32 erg/s, while Source F has L_X (0.5-10 keV) ~ 1*10^32 erg/s. Their relatively hard X-ray spectra are well-fit by power-law or plasma emission models. Five newly detected fainter sources have luminosities between 1-5*10^31 erg/s. 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.