The MAVERIC Survey: Chandra/ACIS Catalog of Faint X-ray sources in 38 Galactic globular clusters

TL;DR

This paper presents a comprehensive catalog of over 1100 faint X-ray sources in 38 Galactic globular clusters observed by Chandra, analyzing their properties, distribution, and implications for X-ray binary populations.

Contribution

It provides the first extensive catalog of faint X-ray sources in multiple globular clusters with detailed spectral and variability analysis, revealing new insights into their populations and characteristics.

Findings

Discovered a new candidate black hole or transitional pulsar in NGC 6539.

Found that neutron star LMXBs may spend ~2% of their lives as transitional millisecond pulsars.

Identified a significant under-abundance of bright intermediate polars in globular clusters.

Abstract

Globular clusters host a variety of lower-luminosity ($L_X<10^{35}$ erg s$^{-1}$) X-ray sources, including accreting neutron stars and black holes, millisecond pulsars, cataclysmic variables, and chromospherically active binaries. In this paper, we provide a comprehensive catalog of more than 1100 X-ray sources in 38 Galactic globular clusters observed by the Chandra X-ray Observatory's ACIS detector. The targets are selected to complement the MAVERIC survey's deep radio continuum maps of Galactic globular clusters. We perform photometry and spectral analysis for each source, determine a best-fit model, and assess the possibility of it being a foreground/background source based on its spectral properties and location in the cluster. We also provide basic assessments of variability. We discuss the distribution of X-ray binaries in globular clusters, their X-ray luminosity function, and carefully analyze systems with $L_X > 10^{33}$ erg s$^{-1}$. Among these moderately bright systems, we discover a new source in NGC 6539 that may be a candidate accreting stellar-mass black hole or a transitional millisecond pulsar. We show that quiescent neutron star LMXBs in globular clusters may spend ~2% of their lifetimes as transitional millisecond pulsars in their active ($L_X>10^{33}$ erg s$^{-1}$) state. Finally, we identify a substantial under-abundance of bright ($L_X>10^{33}$ erg s$^{-1}$) intermediate polars in globular clusters compared to the Galactic field, in contrast with the literature of the past two decades.