Searching for Black Hole Candidates in Quiescence by Using Multi-band Observations in Globular Cluster M22 (NGC6656)

TL;DR

This study identifies a promising stellar-mass black hole candidate in globular cluster M22 through multi-wavelength observations, supporting models of black hole retention in such clusters and providing insights into their radio and X-ray properties.

Contribution

The paper reports the discovery of a new black hole candidate in M22 using combined radio, optical, and X-ray data, demonstrating the effectiveness of multi-band observations in identifying black holes in globular clusters.

Findings

VLA22 is a strong black hole candidate based on luminosity correlations.

VLA19 shows an inverted radio spectrum indicative of a compact jet.

VLA40 aligns with black hole luminosity tracks, suggesting potential black hole nature.

Abstract

We present a multi-wavelength investigation of radio sources in the globular cluster M22 (NGC6656) using VLA, HST, and Chandra observations. Among the eight identified counterparts, we highlight VLA22 as the most promising stellar-mass black hole (BH) candidate. Its radio and X-ray luminosities follow the established $L_{R}-L_{X}$ correlation for quiescent black hole low-mass X-ray binaries (BH-LMXBs), while its moderately steep radio spectrum and X-ray spectral hardening further support this classification. Analysis of two potential optical counterparts-a bright main-sequence star and a faint subgiant/red giant-suggests a binary system with a relatively long orbital period. The discovery of VLA22 consistent with recent retention models that stellar-mass BH can be retained within globular clusters over Hubble timescales. Additionally, VLA19 exhibits a characteristically inverted radio spectrum ($\alpha = 0.79 \pm 0.39, S_\nu \propto \nu^\alpha$) indicative of a compact jet, while VLA40 also aligns with the BH $L_{R}-L_{X}$ track, though both require further observations to definitively confirm their nature.