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

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.