Searching for Black Hole Candidates in Quiescence by Using Multiband Observations in Globular Cluster M22 (NGC 6656)

We present a multiwavelength investigation of radio sources in the globular cluster M22 (NGC 6656) using the Karl G. Jansky Very Large Array, Chandra, and Hubble Space Telescope. By crossmatching the radio and X-ray source catalogs, we identify eight radio/X-ray counterparts, of which VLA22 is the most promising stellar-mass black hole (BH) candidate. Its radio and X-ray luminosities are consistent with the established L X − L R correlation for BH low-mass X-ray binaries. The observed X-ray variability supports an accreting nature. One possible optical counterpart ( M 814 ≈ 16.695 ± 0.011 mag) is identified. Based on its inferred stellar parameters, the estimated orbital period of P orb ∼ 16 ± 6 h places the proposed counterpart predominantly in the BH region rather than in the neutron star (NS) region in the L X − P orb plane. These results demonstrate the effectiveness of joint radio, X-ray, and optical observations in identifying quiescent BH candidates in globular clusters.