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

Abstract

We present a multi-wavelength 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 cross-matching 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 $L_{\mathrm{X}}-L_{\mathrm{R}}$ correlation for BH low-mass X-ray binaries. The observed X-ray variability supports an accreting nature. One possible optical counterpart ($M_{814}\approx 16.695\pm 0.011$ mag) is identified. Based on its inferred stellar parameters, the estimated orbital period of $P_{\mathrm{orb}}\sim 16\pm 6\mathrm{h}$ places the proposed counterpart predominantly in the BH region rather than in the NS region in the $L_{\mathrm{X}}-P_{\mathrm{orb}}$ plane. These results demonstrate the effectiveness of joint radio, X-ray, and optical observations in identifying quiescent BH candidates in globular clusters.

arXiv comment14 pages, 6 figures, 4 table, accepted for publication in ApJ