2024ApJ...976..131B

Black hole (BH) demographics in different environments is critical in view of recent results on massive star binarity, and of the multimessenger detectability of compact object mergers. But the identification and characterization of noninteracting BHs are elusive, especially in the sparse field stellar population. A candidate noninteractive BH + red giant (RG) binary system, 2MASS J05215658+4359220, was identified by T. A. Thompson et al. We obtained Astrosat/UVIT far-ultraviolet (FUV) imaging and Hubble Space Telescope (HST) UV-optical imaging and spectroscopy of the source to test possible scenarios for the optically elusive companion. HST/STIS spectra from ≈1600 to 10230 Å are best fit by the combination of two stellar sources, a RG with T_eff = 4250 ± 150 K, log g = 2.0, R_RG ∼ 27.8 R_⊙ (assuming a single-temperature atmosphere), and a subgiant companion with T_eff = 6000 K, R_comp = 2.7 R_⊙, or T_eff = 5270 K, R_comp = 4.2 R_⊙ using models with one-tenth or one-third solar metallicity, respectively, log g = 3.0, extinction E_B–V = 0.50 ± 0.2, adopting the Data Release 3 Gaia distance D = 2463 ± 120 pc. No FUV data existed prior to our programs. STIS spectra give an upper limit of 10^–17 erg cm^–2 s^–1 Å^–1 shortwards of 2300 Å; an upper limit of ≳25.7 ABmag was obtained in two UVIT FUV broad bands. The nondetection of FUV flux rules out a compact companion such as a hot white dwarf. The STIS spectrum shows strong Mg IIλ2800 Å emission, typical of chromospherically active RGs. The masses inferred by comparison with evolutionary tracks, ∼1M_⊙ for the RG and between 1.1 and 1.6 M_⊙ for the subgiant companion, suggest past mass transfer, although the RG currently does not fill its Roche lobe. WFC3 imaging in F218W, F275W, F336W, F475W, and F606W shows an unresolved source in all filters.