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Abstract 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 withT_eff= 4250 ± 150 K, logg= 2.0,R_RG∼ 27.8R_⊙(assuming a single-temperature atmosphere), and a subgiant companion withT_eff= 6000 K,R_comp= 2.7R_⊙, orT_eff= 5270 K,R_comp= 4.2R_⊙using models with one-tenth or one-third solar metallicity, respectively, logg= 3.0, extinctionE_B−V= 0.50 ± 0.2, adopting the Data Release 3 Gaia distanceD= 2463 ± 120 pc. No FUV data existed prior to our programs. STIS spectra give an upper limit of 10⁻¹⁷erg cm⁻²s⁻¹Å⁻¹shortwards 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 Mgiiλ2800 Å emission, typical of chromospherically active RGs. The masses inferred by comparison with evolutionary tracks, ∼1M_⊙for the RG and between 1.1 and 1.6M_⊙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. 

Stellar evolution predicts the existence of a mass gap for black hole remnants produced by pair-instability supernova dynamics, whose lower and upper edges are very uncertain. We study the possibility of constraining the location of the upper end of the pair-instability mass gap, which is believed to appear around $m_{\min }\sim 130M_{\odot }$, using gravitational wave observations of compact binary mergers with next-generation ground-based detectors. While high metallicity may not allow for the formation of first-generation black holes on the “far side” beyond the gap, metal-poor environments containing population III stars could lead to such heavy black hole mergers. We show that, even in the presence of contamination from other merger channels, next-generation detectors will measure the location of the upper end of the mass gap with a relative precision close to $\mathrm{\Delta }{m}_{\min }/m_{\min }\simeq 4\%(N_{\det }/100{)}^{-1/2}$at 90% CL, where $N_{\det }$is the number of detected mergers with both members beyond the gap. These future observations could reduce current uncertainties in nuclear and astrophysical processes controlling the location of the gap. Published by the American Physical Society2024