16654

Black hole (BH) demographics in binaries is critical in view of recent results on massive stars binarity comma and the proven multi-messenger detectability of compact objects mergers. Close systems are relatively simple to identify and study as the black hole accretes mass from an evolving companion comma and develops an X-ray emitting accretion disk. However comma identification and characterization of non-interacting BHs is elusive comma but may represent a significant unstudied population. 2MASS-J05215658+4359220 is the only candidate binary presumed to harbour a non-interacting stellar-mass black holedoublePoint it consists of a red giant and an optically undetected comma compact companion of about 3.3 solar masses. In the scenarios proposed so far comma based on a broad ensemble of data comma the origin of near-UV flux detected by GALEX and Swift remains unexplained and may be the key to understanding the system. We propose STIS UV spectroscopy and WFC3 multi-band imaging (including UV) comma that uniquely will reveal the origin of the NUV flux comma and hence provide a decisive clue to the nature of the system. Compatible scenarios include the presence of a white dwarf (WD) or tight WD pair or a low-mass stellar close pair orbiting the red-giant primary at a larger separation comma an accretion disk comma or chromospheric-TZ line emission from the red giant. The HST UV data will uniquely allow us to conclusively discern the source of the UV emission comma to detect a stellar companion (or pair) or accretion disk if present comma derive physical parameters comma characterize the entire system also refining the red-giant parameters comma and to conclusively address the BH companion scenario postulated in recent literature.