Discovery of a Candidate Black Hole - Giant Star Binary System in the Galactic Field

Abstract: We report the discovery of the first likely black hole in a non-interacting binary system with a field red giant. By combining radial velocity measurements from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) with photometric variability data from the All-Sky Automated Survey for Supernovae (ASAS-SN), we identified the bright rapidly-rotating giant 2MASS J05215658+4359220 as a binary system with a massive unseen companion. Subsequent radial velocity measurements reveal a system with an orbital period of 83 days and near-zero eccentricity. The photometric variability period of the giant is consistent with the orbital period, indicative of star spots and tidal synchronization. Constraints on the giant's mass and radius from its luminosity, surface gravity, and temperature imply an unseen companion with mass of $3.3^{+2.8}_{-0.7}$ M$_\odot$, indicating a low-mass black hole or an exceedingly massive neutron star. Measurement of the astrometric binary motion by {\it Gaia} will further characterize the system. This discovery demonstrates the potential of massive spectroscopic surveys like APOGEE and all-sky, high-cadence photometric surveys like ASAS-SN to revolutionize our understanding of the compact object mass function, and to test theories of binary star evolution and the supernova mechanism.

• The paper identifies a candidate low-mass black hole in a red giant binary system with an ~83-day, near-circular orbit.
• It employs APOGEE radial velocity measurements and ASAS-SN photometry to constrain the companion’s mass and validate tidal synchronization.
• The study underscores the value of multi-survey data for binary evolution models and anticipates Gaia and gravitational wave follow-ups to refine compact object classifications.

Discovery of a Candidate Black Hole–Giant Star Binary System in the Galactic Field

This paper presents the identification of a candidate black hole (BH) in a non-interacting binary system with a red giant star located in the Galactic field. Using a combination of radial velocity (RV) measurements from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and photometric data from the All-Sky Automated Survey for Supernovae (ASAS-SN), the authors detected the giant 2MASS J05215658+4359220 as part of a binary system with an unseen, massive companion. The system exhibits an orbital period of approximately 83 days and near-zero eccentricity, suggesting tidal synchronization of the star.

A significant outcome of this research is the potential identification of a low-mass black hole, as the inferred mass of the companion, based on constraints from the star's luminosity, surface gravity, and temperature, is $3.3^{+2.8}_{-0.7}$ $M_\odot$. This places the companion’s mass at a boundary where it could plausibly either be a BH or an unusually massive neutron star (NS). The forthcoming astrometric data from the Gaia mission is expected to provide further characterization of this binary, facilitating a more precise determination of the companion's nature.

The outcomes from this study underscore the efficacy of large spectroscopic and photometric surveys for probing the mass distribution of compact objects in binary systems, thereby enriching our understanding of binary evolution and the supernova mechanisms underlying BH and NS formation. Such systems can offer an unbiased view of BH and NS mass functions, crucial for resolving ambiguities in the supernova core-collapse paradigm and investigating mass discrepancies observed in merging binaries detected by LIGO.

From a theoretical standpoint, this discovery provides an essential data point, testing our understanding of the boundaries between NS and BH formation and the associated demographic implications. The apparent discovery of a low-mass black hole opens up new lines of inquiry into the lower mass limits of black holes, potentially reevaluating predictions made by current models of stellar evolution and remnant formation.

In the future, high-precision astrometric surveys like Gaia and targeted spectroscopic follow-up will be pivotal in refining the parameters of this system. Additionally, gravitational wave surveys could complement these findings by providing mass estimates for dynamically evolving systems outside traditional EM spectrum limitations. This paper thus contributes significantly to the corpus of knowledge required to elucidate the complex interplay of stellar evolution, binary interactions, and compact object formation, highlighting important avenues for future exploration in astrophysical research.