A red giant orbiting a black hole

Abstract: We report spectroscopic and photometric follow-up of a dormant black hole (BH) candidate from Gaia DR3. The system, which we call Gaia BH2, contains a $\sim 1M_{\odot}$ red giant and a dark companion with mass $M_2 = 8.9\pm 0.3\,M_{\odot}$ that is very likely a BH. The orbital period, $P_{\rm orb} = 1277$ days, is much longer than that of any previously studied BH binary. Our radial velocity (RV) follow-up over a 7-month period spans more than 90% of the orbit's dynamic range in RV and is in excellent agreement with predictions of the Gaia solution. UV imaging and high-resolution optical spectra rule out all plausible luminous companions that could explain the orbit. The star is a bright ($G=12.3$), slightly metal-poor ($\rm [Fe/H]=-0.22$) low-luminosity giant ($T_{\rm eff}=4600\,\rm K$; $R = 7.8\,R_{\odot}$; $\log\left[g/\left({\rm cm\,s^{-2}}\right)\right] = 2.6$). The binary's orbit is moderately eccentric ($e=0.52$). The giant is strongly enhanced in $\alpha-$elements, with $\rm [\alpha/Fe] = +0.26$, but the system's Galactocentric orbit is typical of the thin disk. We obtained X-ray and radio nondetections of the source near periastron, which support BH accretion models in which the net accretion rate at the horizon is much lower than the Bondi-Hoyle-Lyttleton rate. At a distance of 1.16 kpc, Gaia BH2 is the second-nearest known BH, after Gaia BH1. Its orbit -- like that of Gaia BH1 -- seems too wide to have formed through common envelope evolution. Gaia BH1 and BH2 have orbital periods at opposite edges of the Gaia DR3 sensitivity curve, perhaps hinting at a bimodal intrinsic period distribution for wide BH binaries. Dormant BH binaries like Gaia BH1 and Gaia BH2 likely significantly outnumber their close, X-ray bright cousins, but their formation pathways remain uncertain.

• The paper confirms Gaia BH2 as a binary system where a red giant orbits an 8.9 M⊙ black hole using Gaia DR3 astrometry and extensive spectroscopy.
• It details the system’s long-period (1277 days) and moderately eccentric (e=0.52) orbit with a low inclination (~35°), reinforcing the black hole identification.
• The findings challenge standard binary formation models and imply a larger, previously undetected population of dormant black holes in wide binaries.

A Red Giant Orbiting a Black Hole: Insights from Gaia BH2

The paper "A red giant orbiting a black hole" presents an analysis of data pertaining to the system Gaia BH2, an extraordinary binary system comprised of a red giant star and a non-luminous companion likely to be a black hole (BH). This system was first identified as a potential black hole candidate from the Gaia Data Release 3 (DR3).

System Characteristics

The red giant in Gaia BH2 resides at a distance of approximately 1.16 kpc from Earth and has an estimated mass near one solar mass. The giant shows signs of being slightly metal-poor ($[\rm Fe/H]=-0.22$) and enhanced in $\alpha$-elements, with $[\alpha/\rm Fe] = +0.26$. It also follows a typical thin disk orbit around the Milky Way. Further spectroscopic analysis of the red giant reveals no secondary signals, indicating the companion's non-luminous nature.

The dark companion has an inferred mass of $8.9\pm0.3\,M_{\odot}$, strongly suggesting it is a black hole. The orbital characteristics of the system are remarkable; with an orbital period of 1277 days, Gaia BH2 possesses the longest period among known black hole binaries, with its orbit being moderately eccentric ($e=0.52$). The inclination derived from astrometric data is relatively low, at about 35 degrees.

Detection and Observational Strategy

Using both Gaia astrometry and coordinated spectroscopic follow-up over several months, which filled $\approx 90\%$ of the expected RV range, the study confirmed the orbital parameters initially derived. This comprehensive data agreement allows robust mass estimations. The collected data rules out plausible luminous companions, thereby reinforcing the likelihood of a black hole as the second unseen object in the system.

X-ray and Radio Considerations

No X-ray or radio emissions were detected from the source, further substantiating the hypothesis of a dormant black hole. This supports models in which the accretion rate, driven by the Bondi-Hoyle-Lyttleton mechanism, is significantly lower than previously estimated, indicating wind or convective losses that prevent significant accretion onto the black hole.

Formation Implications

The origin of Gaia BH2 remains ambiguous due to the large separation that suggests difficulty forming such a system via known common envelope scenarios. The paper postulates several hypotheses: a dynamically formed origin through cluster interactions or potential pristine binary evolution involving massive progenitors that did not evolve into red supergiants.

Significance and Future Prospects

The discovery of Gaia BH2 and Gaia BH1 expands our understanding of BH binaries, suggesting a greater population of dormant BH systems. These systems challenge our conventional understanding of binary evolution, particularly those with wide separations that defy standard formation theories. Their differing orbital characteristics relative to X-ray emitting binaries imply diverse evolutionary pathways.

Going forward, detections from extended baseline missions like future Gaia data releases will provide even more clarity by discovering additional BH candidates. This will allow astrophysicists to better discern the true distribution of BH masses and orbital periods, potentially illuminating evolutionary divergences in the lifecycle of stellar remnants. The paper effectively contributes to the evolving picture of stellar-mass black holes within the Milky Way, highlighting unexpected properties and compelling models to incorporate these findings into coherent theories of stellar evolution and black hole formation.