Gaia BH2

Gaia BH2 (Gaia DR3 5870569352746779008) is a binary system consisting of a red giant and a stellar-mass black hole. Gaia BH2 is located about 3,800 light years away (1.16 kpc away) in the constellation of Centaurus, making it as of 2024^[update] the third-closest known black hole system to Earth. Gaia BH2 is the second black hole discovered from Gaia DR3 astrometric data.^[4]

Gaia BH2

DECaPS image of Gaia BH2 (star at the center)
Observation data Epoch J2000      Equinox J2000
Constellation	Centaurus[1]
Right ascension	13h 50m 16.748s[2]
Declination	−59° 14′ 20.33″[2]
Apparent magnitude (V)	12.711[3]
Characteristics
Red giant
Evolutionary stage	Red giant branch[2]
Black hole
Evolutionary stage	Stellar black hole[4]
Astrometry
Radial velocity (Rv)	−5.04±2.08[2] km/s
Proper motion (μ)	RA: −10.48±0.1 mas/yr[4] Dec.: −4.61±0.06 mas/yr[4]
Parallax (π)	0.859±0.018 mas[4]
Distance	3,800 ± 80 ly (1,160 ± 20 pc)
Orbit[4]
Period (P)	1,276.7±0.6 d
Semi-major axis (a)	4.96±0.08 AU
Eccentricity (e)	0.5176±0.0009
Inclination (i)	34.87±0.34°
Longitude of the node (Ω)	266.9±0.5°
Periastron epoch (T)	2457438.3±1.4
Argument of periastron (ω) (secondary)	130.9±0.4°
Semi-amplitude (K1) (primary)	25.23±0.04 km/s
Details
Red giant
Mass	1.17±0.08[5] M☉
Radius	8.55+0.20 −0.15[5] R☉
Luminosity (bolometric)	24.6±1.6[4] L☉
Surface gravity (log g)	2.71±0.24[4] cgs
Temperature	4,604±87[4] K
Metallicity [Fe/H]	−0.22±0.02[4] dex
Rotation	398±5[5] days (preliminary estimate)
Rotational velocity (v sin i)	<1.5[4] km/s
Age	5.1+1.2 −1.8[5] Gyr
Black hole
Mass	8.94±0.34[4] M☉
Other designations
Gaia BH2, UCAC4 154-126202, 2MASS J13501675-5914203, Gaia DR3 5870569352746779008
Database references
SIMBAD	data

The black hole and red giant orbit the system barycentre every 1,277 days, or around 3.5 years, with a moderate eccentricity of 0.518. The black hole's mass is around 8.94 M_☉, which means its Schwarzschild radius should be about 26.4 km (16.4 mi).^[4] The red giant has a mass of 1.17 M_☉ and a radius of 8.6 R_☉.^[5] Its temperature is estimated at 4,604 K (4,331 °C; 7,828 °F).^[4] The star is enriched in alpha elements, thus is believed to have undergone mass transfer with another star.^[4]

Discovery

Gaia BH2 was originally discovered as a black hole binary candidate in 2022, found via astrometric observations with Gaia, along with Gaia BH1. At that time it was not clear if Gaia BH2 did definitely harbour a black hole, but it was the only plausible candidate in the Gaia data other than Gaia BH1.^[6][7] Later radial velocity observations confirmed this black hole system and refined its orbital parameters.^[4]

See also

• List of nearest known black holes
• Gaia BH1
• Gaia BH3