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 ∼1 M⊙ red giant and a dark companion with mass $M_2 = 8.9\pm 0.3\, {\rm M}_{\odot }$ that is very likely a BH. The orbital period, Porb = 1277 d, is much longer than that of any previously studied BH binary. Our radial velocity (RV) follow-up over a 7-month period spans >90 per cent of the orbit’s RV range and is in excellent agreement with the Gaia solution. UV imaging and high-resolution optical spectra rule out 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 enhanced in α-elements, with $\rm [\alpha /Fe] = +0.26$, but the system’s Galactocentric orbit is typical of the thin disc. We obtained X-ray and radio non-detections 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 significantly outnumber their close, X-ray bright cousins, but their formation pathways remain uncertain.

Constraints on the Cosmological Coupling of Black Holes from the Globular Cluster NGC 3201

“…I have focused so far only on the two (or possibly three) BHs in NGC 3201 since the small masses and old GC age make them ideal tests of cosmological BH growth. This ignores the potential limits that could be imposed by including additional BHs with known masses from the galactic field, such as the two detached BH-star binaries identified in Gaia Data Release 3 by El-Badry et al (2023a. Unlike the two NGC 3201 BHs, Gaia BH1 and BH2 have measured masses with upper bounds (at 9.62 ± 0.18 M e and 8.9 ± 0.3 M e , respectively) derived from the combination of RV and astrometric observations.…”

A Reanalysis of the Isolated Black Hole Candidate OGLE-2011-BLG-0462/MOA-2011-BLG-191

“…The first BH in a noninteracting system was found to have a somewhat surprising low mass of 3 M e , in the "lower mass gap" where neutron stars and BHs had not previously been observed electromagnetically (Thompson et al 2019). Since then, two more BHs in noninteracting binary systems have been found, with masses of 9-10 M e (Chakrabarti et al 2023;El-Badry et al 2023a, 2023b. This is somewhat higher than the average observed BH mass in an LMXB, but falls within the typical mass range.…”

“…Around 20%-30% of O stars are expected to merge and form a single, even more massive star (Sana et al 2012), and many of the remaining binary systems are disrupted before, during, or after the formation of the BH due to natal kicks or mass loss. Despite this, nearly all known Galactic BHs are in binary systems (Corral-Santana et al 2016;Thompson et al 2019;Chakrabarti et al 2023;El-Badry et al 2023a, 2023b. This detection bias exists because, unlike BH binaries, isolated BHs do not have a companion that can electromagnetically identify their presence, making them particularly elusive.…”

Supernovae Origin for the Low-latitude Intermediate-velocity Arch and the North Celestial Pole Loop

“…To circumvent this problem, we need to invoke an asymmetric explosion, imparting a kick to both the exploding star, which strongly modifies the orbital properties like period and eccentricity (Postnov & Yungelson 2014;Escorza et al 2023), and the center of mass of the system (Brandt & Podsiadlowski 1995;Kalogera 1996). It is worth mentioning that the two black hole binaries discovered so far using Gaia DR3 data also have low-mass primaries and orbital parameters outside the theoretically expected ranges (Chakrabarti et al 2023;El-Badry et al 2023a). Their properties (Tables 2-4) are quite different from previously known black hole binaries, and explaining their formation, as well as currently observed properties like their periods, eccentricities, and space velocities, seems challenging as well.…”