ESPRESSO observations of Gaia BH1: high-precision orbital constraints and no evidence for an inner binary

Abstract

We present high-precision radial velocity (RV) observations of Gaia BH1, the nearest known black hole (BH). The system contains a solar-type G star orbiting a massive dark companion, which could be either a single BH or an inner BH + BH binary. A BH + BH binary is expected in some models where Gaia BH1 formed as a hierarchical triple, which are attractive because they avoid many of the difficulties associated with forming the system through isolated binary evolution. Our observations test the inner binary scenario. We have measured 115 precise RVs of the G star, including 40 from ESPRESSO with a precision of $3$-$5$ m s${}^{-1}$, and 75 from other instruments with a typical precision of $30$-$100$ m s${}^{-1}$. Our observations span $2.33$ orbits of the G star and are concentrated near a periastron passage, when perturbations due to an inner binary would be largest. The RVs are well-fit by a Keplerian two-body orbit and show no convincing evidence of an inner binary. Using REBOUND simulations of hierarchical triples with a range of inner periods, mass ratios, eccentricities, and orientations, we show that plausible inner binaries with periods $P_{\text{inner}}\gtrsim 1.5$ days would have produced larger deviations from a Keplerian orbit than observed. Binaries with $P_{\text{inner}}\lesssim 1.5$ days are consistent with the data, but these would merge within a Hubble time and would thus imply fine-tuning. We present updated parameters of Gaia BH1's orbit. The RVs yield a spectroscopic mass function $f\left(M_{\text{BH}}\right)=3.9358\pm 0.0002M_{\odot }$ - about $7000\sigma$ above the $\sim 2.5M_{\odot }$ maximum neutron star mass. Including the inclination constraint from Gaia astrometry, this implies a BH mass of $M_{\text{BH}}=9.27\pm 0.10M_{\odot }$.

26 pages, 14 figures, Accepted to PASP. Github repository at https://github.com/pranav-nagarajan/Gaia-BH1-Precision-RVs

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Cited by in corpus (6)

• Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry
• A generative model for Gaia astrometric orbit catalogs: selection functions for binary stars, giant planets, and compact object companions
• Gaia BH1 and BH2 -- evolutionary models with overshooting of the black hole progenitors within the present-day binary separation
• Validation of Gaia DR3 orbital and acceleration solutions with hierarchical triples
• Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects
• Exciting Stellar Eccentricity in Gaia BH3 via a Hidden Black Hole Binary