Constraining Wind-Driven Accretion Onto Gaia BH3 With Chandra

TL;DR

This study constrains the low-level wind-driven accretion onto Gaia BH3, the most massive stellar black hole in the Milky Way, using Chandra X-ray observations and theoretical models, revealing radiatively inefficient accretion modes.

Contribution

It provides the first sensitive upper bounds on Gaia BH3's X-ray flux and accretion rate, combining observational data with stellar wind and ADAF models to understand accretion in this unique system.

Findings

Accretion rate constrained to $f_{Edd}<4.91 imes 10^{-7}$ at apastron.

X-ray flux upper bound set at $F_X<3.25 imes 10^{-15}$ erg s$^{-1}$ cm$^{-2}$.

Accretion did not significantly grow the black hole over its lifetime.

Abstract

Gaia BH3 is the most massive known stellar-origin black hole in the Milky Way, with a mass $M_\bullet \approx 33 \, \rm M_\odot$. Detected from Gaia's astrometry, this black hole is in the mass range of those observed via gravitational waves, whose nature is still highly debated. Hosted in a binary system with a companion giant star that is too far away for Roche-lobe mass transfer, this black hole could nonetheless accrete at low levels due to wind-driven mass-loss from its companion star, thus accreting in advection-dominated accretion flow, or ADAF, mode. Using stellar wind models, we constrain its Eddington ratio in the range $10^{-9} < f_{\rm Edd} < 10^{-7}$, corresponding to radiative efficiencies $5\times10^{-5} < \epsilon < 10^{-3}$, compatible with radiatively inefficient accretion modes. Chandra ACIS-S observed this object and obtained the most sensitive upper bound of its [2-10] keV flux: $F_X < 3.25 \times 10^{-15} \, \rm erg \, s^{-1} \, cm^{-2}$ at $90\%$ confidence level corresponding to $L_{[2-10]}< 2.10 \times 10^{29} \, \rm erg \, s^{-1}$. Using ADAF emission models, we constrained its accretion rate to $f_{\rm Edd}< 4.91 \times 10^{-7}$ at the apastron, in agreement with our theoretical estimate. At the periastron, we expect fluxes $\sim 50$ times larger. Because of the inferred low rates, accretion did not significantly contribute to black hole growth over the system's lifetime. Detecting the electromagnetic emission from Gaia BH3 will be fundamental to informing stellar wind and accretion disk models.