VLTI/GRAVITY upper limit on near-infrared emission from the nearby 33 Msun black hole Gaia BH3

TL;DR

This study used the VLTI/GRAVITY instrument to set an upper limit on near-infrared emission from the black hole Gaia BH3, indicating very low accretion activity and providing constraints on its emission properties.

Contribution

First interferometric upper limit on near-infrared emission from Gaia BH3, constraining its accretion rate and emission characteristics.

Findings

No near-infrared emission detected from Gaia BH3 at the current separation.

Upper limit on the black hole's flux density in the K band is 1.9 x 10^{-16} W/m^2/μm.

Accretion activity is very low, consistent with an advection-dominated flow.

Abstract

The recent astrometric discovery of the nearby (590 pc) massive ($33 M_\odot$) dormant black hole candidate Gaia BH3 offers the possibility to angularly resolve the black hole from its companion star by using optical interferometry. Our aim is to detect emission in the near-infrared K band from the close-in environment of Gaia BH3 caused by accretion. Gaia BH3 was observed with the GRAVITY instrument using the four 8-meter Unit Telescopes of the VLT Interferometer. We searched for the signature of emission from the black hole in the interferometric data using the CANDID, PMOIRED, and exoGravity tools. With a present separation of 18 mas, the Gaia BH3 system can be well resolved angularly by GRAVITY. We did not detect emission from the black hole at a contrast level of $\Delta m = 6.8$ mag with respect to the companion star, that is, $f_\mathrm{BH}/f_* < 0.2\%$. This corresponds to an upper limit on the continuum flux density of $f_\mathrm{BH} < 1.9 \times 10^{-16}$ W m$^{-2}$ ${\mu}$m$^{-1}$ in the K band. In addition, we did not detect emission from the black hole in the hydrogen Br{\gamma} line. The non-detection of near-infrared emission from the black hole in Gaia BH3 indicates that its accretion of the giant star wind is presently occurring at most at a very low rate. This is consistent with the limit of $f_\mathrm{Edd} < 4.9 \times 10^{-7}$ derived previously on the Eddington ratio for an advection-dominated accretion flow. Deeper observations with GRAVITY may be able to detect the black hole as the companion star approaches periastron around 2030.