No evidence for accretion around the intermediate-mass black hole in Omega Centauri

TL;DR

This study used ultra-deep radio observations to search for signs of accretion onto the proposed intermediate-mass black hole in Omega Centauri, finding no evidence and constraining accretion efficiency to be very low.

Contribution

The paper provides the most sensitive radio observations to date of Omega Centauri's core, setting new upper limits on accretion activity around the IMBH candidate.

Findings

No radio emission detected at the cluster center.

Accretion efficiency constrained to be less than approximately 4×10⁻³.

Results challenge previous evidence for active accretion onto the IMBH.

Abstract

For over a decade, both theoretical predictions and observational studies have suggested that $\omega$ Centauri ($\omega$ Cen), the most massive Milky Way globular cluster, might harbor an intermediate-mass black hole (IMBH). Recently, identification of fast-moving stars in the core of $\omega$ Cen provided the strongest evidence to date for the presence of such an IMBH. One of the key questions in the study of IMBHs is their accretion efficiency, which determines their radio and X-ray signatures. We investigate the accretion signature of the IMBH in $\omega$ Cen with ultra-deep radio continuum observations of the central region of the cluster. Using approximately 170 hours of Australia Telescope Compact Array observations, we achieve a root mean square noise of 1.1 $\mu$Jy at 7.25 GHz, making this the most sensitive radio image of the cluster to date. We detect no radio emission at any of the proposed centers of the cluster, imposing stringent constraints on the presence of an accreting IMBH in $\omega$ Cen. Considering the fundamental plane of black hole activity, our findings indicate that the accretion efficiency around the black hole is exceptionally low (with a conservative 3-$\sigma$ upper limit of $\epsilon \lesssim 4\times10^{-3}$).