A Deep Chandra X-Ray Limit on the Putative IMBH in Omega Centauri

We report a sensitive X-ray search for the proposed intermediate-mass black hole (IMBH) in the Galactic cluster, $\omega$ Centauri. Combining Chandra X-ray data from Cycles 1 and 13, we obtain a deep ($\sim$291 ks) exposure of the central regions of the cluster. We find no evidence for an X-ray point source near any of the cluster's proposed dynamical centers, and place an upper limit on the X-ray flux from a central source of f$_X$(0.5-7.0 keV) $\le$ 5.0 $\times$ 10$^{-16}$ erg cm$^{-2}$ s$^{-1}$. This corresponds to an unabsorbed X-ray luminosity of L$_X$(0.5-7.0 keV) $\le$1.6 $\times$ 10$^{30}$ erg s$^{-1}$, for a cluster distance of 5.2 kpc, Galactic column density N$_H$ = 1.2 $\times$ 10$^{21}$ cm$^{-2}$, and power-law spectrum with $\Gamma = 2.3$. If a $\sim 10^4$ M$_{sum}$ IMBH resides in the cluster's core, as suggested by some stellar dynamical studies, its Eddington luminosity would be L$_{Edd} \sim 10^{42}$ erg s$^{-1}$. The new X-ray limit would then suggest an Eddington ratio $\sim$10 lower than even the quiescent state of our Galaxy's notoriously inefficient supermassive black hole Sgr A*. This study leaves open three possibilities: either $\omega$ Cen does not harbor an IMBH or, if an IMBH does exist, it must experience very little or very inefficient accretion.