2013ApJ...773L..31H

We report a sensitive X-ray search for the proposed intermediate-mass black hole (IMBH) in the massive Galactic cluster, ω Centauri (NGC 5139). Combining Chandra X-ray Observatory data from Cycles 1 and 13, we obtain a deep (∼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) ≤ 5.0x10^–16 erg/cm2/s, after correcting for absorption. This corresponds to an unabsorbed X-ray luminosity of L_X(0.5-7.0 keV) ≤ 1.6 x10³⁰ erg/s, for a cluster distance of 5.2 kpc, Galactic column density N_H= 1.2x10²¹/cm2, and power-law spectrum with Γ = 2.3. If a ∼10⁴ M_☉ IMBH resides in the cluster's core, as suggested by some stellar dynamical studies, its Eddington luminosity would be L_Edd∼10⁴² erg/s. The new X-ray limit would then establish an Eddington ratio of L_X/L_Edd ≲ 10^–12, a factor of ∼10 lower than even the quiescent state of our Galaxy's notoriously inefficient supermassive black hole Sgr A*, and imply accretion efficiencies as low as η ≲ 10^–6-10^–8. This study leaves open three possibilities: either ω Cen does not harbor an IMBH or, if an IMBH does exist, it must experience very little or very inefficient accretion.