A deep Chandra x-ray limit on the putative IMBH in omega centauri

Abstract

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) ≤5.0 × 10-16 erg cm-2 s-1, after correcting for absorption. This corresponds to an unabsorbed X-ray luminosity of L X(0.5-7.0 keV) ≤1.6 × 1030 erg s-1, for a cluster distance of 5.2 kpc, Galactic column density N H = 1.2 × 1021 cm-2, and power-law spectrum with Γ = 2.3. If a ∼104 IMBH resides in the cluster's core, as suggested by some stellar dynamical studies, its Eddington luminosity would be L Edd ∼1042 erg s -1. 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. © 2013. The American Astronomical Society. All rights reserved.