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 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 fX(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 LX(0.5–7.0 keV) ⩽1.6 × 1030 erg s−1, for a cluster distance of 5.2 kpc, Galactic column density NH = 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 LEdd ∼1042 erg s−1. The new X-ray limit would then establish an Eddington ratio of LX/LEdd ≲ 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.