A Stringent Limit on the Accretion Luminosity of the Possible Central Black Hole in the Globular Cluster M15

The globular cluster M15 has recently been found to host a possible central black hole with a mass of ~2000 solar masses. A deep, high-resolution Chandra image failed to detect the "nucleus" of the cluster in X-rays. The upper limit on the X-ray luminosity (L_x < 5.6x10^32 erg/s) corresponds to a bolometric Eddington ratio of L_bol/L_Edd < (2-4)x10^-8. Combining this limit with an estimate of the electron density of the intracluster ionized plasma derived from pulsar dispersion measures, we show that the radiative efficiency of the accretion flow, if it accretes at the Bondi rate, must be much lower than that of a standard optically thick, geometrically thin disk.