The globular cluster M15 has recently been found to host a possible central black hole with a mass of \sim 2000 M _ { \odot } .
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 } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 5.6 \times 10 ^ { 32 } erg s ^ { -1 } ) corresponds to a bolometric Eddington ratio of L _ { bol } / L _ { Edd } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } ( 2 - 4 ) \times 10 ^ { -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 .