2006MNRAS.368L..43D

We present upper limits on the masses of the putative central intermediate-mass black holes in two nearby Galactic globular clusters: 47Tuc (NGC 104), the second brightest Galactic globular cluster, and NGC 6397, a core-collapse globular cluster and, with a distance of 2.7 kpc, quite possibly the nearest globular cluster. These upper limits are obtained using a technique suggested by T. Maccarone. These mass estimates have been derived from 3σ upper limits on the radio continuum flux at 1.4 GHz, assuming that the putative central black hole accretes the surrounding matter at a rate of between 0.1 and 1 per cent of the Bondi accretion rate. For 47Tuc, we find a 3σ upper limit of 2060-670 M_☉, depending on the actual accretion rate of the black hole and the distance to 47Tuc. For NGC 6397, which is closer to us, we derive a 3σ upper limit of 1290-390 M_☉. While estimating mass upper limits based on radio continuum observations requires making assumptions about the gas density and the accretion rate of the black hole, their derivation does not require complex and time-consuming dynamical modelling. Thus, this method offers an independent way of estimating black hole masses in nearby globular clusters. If, generally, central black holes in stellar systems accrete matter faster than 0.1 per cent of the Bondi accretion rate, then these results indicate the absence of black holes in these globular clusters with masses as predicted by the extrapolated M_ø-σ_crelation.