We present upper-limits on the masses of the putative central intermediate-mass black holes in two nearby Galactic globular clusters : 47Tuc ( NGC104 ) , the second brightest Galactic globular cluster , and NGC6397 , a core-collapse globular cluster and , with a distance of 2.7 kpc , quite possibly the nearest globular cluster , using a technique suggested by T. Maccarone .
These mass estimates have been derived from 3 \sigma 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 between 0.1 % and 1 % of the Bondi accretion rate .
For 47Tuc , we find a 3 \sigma upper limit of 2060 - 670 M _ { \odot } , depending on the actual accretion rate of the black hole and the distance to 47Tuc .
For NGC6397 , which is closer to us , we derive a 3 \sigma upper limit of 1290 - 390 M _ { \odot } .
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 modeling .
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 % 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 _ { \bullet } - \sigma _ { c } relation .