Abstract
We present the results of deep radio observations with the Australia Telescope Compact Array (ATCA) of the globular cluster NGC 6388. We show that there is no radio source detected (with a r.m.s. noise level of 27 uJy) at the cluster centre of gravity or at the locations of the any of the Chandra X-ray sources in the cluster. Based on the fundamental plane of accreting black holes which is a relationship between X-ray luminosity, radio luminosity and black hole mass, we place an upper limit of 1500 M_sun on the mass of the putative intermediate-mass black hole located at the centre of NGC 6388. We discuss the uncertainties of this upper limit and the previously suggested black hole mass of 5700 M_sun based on surface density profile analysis.
Highlights
Following the early discoveries of X-ray sources in globular clusters in the mid-1970s (Clark 1975; Clark, Markert & Li 1975), it was proposed that the X-ray emission of these clusters was due to accretion of intracluster material released by stellar mass loss onto central black holes (Bahcall & Ostriker 1975; Silk & Arons 1975)
We report on radio observations with the Australia Telescope Compact Array (ATCA) of NGC 6388 that led to an upper limit on the mass of the putative intermediate-mass black hole located at the centre of NGC 6388
After reduction of the ATCA radio data, we found there are no radio sources detected in association with the cluster centre of gravity nor at the locations of any of the Chandra X-ray sources within the cluster (Fig. 2)
Summary
Following the early discoveries of X-ray sources in globular clusters in the mid-1970s (Clark 1975; Clark, Markert & Li 1975), it was proposed that the X-ray emission of these clusters was due to accretion of intracluster material released by stellar mass loss onto central black holes (Bahcall & Ostriker 1975; Silk & Arons 1975). This started a debate about whether globular clusters contain black holes of intermediate masses (i.e. greater than the ∼ 30 M⊙ limit for black holes formed through normal single star evolution, but less than the 105 M⊙ seen in the smallest galactic nuclei). The relation between black hole mass and X-ray and radio luminosity empirically appears to follow a ”fundamental
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