Radio observations of NGC 6388: an upper limit on the mass of its central black hole

TL;DR

This study used deep radio observations to search for an intermediate-mass black hole in NGC 6388, setting an upper mass limit of 1500 solar masses based on non-detection and the fundamental plane of black hole accretion.

Contribution

The paper provides the first radio-based upper limit on the black hole mass in NGC 6388, refining previous estimates from surface density profiles.

Findings

No radio source detected at the cluster center.

Upper limit of 1500 solar masses on the black hole mass.

Discussion of uncertainties and comparison with previous estimates.

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.