The globular cluster NGC 6388: $XMM$-Newton and $Chandra$ observations

TL;DR

This study uses X-ray observations from XMM-Newton and Chandra to investigate the presence of an intermediate-mass black hole in globular cluster NGC 6388 by analyzing X-ray emissions and setting upper limits on accretion activity.

Contribution

It provides the first X-ray observational constraints on the hypothesized intermediate-mass black hole in NGC 6388, establishing upper limits on its accretion efficiency.

Findings

Detected unresolved X-ray sources at the cluster center.

Measured an upper limit on accretion efficiency of 3×10⁻⁹.

Identified multiple point-like X-ray sources within the cluster.

Abstract

By studying the optical brightness surface density of the globular cluster NGC 6388, it has been recently proposed that it harbors a central intermediate-mass black hole with mass $\simeq 5.7\times 10^3$ M$_{\odot}$. We expect that the compact object in the center of NGC 6388 emits radiation in the $X$-ray band as a consequence of the accretion from the surrounding matter. We searched for $XMM$-Newton and $Chandra$ observations towards NGC 6388 to test this hypothesis. The $Chandra$ satellite disentangles several point-like $X$-ray sources, probably low mass $X$-ray binaries, well within the core radius of the globular cluster. However, three of them, coinciding with the cluster center of gravity, remain unresolved. Their total luminosity is $L_X^{Obs}\simeq 2.7\times 10^{33}$ erg s$^{-1}$. If one of these sources is the $X$-ray counterpart of the intermediate-mass black hole in NGC 6388, the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is $3\times 10^{-9}$. This measurement could be tightened if moderately deep radio observations of the field were performed.