XMM-Newton and Chandra observations of the globular cluster NGC 6388

¹ XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 78, 28691 Villanueva de la , Madrid, Spain
² Dipartimento di Fisica, Università del Salento, and , Sezione di Lecce, CP 193, 73100 Lecce, Italy e-mail: nucita@le.infn.it

Received: 15 October 2007
Accepted: 27 November 2007

Abstract

Context.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 5.710³ .

Aims.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.

Methods.We determine both the hardness ratios and luminosity with a minimum set of assumptions for each of the identified field sources.

Results.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 2.710³³ 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 310^-9. This measurement could be tightened if moderately deep radio observations of the field were performed.