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

Free access article

Issue		A&A Volume 478, Number 3, February II 2008


Page(s)		763 - 768
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20078850

A&A 478, 763-768 (2008)
DOI: 10.1051/0004-6361:20078850

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

A. A. Nucita¹, F. De Paolis², G. Ingrosso², S. Carpano¹, and M. Guainazzi¹

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 $\simeq$ 5.7 $\times$ 10³ $M_{\odot}$ .
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 $L_{\rm X}^{\rm Obs}$ $\simeq$ 2.7 $\times$ 10³³ 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.

Key words: Galaxy: globular clusters: general -- Galaxy: globular clusters: individual: NGC 6388

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.