An XMM-Newton search for X-ray sources in the Fornax dwarf galaxy

TL;DR

This study used deep XMM-Newton observations to analyze X-ray sources in the Fornax dwarf galaxy, identifying potential associations with stars and globular clusters, and constraining the properties of a possible intermediate-mass black hole.

Contribution

First comprehensive X-ray source catalog for the Fornax galaxy, including potential black hole constraints and associations with stellar objects.

Findings

Detected X-ray sources possibly linked to galaxy's stars and clusters.

Placed upper limits on accretion efficiency of a potential central black hole.

Identified candidate sources for further follow-up studies.

Abstract

We report the results of a deep archive XMM-Newton observation of the Fornax spheroidal galaxy that we analyzed with the aim of fully characterizing the X-ray source population (in most of the cases likely to be background active galactic nuclei) detected towards the target. A cross correlation with the available databases allowed us to find a source that may be associated with a variable star belonging to the galaxy. We also searched for X-ray sources in the vicinity of the Fornax globular clusters GC 3 and GC 4 and found two sources probably associated with the respective clusters. The deep X-ray observation was also suitable for the search of the intermediate-mass black hole (of mass $\simeq 10^{4}$ M$_{\odot}$) expected to be hosted in the center of the galaxy. In the case of Fornax, this search is extremely difficult since the galaxy centroid of gravity is poorly constrained because of the large asymmetry observed in the optical surface brightness. Since we cannot firmly establish the existence of an X-ray counterpart of the putative black hole, we put constraints only on the accretion parameters. In particular, we found that the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is as low as a few $10^{-5}$.