A XMM-Newton observation of a sample of four close dwarf spheroidal galaxies

Abstract

We present the results of the analysis of deep archival <it>XMM</it>-Newton observations towards the dwarf spheroidal galaxies Draco, Leo I, Ursa Major II (UMa II) and Ursa Minor (UMi) in the Milky Way neighbourhood. The X-ray source population is characterized and cross-correlated with available databases to infer their nature. We also investigate if intermediate-mass black holes are hosted in the centre of these galaxies. For Draco, we detect 96 high-energy sources, two of them possibly being local stars, while no evidence for any X-ray emitting central compact object is found. Towards the Leo I and UMa II fields of view, we reveal 116 and 49 X-ray sources, respectively. None of them correlates with the putative central black holes and only one is likely associated with a UMa II local source. The study of the UMi dwarf galaxy found 54 high-energy sources and a possible association with a source at the dwarf spheroidal galaxy centre. We put an upper limit on the luminosity of the central compact object of 4.02 × 1033 erg s−1. Furthermore, via the correlation with a radio source near the galactic centre, the putative black hole should have a mass of <f>$\\left(2.76^{+32.00}_{-2.54}\\right)\\times 10^6\\,\\mathrm{M}_{{{\\odot }}}$</f> and be radiatively inefficient. This confirms a previous result obtained using <it>Chandra</it> data alone.