Hint for a faint intermediate mass black hole in the Ursa Minor dwarf galaxy

Abstract

We report the results of the analysis of an archive Chandra observation of the Ursa Minor spheroidal galaxy, one of the closest Milky Way satellites, searching for signatures from the intermediate mass black hole possibly hosted in the center of the galaxy. We identified an X-ray source with a detection confidence as low as ≃2.5σ and with an estimated unabsorbed flux in the 0.5–7keV band of ≃4.9×10-15ergs−1cm−2 and at a few arcseconds from the reported center of the galaxy. The source is spatially coincident with a radio object (having flux density of ≃7.1mJy at 1.4GHz) already observed in the NRAO VLA Sky Survey. In the accreting black hole scenario, depending on the used fundamental plane relation, one estimates an accretor mass of (2.9-2.7+33.6)×106M⊙ or (11.7-9.7+57.1)×106M⊙. Relaxing the assumption for a flat radio spectrum, the minimum black hole mass would result in the range 5×104M⊙-5×105M⊙, i.e. still consistent with an intermediate mass black hole scenario. The compact object seems to radiate at a very tiny fraction of the associated Eddington luminosity.