2006MNRAS.373L...1M

We report the XMM-Newton detection of a moderately bright X-ray source (F_0.5–7∼ 8.2x10^–14erg/cm2/s) superimposed on the outer arms of the inactive spiral galaxy MCG-03-34-63 (z = 0.0213). It is clearly offset from the nucleus (by about 19 arcsec) but well within the D₂₅ellipse of the galaxy, just along its bar axis. The field has also been observed with the Hubble Space Telescope (HST), enabling us to compute a lower limit of >94 on the X-ray-to-optical flux ratio which, together with the X-ray spectrum of the source, argues against a background active galactic nucleus. On the other hand, the detection of excess X-ray absorption and the lack of a bright optical counterpart argue against foreground contamination. Short time-scale variability is observed, ruling out the hypothesis of a particularly powerful supernova. If it is associated with the apparent host galaxy, the source is the most powerful ultraluminous X-ray source detected so far, with a peak luminosity of ∼1.35x10⁴¹ erg/s in the 0.5-7 keV band. If confirmed by future multi-wavelength observations, the inferred bolometric luminosity (∼3x10⁴¹ erg/s) requires a rather extreme beaming factor (larger than 115) to accommodate accretion on to a stellar-mass black hole of 20M_☉and the source could instead represent one of the best intermediate-mass black hole candidate so far. If beaming is excluded, the Eddington limit implies a mass of >2300M_☉for the accreting compact object.