Have we detected the most luminous ULX so far?

We report the XMM–Newton detection of a moderately bright X‐ray source ( F 0.5−7 ∼ 8.2 × 10 −14  erg cm −2  s −1 ) 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 D 25 ellipse 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 ∼1.35 × 10 41 erg s −1 in the 0.5–7 keV band. If confirmed by future multi‐wavelength observations, the inferred bolometric luminosity (∼3 × 10 41 erg s −1 ) requires a rather extreme beaming factor (larger than 115) to accommodate accretion on to a stellar‐mass black hole of 20 M ⊙ 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 >2300 M ⊙ for the accreting compact object.