Have we detected the most luminous ULX so far

We report the XMM‐Newtondetection of a moderately bright X‐ray source (F0.5−7 � 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”) but well within the D25 ellipse of the galaxy, just along its bar axis. The field has al so 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 AGN. On the other hand, the detection of excess X‐ray absorption and the lack of a bright optical counterpart argue against foregrou nd contamination. Short‐timescale variability is observed, ruling out the hypothesis of a part icularly powerful supernova. If it is associated with the apparent host galaxy, the source is the most powerful Ultra‐Luminous X‐ ray source (ULX) 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 observat ions, the inferred bolometric luminosity (� 3 × 10 41 erg s −1 ) requires a rather extreme beaming factor (larger than 115) to accommodate accretion onto a stellar‐mass black hole of 20 M⊙ and the source could represent instead 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.