We report the XMM–Newton detection of a moderately bright X–ray source ( F _ { 0.5 - 7 } \sim 8.2 \times 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 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 AGN .
On the other hand , the detection of excess X–ray absorption and the lack of a bright optical counterpart argue against foreground contamination .
Short–timescale 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 Ultra–Luminous X–ray source ( ULX ) detected so far with a peak luminosity of \sim 1.35 \times 10 ^ { 41 } erg s ^ { -1 } in the 0.5–7 keV band .
If confirmed by future multi–wavelength observations , the inferred bolometric luminosity ( \sim 3 \times 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 _ { \odot } 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 _ { \odot } for the accreting compact object .