Have we detected the most luminous ULX so far?

Abstract

abstract We report the XMM–Newton detection 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 arcsec) but well within the D25 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 × 1041 erg s−1 in the 0.5–7 keV band. If confirmed by future multi-wavelength observations, the inferred bolometric luminosity (∼3 × 1041 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.