An ultraluminous supersoft source with a 4 hour modulation in NGC 4631

Context. Supersoft X-ray sources (SSSs) are characterised by very low temperatures ( 99%. Besides dips observed in the light curve, the flux decreased by a factor of 3 within ∼ 10 h. The spectrum can be described with an absorbed blackbody model with kT ∼ 67 eV. The absorbed luminosity in the 0.2‐2 keV energy band was 2.7× 10 38 erg s −1 while the bolometric luminosity was a hundred time higher (3.2× 10 40 erg s −1 ), making the source one of the most luminous of its class, assuming the best fit model is correct. Conclusions. This source is another very luminous SSS for which the standard white dwarf interpretation cannot be applied, unless a strong beaming factor is considered. A stellar-mass black hole accreting at a super Eddington rate is a more likely interpretation, where the excess of accreted matter is ejected through a strong optically-thick outflow. The 4 h modulation could either b e an eclipse from the companion star or the consequence of a warped accretion disk.