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We studied the colliding galaxy NGC 7714 with two XMM-Newton observations,six months apart. The galaxy contains two bright X-ray sources: we show thatthey have different physical nature. The off-nuclear source is an accretingcompact object, one of the brightest ultraluminous X-ray sources (ULXs) foundto date. It showed spectral and luminosity changes between the twoobservations, from a low/soft to a high/hard state; in the high state, itreached L_x ~ 6 x 10^{40} erg/s. Its lightcurve in the high state suggestsvariability on a ~ 2-hr timescale. Its peculiar location, where the tidalbridge between NGC 7714 and NGC 7715 joins the outer stellar ring of NGC 7714,makes it an interesting example of the connection between gas flows incolliding galaxies and ULX formation. The nuclear source (L_x ~ 10^{41} erg/s)coincides with a starburst region, and is the combination of thin thermalplasma emission and a point-source contribution (with a power-law spectrum).Variability in the power-law component between the two observations hints atthe presence of a single, bright point source (L_x >~ 3 x 10^{40} erg/s):either a hidden AGN or another ULX.Comment: submitted to A&A, 10 pages (2 MB

A variable ultra-luminous X-ray source in the colliding galaxy NGC 7714