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

Abstract

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 that they have different physical nature. The off-nuclear source is an accreting compact object, one of the brightest ultraluminous X-ray sources (ULXs) found to date. It showed spectral and luminosity changes between the two observations, from a low/soft to a high/hard state; in the high state, it reached L_x ~ 6 x 10^{40} erg/s. Its lightcurve in the high state suggests variability on a ~ 2-hr timescale. Its peculiar location, where the tidal bridge 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 in colliding galaxies and ULX formation. The nuclear source (L_x ~ 10^{41} erg/s) coincides with a starburst region, and is the combination of thin thermal plasma emission and a point-source contribution (with a power-law spectrum). Variability in the power-law component between the two observations hints at the presence of a single, bright point source (L_x >~ 3 x 10^{40} erg/s): either a hidden AGN or another ULX.