Newton observations of the interacting galaxy pairs NGC 7771 / 0 and NGC 2342 / 1

We present XMM-Newton X-ray observations of the interacting galaxy pairs NGC 7771/7770 & NGC 2342/2341. In NGC 7771, for the first time we are able to resolve the X-ray emission into a bright central source (L X ∼ 10 41 erg s −1), plus two bright (L X > 10 40 erg s −1) ultraluminous X-ray sources (ULXs) located either end of the bar. The soft emission of the central source is well-modelled by a two-temperature thermal plasma with kT =0.4/0.7 keV. The hard emission is modelled with a flat absorbed power-law (Γ ∼ 1.7, N H ∼ 10 22 cm −2), and this together with a low-significance (1.7σ) ∼ 300 eV equivalent width emission line at ∼6 keV are the first indications that NGC 7771 may host a low-luminosity AGN. For the bar ULXs, a power-law fit to X-1 is improved at the 2.5σ level with the addition of a thermal plasma component (kT ∼ 0.3 keV), while X-2 is improved only at the 1.3σ level with the addition of a disc blackbody component with T in ∼ 0.2 keV. Both sources are variable on short timescales implying that their emission is dominated by single accreting X-ray binaries (XRBs). The three remaining galaxies, NGC 7770, NGC 2342 and NGC 2341, have observed X-ray luminosities of 0.2, 1.8 & 0.9×10 41 erg s −1 respectively (0.3–10 keV). Their integrated spectra are also well-modelled by multi-temperature thermal plasma components with kT =0.2–0.7 keV, plus power-law continua with slopes of Γ=1.8– 2.3 that are likely to represent the integrated emission of populations of XRBs as observed in other nearby merger systems. A comparison with other isolated, interacting and merging systems shows that all four galaxies follow the established correlations for starburst galaxies between X-ray, far-infrared and radio luminosities, demonstrating that their X-ray outputs are dominated by their starburst components.