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

Abstract

We present XMM‐Newton X-ray observations of the interacting galaxy pairs NGC 7771/7770 and NGC 2342/2341. In NGC 7771, for the first time we are able to resolve the X-ray emission into a bright central source plus two bright (L X > 10 40 erg s −1 ) ultraluminous X-ray sources (ULXs) located either end of the bar. In the bright central source (L X ∼ 10 41 erg s −1 ), the soft emission 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 (� ∼ 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 time-scales 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 and 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 � = 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.