ROSAT HRI and ASCA Observations of the Spiral Galaxy NGC 6946 and Its Northeast Complex of Luminous Supernova Remnants

Analysis of 80 ks ASCA and 60 ks ROSAT HRI observations of the face-on spiral galaxy NGC 6946 are presented. The ASCA image is the first observation of this galaxy above ∼2 keV. Diffuse emission may be present in the inner ∼4′ extending to energies above ∼2–3 keV. In the HRI data, 14 pointlike sources are detected, the brightest two being a source very close to the nucleus and a source to the northeast that corresponds to a luminous complex of interacting supernova remnants (SNRs). We detect a point source that lies ∼30″ west of the SNR complex but with a luminosity ∼1/15 of the SNR complex. None of the point sources show evidence of strong variability; weak variability would escape our detection. The ASCA spectrum of the SNR complex shows evidence for an emission line at ∼0.9 keV that could be either Ne IX at ∼0.915 keV or a blend of ion stages of Fe L-shell emission if the continuum is fitted with a power law. However, a two-component, Raymond-Smith thermal spectrum with no lines gives an equally valid continuum fit and may be more physically plausible given the observed spectrum below 3 keV. Adopting this latter model, we derive a density for the SNR complex of 10–35 cm-3, consistent with estimates inferred from optical emission-line ratios. The complex's extraordinary X-ray luminosity may be related more to the high density of the surrounding medium than to a small but intense interaction region where two of the complex's SNRs are apparently colliding.