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 spir al galaxy NGC 6946 are presented. The ASCA image is the first observation o f this galaxy above similar to 2 keV. Diffuse emission may be present in th e inner similar to 4' extending to energies above similar to 2-3 keV. In th e HRI data, 14 pointlike sources are detected, the brightest two being a so urce very close to the nucleus and a source to the northeast that correspon ds to a luminous complex of interacting supernova remnants (SNRs). We detec t a point source that lies similar to 30 " west of the SNR complex but with a luminosity similar to 1/15 of the SNR complex. None of the point sources show evidence of strong variability; weak variability would escape our det ection. The ASCA spectrum of the SNR complex shows evidence for an emission line at similar to 0.9 keV that could be either Ne nr at similar to 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 p lausible given the observed spectrum below 3 keV. Adopting this latter mode l, 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 extrao rdinary X-ray luminosity may be related more to the high density of the sur rounding medium than to a small but intense interaction region where two of the complex's SNRs are apparently colliding.