SIMBAD references

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.