Abstract
We present an analysis of the ROSAT observations of the massive spiral galaxy NGC 1961 and surrounding small group of spiral galaxies. Contrary to the earlier Einstein observation which suggested that the X-ray emission comes mainly from an arc-shaped region on one side of NGC 1961, the ROSAT observations show that the X-ray emission is fairly typical for a spiral galaxy and comes mainly from the starburst nucleus with some contribution from the surrounding spiral arm regions. The total X-ray flux of 5.1 × 1040 ergs s-1 is normal, but the spectrum has an unusually narrow peak at 0.9 keV that can only be fit by models that have high absorption and low characteristic temperatures. A nearby bright X-ray source (LX = 1.7 × 1040 ergs s-1) at a projected distance of 44 kpc from NGC 1961 has no optical or radio counterpart and may be an example of the rare high-luminosity X-ray sources seen in a few other galaxies. There is no detectable diffuse X-ray emission in this group of spiral galaxies, but this is not unexpected because recent statistical studies show that groups with a hot intragroup medium (IGM) tend to have a higher velocity dispersion and contain a large fraction of elliptical and S0 galaxies. If there is a substantial IGM in this group, as is suggested by the head-tail distribution of H I gas around NGC 1961, then it must be cooler than ≈ 0.3 keV and thus invisible to the ROSAT detectors.
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