1994A&A...284..573A

The supernova remnant IC443 and the surrounding field have been imaged for the first time in X-rays during the ROSAT all-sky survey. Radio and infrared observations had suggested that IC443 consists of three connected, partially incomplete subshells. Whereas two of the three sub-shells have been known in X-rays before, the full extent and the structure of the third sub-shell have been uncovered by ROSAT. Unlike in the radio range, the X-ray emission region is not restricted to a small shell fragment but it forms a complete shell, the western section of which overlaps with the other two bright sub-shells of IC443. Both the morphology and the very soft X-ray spectrum suggest that this shell is a separate ∼10⁵ yr old supernova remnant, which we call G189.6+3.3. The age of IC443, on the other hand, has been estimated to ∼10³ yr. This is supported by the joint analysis of the ROSAT and Ginga energy spectra, which shows that high energy X-rays (>2keV) are emitted only from the smaller two shells (IC443) but not from the large shell of G189.6+3.3. Spatially resolved spectroscopy of a ROSAT pointed observation of the IC443 field reveals that G189.6+3.3 and IC443 are separated along the line of sight by a well-known CO molecular cloud. Only the IC443 spectrum is affected by low energy X-ray absorption in the molecular cloud. The spectral analysis shows also that the previously known lane-like structure of reduced X-ray surface brightness across IC443 is due to absorption of the background X-rays from IC443 by a dense shell of cold matter associated with G189.6+3.3 in the foreground.