Einstein X-ray observations of the supernova remnant HB 21.

In radio, HB21 is a large (≡2°diameter) supernova remnant. It has been observed with incomplete coverage by five Einstein IPC fields. Here is reported the first detection of X-ray emission from HB21. The 0.2 - 4 keV X-ray image of HB21 does not correlate well with radio maps. HB21 has an X-ray temperature of 7(+6, -2)×10⁶K and a luminosity of 1.9(+7.6, -1.2)×10³⁴erg s^-1. A Sedov model applied to HB21 gives too large a temperature and shock velocity. Supernova remnant models for expansion into a 3-component ISM can fit HB21, from which an age of 8000 - 15000 yr is inferred.