AAS Meeting #193 - Austin, Texas, January 1999
Session 74. Supernova Remnants
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [74.01] ASCA observations of the supernova remnant HB 21

J. Rho (UCSB), A. Decourchelle (CEA-Saclay, France), R. Petre (NASA/GSFC)

We report preliminary results of ASCA observations of the mature supernova remnant HB21. While ROSAT observations showed the X-ray emission is center-filled, the PSPC spectra could not distinguish whether the emission is from shock-heated plasma or synchrotron radiation. Our ASCA spectra reveal that the emission is primarily thermal, with strong Ne, Si, and S lines and weak Mg and Ar lines. The best-fit thermal model yields a line of sight absorption of NH~3\times1021 cm-2, and a temperature kT~0.6 keV. No emission above 6 keV was detected from HB21. The ASCA image confirms that the X-ray morphology is centrally concentrated and the emission smoothly filled within the well-defined radio shell. The center-filled X-ray and shell-like radio morphology, and thermal X-ray spectrum, show that HB21 belongs to the newly-classified mixed-morphology supernova remnants'' (Rho & Petre 1998, ApJL, 503, 167). The fact that this remnant is known to be interacting with a molecular cloud is a common characteristic of mixed-morphology supernova remnants, suggesting that HB 21 also evolved in a dense interstellar medium.