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H.-G. Lee (Seoul National University, Korea), J. Rho (IPAC/CalTech), B.-C. Koo (SNU, Korea), R. Petre (NASA/GSFC), A. Decourchelle (CEA-Saclay, France)
We present ASCA and ROSAT observations of the supernova remnant (SNR) HB21. ROSAT observations show centrally brightened X-ray emission within a well-defined radio shell, and the X-ray bright emission is elongated from the northwest to the southeast where the remnant angular size is ~120\prime \times 90\prime. ASCA observations were performed toward each of two bright X-ray peaks; one in the northwest and the other in the southeast. The GIS image is globally similar to the ROSAT image, and hard emission is not detected from the remnant.
Two sets of ASCA spectra were extracted from bright interior emission, toward the northwest and the southeast. The spectra show Si and weak Mg and S lines, and no hard emission above 5 keV is detected. We simultaneously fit the GIS/SIS/ROSAT spectra for each of two regions. Both northwestern and southeastern spectra show thermal emission, but there is a slight difference between the two; while the northwestern spectrum is well fit by ionization equilibrium model, the southeastern spectrum is requiring either ionization non-equilibrium or slightly enhanced Mg and Si abundances. A one-temperature, thermal plasma model with NH = 3.5\times1021 cm-2, kT = 0.6 keV, and solar metal abundances, yielded an acceptable fit to the northwestern spectrum. The best fit for the southeastern spectrum using an ionization non-equilibrium model yielded a temperature of 0.67 keV and an ionization age of 2.5\times1013 cm-3 sec, which is a slight departure from ionization equilibrium. Our observations show HB21 has center-filled X-ray morphology and the ASCA spectra unambiguously confirmed its thermal nature. We conclude HB21 is a typical mixed-morphology SNR along with recent evidence for interaction of HB21 with molecular clouds, and is similar to other mixed-morphology SNRs such as W44 and 3C391. We discuss a few possibilities to explain the observed morphology and temperature structure of HB21 related to the models of mixed-morphology SNRs.