Mixed-morphology supernova remnants in x-rays: isothermal plasma in HB21 and probable oxygen-rich ejecta in CTB 1

Abstract

We present an analysis of X-ray observations of the Galactic supernova remnants (SNRs) HB21 (G89.0+4.7) and CTB 1 (G116.9+0.2), two well-known members of the class of mixed-morphology (MM) SNRs. Our analysis draws upon observations of both SNRs made with the Advanced Satellite for Cosmology and Astrophysics (ASCA): we have also used an archived Chandra observation of CTB 1 as part of this effort. We find a marked contrast between the X-ray properties of HB21 and CTB 1: in the case of HB21, the extracted spectra of the northwest and southeast regions of the X-ray-emitting plasma associated with the SNR can be fit with a single thermal model with marginally enhanced silicon and sulfur abundances. For both of these regions, the derived column density and temperature are N_H ~ 0.3 × 10^(22) cm^(−2) and kT ~ 0.7 keV, respectively. No significant spatial differences in temperature or elemental abundances between the two regions are detected and the X-ray-emitting plasma for both regions is close to ionization equilibrium. Our Chandra spectral analysis of CTB 1 reveals that this source is likely an oxygen-rich SNR with enhanced abundances of oxygen and neon: this result is quite surprising for an evolved SNR like CTB 1. The high angular resolution Chandra observation of CTB 1 reveals spectral variations across this SNR: in particular, we have detected localized hard emission with an angular extent of ~1'. The extracted ASCA spectra for both the southwest and northeastern regions of CTB 1 cannot be fit with a single thermal component and instead an additional component is required to account for the presence of excess emission seen at higher energies. Based on our fits to the extracted ASCA spectra, we derive a column density NH ~ 0.6 × 10^(22) cm^(−2) and a temperature for the soft thermal component of kT_(soft) ~ 0.28 keV for both regions. The hard emission from the southwest region may be modeled with either a thermal component with a temperature kT_(hard) ~ 3 keV or by a power-law component with a photon index Γ ~ 2–3; for the northeast region, the hard emission may be modeled with a power-law component with a photon index Γ = 1.4. The detection of center-filled ejecta-dominated X-ray emission from HB21 and CTB 1 as well as other MM SNRs suggests a new scenario for the origin of the contrasting X-ray and radio morphologies of this class of sources. Lastly, we have analyzed the properties of the discrete hard X-ray source 1WGA J0001.4+6229 which is seen in projection just inside the northeastern shell of CTB 1. Our extracted ASCA GIS spectra of this source are best fit using a power-law model with a photon index Γ = 2.2^(+0.5)_(−1.2): this slope is typical for featureless power-law continua produced by rotation-powered pulsars. This source may be a neutron star associated with CTB 1. We find marginal evidence for X-ray pulsations from this source with a period of 47.6154 ms. A deep radio observation of this source failed to reveal any pulsations.

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