Chandra Observations of the Northeastern Rim of the Cygnus Loop

TL;DR

This study uses Chandra's high-resolution imaging to analyze the northeastern rim of the Cygnus Loop supernova remnant, revealing localized abundance enhancements likely originating from the interstellar medium rather than ejecta.

Contribution

First spatially resolved spectral analysis of the NE rim of the Cygnus Loop using Chandra, distinguishing between abundance-enhanced and normal regions and exploring their origins.

Findings

Abundance-enhanced region is about 200" behind the shock front.

Metal abundances are close to solar values within a factor of 2.

Abundance depletion in normal regions is not explained by non-thermal emission.

Abstract

We present results from spatially resolved spectral analyses of the northeastern (NE) rim of the Cygnus Loop supernova remnant (SNR) based on two Chandra observations. One pointing includes northern outermost abundance-enhanced regions discovered by recent Suzaku observations, while the other pointing is located on regions with "normal" abundances in the NE rim of the Cygnus Loop. The superior spatial resolving power of Chandra allows us to reveal that the abundance-enhanced region is concentrated in an about 200"-thickness region behind the shock front. We confirm absolute metal abundances (i.e., relative to H) as well as abundance ratios between metals are consistent with those of the solar values within a factor of about 2. Also, we find that the emission measure in the region gradually decreases toward the shock front. These features are in contrast with those of the ejecta fragments around the Vela SNR, which leads us to believe that the abundance enhancements are not likely due to metal-rich ejecta. We suggest that the origin of the plasma in this region is the interstellar medium (ISM). In the "normal" abundance regions, we confirm that abundances are depleted to the solar values by a factor of about 5 that is not expected in the ISM around the Cygnus Loop. Introduction of non-thermal emission in our model fitting can not naturally resolve the abundance-depletion problem. The origin of the depletion still remains as an open question.