Multi-year X-Ray Variations of Iron-K and Continuum Emissions in the Young Supernova Remnant Cassiopeia A

We found a simultaneous decrease of the Fe–K line and 4.2–6 keV continuum of Cassiopeia A with the monitoring data taken by the Chandra X-ray Observatory in 2000–2013. The flux change rates in the whole remnant are −0.65 ± 0.02% yr⁻¹ in the 4.2–6.0 keV continuum and −0.6 ± 0.1% yr⁻¹ in the Fe–K line. In the eastern region where the thermal emission is considered to dominate, the variations show the largest values: −1.03 ± 0.05% yr⁻¹ (4.2–6 keV band) and −0.6 ± 0.1% yr⁻¹ (Fe–K line). In this region, the time evolution of the emission measure and the temperature have a decreasing trend. This could be interpreted as adiabatic cooling with the expansion of m = 0.66. On the other hand, in the non-thermal emission dominated regions, variations of the 4.2–6 keV continuum show smaller rates: −0.60 ± 0.04% yr⁻¹ in the southwestern region, −0.46 ± 0.05% yr⁻¹ in the inner region, and +0.00 ± 0.07% yr⁻¹ in the forward shock region. In particular, flux does not show significant change in the forward shock region. These results imply that strong braking in shock velocity has not been occurring in Cassiopeia A (<5 km s⁻¹ yr⁻¹). All of our results support the idea that X-ray flux decay in the remnant is mainly caused by thermal components.