Asymmetric expansion of the Fe ejecta in Kepler's supernova remnant

The ejecta kinematics of supernova remnants (SNRs) is one of the crucial clues to understanding the explosion mechanism of type Ia supernovae (SNe). In particular, the kinematic asymmetry of iron-peak elements provides the key to understanding the physical processes taking place in the core of the exploding white dwarfs (WDs), although it has been poorly understood by observations. In this paper, we show for the first time the asymmetric expansion structure in the line-of-sight direction of Fe ejecta in Kepler's SNR revealed by spectral and imaging analysis using the Chandra archival data. We found that the Kα line centroid energy and line width is relatively lower (<6.4 keV) and narrower (∼80 eV) around the center of the remnant, which implies that the majority of the Fe ejecta in the central region is redshifted. At the outer regions, we identify bright blueshifted structures as have been ejected as high-velocity dense clumps. Taking into account the broad population of the Fe charge states, we estimate the redshifted velocity of ∼2000 km s^-1 and the blueshifted velocity of ∼3000 km s^-1 for each velocity structure. We also present the possibility that a portion of the Fe ejecta near the center are interacting with the dense circumstellar medium (CSM) on the near side of the remnant. For the origin of the asymmetric motion of the Fe ejecta, we suggest three scenarios; (1) the asymmetric distribution of the CSM, (2) the "shadow" in Fe cast by the companion star, and (3) the asymmetric explosion.