Spectral Index Distribution of Various Scale Components in Supernova Remnant Cassiopeia A

Abstract

Cassiopeia A (Cas A) is a well-known candidate for studying cosmic-ray acceleration, in which compact features of various scales have attracted much attention. Based on observations by the Very Large Array of Cas A at 6 cm and 21 cm, we measure the spectral index distribution of various scale components using the observation of the 1998 epoch. We decompose its total density image into nine scale components, and map the temperature spectral index distribution of each component, which ranges from −2.48 ± 0.01 to −2.91 ± 0.05. We find that the spectral indices increase from the small scale to large scale components. A damped post-shock magnetic field model with a strength larger than ∼200 μG and a damping length scale less than ∼10% of the remnant radius can account for the spectral index variation naturally.