A Comparative Study of the Supernova Remnant Cassiopeia A from 2013--2020 Deep [Fe II]+[Si I] Images

Abstract

We present a comparative analysis of supernova remnant Cassiopeia A based on two deep, narrow-band images covering the [Fe II] 1.644um + [Si I] 1.645um lines obtained in 2013 and 2020 with the same instruments on the UKIRT 3.8m telescope. The identical setup and observing procedure allow for direct, accurate measurements of morphological and kinematic changes over a seven-year baseline. We identified 263 compact knots in the 2020 image and, through comparison with the 2013 catalog of Koo et al. 2018 (arXiv:1809.07935), classified them into quasi-stationary circumstellar knots and fast-moving knots (FMKs) of supernova ejecta. The FMKs show significant flux fluctuations, and many of those detected in 2013 are absent in the 2020 image. Proper-motion measurements derived from cross-correlation analysis indicate that most FMKs follow nearly ballistic expansion, whereas some, particularly those just beyond the eastern Fe-rich, X-ray emitting ejecta region, exhibit noticeable deceleration. The proper motions of the main ejecta shell were also measured and modeled as a uniformly expanding shell with a systemic motion, which reproduces the observed geometric and kinematic asymmetries of the remnant.

Figures (12)

• Figure 1: Top: The 2020 deep [Fe2]+[Si1] image of Cas A obtained from the UKIRT 3.8 m telescope. North is up, and east is to the left. Stellar sources have been removed. The gray scale varies linearly from $-2\times10^{-18}$ to $2\times10^{-17}$ erg cm$^{-2}$ s$^{-1}$ pixel$^{-1}$. QSFs and FMKs are marked by magenta and cyan contours, respectively. Yellow contours mark the radio boundary of the SNR from the VLA 6 cm image with a threshold level 0.3 mJy beam$^{-1}$delaney04. The red star represents the explosion center at $(\alpha, \delta)_{\rm J2000} =$$(23^{\rm h}23^{\rm m}27\fs77,\ +58\arcdeg48\arcmin49\farcs4)$thorstensen01 Bottom: Image difference created by subtracting the 2013 deep [Fe2]+[Si1] image from the 2020 deep [Fe2]+[Si1] image. The gray scale varies linearly from $-2\times10^{-17}$ to $2\times10^{-17}$ erg cm$^{-2}$ s$^{-1}$ pixel$^{-1}$.
• Figure 2: Left: A revised finding chart of QSFs. QSFs colored blue indicate darkening in 2020, and red indicates brightening. The blue contour shows the main ejecta shell in 2020 deep [Fe2]+[Si1] image. The red star represents the explosion center. Right: Example of QSFs with significant flux change. From left to right, the images correspond to the 2022 JWST image, the 2020 UKIRT image, and the 2013 UKIRT image. Knot numbers are shown on the left, and the red circle has a radius of 1.
• Figure 3: (a) A comparison plot of QSF flux between 2013 and 2020. (b) The same as (a) but for FMKs. The gray area indicates the 1$\sigma$ confidence band of unchanged synthetic knots for reference. Knot numbers with flux variations exceeding a factor of 4 are labeled.
• Figure 4: Magnified image of the NE jet region. On the left is the 2013 image, while on the right is the 2020 image. Both images are displayed in grayscale with an intensity range from $0$ to $2 \times 10^{-17}$ erg cm$^{-2}$ s$^{-1}$ pixel$^{-1}$ , and red arrows mark knots that appear bright in only one of the two epochs.
• Figure 5: Left: Distribution of the identified FMKs in the 2020 deep [Fe2]+[Si1] image. Red contours indicate the positions of the FMKs. North is up, and east is to the left. The dark yellow contour marks the approximate boundary of the SNR in radio in 2003 delaney04. Right: Same FMKs overlaid on the Chandra X‐ray image in the 6.52--6.94 keV band. The FMKs are divided into four groups for reference.