Speaker
Description
The core-collapse supernova remnant, Cassiopeia A (Cas A), is the youngest galactic remnant (~350 yrs) and is amongst the closest known (~ 3.4 kpc). Moreover, it has a secure supernova classification using light echo spectroscopy analysis. These make it an ideal source for studying supernova explosion mechanisms and progenitor systems. The recent James Webb Space Telescope (JWST) survey using its MIRI instrument has revealed a large structure in the interior region of Cas A, referred to as the "Green Monster''. Although its central location suggests an ejecta association, the infrared properties of the “Green Monster” hint at a circumstellar medium (CSM) origin. We present the analysis of X-ray properties of emission of this region using the archival Chandra X-ray Observatory data. We extracted spectra along the “Green Monster” as well as from shocked CSM regions. Both the extracted spectra and a principal component analysis show that the “Green Monster” emission properties are similar to those of the shocked CSM. We employ a Bayesian scheme to fit the spectra with a model consisting of a combination of a non-equilibrium-ionization model and a power-law component, modified by Galactic absorption. All the “Green Monster” spectra show a blueshift corresponding to a radial velocity of around −2300 km s−1, suggesting that the structure is on the near side of Cas A. The ionization age is around net ≈ 1.5 × 10^11 cm−3 s. This translates into a pre-shock density of ~ 12 cm−3, higher than previous estimates of the unshocked CSM. The relatively high net and relatively low radial velocity suggest that this structure has a relatively high density compared to other shocked CSM plasma. The infrared and X-ray properties of the "Green Monster" provide yet another piece of evidence that the CSM around Cas A’s progenitor was not that of a smooth steady wind profile. This will help to reconstruct the mass-loss properties of the progenitor star.
