Supernova Remnant Cassiopeia A

Abstract

Cassiopeia A (Cas A) is one of the youngest supernova remnants (SNRs) in the Milky Way with the estimated date of the supernova (SN) explosion being AD 1681 ± 19. The light ‘echo’ scattered from the flash of the SN explosion has been detected after ∼ 330 years (i.e., 2008), which confirmed that Cas A is a remnant of Type IIb SN, which probably resulted from the explosion of a massive (\(\gtrsim 15\ M_{\odot }\)) star. It is also relatively nearby, that is, at 3.4 kpc. Cas A, therefore, has been the subject of extensive studies since its discovery, revealing to us the great details of physical and chemical processes in massive star explosions. There is, however, no unambiguous historical record of the SN event, which remains as a puzzle. Here we first explore the Cas A SNR in its astrophysical context: its physical and spectral properties over all wavelengths, its dynamical structure and evolution, and its connection to its supernova type and its progenitor. We also survey the date and position of the SN event estimated from proper motions. We then describe historical records that could be possibly related to the Cas A SN event and discuss hypotheses to explain why clear observations of the ‘historical’ SN explosion are missing.