Infrared Echoes Reveal the Shock Breakout of the Cas A Supernova
  (Condensed form) - The serendipitous discovery of infrared echoes around the supernova remnant of Cas A by the Spitzer satellite has provided astronomers with a unique opportunity to study the properties of the echoing material and the history and nature of the outburst that generated these echoes.   All the echoes located within a distance of ~15 arcmin from the SN are caused by the delayed arrival of thermal emission from dust located at a distance of 160 lyr (corresponding to half the adopted age of the remnant) directly behind the origin of the explosion. The spectra of the echoes are distinct from that of the general diffuse interstellar medium (ISM) revealing hot silicate grains that are either stochastically heated to temperatures in excess of ~150 K, or radiating at an equilibrium temperature of this value. We show that the optical light curve from the supernova, is not capable of producing such spectra, and could therefore not have given rise to the echoes. Instead, we find that the echoes were generated by an intense and short burst of EUV-UV radiation with a luminosity of ~ 1.5E11 Lsun. The average H-column density of the IR emitting region in the echoing clouds is about 5E17 cm-2. Taking a burst time of ~1 d gives a cloud density of ~200 cm-3, typical of dense IR cirrus. 
Eli Dwek and Richard G. Arendt