Infrared Echoes Reveal the Shock Breakout of the Cas a Supernova

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. In retrospect, the echoes are also clearly visible as infrared “hot spots” in IRAS images of the region. In this paper we present a detailed analysis of the geometry and spectra of the echoes, and the nature of the radiation source that that created them. We rule out a suggestion, based on early and spatially-limited data, that the echoes were generated by a recent pulse of radiation that occurred around 1950. All the echoes located within a distance of ∼ 15 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 associated with the breakout of the shock through the surface of the exploding star. The inferred luminosity of the burst depends on the amount of attenuation in the intervening medium to the clouds, and we derive a burst luminosity of ∼ 1.5 × 10 L⊙ for an assumed H-column density of 1.5 × 10 cm. The average H-column density of the IR emitting region in the echoing clouds is about 5 × 10 cm. Derivation of their density requires knowledge of the width of the echo that is sweeping though the ISM, which in Observational Cosmology Lab., Code 665; NASA Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A., e-mail: [email protected] University of Maryland Baltimore County (UMBC), Code 665, NASA Goddard Space Flight Center, Greenbelt MD, 20771, U.S.A., e-mail: [email protected]