  (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.
