We present the analysis of an IRS 5-38 \mu m spectrum and MIPS photometric measurements of an infrared echo near the ( catalog Cassiopeia A ) supernova remnant observed with the Spitzer Space Telescope .
We have modeled the recorded echo accounting for PAHs , quantum-heated carbon and silicate grains , as well as thermal carbon and silicate particles .
Using the fact that optical light echo spectroscopy has established that Cas A originated from a type IIb supernova explosion showing an optical spectrum remarkably similar to the prototypical type IIb ( catalog SN~1993J ) , we use the latter to construct template data input for our simulations .
We are then able to reproduce the recorded infrared echo spectrum by combining the emission of dust heated by the UV burst produced at the shock breakout after the core-collapse and dust heated by optical light emitted near the visual maximum of the supernova light curve , where the UV burst and optical light curve characteristics are based on SN 1993J .
We find a mean density of \sim 680 H cm ^ { -3 } for the echo region , with a size of a few light years across .
We also find evidence of dust processing in the form of a lack of small PAHs with less than \sim 300 carbon atoms , consistent with a scenario of PAHs destruction by the UV burst via photodissociation at the estimated distance of the echo region from Cas A .
Furthermore , our simulations suggest that the weak 11 \mu m features of our recorded infrared echo spectrum are consistent with a strong dehydrogenated state of the PAHs .
This exploratory study highlights the potential of investigating dust processing in the interstellar medium through infrared echoes .