Observations of infrared echoes around Cassiopeia A with WISE

Context. Light echoes of supernovae have proven to be a viable tool for characterizing both the supernova explosion mechanism and the surrounding circumstellar and interstellar medium. Aims. We explore new possibilities for identifying infrared echoes with WISE data and extend the area of known echoes around the supernova remnant Cassiopeia A in different illumination conditions. Methods. Preliminary release WISE data around Cassiopeia A is analyzed and compared to Spitzer-MIPS data. While Spitzer-WISE difference images directly show evidence of infrared light echoes, they can also be identified according to their 22/12 mu m flux ratio. Results. We find that WISE allows the detection of previously unknown infrared echoes in various illumination conditions. For the first time, infrared echoes at distances greater than 800 pc from a supernova remnant have been found. The 22/12 mu m flux ratio criterion can be used as a conceptual factor for identifying infrared echoes around other supernovae as well as providing insights into the dust processing conditions of the interstellar medium.