Geometric distance measurement using light echo polarization

By imaging the polarization distribution of a light echo and finding the angular size of the ring of maximum polarization, the geometric distance to the echo may be inferred. The technique may be used for extragalactic geometric distance measurement with observations of supernova light echoes. We have obtained such observations using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The recent spectacular light echo around the Galactic nova V838 Mon has also been observed with ACS and is providing valuable insights into the technique as well as an estimate of the distance of that object.