We present the discovery of a light echo from SN 2007af, a normal Type Ia supernova (SN Ia)
in NGC 5584. Hubble Space Telescope (HST) images taken three years post explosion reveal two
separate echoes; an outer echo and extended central region, which we propose as an unresolved
inner echo. Multiple images were obtained in the F160W, F350LP, F555W, and F814W using the
Wide Field Camera 3. If the outer echo is produced by an interstellar dust sheet perpendicular
to the line of sight, it is located ∼800 pc in front of the SN. The dust for the inner echo is 0.45 pc
< d < 90 pc away from the SN. The inner echo color is consistent with typical interstellar dust
wavelength-dependent scattering cross-sections, while the outer echo color does not match the
predictions. Both dust sheets, if in the foreground, are optically thin for scattering, with the outer
echo sheet thickness consistent with the inferred extinction from peak brightness. Whether the
inner echo is from interstellar or circumstellar dust is ambiguous. Overall, the echo characteristics
are quite similar to previously observed SN Ia echoes.
Authors: D. Drozdov, M. D. Leising, P. A. Milne, J. Pearcy, A. G. Riess, L. M. Macri, G. L. Bryngelson, P. M. Garnavich