Detection of a Light Echo from the Otherwise Normal SN 2007af

TL;DR

This paper reports the discovery of two light echoes from SN 2007af, a Type Ia supernova, revealing details about the surrounding interstellar dust and echo geometry using HST imaging.

Contribution

It provides the first detailed observation of multiple light echoes from SN 2007af, including their locations, colors, and implications for dust distribution.

Findings

Outer echo is ~800 pc in front of the SN

Inner echo is 0.45 to 90 pc from the SN

Echo characteristics resemble previous SN Ia echoes

Abstract

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.