Discovery of a Scattered-Light Echo around SN 2016adj

ATel #8890; Ben Sugerman (Goucher College), Stephen Lawrence (Hofstra University)
on 1 Apr 2016; 02:36 UT
Credential Certification: Ben Sugerman (ben.sugerman@gmail.com)

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SN 2016adj was imaged with the Hubble Space Telescope (HST) WFC3 UVIS camera on UT 2016 Mar 19.97-20.10 as part of DD-14487 (P.I. Sugerman). A four-point sub-pixel dither pattern was used to image the SN in F438W, F547M, and F814W with total exposure times of 1400 s, 1600 s, and 1320 s, respectively. Pipeline calibrated images were combined using AstroDrizzle to an enhanced resolution of 0.03 arcsec/pix. Empirical point-spread functions (PSF) were used to measure the photometry of the supernova, yielding approximate magnitudes of B=20.25 +/- 0.04, V=17.40 +/- 0.04, and I_c=13.68 +/- 0.15. These uncertainties do not include errors from converting HST fluxes to Johnson/Cousins magnitudes; including these errors, we estimate all B and V photometry to be +/- 0.1 mag. Subtracting away the PSF in the F438W and F547M images reveals a nearly complete ring 0.18 arcsec from the SN; a clean PSF subtraction was not possible in the F814W images. This ring has an average surface brightness of 20.1 mag/arcsec^2 in B and 18.9 mag/arcsec^2 in V, and integrated fluxes of B=22.4 and V=21.2 mags. Near discovery, Kiyota et al. (ATEL #8654) reported the SN photometry as B=17.3 and V=15.2, while Stritzinger et al. (ATEL #8657) reported B=17.40 and V=15.70. This ring is therefore 5-6 mags fainter, and 0.5-0.9 mags bluer, than the SN at very early times. We propose that the ring is an echo of the early SN light pulse scattered off of pre-existing interstellar material along the line of sight. Adopting a distance of 3.42 Mpc (Ferrarese et al. 2007, ApJ, 654, 186), the ring is located 2.9 pc from the SN on the plane of the sky. SN 2016adj was discovered about 40 days before these observations (Marples et al., ATEL #8651), and the epoch of maximum light has been estimated as one week earlier by Stritzinger et al. This allows us to calculate that the ring material is located 105-125 pc in front of the SN. With this geometry, the color shift is consistent with the scattering properties of Galactic dust (Mathis et al., 1977, ApJ, 217, 425) although smaller grains cannot be ruled out (Sugerman, 2003, AJ, 126, 1939). There may also be a brighter echo 0.08 arcsec from the SN, however since this region may be confused with PSF subtraction residuals, we await confirmation from forthcoming observations as the SN continues to fade.