Swift Ultraviolet Observations of Supernova 2014J in M82: Large Extinction from Interstellar Dust

TL;DR

This study uses Swift UV and optical data to analyze the extinction law of supernova 2014J, concluding that interstellar dust causes the high reddening and that the reddening law depends on dust geometry and properties.

Contribution

It demonstrates that the high reddening of SN 2014J is due to interstellar dust and explores how dust geometry affects the reddening law, challenging assumptions of circumstellar scattering.

Findings

Reddening law with R_V~1.4 consistent with interstellar dust

Light curve evolution shows no flattening from scattered photons

Reddening law varies with dust geometry, density, and epoch

Abstract

We present optical and ultraviolet (UV) photometry and spectra of the very nearby and highly reddened supernova (SN) 2014J in M82 obtained with the Swift Ultra-Violet/Optical Telescope (UVOT). Comparison of the UVOT grism spectra of SN~2014J with Hubble Space Telescope observations of SN2011fe or UVOT grism spectra of SN~2012fr are consistent with an extinction law with a low value of R_V~1.4. The high reddening causes the detected photon distribution in the broadband UV filters to have a much longer effective wavelength than for an unreddened SN. The light curve evolution is consistent with this shift and does not show a flattening due to photons being scattered back into the line of sight. The light curve shapes and color evolution are inconsistent with a contribution scattered into the line of sight by circumstellar dust. We conclude that most or all of the high reddening must come from interstellar dust. We show that even for a single dust composition, there is not a unique reddening law caused by circumstellar scattering. Rather, when considering scattering from a time-variable source, we confirm earlier studies that the reddening law is a function of the dust geometry, column density, and epoch. We also show how an assumed geometry of dust as a foreground sheet in mixed stellar/dust systems will lead to a higher inferred R_V. Rather than assuming the dust around SNe is peculiar, SNe may be useful probes of the interstellar reddening laws in other galaxies.