On the Time Variation of Dust Extinction and Gas Absorption for Type Ia Supernovae Observed through a Nonuniform Interstellar Medium

Abstract

For Type Ia supernovae (SNe Ia) observed through a non-uniform interstellar medium (ISM) in its host galaxy, we investigate whether the non-uniformity can cause observable time variations in dust extinction and in gas absorption due to the expansion of the SN photosphere with time. We show that, owing to the steep spectral index of the ISM density power spectrum, sizable density fluctuation amplitudes at the length scale of typical ISM structures ( 10 pc) will translate to much smaller fluctuations on the scales of a SN photosphere. Therefore the typical amplitude of time variation due to non-uniform ISM, of absorption equivalent widths and of extinction, would be small. As a result, we conclude that non-uniform ISM density should not impact cosmology measurements based on SNe Ia. We apply our predictions based on the ISM density power law power spectrum to the observations of two highly reddened SNe Ia, SN 2012cu and SN 2014J.

Shedding light on the Type Ia supernova extinction puzzle: dust location found

“…We note that -with the exception of SN 2006X -all the detection of time-variable absorption features in the literature are also compatible with geometric effects. In particular, the observed changes in the equivalent widths of these lines are of the order of those induced by the SN photosphere expanding in a fractal patchy IS medium ( 50 m Å for Na I D, Patat et al 2010;Huang et al 2017). If attributed to geometric effects, changes observed in SN 2007le and SN 2014J would be qualitatively consistent with our findings of material at 5.6 and 44.6 pc, respectively.…”

“…As discussed in Section 5.1, the observed timevariabilities in Na and K features can only be explained in the IS framework by invoking some geometric effects. While most of the variations seen in SNe Ia are compatible with the projected SN photosphere expanding in a fractal patchy IS medium (Patat et al 2010;Huang et al 2017), those observed in SN 2006X would require a highly non-homogeneous IS medium (see also Chugai 2008). It is again encouraging that all the normal SNe Ia showing time-variability in either Na or K (SN 2006X, SN 2007le, SN 2014J) have dust in relatively nearby interstellar clouds according to our analysis, at distances where the covering factor of IS clouds can be significant and rapidly varying with time.…”

A Probabilistic Autoencoder for Type Ia Supernova Spectral Time Series

“…The Milky Way extinction can be determined independently and removed from the observed spectrum. Any optical depth variations along the line of sight should be slowly varying with respect to the ∼50 day observation window of a supernovae (Huang et al 2017), and therefore should be dependent only on the wavelength of observation. This color law is generally an input to the model (Guy et al 2007;Saunders et al 2018;Mandel et al 2020;Boone et al 2021a), and any time-dependent color variation should be captured by the variations to the global model.…”