tokens: SNe Ia are monitored only for a few months; if CSM is more prevalent or typically at a larger distance from the WD, we might be systematically missing the signatures of this interaction. There are additional good reasons to suspect that we might be systematically missing the interaction of CSM with SN Ia ejecta. For example, SN Ia 2006X exhibited evolution in its Na i D absorption line that was interpreted as the photoionization of more distant CSM (Patat et al. 2007 ). In a compilation of 17 SNe Ia with multi-epoch spectra, Sternberg et al. ( 2014 ) found that ∼18% exhibit time-variable Na i D. Additionally, studies of large samples of SNe Ia (20–35 events) found Na i D to be blueshifted ∼20% more often than redshifted, and argued that this was best explained by outflowing CSM from the progenitor system (Sternberg et al. 2011 ; Maguire et al. 2013b ), but see Bulla et al. ( 2018 ) for an alternative explanation. In these SNe Ia, the CSM is estimated to be 5 × 10 16 –10 18 cm away from the explosion, which means that the fastest SN ejecta would not interact until >1 yr. Additional support for late-time CSM interaction is presented by Fox Filippenko ( 2013 ), who interpreted an increase in Spitzer mid-infrared photometry for SN Ia 2005gj at 500–1000 days past peak brightness as dust heated by hot blackbody emission from new or renewed CSM interaction. Additional evidence for nondegenerate companion stars comes from detections of, e.g., “blue bumps” in the light curve in the week after explosion (Hosseinzadeh et al. 2017 ) and/or late-time H α emission from material swept off the companion star and embedded in the ejecta (Maguire et al. 2016 ; Sand et al. 2018 ). It is important to note that there is plenty of evidence that many—or even most—SN Ia progenitor systems host a degenerate binary companion and/or do not contain CSM (e.g., Hayden et al. 2010 ; Maoz et al. 2014 ; Chomiuk et al. 2016 ). In this work, we aim to put upper limits on the fraction of SNe Ia with CSM, and we are not suggesting that the systematic oversight of CSM interaction extends to all SNe Ia. There is further motivational support in searching for late-onset CSM interaction in SNe Ia from recent observations of delayed CSM interaction in some core-collapse SNe. For example, Milisavljevic et al. ( 2015 ) presented observations of SN 2014C, an SN Ib that appeared to explode into an empty cavity surrounded by CSM, which transitioned to an SN IIn as the ejecta caught up with the previously released material. Similarly, Mauerhan et al. ( 2018 ) found H α emission from the onset of CSM interaction in SN Ib 2004dk 13 years after the original explosion. Vinko et al. ( 2017 ) presented the results of a systematic narrow-band imaging survey of Type Ibc, IIb, and Ia SNe to look for H α emission due to CSM interaction years to decades after explosion. Most of their potential detections are associated with core-collapse events, but of particular relevance to this work is their
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