tokens: while the other segments have been fading throughout the observations. This supports and quantifies the apparent asymmetries in the images in Figure 4 . Figure 6. The 3.6 μ m light curves of the four ER segments and the 3 surrounding stars, derived from the decomposition of the 04 mosaics at three epochs. Formal uncertainties on the flux densities are ≲2%. The mapping and decomposition procedures were also applied to the individual epochs of targeted observation without averaging into broader intervals. In this case, ∼25% of the 04 pixels in the images contain no data, and the decomposition is applied giving no weight to these pixels. The results are, as expected, much noisier than those illustrated above, but the same trends do emerge when similar averaging is applied after, rather than before, the decomposition. 5. Discussion Across all IR wavelengths, the light curves are reasonably well fitted by the model of Equation ( 1 ) with similar parameters (Figure 2 ). This empirical model is similar in form to those developed by Lundqvist Fransson ( 1991 ) and Dwek Felten ( 1992 ) to explain the prompt line emission from the ER. In those models, the light travel time across the ER (326 d for an ER with radius R = 239 lt-days and inclination i = 43°; Dwek Felten 1992 ) is explicitly modeled. Here, the light travel delays can be assumed to be a contributing factor to the width of the Gaussian component of our model. Irregularities in the radius and density distribution of the ER and asymmetries in the radius (or velocity) of the blast wave would be other contributing factors. The X-ray light curve is also well fitted by this model, although the date, d 0 , of the peak of the Gaussian function is somewhat later, and the exponential timescale, τ d , is significantly longer than at the IR wavelengths. Both these differences act to shift the peak of the X-ray light curve to a later date. In a very simple interpretation, and assuming that the IR emission is from dust that is collisionally heated by the X-ray emitting gas (Dwek 1986 ; Bocchio et al. 2013 ), the shorter exponential timescale for the IR emission compared to that of the X-ray emission, may suggest that dust is being destroyed faster than the gas is cooling. However, additional factors such as the development of a reverse shock, and variations in the density (and consequent post-shock temperature) of the CSM may also contribute to the differences in d 0 and τ d between the IR and X-ray light curves. Systematic differences in the IR and X-ray trends that may be indicative of dust destruction had been much more difficult to identify prior to about Day 8000 while both light curves were still rising (e.g., Dwek et al. 2008 , 2010 ). As shown in Figure 2 , we also find that the model of Equation ( 1 ) provides a good fit to the evolution of the B, R, and F502N band optical emission reported by Larsson et al. ( 2019 ) if restricted to dates after Day 5500 when the interaction with the ER is
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