Top: Temporal evolution of τ_γγ for photons of 0.1, 1, and 10 TeV. Vertical gray lines represent the Bayesian block interval identifying the LAT emission, while the horizontal dashed line (τ_γγ = 1) represents the transition to a transparent medium (τ_γγ ≲ 1). Shaded areas represent the error propagation from temperature and luminosity measurements in Zhu et al. (2023). Bottom: Absorption above 10 GeV, averaged over the Bayesian block time interval in Fig. C.1. The overall shape is a result of the peaked SN blackbody spectrum and the cross section σ_γγ.