2019ApJ...874...62S

I show that the characteristic diffusion timescale and the gamma-ray escape timescale, of SN Ia ejecta, are related with each other through the time when the bolometric luminosity, L_bol, intersects with instantaneous radioactive decay luminosity, L_γ, for the second time after the light-curve peak. Analytical arguments, numerical radiation-transport calculations, and observational tests show that L_bol generally intersects L_γ at roughly 1.7 times the characteristic diffusion timescale of the ejecta. This relation implies that the gamma-ray escape timescale is typically 2.7 times the diffusion timescale, and also implies that the bolometric luminosity 15 days after the peak, L_bol(t₁₅), must be close to the instantaneous decay luminosity at that time, L_γ (t₁₅). With the employed calculations and observations, the accuracy of L_bol = L_γ at t = t₁₅ is found to be comparable to the simple version of "Arnett's rule" (L_bol = L_γ at t = t_peak). This relation aids the interpretation of SN Ia light curves and may also be applicable to general hydrogen-free explosion scenarios powered by other central engines.