High $\gamma$-ray escape time in 2003fg-like supernovae: A challenge to proposed models

A rare subclass of Type Ia supernovae (SNe Ia), named after the prototype SN 2003fg, includes some of the brightest SNe Ia, often called "super Chandrasekhar-mass" SNe Ia. We calculate the $\gamma$-ray deposition histories and the $^{56}$Ni mass synthesized in the explosion, $M_\mathrm{Ni56}$, for eight 2003fg-like SNe. Our findings reveal that the $\gamma$-ray escape time, $t_0$, for these objects is $t_0\approx45\text{-}60 \,$ days, significantly higher than that of normal SNe Ia. 2003fg-like SNe are distinct from normal SNe Ia in the $t_0$-$M_\mathrm{Ni56}$ plane, with a noticeable gap between the two populations. The observed position of 2003fg-like SNe in this plane poses a significant challenge for theoretical explosion models. We demonstrate that the merger of two white dwarfs (WDs) and a single star exceeding the Chandrasekhar limit fail to reproduce the observed $t_0$-$M_\mathrm{Ni56}$ distribution. However, preliminary calculations of head-on collisions of massive WDs show agreement with the observed $t_0$-$M_\mathrm{Ni56}$ distribution.