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The Type Ia supernova (SN Ia) LSQ14fmg exhibits exaggerated properties whichmay help to reveal the origin of the "super-Chandrasekhar" (or 03fg-like)group. The optical spectrum is typical of a 03fg-like SN Ia, but the lightcurves are unlike those of any SNe Ia observed. The light curves of LSQ14fmgrise extremely slowly. At -23 rest-frame days relative to B-band maximum,LSQ14fmg is already brighter than $M_V$=-19 mag before host extinctioncorrection. The observed color curves show a flat evolution from the earliestobservation to approximately one week after maximum. The near-infrared lightcurves peak brighter than -20.5 mag in the J and H bands, far more luminousthan any 03fg-like SNe Ia with near-infrared observations. At one month pastmaximum, the optical light curves decline rapidly. The early, slow rise andflat color evolution are interpreted to result from an additional excess fluxfrom a power source other than the radioactive decay of the synthesized$^{56}Ni$. The excess flux matches the interaction with a typical superwind ofan asymptotic giant branch (AGB) star in density structure, mass-loss rate, andduration. The rapid decline starting at around one month past B-band maximummay be an indication of rapid cooling by active carbon monoxide (CO) formation,which requires a low temperature and high density environment. Thesepeculiarities point to an AGB progenitor near the end of its evolution and thecore degenerate scenario as the likely explosion mechanism for LSQ14fmg.

Carnegie Supernova Project II: The slowest rising Type Ia supernova LSQ14fmg and clues to the origin of super-Chandrasekhar/03fg-like events