Carnegie Supernova Project II: The slowest rising Type Ia supernova
  LSQ14fmg and clues to the origin of super-Chandrasekhar/03fg-like events
  The Type Ia supernova (SN Ia) LSQ14fmg exhibits exaggerated properties which
may 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 light
curves are unlike those of any SNe Ia observed. The light curves of LSQ14fmg
rise extremely slowly. At -23 rest-frame days relative to B-band maximum,
LSQ14fmg is already brighter than $M_V$=-19 mag before host extinction
correction. The observed color curves show a flat evolution from the earliest
observation to approximately one week after maximum. The near-infrared light
curves peak brighter than -20.5 mag in the J and H bands, far more luminous
than any 03fg-like SNe Ia with near-infrared observations. At one month past
maximum, the optical light curves decline rapidly. The early, slow rise and
flat color evolution are interpreted to result from an additional excess flux
from a power source other than the radioactive decay of the synthesized
$^{56}Ni$. The excess flux matches the interaction with a typical superwind of
an asymptotic giant branch (AGB) star in density structure, mass-loss rate, and
duration. The rapid decline starting at around one month past B-band maximum
may be an indication of rapid cooling by active carbon monoxide (CO) formation,
which requires a low temperature and high density environment. These
peculiarities point to an AGB progenitor near the end of its evolution and the
core degenerate scenario as the likely explosion mechanism for LSQ14fmg.
