  We present optical observations and Monte Carlo radiative transfer modeling
of the Type Ia supernova SN 2011aa. With a $\Delta m_{15} (B)$ of $0.59 \pm
0.07$ mag and a peak magnitude $M_{\rm B}$ of $-19.30 \pm 0.27$ mag, SN 2011aa
has the slowest decline rate among SNe Ia. The secondary maximum in the
$I$-band is absent or equally bright as the primary maximum. The velocity of C
II is lower than the velocity of Si II. This indicates either presence of C at
lower velocities than Si or a line of sight effect. Application of Arnett's
radiation diffusion model to the bolometric light curve indicates a massive
ejecta $M_{\rm{ej}} ~ 1.8 - 2.6~M_{\odot}$. The slow decline rate and large
ejecta mass, with a normal peak magnitude, are well explained by double
degenerate, violent merger explosion model. The synthetic spectra and light
curves generated with SEDONA considering a violent merger density profile match
the observations.
