  Double-degenerate (DD) mergers of carbon-oxygen white dwarfs have recently
emerged as a leading candidate for normal Type Ia supernovae (SNe Ia). However,
many outstanding questions surround DD mergers, including the characteristics
of their light curves and spectra. We have recently identified a spiral
instability in the post-merger phase of DD mergers and demonstrated that this
instability self-consistently leads to detonation in some cases. We call this
the spiral merger SN Ia model. Here, we utilize the SuperNu radiative transfer
software to calculate three-dimensional synthetic light curves and spectra of
the spiral merger simulation with a system mass of 2.1 $M_\odot$ from Kashyap
et al. Because of their large system masses, both violent and spiral merger
light curves are slowly declining. The spiral merger resembles very slowly
declining SNe Ia, including SN 2001ay, and provides a more natural explanation
for its observed properties than other SN Ia explosion models. Previous
synthetic light curves and spectra of violent DD mergers demonstrate a strong
dependence on viewing angle, which is in conflict with observations. Here, we
demonstrate that the light curves and spectra of the spiral merger are less
sensitive to the viewing angle than violent mergers, in closer agreement with
observation. We find that the spatial distribution of 56Ni and IMEs follows a
characteristic hourglass shape. We discuss the implications of the asymmetric
distribution of 56Ni for the early-time gamma-ray observations of 56Ni from SN
2014J. We suggest that DD mergers that agree with the light curves and spectra
of normal SNe Ia will likely require a lower system mass.
