  In a companion paper, Seitenzahl et al. (2013) presented a set of
three-dimensional delayed detonation models for thermonuclear explosions of
near-Chandrasekhar mass white dwarfs (WDs). Here, we present multi-dimensional
radiative transfer simulations that provide synthetic light curves and spectra
for those models. The model sequence explores both changes in the strength of
the deflagration phase (controlled by the ignition configuration) and the WD
central density. In agreement with previous studies, we find that the strength
of the deflagration significantly affects the explosion and the observables.
Variations in the central density also have an influence on both brightness and
colour, but overall it is a secondary parameter in our set of models. In many
respects, the models yield a good match to normal Type Ia supernovae (SNe Ia):
peak brightness, rise/decline time scales and synthetic spectra are all in
reasonable agreement. There are, however, several differences. In particular,
the models are too red around maximum light, manifest spectral line velocities
that are a little too high and yield I-band light curves that do not match
observations. Although some of these discrepancies may simply relate to
approximations made in the modelling, some pose real challenges to the models.
If viewed as a complete sequence, our models do not reproduce the observed
light-curve width-luminosity relation (WLR) of SNe Ia: all our models show
similar B-band decline rates, irrespective of peak brightness. This suggests
that simple variations in the strength of the deflagration phase in
Chandrasekhar-mass deflagration-to-detonation models do not readily explain the
observed diversity of normal SNe Ia. This may imply that some other parameter
within the Chandrasekhar-mass paradigm is key to the WLR, or that a substantial
fraction of normal SNe Ia arise from an alternative explosion scenario.
