  Currently popular models for Type Ia supernovae (SNe Ia) fall into two
general classes. The first comprises explosions of nearly pure carbon/oxygen
(C/O) white dwarfs at the Chandrasekhar limit which ignite near their centers.
The second consists of lower-mass C/O cores which are ignited by the detonation
of an accreted surface helium layer. Explosions of the latter type produce
copious Fe, Co and Ni K-alpha emission from 56Ni and 56Co decay in the
detonated surface layers, emission which is much weaker from Chandrasekhar-mass
models. The presence of this emission provides a simple and unambiguous
discriminant between these two models for SNe Ia. Both mechanisms may produce
0.1-0.6 solar masses of 56Ni, making them bright gamma-ray line emitters. The
time to maximum brightness of 56Ni decay lines is distinctly shorter in the
sub-Chandrasekhar mass class of model (approximately 15 days) than in the
Chandrasekhar mass model (approximately 30 days), making gamma-ray line
evolution another direct test of the explosion mechanism. It should just be
possible to detect K-shell emission from a sub-Chandrasekhar explosion from SNe
Ia as far away as the Virgo cluster with the XMM Observatory. A 1 to 2 square
meter X-ray telescope such as the proposed Con-X Observatory could observe
K-alpha emission from sub-Chandrasekhar mass SNe Ia in the Virgo cluster,
providing not just a detection, but high-accuracy flux and kinematic
information.
