A test for the nature of the type Ia supernova explosion mechanism

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 (WDs) 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 Ni-56 and Co-56 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 M . of Ni-56, making them bright gamma -ray line emitters. The time to maximum brightness of Ni-56 decay lines is distinctly shorter in the M < M-ch class of model (<similar to>15 days) than in the M-ch model (similar to 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-M-ch explosion from SNe Ia as far away as the Virgo cluster with the XMM observatory. A 1-2 m(2) X-ray telescope such as the proposed Constellation-X observatory could observe K alpha emission from M < M-ch SNe Ia in the Virgo cluster, providing not just a detection but high- accuracy flux and kinematic information.