Using gamma ray monitoring to avoid missing the next Milky Way Type Ia supernova

A Milky Way Type Ia supernova (SNIa) could be unidentified or even initially unnoticed, being dim in radio, X-rays, and neutrinos, and suffering large optical/IR extinction in the Galactic plane. But SNIa emit nuclear gamma-ray lines from ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe radioactive decays. These lines fall within the Fermi/GBM energy range, and the ⁵⁶Ni 158 keV line is detectable by Swift/BAT. Both instruments frequently monitor the Galactic plane, which is transparent to gamma rays. Thus GBM and BAT are ideal Galactic SNIa early warning systems. We simulate SNIa MeV light curves and spectra to show that GBM and BAT could confirm a Galactic SNIa explosion, followed by Swift localization and observation in X-rays and UVOIR band. The time of detection depends sensitively on the ⁵⁶Ni distribution, and can be as early as a few days if {≳ } 10{{ per cent}} of the ⁵⁶Ni is present in the surface as suggested by SN2014J gamma data.