Fermi and Swift as supernova alarms: Alert, localization, and diagnosis of future Galactic Type Ia explosions

A Galactic SNIa event could go entirely unnoticed due to the large optical and near-IR extinction in the Milky Way plane, low radio and X-ray luminosities, and a weak neutrino signal. But the recent SN2014J confirms that Type Ia supernovae emit nuclear γ- ray lines, from the ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe radioactive decay. The energy released in these decays powers the SNIa UVOIR light curve at times after ~1 week, leading to an exponential decline. Importantly for Swift and Fermi, these decays are accompanied by γ-ray line emission, with distinct series of lines for both the ⁵⁶Ni and ⁵⁶Co decays, spanning 158 keV to 2.6 MeV. These lines are squarely within the Fermi/GBM energy range, and the ⁵⁶Ni 158 keV line is detectable by Swift/BAT. The Galaxy is optically thin to γ-rays, so the supernova line flux will suffer negligible extinction. Both GBM and BAT have continuous and nearly all-sky coverage. Thus GBM and BAT are ideal Galactic SNIa monitors and early warning systems. We will illustrate expected GBM and BAT light curves and spectra, based on our model for SNIa γ-ray emission and transfer. We show that the supernova signal emerges as distinct from the GBM background within days after the explosion in the SN2014J shell model. Therefore, if a Galactic SNIa were to explode, there are two possibilities of confirming and sounding the alert: 1) Swift/BAT discovers the SNIa first and localizes it within arcminutes; 2) Fermi/GBM finds the SNIa first and localizes it to within ~1 degree, using the Earth occultation technique, followed up by BAT to localize it within arcminutes. After the alert of either BAT or GBM, Swift localizes it to take spectra in optical, UV, soft and hard X-rays simultaneously with both XRT and UVOT instruments.