  In the inner region of the disc of an active galactic nucleus (AGN), the
collision of two white dwarfs (WDs) through Jacobi capture might be inevitable,
leading to a Type Ia supernova (SN Ia) explosion. This transient event,
influenced by the disc gas and the gravity of the supermassive black hole
(SMBH), exhibits distinct characteristics compared to normal SNe Ia. The energy
of the explosion is mainly stored in the ejecta in the form of kinetic energy.
Typically, the ejecta is not effectively decelerated by the AGN disc and rushes
rapidly out of the AGN disc. However, under the influence of the SMBH, most of
the ejecta falls back toward the AGN disc. As the fallback ejecta becomes more
dispersed, it interacts with the disc gas, converting its kinetic energy into
thermal energy. This results in a high-energy transient characterized by a
rapid initial rise followed by a decay with $L\propto t^{-2.8}$. The time-scale
of the transient ranges from hours to weeks, depending on the mass of the SMBH.
This process generates high-energy radiation spanning from hard X-rays to the
soft $\gamma$ range. Additionally, the subsequent damage to the disc may result
in changing-look AGNs. Moreover, the falling back of SNe Ia ejecta onto the AGN
disc significantly increases the metallicity of the AGN and can even generate
heavy elements within the AGN discs.

High Energy Astrophysical Phenomena;Astrophysics of Galaxies