Scientific potentials for MeV polarimetry

Magnetic field plays an essential role in the plasma dynamics and particle acceleration in high-energy astrophysical systems. Polarization offers a direct view of the magnetic field morphology and evolution in these systems. A regime yet to be explored is the high-energy polarimetry. MeV polarimetry offers unique insights into the origin of extragalactic cosmic rays and neutrinos, as well as extreme particle acceleration processes therein. In this presentation, I will talk about scientific potentials for MeV polarimetry, focusing on blazars, gamma-ray bursts, and pulsars. Future gamma-ray mission with polarimetry capability, such as the newly selected gamma-ray small explorer COSI, and the AMEGO-X that is under proposal phase, will open up this new window, bringing unprecedented scientific discoveries.