High-Energy Polarimetry - a new window to probe extreme physics in AGN jets

The constantly improving sensitivity of ground-based and space-borne observatories has made possible the detection of high-energy emission (X-rays and gamma-rays) from several thousands of extragalactic sources. Enormous progress has been made in measuring the continuum flux enabling us to perform imaging, spectral and timing studies. An important remaining challenge for high-energy astronomy is measuring polarization. The capability to measure polarization is being realized currently at X-ray energies (e.g. with IXPE), and sensitive gamma-ray telescopes capable of measuring polarization, such as AMEGO, AdEPT, e-ASTROGAM, etc., are being developed. These future gamma-ray telescopes will probe the radiation mechanisms and magnetic fields of relativistic jets from active galactic nuclei at spatial scales much smaller than the angular resolution achieved with continuum observations of the instrument. In this white paper, we discuss the scientific potentials of high-energy polarimetry, especially gamma-ray polarimetry, including the theoretical implications, and observational technology advances being made. In particular, we will explore the primary scientific opportunities and wealth of information expected from synergy of multi-wavelength polarimetry that will be brought to multi-messenger astronomy.
Comment: submitted to Astro2020 (Astronomy and Astrophysics Decadal Survey)