Probing the origin of intergalactic magnetic fields and cosmic rays with gamma-ray cascades

ended

The existence of an intergalactic magnetic field (IGMF) remains elusive and only upper limits on its strength are known from limits on the rotation of polarization angles of radio waves. Its measurement would provide crucial information about large scale structure formation, since the IGMF is thought to act as a seed field for magnetic fields in galaxies and galaxy clusters, how the Universe evolved, and how charged cosmic rays and electromagnetic waves propagate through intergalactic space. Here, I propose a new search for IGMF signatures using observations of a high-energy gamma-ray cascades from distant galaxies. Gamma rays interact with background radiation fields to produce electron-positron pairs. These pairs up-scatter cosmic-microwave photons to gamma-ray energies, initiating the cascade. The IGMF morphology is imprinted on the cascade through a deflection of the pairs in the IGMF. A novel combination of observations with imaging air Cherenkov telescopes and the Fermi Large Area Telescope, using both spectral and spatial information, combined with precise model predictions of the cascade, promise an unprecedented sensitivity for detection of the cascade signal and the IGMF. Strong constraints on the IGMF strength will be possible if no cascade is detected. Furthermore, I will also search for a gamma-ray and neutrino signal from cascades initiated by cosmic rays from active galaxies. This will yield an independent probe of the IGMF and, additionally, constraints on the cosmic-ray acceleration power of such galaxies. As part of this work, I will also make predictions how the future Cherenkov Telescope Array (CTA) can further improve the detection of or constraints on the IGMF and the cosmic-ray acceleration power of active galaxies. This will result in an optimized observation strategy for the extragalactic survey and the blazar monitoring program, which are both planned with CTA.