The corona of an active galactic nucleus is a magnetized, turbulent plasma hovering above the accretion disk around a supermassive black hole. Light from this region is reprocessed, scattered, and absorbed before reaching us. The electromagnetic signal is an edited version of what happens inside.

Neutrinos pass through everything.

IceCube's detection of TeV neutrinos from NGC 1068 opened a direct channel into the corona. The neutrinos originate from hadrons accelerated by magnetized turbulence — protons gaining energy from the chaotic magnetic fields, then colliding to produce pions that decay into neutrinos. The neutrino intensity and spectrum encode the magnetic field strength, the turbulence injection scale, and the corona's physical size.

Combining IceCube neutrino data with Fermi-LAT gamma-ray observations constrains the corona's properties for two galaxies. NGC 1068's strong neutrino signal implies a compact, highly magnetized corona. NGC 7469, with roughly 100 TeV neutrino events, points to a larger corona with different turbulence characteristics. The two galaxies probe different regimes of the same physics.

The constraining power is remarkable. Neither neutrinos nor gamma rays alone would suffice — neutrinos give the hadronic acceleration rate, gamma rays give the photon field density, and together they triangulate the magnetic environment. Diffuse neutrino emission predictions from Seyfert galaxy populations follow, testable with next-generation detectors.

Ghost particles from the hearts of distant galaxies, carrying unscattered information about magnetic storms that light cannot faithfully report.