Magnetars and Physics in Strong Magnetic Fields

Neutron stars serve as useful laboratories to study physics under conditions of extreme density, gravity, and magnetic fields inaccessible terrestrially. Magnetars are a topical subclass of neutron stars with surface fields exceeding 10^10 Tesla, a regime where exotic untested QED processes may operate. Magnetars in our galaxy are largely observed through their X-ray/gamma-ray emission via bursts and persistent emission. In magnetars, the magnetic field is in the quantum electrodynamic (QED) domain where the cyclotron energy approaches or exceeds the electron rest mass. This defines the Schwinger or critical field of 4.41x10^9 Tesla and is a regime where exotic aspects of standard (but nonlinear and nonperturbative) QED are important. In this talk I will detail advanced spectral modeling of magnetars. Principal QED opacity processes will be discussed, namely magnetic pair production and photon splitting, which attenuate spectra at hard X-ray and higher energies. Opacity to photon splitting can be significant above 100 keV, introducing strong polarization that can be probed by future hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope like AMEGO and COSI. Such instruments may finally confirm photon splitting can occur in nature, and inform on its selection rules and the non-pertubative regime of QED.