2020ApJ...904..184T

The photon spectrum emitted by a transrelativistic pair plasma is calculated in the presence of an ultrastrong magnetic field, and is shown to bear a remarkable resemblance to the rising hard X-ray spectra of quiescent magnetars. This emission is powered by pair annihilation which, in contrast with a weakly magnetized pair plasma, shows an extended low-frequency tail similar to bremsstrahlung. Cross sections for electron-positron annihilation/scattering, two-photon pair creation, and photon-e^± scattering are adopted from our earlier ab initio quantum electrodynamic calculations in the regime 10α_em^–1B_Q ≫ B ≫ B_Q. Careful attention is given to the u-channel scattering resonance. Magnetospheric arcades anchored in zones of intense crustal shear and reaching about twice the magnetar radius are identified as the sites of the persistent hard X-ray emission. We deduce a novel and stable configuration for the magnetospheric circuit, with a high plasma density sustained by ohmic heating and in situ pair creation. Pairs are sourced nonlocally by photon collisions in zones with weak currents, such as the polar cap. Annihilation bremsstrahlung extends to the optical-IR band, where the plasma cutoff is located. The upper magnetar atmosphere experiences strong current-driven growth of ion-acoustic turbulence, which may limit positron diffusion. Coherent optical-IR emission is bounded near the observed flux by induced scattering. This model accommodates the rapid X-ray brightening of an activating magnetar, concentrated thermal hotspots, and the subdominant thermal X-ray emission of some active magnetars. Current injection is ascribed to continuous magnetic braiding, as seen in the global yielding calculations of Thompson, Yang, and Ortiz.