  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-$e^\pm$ scattering are adopted from our
earlier ab initio QED calculations in the regime $10\alpha_{\rm em}^{-1}B_{\rm
Q} \gg B \gg B_{\rm Q}$. Careful attention is given to the $u$-channel
scattering resonance. Magnetospheric arcades anchored in zones of intense
crustal shear and extending to 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
non-locally 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 & Ortiz.
