Pair Plasma in Super-QED Magnetic Fields and the Hard X-ray/Optical Emission of Magnetars

Abstract

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 $10{\alpha }_{\mathrm{em}}^{-1}{B}_{\mathrm{Q}}\gg B\gg B_{\mathrm{Q}}$. 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.

28 pages, 18 figures, accepted for publication in the Astrophysical Journal after minor revisions

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• Polarized x-rays from a magnetar
• IXPE Detection of Polarized X-rays from Magnetars and Photon Mode Conversion at QED Vacuum Resonance
• Probing magnetar emission mechanisms with spectropolarimetry
• Direct Emission of Strong Radio Pulses During Magnetar Flares
• Relativistic Alfvén Waves Entering Charge Starvation in the Magnetospheres of Neutron Stars
• Radio Emission of Pulsars. I. Slow Tearing of a Quantizing Magnetic Field
• Photon-axion mixing in thermal emission of isolated neutron stars
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• Collisionless tearing instability in a relativistic pair plasma with a power-law distribution function
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