Pair Plasma in Super-QED Magnetic Fields and the Hard X-ray/Optical Emission of Magnetars
| Autor: | Thompson, Christopher, Kostenko, Alexander |
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| Rok vydání: | 2020 |
| Předmět: | |
| Druh dokumentu: | Working Paper |
| DOI: | 10.3847/1538-4357/abbe87 |
| Popis: | 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. Comment: 28 pages, 18 figures, accepted for publication in the Astrophysical Journal after minor revisions |
| Databáze: | arXiv |
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