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 ± scattering are adopted from our earlier ab initio quantum electrodynamic calculations in the regime ≫ 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.

Radio Emission of Pulsars. I. Slow Tearing of a Quantizing Magnetic Field

“…The state of the e ± plasma is trans-relativistic, quasi-thermal and stable, with density n ± ∼ 15 |∇ × B|/4πe and magnetization about 10 2 times higher than in the relativisitic double-layer model. One also obtains a direct explanation for the presence of a hard X-ray component of the magnetar spectrum: the annihilation of trans-relativisitic e + − e − pairs in magnetic fields B 10 14 G produces a broad, bremsstrahlung-like spectrum of X-rays (Thompson & Kostenko 2020), similar to that observed. The higher density in this state also provides a promising context for collective plasma emission in the IR-optical band, which is observed from quiescent magnetars at a rate far exceeding the expected surface blackbody flux (Kaspi & Beloborodov 2017).…”

“…A recent re-examination of QED processes in ultrastrong magnetic fields has revealed the possibility of a collisional plasma state with a high resistivity, unlike the most common picture of a pulsar magnetosphere (Thompson & Kostenko 2020). This collisional state would be sustained by frequent pair annihilation and reconversion, e + +e − → γ → e + +e − .…”

Probing magnetar emission mechanisms with spectropolarimetry

“…Polarization will also allow us to discriminate between different mechanisms for the non-thermal emission below 10 keV. At higher energies, alternative models beyond those discussed here generally predict that the polarization should be dominated by the ordinary mode even up to high energies (e.g Thompson & Beloborodov 2005b;Heyl & Hernquist 2005;González-Caniulef et al 2019a;Thompson & Kostenko 2020) in contrast to the RCS model, which predicts the dominance of the extraordinary mode. If the surface turns out to be condensed, the lack of polarization at low energies will deprive observers of a lodestone from which to orient the measurements at higher energies to verify the underlying processes for the high-energy emission.…”

“…The "soft excess", on the other hand, might be produced by a number of processes, such as a hot-spot or RCS or Comptonization, that to a varying degree are linked to the magnetospheric currents. Many persistent sources also show a hard power-law, with a positive slope, that extends to hundreds of keV, for which proposed mechanisms range from thermal bremsstrahlung in the surface layers of the star, heated by a downward beam of charges, to emission from pairs created in the magnetosphere (Thompson & Beloborodov 2005a;Thompson & Kostenko 2020), to resonant Compton scattering (RCS) of seed photons on a population of highly relativistic charges (Baring et al 2005;Fernández & Thompson 2007;Nobili et al 2008a;Baring & Harding 2008;Zane et al 2009;Beloborodov 2013). In this work, we will only consider RCS as the emission mechanism for the hard power law.…”

Radio Emission of Pulsars. II. Coherence Catalyzed by Cerenkov-Unstable Shear Alfvén Waves

“…Observations of persistent hard X-ray emission from magnetars point to dissipation rates exceeding 10 3 times the spindown power of the neutron star in some cases (Kaspi & Beloborodov 2017). The plasma state of the magnetosphere has been debated: alternative models include (i) a relativistic double layer supported by counterstreaming e − and e + with γ0 ∼ 10 3 (Beloborodov & Thompson 2007) and (ii) a transrelativistic, collisional plasma that is sustained by intense ohmic heating in localized dissipative structures (Thompson & Kostenko 2020). The second option more directly accounts for the hard X-ray spectra through soft-photon emission associated with e + −e + annihilation.…”