Electrodynamics of Magnetars. III. Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere

Abstract

We first examine the QED processes that create electron-positron pairs in magnetic fields approaching and exceeding 10 14 G. The formation of free and bound pairs is addressed, and the importance of positronium dissociation by thermal X-rays is noted. We calculate the collision cross section between an X-ray and a gamma ray, and point out a resonance in the cross section when the gamma ray is close to the threshold for pair conversion. We then show how the pair creation rate in the open-field circuit and the outer magnetosphere can be strongly enhanced by current-driven instabilities near the light cylinder. A cascade develops when the current has a strong fluctuating component. We examine the mechanism of particle heating and show that a high rate of pair creation can be sustained close to the star, but only if the spin period is shorter than several seconds. The dissipation rate in this turbulent state can easily accommodate the observed radio output of the transient radio-emitting magnetars and even their infrared emission. Finally, we outline how a very high rate of pair creation on the open magnetic field lines can help to stabilize a static twist in the closed magnetosphere and to regulate the loss of magnetic helicity by reconnection at the light cylinder.

Pair cascades in the magnetospheres of strongly magnetized neutron stars

“…High‐multiplicity cascades due to RICS may still occur along twisted lines in the closed‐field region (where the primary electrons never reach Lorentz factors larger than γ 0 ∼10 3 ), as they do in magnetars (e.g. Thompson et al 2002; Beloborodov & Thompson 2007; Thompson 2008a,b; Beloborodov 2009).…”

“…More recently it has been suggested that the pair cascade is also necessary for non‐thermal emission from magnetars (e.g. Beloborodov & Thompson 2007; Thompson 2008a,b; see Woods & Thompson 2006 for a review of magnetars). The basic pair cascade involves several steps: (i) acceleration of primary particles by an electric field parallel to the magnetic field; (ii) gamma‐ray emission by the accelerated particles moving along the magnetic field lines (either by curvature radiation or by inverse Compton upscattering of surface photons); (iii) field‐assisted photon decay into electron–positron pairs as the angle between the photon and the magnetic field line becomes sufficiently large, or pair production by two‐photon annihilation in weak‐field regimes; (iv) gamma‐ray emission by the newly created particles as they lose their transverse energy through synchrotron emission; (v) further pair production and gamma‐ray emission via steps (iii) and (iv).…”

“…11). The magnetosphere acceleration zone in the superstrong, twisted field regime of magnetars is investigated analytically by Beloborodov & Thompson (2007) for cascades occurring in the closed‐field‐line region of the magnetosphere and by Thompson (2008a,b) in the open‐field‐line region.…”

“…A number of models have been proposed for the location of the gap, from inner magnetosphere accelerators (both ‘vacuum’ and ‘space‐charge‐limited flow’ types; see e.g. Ruderman & Sutherland 1975; Arons & Scharlemann 1979; Muslimov & Tsygan 1992; Hibschman & Arons 2001a; Medin & Lai 2007; Thompson 2008a,b) to outer magnetosphere accelerators (e.g. Cheng et al 1986a,b; Romani 1996; Cheng et al 2000; Takata et al 2006) and hybrid inner–outer magnetosphere accelerators (‘slot’ gaps and extended outer gaps; e.g.…”

Pair production in a plane wave by thermal background photons

“…Creation of electron-positron pairs is predicted to be triggered in a variety of ways: in intense electromagnetic backgrounds such as two plane waves propagating in different directions [16,17], in a similar setup with the inclusion of the Coulomb field of a nucleus [18], but also in more exotic contexts, such as photons decaying in magnetic fields of magnetars [19]. Another example is through thermal radiation, with pair-creation rates having been calculated in a constant electric field in various formalisms [20][21][22], also including stimulated pair creation [23].…”