Corona of Magnetars

We develop a theoretical model that explains the formation of hot coronae around strongly magnetized neutron stars — magnetars. The starquakes of a magnetar shear its external magnetic field, which becomes non-potential and is threaded by an electric current. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The self-induction electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by curlB and regulates the self-induction e.m.f. by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light-crossing time ∼ 10−4 s and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. Using a direct numerical experiment, we show that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage eΦe ∼ 1 GeV along the magnetic lines. The voltage is maintained near threshold for ignition of pair production, and pair production occurs at a rate just enough to feed the electric current. The heating rate of the corona is ∼ 10 erg/s, in agreement with the observed persistent, high-energy output of magnetars. We deduce that a static twist that is suddenly implanted into the magnetosphere will decay on a timescale of 1-10 yrs. The particles accelerated in the corona impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and the corona may be hot enough to create additional e± pairs. This layer is the likely source of the observed 100 keV emission from magnetars. The corona emits curvature radiation up to 10 Hz and can supply the observed IR-optical luminosity. We outline the implications of our results for the non-thermal radiation in other bands, and for the post-burst evolution of magnetars. Also at Astro-Space Center of Lebedev Physical Institute, Profsojuznaja 84/32, Moscow 117810, Russia