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ABSTRACT J191213.72 − 441045.1 is a binary system composed of a white dwarf and an M-dwarf in a 4.03-h orbit. It shows emission in radio, optical, and X-ray, all modulated at the white dwarf spin period of 5.3 min, as well as various orbital sideband frequencies. Like in the prototype of the class of radio-pulsing white dwarfs, AR Scorpii, the observed pulsed emission seems to be driven by the binary interaction. In this work, we present an analysis of far-ultraviolet spectra obtained with the Cosmic Origins Spectrograph at the Hubble Space Telescope, in which we directly detect the white dwarf in J191213.72 − 441045.1. We find that the white dwarf has a temperature of Teff = 11485 ± 90 K and mass of 0.59 ± 0.05 M⊙. We place a tentative upper limit on the magnetic field of ≈50 MG. If the white dwarf is in thermal equilibrium, its physical parameters would imply that crystallization has not started in the core of the white dwarf. Alternatively, the effective temperature could have been affected by compressional heating, indicating a past phase of accretion. The relatively low upper limit to the magnetic field and potential lack of crystallization that could generate a strong field pose challenges to pulsar-like models for the system and give preference to propeller models with a low magnetic field. We also develop a geometric model of the binary interaction which explains many salient features of the system. 

ABSTRACT We find a class of twisted and differentially rotating neutron star magnetospheres that do not have a light cylinder, generate no wind, and thus do not spin-down. The magnetosphere is composed of embedded differentially rotating flux surfaces, with the angular velocity decreasing as Ω ∝ 1/r (equivalently, becoming smaller at the foot-points closer to the axis of rotation). For each given North–South self-similar twist profile there is a set of self-similar angular velocity profiles (limited from above) with a ‘smooth’, dipolar-like magnetic field structure extending to infinity. For spin parameters larger than some critical value, the light cylinder appears, magnetosphere opens up, and the wind is generated. 

ABSTRACT We consider conditions for jet breakout through ejecta following mergers of neutron stars and provide simple relations for the breakout conditions. We demonstrate that: (i) break-out requires that the isotropic-equivalent jet energy Ej exceeds the ejecta energy Eej by Ej ≥ Eej/βej, where βej = Vej/c, Vej is the maximum velocity of the ejecta. If the central engine terminates before the breakout, the shock approaches the edge of the ejecta slowly ∝ 1/t; late breakout occurs only if at the termination moment the head of the jet was relatively close to the edge. (ii) If there is a substantial delay between the ejecta’s and the jet’s launching, the requirement on the jet power increases. (iii) The forward shock driven by the jet is mildly strong, with Mach number M ≈ 5/4 (increasing with time delay td); (iii) the delay time td between the ejecta and the jet’s launching is important for $$t_\mathrm{ d} \gt t_0= ({3 }/{16}) {c M_{\mathrm{ ej}} V_{\mathrm{ ej}}}/{L_\mathrm{ j}} = 1.01 {\rm \mathrm{ s}} M_{\mathrm{ ej}, -2} L_{\mathrm{ j}, 51} ^{-1} \left({\beta _{\mathrm{ ej}}} /{0.3} \right)$$, where Mej is ejecta mass, Lj is the jet luminosity (isotropic equivalent). For small delays, t0 is also an estimate of the break-out time. 

We first derive a set of equations describing general stationary configurations of relativistic force-free plasma, without assuming any geometric symmetries. We then demonstrate that electromagnetic interaction of merging neutron stars is necessarily dissipative due to the effect of electromagnetic draping—creation of dissipative regions near the star (in the single magnetized case) or at the magnetospheric boundary (in the double magnetized case). Our results indicate that even in the single magnetized case we expect that relativistic jets (or "tongues") are produced, with correspondingly beamed emission pattern. 

Abstract We consider the propagation of polarization in the inner parts of pair-symmetric magnetar winds, close to the light cylinder. Pair plasmas in magnetic field is birefringent, a ∝ B 2 effect. As a result, such plasmas work as phase retarders: Stokes parameters follow a circular trajectory on the Poincare sphere. In the highly magnetized regime, ω , ω p ≪ ω B , the corresponding rotation rates are independent of the magnetic field. A plasma screen with dispersion measure DM ∼ 10 −6 pc cm −3 can induce large polarization changes, including large effective rotation measures (RMs). The frequency scaling of the (generalized) RM, ∝ λ α , mimics the conventional RM with α = 2 for small phase shifts, but can be as small as α = 1. In interpreting observations, the frequency scaling of polarization parameters should be fitted independently. The model offers explanations for (i) the large circular polarization component observed in FRBs, with right–left switching; (ii) large RM, with possible sign changes (when the observation bandwidth is small); and (iii) time-dependent variable polarization. A relatively dense and slow wind is needed—the corresponding effect in regular pulsars is small. 

Abstract We consider topological configurations of the magnetically coupled spinning stellar binaries (e.g., merging neutron stars or interacting star–planet systems). We discuss conditions when the stellar spins and the orbital motion nearly “compensate” each other, leading to very slow overall winding of the coupled magnetic fields; slowly winding configurations allow gradual accumulation of magnetic energy, which is eventually released in a flare when the instability threshold is reached. We find that this slow winding can be global and/or local. We describe the topology of the relevant space F = T 1 S 2 as the unit tangent bundle of the two-sphere and find conditions for slowly winding configurations in terms of magnetic moments, spins, and orbital momentum. These conditions become ambiguous near the topological bifurcation points; in certain cases, they also depend on the relative phases of the spin and orbital motions. In the case of merging magnetized neutron stars, if one of the stars is a millisecond pulsar, spinning at ∼10 ms, the global resonance ω 1 + ω 2 = 2Ω (spin-plus beat is two times the orbital period) occurs approximately one second before the merger; the total energy of the flare can be as large as 10% of the total magnetic energy, producing bursts of luminosity ∼10 44 erg s −1 . Higher order local resonances may have similar powers, since the amount of involved magnetic flux tubes may be comparable to the total connected flux. 

We find a new family of solutions for force-free magnetic structures in cylindrical geometry. These solutions have radial power-law dependence and are periodic but non-harmonic in the azimuthal direction; they generalize the vacuum $$z$$ -independent potential fields to current-carrying configurations.