Three-dimensional Analytical Description of Magnetised Winds from Oblique Pulsars

Rotating neutron stars, or pulsars, are plausibly the source of power behind many astrophysical systems, such as gamma-ray bursts, supernovae, pulsar wind nebulae and supernova remnants. In the past several years, 3D numerical simulations made it possible to compute pulsar spindown luminosity from first principles and revealed that oblique pulsar winds are more powerful than aligned ones. However, what causes this enhanced power output of oblique pulsars is not understood. In this work, using time-dependent 3D magnetohydrodynamic (MHD) and force-free simulations, we show that, contrary to the standard paradigm, the open magnetic flux, which carries the energy away from the pulsar, is laterally non-uniform. We argue that this non-uniformity is the primary reason for the increased luminosity of oblique pulsars. To demonstrate this, we construct simple analytic descriptions of aligned and orthogonal pulsar winds and combine them to obtain an accurate 3D description of the pulsar wind for any obliquity. Our approach describes both the warped magnetospheric current sheet and the smooth variation of pulsar wind properties outside of it. We find that generically the magnetospheric current sheet separates plasmas that move at mildly relativistic velocities relative to each other. This suggests that the magnetospheric reconnection is a type of driven, rather than free, reconnection. The jump in magnetic field components across the current sheet decreases with increasing obliquity, which could be a mechanism that reduces dissipation in near-orthogonal pulsars. Our analytical description of the pulsar wind can be used for constructing models of pulsar gamma-ray emission, pulsar wind nebulae, and magnetar-powered core-collapse gamma-ray bursts and supernovae.Comment: Submitted to MNRAS, comments welcome. 12 pages, 16 figures, uses mn2e.cl

Ab-initio pulsar magnetosphere: three-dimensional particle-in-cell simulations of axisymmetric pulsars

We perform first-principles relativistic particle-in-cell simulations of aligned pulsar magnetosphere. We allow free escape of particles from the surface of a neutron star and continuously populate the magnetosphere with neutral pair plasma to imitate pair production. As pair plasma supply increases, we observe the transition from a charge-separated electrosphere solution with trapped plasma and no spin-down to a solution close to the ideal force-free magnetosphere with electromagnetically-dominated pulsar wind. We calculate the magnetospheric structure, current distribution and spin-down power of the neutron star. We also discuss particle acceleration in the equatorial current sheet.Comment: 6 pages, 5 figures, published in ApJ Letter

Effects of synchrotron cooling and pair production on collisionless relativistic reconnection

High energy radiation from nonthermal particles accelerated in relativistic magnetic reconnection is thought to be important in many astrophysical systems, ranging from blazar jets and black hole accretion disk coronae to pulsars and magnetar flares. The presence of a substantial density of high energy photons ($>$MeV) in these systems can make two-photon pair production ($\gamma\gamma\to e^-e^+$) an additional source of plasma particles and can affect the radiative properties of these objects. We present the results of novel particle-in-cell simulations that track both the radiated synchrotron photons and the created pairs, with which we study the evolution of a two-dimensional reconnecting current sheet in pair plasma. Synchrotron radiation from accelerated particles in the current sheet produces hot secondary pairs in the upstream which are later advected into the current sheet where they are reaccelerated and produce more photons. In the optically thin regime, when most of the radiation is leaving the upstream unaffected, this process is self-regulating and depends only on the background magnetic field and the optical depth of photons to pair production. The extra plasma loading also affects the properties of reconnection. We study how the inflow of the secondary plasma, with multiplicities up to several hundred, reduces the effective magnetization of the plasma, suppressing the acceleration and thus decreasing the high energy photon spectrum cutoff. This offers an explanation for the weak dependence of the observed gamma-ray cutoff in pulsars on the magnetic field at the light cylinder.Comment: 22 pages, 15 figures, submitted to Ap

Ab-initio pulsar magnetosphere: the role of general relativity

It has recently been demonstrated that self-consistent particle-in-cell simulations of low-obliquity pulsar magnetospheres in flat spacetime show weak particle acceleration and no pair production near the poles. We investigate the validity of this conclusion in a more realistic spacetime geometry via general-relativistic particle-in-cell simulations of the aligned pulsar magnetospheres with pair formation. We find that the addition of frame-dragging effect makes local current density along the magnetic field larger than the Goldreich-Julian value, which leads to unscreened parallel electric fields and the ignition of a pair cascade. When pair production is active, we observe field oscillations in the open field bundle which could be related to pulsar radio emission. We conclude that general relativistic effects are essential for the existence of pulsar mechanism in low obliquity rotators.Comment: 5 pages, 4 figure, submitted to ApJLetter

Inclined Pulsar Magnetospheres in General Relativity: Polar Caps for the Dipole, Quadrudipole and Beyond

In the canonical model of a pulsar, rotational energy is transmitted through the surrounding plasma via two electrical circuits, each connecting to the star over a small region known as a "polar cap." For a dipole-magnetized star, the polar caps coincide with the magnetic poles (hence the name), but in general, they can occur at any place and take any shape. In light of their crucial importance to most models of pulsar emission (from radio to X-ray to wind), we develop a general technique for determining polar cap properties. We consider a perfectly conducting star surrounded by a force-free magnetosphere and include the effects of general relativity. Using a combined numerical-analytical technique that leverages the rotation rate as a small parameter, we derive a general analytic formula for the polar cap shape and charge-current distribution as a function of the stellar mass, radius, rotation rate, moment of inertia, and magnetic field. We present results for dipole and quadrudipole fields (superposed dipole and quadrupole) inclined relative to the axis of rotation. The inclined dipole polar cap results are the first to include general relativity, and they confirm its essential role in the pulsar problem. The quadrudipole pulsar illustrates the phenomenon of thin annular polar caps. More generally, our method lays a foundation for detailed modeling of pulsar emission with realistic magnetic fields.Comment: 12 pages, 4 figures. v2: minor edits, matches published version. Related videos available at https://youtu.be/M_ruTbM8YN