Iwona Mochol

Radiative damping and emission signatures of strong superluminal waves in pulsar winds

We analyze the damping of strong, superluminal electromagnetic waves by radiation reaction and Compton drag in the context of pulsar winds. The associated radiation signature is found by estimating the efficiency and the characteristic radiation frequencies. Applying these estimates to the gamma-ray binary containing PSR B1259–63, we show that the GeV flare observed by the Fermi Large Area Telescope can be understood as inverse-Compton emission by particles scattering photons from the companion star, if the pulsar wind termination shock acquires a precursor of superluminal waves roughly 30 days after periastron. This requirement constrains the mass-loading factor of the wind μ=L/ N-dot mc{sup 2}, where L is the luminosity and N-dot is the rate of loss of electrons and positrons, to be roughly 6 × 10{sup 4}.

Propagation and Stability of Superluminal Waves in Pulsar Winds

Nonlinear electromagnetic waves with superluminal phase velocity can propagate in the winds around isolated pulsars, and around some pulsars in binary systems. Using a short-wavelength approximation, we find and analyze an integrable system of equations that govern their evolution in spherical geometry. A confined mode is identified that stagnates to finite pressure at large radius and can form a precursor to the termination shock. Using a simplified criterion, we find this mode is stable for most isolated pulsars, but may be unstable if the external pressure is high, such as in the pulsar wind nebulae in starburst galaxies and in W44. Pulsar winds in eccentric binary systems, such as PSR 1259-63, may go through phases with stable and unstable electromagnetic precursors, as well as phases in which the density is too high for these modes to propagate.

The stability of strong waves and its implications for pulsar wind shocks

Strong waves can mediate a shock transition between a pulsar wind and its surroundings, playing the role of an extended precursor, in which the energy is effectively transferred from fields to non-thermal particles. The damping of such precursors results in an essentially unmagnetized shock near the equator. In this context, we discuss the stability of strong waves and its implications for the properties of shocks. Those with stable precursors can exist in the winds of most of isolated pulsars, but the precursors may be unstable if the external pressure in the nebula is high, as in Vela-like pulsars. Pulsar wind shocks in eccentric binary systems, such as B1259–63, can acquire precursors only at certain orbital phases, and this process should be accompanied by enhanced synchro-Compton and inverse Compton emission from the precursor. The same scenario may be at work in the binary HESS J0632+057. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-energy emission from pulsar binaries

Unpulsed, high-energy emission from pulsar binaries can be attributed to the interaction of a pulsar wind with that of a companion star. At the shock between the outflows, particles carried away from the pulsar magnetosphere are accelerated and radiate both in synchrotron and inverse Compton processes. This emission constitutes a significant fraction of the pulsar spin-down luminosity. It is not clear however, how the highly magnetized pulsar wind could convert its mainly electromagnetic energy into the particles with such high efficiency. Here we investigate a scenario in which a pulsar striped wind converts into a strong electromagnetic wave before reaching the shock. This mode can be thought of as a shock precursor that is able to accelerate particles to ultrarelativistic energies at the expense of the electromagnetic energy it carries. Radiation of the particles leads to damping of the wave. The efficiency of this process depends on the physical conditions imposed by the external medium. Two regimes c...

Charge-starved jets and rapid variability in blazars

Very rapid variations of the gamma-ray flux from blazars sugg est that there is a mechanism at work which modulates blazar emission on timescales much sma ller than the light-crossing time of the black hole’s event horizon. We propose a scenario in wh ich blazar photons are modulated at the frequencyω of a large-amplitude wave that is launched in the polar regio n of the central, rotating black hole, and propagates in a charge-starved jet . Using a two-fluid ( e±) description, we find the outflow exhibits a delayed acceleration phase that st arts when the inertia associated with the wave currents becomes important. The modulation of the e mission from the accelerating jet is preserved for the observer provided that the density of pa irs, produced in an electromagnetic cascade, is sufficiently low.

Waves in Poynting-flux dominated jets

High-energy emission from blazars is thought to arise in a relativistic jet launched by a supermassive black hole. The rapid variability of the emission suggests that structure of length scale smaller than the gravitational radius of the central black hole is imprinted on the jet as it is launched, and modulates the radiation released after it has been accelerated to high Lorentz factor. We describe a mechanism which can account for the acceleration of the jet, and for the rapid variability of the radiation, based on the propagation characteristics of nonlinear waves in charge-starved, polar jets. These exhibit a delayed acceleration phase, that kicks-in when the inertia associated with the wave currents becomes important. The time structure imprinted on the jet at launch modulates the photons produced by the accelerating jet provided that the electromagnetic cascade in the black-hole magnetosphere is not prolific.