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We study the transport of high-energy particles in pulsar wind nebulae (PWN) using threedimensional magnetohydrodynamic (MHD) and test-particle simulations, as well as a Fokker- Planck particle transport model. The latter includes radiative and adiabatic losses, diffusion, and advection on the background flow of the simulated MHD nebula. By combining the models, the spatial evolution of flux and photon index of the X-ray synchrotron emission is modelled for the three nebulae G21.5-0.9, the inner regions of Vela, and 3C 58, thereby allowing us to derive governing parameters: the magnetic field strength, average flow velocity, and spatial diffusion coefficient. For comparison, the nebulae are also modelled with the semi-analytic Kennel & Coroniti model but the Porth et al. model generally yields better fits to the observational data. We find that high velocity fluctuations in the turbulent nebula (downstream of the termination shock) give rise to efficient diffusive transport of particles, with average Péclet number close to unity, indicating that both advection and diffusion play an important role in particle transport. We find that the diffusive transport coefficient of the order of ~2 ? 1027(Ls/0.42 Ly) cm² s-1 (Ls is the size of the termination shock) is independent of energy up to extreme particle Lorentz factors of γp ~ 1010. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of bow shock nebulae created by pulsars moving supersonically through a partially ionized interstellar medium. A fraction of interstellar neutral hydrogen atoms penetrating into the tail region of a pulsar wind will undergo photoionization due to the ultraviolet light emitted by the nebula, with the resulting mass loading dramatically changing the flow dynamics of the light leptonic pulsar wind. Using a quasi-1D hydrodynamic model of both non-relativistic and relativistic flow, and focusing on scales much larger than the stand-off distance, we find that if a relatively small density of neutral hydrogen, as low as 10-4 cm-3, penetrate inside the pulsar wind, this is sufficient to strongly affect the tail flow. Mass loading leads to the fast expansion of the pulsar wind tail, making the tail flow intrinsically nonstationary. The shapes predicted for the bow shock nebulae compare well with observations, both in Ha and X-rays. [ABSTRACT FROM AUTHOR]

We discuss a novel instability in inertia-less electron magneto-hydrodynamics (EMHD), which arises from a combination of electron velocity shear and electron density gradients. The unstable modes have a lengthscale longer than the transverse density scale, and a growth-rate of the order of the inverse Hall timescale. We suggest that this density-shear instability may be of importance in magnetic reconnection regions on scales smaller than the ion skin depth, and in neutron star crusts. We demonstrate that the so-called Hall drift instability, previously argued to be relevant in neutron star crusts, is a resistive tearing instability rather than an instability of the Hall term itself. We argue that the density-shear instability is of greater significance in neutron stars than the tearing instability, because it generally has a faster growth-rate and is less sensitive to geometry and boundary conditions. We prove that, for uniform electron density, EMHD is "at least as stable" as regular, incompressible MHD, in the sense that any field configuration that is stable in MHD is also stable in EMHD. We present a connection between the density-shear instability in EMHD and the magneto-buoyancy instability in anelastic MHD. [ABSTRACT FROM AUTHOR]

There is growing evidence that the pulsar high-energy emission is generated by the inverse Compton effect. The particles producing the Crab nebula flares, and possibly most of the Crab Nebula's high-energy emission, are accelerated by reconnection events, and not by a shock according to the Fermi mechanism. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

ABSTRACT The unusually short durations, high luminosities and high photon energies of the Crab nebula gamma-ray flares require the relativistic bulk motion of the emitting plasma. We explain the Crab flares as a result of randomly oriented relativistic 'minijets' originating from reconnection events in a magnetically dominated plasma. We develop a statistical model of the emission from Doppler boosted reconnection minijets and find analytical expressions for the moments of the resulting nebula light curve (e.g. time average, variance, skewness). The light curve has a flat power spectrum that transitions at short time-scales to a decreasing power law of index 2. The flux distribution from minijets follows a decreasing power law of index ∼1, implying that the average flux from flares is dominated by bright rare events. The predictions for the flares' statistics can be tested against forthcoming observations. We find that the observed flare spectral energy distributions (SEDs) have several notable features: a hard power-law index of p ≲ 1 for accelerated particles that is expected in various reconnection models, including some evidence of a pile-up near the radiation reaction limit. Also, the photon energy at which the SED peaks is higher than that implied by the synchrotron radiation reaction limit, indicating that the flare emission regions' Doppler factors are ≳ a few. We conclude that magnetic reconnection can be an important, if not dominant, mechanism of particle acceleration within the nebula. [ABSTRACT FROM AUTHOR]

In the double pulsar system PSR J0737-3039A/B, the strong wind produced by pulsar A distorts the magnetosphere of pulsar B. The influence of these distortions on the orbital-dependent emission properties of pulsar B can be used to determine the location of the coherent radio emission generation region in the pulsar magnetosphere. Using a model of the wind-distorted magnetosphere of pulsar B and the well-defined geometrical parameters of the system, we determine the minimum emission height to be ~20RNS in the two bright orbital longitude regions. We can determine the maximum emission height by accounting for the amount of deflection of the polar field line with respect to the magnetic axis using the analytical magnetic reconnection model of Dungey and the semi-empirical numerical model of Tsyganenko. Both of these models estimate the maximum emission height to be ~2500RNS. The minimum and maximum emission heights we calculate are consistent with those estimated for normal isolated pulsars. [ABSTRACT FROM AUTHOR]

The double pulsar PSR J0737-3039A/B displays short, 30 s eclipses that arise around conjunction when the radio waves emitted by pulsar A are absorbed as they propagate through the magnetosphere of its companion pulsar B. These eclipses offer a unique opportunity to directly probe the magnetospheric structure and the plasma properties of pulsar B. We have performed a comprehensive analysis of the eclipse phenomenology using multi-frequency radio observations obtained with the Green Bank Telescope. We have characterized the periodic flux modulations previously discovered at 820 MHz by McLaughlin et al. and investigated the radio frequency dependence of the duration and depth of the eclipses. Based on their weak radio frequency evolution, we conclude that the plasma in pulsar B's magnetosphere requires a large multiplicity factor (~105). We also found that, as expected, flux modulations are present at all radio frequencies in which eclipses can be detected. Their complex behavior is consistent with the confinement of the absorbing plasma in the dipolar magnetic field of pulsar B as suggested by Lyutikov & Thompson and such a geometric connection explains that the observed periodicity is harmonically related to pulsar B's spin frequency. We observe that the eclipses require a sharp transition region beyond which the plasma density drops off abruptly. Such a region defines a plasmasphere that would be well inside the magnetospheric boundary of an undisturbed pulsar. It is also two times smaller than the expected standoff radius calculated using the balance of the wind pressure from pulsar A and the nominally estimated magnetic pressure of pulsar B. [ABSTRACT FROM AUTHOR]

ABSTRACT We present an analytical class of equilibrium solutions for the structure of relativistic sheared and rotating magnetized jets that contain no boundary current sheets. We demonstrate the overall dynamical stability of these solutions and, most importantly, a better numerical resistive stability than the commonly employed force-free structures which inevitably require the presence of dissipative surface currents. The jet is volumetrically confined by the external pressure, with no pressure gradient on the surface. We calculate the expected observed properties of such jets. Given the simplicity of these solutions, we suggest them as useful initial conditions for relativistic jet simulations. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the emission properties that a large-scale helical magnetic field imprints on active galactic nuclei (AGN) jet synchrotron radiation. A cylindrically symmetric relativistic jet and large-scale helical magnetic field produce significant asymmetrical features in transverse profiles of fractional linear polarization, intensity, the Faraday rotation and spectral index. The asymmetrical features of these transverse profiles correlate with one another in ways specified by the handedness of the helical field, the jet viewing angle ( θob) and the bulk Lorentz factor of the flow (Γ). Thus, these correlations may be used to determine the structure of the magnetic field in the jet. In the case of radio galaxies ( θob∼ 1 rad) and a subclass of blazars with particularly small viewing angles ( θob≪ 1/Γ), we find an edge-brightened intensity profile that is similar to that observed in the radio galaxy M87. We present observations of the AGNs 3C 78 and NRAO 140 that display the type of transverse asymmetries that may be produced by large-scale helical magnetic fields. [ABSTRACT FROM AUTHOR]

The oblique geometry of the pulsar wind termination shock ensures that the Doppler beaming has a strong impact on the shock emission. We illustrate this using the recent relativistic magnetohydrodynamic (MHD) simulations of the Crab nebula and the analysis of oblique shocks. We also show that the observed size, shape and distance from the Crab pulsar of the Crab nebula inner knot are consistent with its interpretation as a Doppler-boosted emission from the termination shock. If the electrons responsible for the synchrotron gamma-rays are accelerated only at the termination shock, then their short lifetime ensures that these gamma-rays originate close to the shock and are also strongly affected by the Doppler beaming. As a result, the bulk of the observed synchrotron gamma-rays of the Crab nebula around 100 MeV may come from its inner knot. This hypothesis is consistent with the observed optical flux of the inner knot, provided its optical-gamma spectral index is the same as the injection spectral index found in the Kennel & Coroniti model of the nebula spectrum. The observed variability of synchrotron gamma-ray emission on the time-scale of the wisp production can be caused by the instability of the termination shock discovered in recent numerical simulations. Given the small size of the knot, it is also possible that the 2010 September gamma-ray flare of the Crab nebula also came from the knot, though the actual mechanism remains unclear. The model predicts correlation of the temporal variability of the synchrotron gamma-ray flux in the Fermi and Astro-revilatore Gamma a Immagini LEggero ( AGILE) windows with the variability of the un-pulsed optical flux from within 1 arcsec of the Crab pulsar. [ABSTRACT FROM AUTHOR]

Many astrophysical sources of high-energy emission such as black hole magnetospheres, superstrongly magnetized neutron stars (magnetars) and probably relativistic jets in active galactic nuclei and gamma-ray bursts involve relativistically magnetically dominated plasma. In such plasma the energy density of magnetic field greatly exceeds the thermal and the rest mass energy density of particles. Therefore, the magnetic field is the main reservoir of energy and its dissipation may power the bursting emission from these sources, in close analogy to solar flares. One of the principal dissipative instabilities that may lead to release of magnetic energy is the tearing instability. In this paper we study, both analytically and numerically, the development of tearing instability in relativistically magnetically dominated plasma using the framework of resistive magnetodynamics. We confirm and elucidate the previously obtained result on the growth rate of the tearing mode: the shortest growth time is the same as in the case of classical non-relativistic magnetohydrodynamics (MHD), namely , where τa is the Alfvén crossing time and τd is the resistive time of a current layer. The reason for this coincidence is the close similarity between the governing equations, especially in the quasi-equilibrium approximation. In particular, the role of the mass density of non-relativistic MHD is played by the mass–energy density of the magnetic field, . [ABSTRACT FROM AUTHOR]

Sharp fronts observed by the Chandra satellite between dense cool cluster cores moving with near-sonic velocity through the hotter intergalactic gas, require strong suppression of thermal conductivity across the boundary. This may be due to magnetic fields tangential to the contact surface separating the two plasma components. We point out that a super-Alfvenic motion of a plasma cloud (a core of a merging galaxy) through a weakly magnetized intercluster medium leads to ‘magnetic draping’, formation of a thin, strongly magnetized boundary layer with a tangential magnetic field. For supersonic cloud motion, , magnetic field inside the layer reaches near-equipartition values with thermal pressure. Typical thickness of the layer is , where L is the size of the obstacle (plasma cloud) moving with Alfvén Mach number . To a various degree, magnetic draping occurs for both subsonic and supersonic flows, random and ordered magnetic fields and it does not require plasma compressibility. The strongly magnetized layer will thermally isolate the two media and may contribute to the Kelvin–Helmholtz stability of the interface. Similar effects occur for radio bubbles, quasi-spherical expanding cavities blown up by active galactic nucleus jets; in this case, the thickness of the external magnetized layer is smaller, . [ABSTRACT FROM AUTHOR]

We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron–positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factorΓ. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. Our computations predict and explain the following behaviour. (i) For jets unresolved in the direction perpendicular to their direction of propagation, the position angle of the electric vector of the linear polarization has a bimodal distribution, being oriented either parallel or perpendicular to the jet. (ii) If an ultra-relativistic jet with whose axis makes a small angle to the line of sight,, experiences a relatively small change in the direction of propagation, velocity or pitch angle of the magnetic fields, the polarization is likely to remain parallel or perpendicular; on the other hand, in some cases, the degree of polarization can exhibit large variations and the polarization position angle can experience abrupt 90° changes. This change is more likely to occur in jets with flatter spectra. (iii) In order for the jet polarization to be oriented along the jet axis, the intrinsic toroidal magnetic field (in the frame of the jet) should be of the order of or stronger than the intrinsic poloidal field; in this case, the highly relativistic motion of the jet implies that, in the observer's frame, the jet is strongly dominated by the toroidal magnetic field. (iv) The emission-weighted average pitch angle of the intrinsic helical field in the jet must not be too small to produce polarization along the jet axis. In force-free jets with a smooth distribution of emissivities, the emission should be generated in a limited range of radii not too close to the jet core. (v) For mildly relativistic jets, when a counter-jet can be seen, the polarization of the counter-jet is preferentially orthogonal to the axis, unless the jet is strongly dominated by the toroidal magnetic field in its rest frame. (vi) For resolved jets, the polarization pattern is not symmetric with respect to jet axis. Under certain conditions, this can be used to deduce the direction of the spin of the central object (black hole or disc), whether it is aligned or anti-aligned with the jet axis. (vii) In resolved‘cylindrical shell’ type jets, the central parts of the jet are polarized along the axis, while the outer parts are polarized orthogonal to it, in accordance with observations. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. Since the typical degrees of polarization are≤15 per cent, the emitting parts of the jets must have comparable rest-frame toroidal and poloidal fields. In this case, most relativistic jets are strongly dominated by the toroidal magnetic field component in the observer's frame,. We also discuss the possibility that relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In this case, dissipation of the toroidal magnetic field (and not fluid shocks) may be responsible for particle acceleration. [ABSTRACT FROM AUTHOR]

We consider the magnetohydrodynamical interaction between the relativistic wind outflowing from pulsar A and the static magnetosphere of pulsar B in the binary pulsar system PSR J07373039. We construct a semi-analytical model describing the form of the interface separating the two pulsars. The assumption of vacuum dipole spin-down for pulsar B leads to a duration of the eclipse 10 times longer than observed. We discuss a possible torque modification for pulsar B and magnetic field estimates due to the interaction with the wind from pulsar A. Unless the orbital inclination is < 86°, the duration of eclipses is typically shorter than that implied by the size of the eclipsing region. We propose that eclipses occur as a result of synchrotron absorption by mildly relativistic particles in the shocked wind from pulsar A. The corresponding optical depth may be high enough if the wind density from pulsar A is at the upper allowed limit. We derive jump conditions at oblique, relativistic, magnetohydrodynamical shocks and discuss the structure of the shocked wind from pulsar A. Finally, we speculate on a possible mechanism of orbital modulation of the radio emission from pulsar B. [ABSTRACT FROM AUTHOR]

ABSTRACT The dynamics of relativistic magnetized mass-loaded outflows carrying the toroidal magnetic field is analysed in the context of pulsar wind nebulae (PWNs). Mass-loading is very efficient in slowing down super-relativistic magnetized flows and weakening relativistic shocks. We suggest that the weakening of relativistic reverse shocks by mass-loading in PWNs is responsible for the low radiative efficiencies of the majority of the PWNs. Mass-loading may also result in a shock transition near the fast magnetosonic point; this is unlikely to happen in the majority of PWNs. The evolution of magnetized mass-loaded flows beyond the reverse shock is complicated: after the initial deceleration to the minimal velocity required to transport the magnetic flux, the mass-loaded flows have to accelerate. This formally precludes mass-loaded flows carrying ordered toroidal magnetic field from reaching infinity. The final outcome of mass loading of magnetized winds remains uncertain and may depend on kinetic details of how mass loading occurs. [ABSTRACT FROM AUTHOR]

We describe the possible electromagnetic signals expected from the magnetospheric interactions of a neutron star binary prior to merger. We find that both radio and X-ray signals of detectable strength are possible. We discuss possible links with the phenomenon of gamma-ray bursts (GRBs) and describe the prospects for direct detection of these signals in searches for radio and X-ray transients. [ABSTRACT FROM AUTHOR]

We analyse mechanisms of the excitation of Alfvén waves in pulsar magnetospheres as a possible source of pulsar radio emission generation. We find that Cherenkov excitation of obliquely propagating Alfvén waves is inefficient, while excitation at the anomalous cyclotron resonance by the particles from the primary beam and from the tail of the bulk distribution function may have a considerable growth rate. The cyclotron instability on Alfvén waves occurs in the kinetic regime still not very closed to the star: r&ges;50 R[sub NS]. We also discuss various mechanisms of conversion of Alfvén waves into escaping radiation. Unfortunately, no decisive conclusion about the effectiveness of such conversion can be made. [ABSTRACT FROM AUTHOR]

A theory of pulsar radio emission generation, in which the observed waves are produced directly by maser-type plasma instabilities operating at the anomalous cyclotron--Cherenkov resonance ο k[sub ∥] v[sub ∥]+ ο[sub B] γ[sub res]=0 and the Cherenkov drift resonance ο - k[sub ∥] v[sub ∥]- k[sub ⊥] u[sub d] =0, is capable of explaining the main observational characteristics of pulsar radio emission. The instabilities are due to the interaction of the fast particles from the primary beam and the tail of the distribution with the normal modes of a strongly magnetized one-dimensional electron--positron plasma. The waves emitted at these resonances are vacuum-like, electromagnetic waves that may leave the magnetosphere directly. In this model, the cyclotron--Cherenkov instability is responsible for the core-emission pattern and the Cherenkov drift instability produces conal emission. The conditions for the development of the cyclotron--Cherenkov instability are satisfied for both typical and millisecond pulsars provided that the streaming energy of the bulk plasma is not very high γ[sub p] ≈ 10. In a typical pulsar the cyclotron--Cherenkov and Cherenkov drift resonances occur in the outer parts of the magnetosphere at r[sub res], ≈ 10[sub 9]cm. This theory can account for various aspects of pulsar phenomenology, including the morphology of the pulses, their polarization properties and their spectral behaviour. We propose several observational tests for the theory. The most prominent prediction is the high altitudes of the emission region and the linear polarization of conal emission in the plane orthogonal to the local osculating plane of the magnetic field. [ABSTRACT FROM AUTHOR]

We observed the Crab pulsar with the 43 m telescope in Green Bank, WV over a timespan of 15 months. In total we obtained 100 hr of data at 1.2 GHz and seven hours at 330 MHz, resulting in a sample of about 95,000 giant pulses (GPs). This is the largest sample, to date, of GPs from the Crab pulsar taken with the same telescope and backend and analyzed as one data set. We calculated power-law fits to amplitude distributions for main pulse (MP) and interpulse (IP) GPs, resulting in indices in the range of 2.1-3.1 for MP GPs at 1.2 GHz and in the range of 2.5-3.0 and 2.4-3.1 for MP and IP GPs at 330 MHz. We also correlated the GPs at 1.2 GHz with GPs from the Robert C. Byrd Green Bank Telescope (GBT), which were obtained simultaneously at a higher frequency (8.9 GHz) over a span of 26 hr. In total, 7933 GPs from the 43 m telescope at 1.2 GHz and 39,900 GPs from the GBT were recorded during these contemporaneous observations. At 1.2 GHz, 236 (3%) MP GPs and 23 (5%) IP GPs were detected at 8.9 GHz, both with zero chance probability. Another 15 (4%) low-frequency IP GPs were detected within one spin period of high-frequency IP GPs, with a chance probability of 9%. This indicates that the emission processes at high and low radio frequencies are related, despite significant pulse profile shape differences. The 43 m GPs were also correlated with Fermi γ-ray photons to see if increased pair production in the magnetosphere is the mechanism responsible for GP emission. A total of 92,022 GPs and 393 γ-ray photons were used in this correlation analysis. No significant correlations were found between GPs and γ-ray photons. This indicates that increased pair production in the magnetosphere is likely not the dominant cause of GPs. Possible methods of GP production may be increased coherence of synchrotron emission or changes in beaming direction. [ABSTRACT FROM AUTHOR]

The Swift X-Ray Telescope often observes a rapidly decaying X-ray emission stretching to as long as after a conventional prompt phase. This component is most likely due to a prompt emission viewed at large observer angles , where is a typical viewing angle of the jet and is the Lorentz factor of the flow during the prompt phase. This can be used to estimate the prompt emission radii, . These radii are much larger than is assumed within the framework of a fireball model. Such large emission radii can be reconciled with a fast variability, on time-scales as short as milliseconds, if the emission is beamed in the bulk outflow frame, e.g. because of a random relativistic motion of ‘fundamental emitters’. This may also offer a possible explanation for X-ray flares observed during early afterglows. [ABSTRACT FROM AUTHOR]