arXiv subjects

John G. Kirk

Publications and source records attributed to John G. Kirk.

At least 37 records · Page 2Linked to original sources

arXiv2013

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 can be distinguished: a high density one, where the EM wave cannot be launched at all and the current sheets in the striped wind are first compressed by an MHD shock and subsequently dissipate by reconnection, and a low density one, where the wind can first convert into an electromagnetic wave in the shock precursor, which then damps and merges into the surroundings. Shocks in binary systems can transit from one regime to another according to binary phase. We discuss possible observational implications for these objects.

astro-ph.HE↗

arXiv2012

Linearly polarized superluminal waves in pulsar winds

Pulsar winds are the ideal environment for the study of non-linear electromagnetic waves. It is generally thought that a pulsar launches a striped wind, a magnetohydrodynamic entropy wave, where plasma sheets carried along with the flow separate regions of alternating magnetic field. But when the density drops below a critical value, or equivalently for distances from the pulsar greater than a critical radius, a strong superluminal wave can also propagate. In this contribution we discuss the conversion of the equatorial striped wind into a linearly polarized superluminal wave, and we argue that this mode is important for the conversion of Poynting flux to kinetic energy flux before the outflow reaches the termination shock.

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arXiv2011

Of Winds and Waves

Recent work on the properties of superluminal waves in pulsar winds is summarized. It is speculated that these waves play an important role in the termination shock that divides the wind from the surrounding nebula.

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arXiv2011

Superluminal waves in pulsar winds

The energy lost by a rotation-powered pulsar is carried by a relativistic flow containing a mixture of electromagnetic fields and particles. In the inner regions, this is thought to be a magnetically dominated, cold, electron-positron wind that is well described by the MHD equations. However, beyond a critical radius r_{cr}, the same particle, energy and momentum fluxes can be transported by a strong, transverse electromagnetic wave with superluminal phase speed. We analyze the nonlinear dispersion relation of these waves for linear and circular polarization, and find the dependence of r_{cr} on the mass-loading, magnetization and luminosity of the flow, as well as on the net magnetic flux. We show that, for most isolated pulsars, the wind lies well outside r_{cr}, and speculate that superluminal modes play an important role in the dissipation of electromagnetic energy into nonthermal particles at the termination shock.

astro-ph.HE↗

arXiv2011

Charge-starved, relativistic jets and blazar variability

High energy emission from blazars is thought to arise in a relativistic jet launched by a supermassive black hole. The emission site must be far from the hole and the jet relativistic, in order to avoid absorption of the photons. In extreme cases, rapid variability of the emission suggests that structures of length-scale smaller than the gravitational radius of the central black hole are imprinted on the jet as it is launched, and modulate the radiation released after it has been accelerated to high Lorentz factor. We propose 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 large-amplitude waves in charge-starved, polar jets. Using a two-fluid (electron-positron) description, we find the outflows exhibit a delayed acceleration phase, that starts at roughly 1pc, where the inertia associated with the wave currents becomes important. The time-structure imprinted on the jet at launch modulates photons produced by the accelerating jet provided the pair multiplicity in the black-hole magnetosphere is sufficiently small, suggesting that very rapid variability is confined to sources in which the electromagnetic cascade in the black-hole magnetosphere is not prolific.

astro-ph.HE↗

arXiv2010

Computation of synthetic spectra from simulations of relativistic shocks

Particle-in-cell (PIC) simulations of relativistic shocks are in principle capable of predicting the spectra of photons that are radiated incoherently by the accelerated particles. The most direct method evaluates the spectrum using the fields given by the Lienard-Wiechart potentials. However, for relativistic particles this procedure is computationally expensive. Here we present an alternative method, that uses the concept of the photon formation length. The algorithm is suitable for evaluating spectra both from particles moving in a specific realization of a turbulent electromagnetic field, or from trajectories given as a finite, discrete time series by a PIC simulation. The main advantage of the method is that it identifies the intrinsic spectral features, and filters out those that are artifacts of the limited time resolution and finite duration of input trajectories.

astro-ph.HE↗

arXiv2010

Linear acceleration emission in pulsar magnetospheres

(abbrev.) Linear acceleration emission occurs when a charged particle is accelerated parallel to its velocity. We evaluate the spectral and angular distribution of this radiation for several special cases, including constant acceleration (hyperbolic motion) of finite duration. Based on these results, we find the following general properties of the emission from an electron in a linear accelerator that can be characterized by an electric field E acting over a distance L: (i) the spectrum extends to a cut-off photon energy ~ LE^2 MeV, where E is in units of the Schwinger critical field and L in units of the Compton wavelength of the electron. (ii) the total energy emitted by a particle traversing the accelerator is in agreement with the standard Larmor formula (iii) the low frequency spectrum is flat for hyperbolic trajectories, but in general depends on the details of the accelerator. We also show that linear acceleration emission complements curvature radiation in the strongly magnetized pair formation regions in pulsar magnetospheres. It dominates when the length L of the accelerator is less than the formation length of curvature photons, which is given by the ratio of the radius of curvature of the magnetic field lines to the Lorentz factor of the particle. In standard static models of pair creating regions linear acceleration emission is negligible, but it is important in more realistic dynamical models in which the accelerating field fluctuates on a short length-scale.

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arXiv2010

Radiative Signatures of Relativistic Shocks

(Abbreviated) Particle-in-cell simulations of relativistic, weakly magnetized collisionless shocks show that particles can gain energy by repeatedly crossing the shock front. This requires scattering off self-generated small length-scale magnetic fluctuations. The radiative signature of this first-order Fermi acceleration mechanism is important for models of both the prompt and afterglow emission in gamma-ray bursts and depends on the strength parameter "a" of the fluctuations. For electrons (and positrons), acceleration saturates when the radiative losses produced by the scattering cannot be compensated by the energy gained on crossing the shock. We show that this sets an upper limit on both the electron Lorentz factor and on the energy of the photons radiated during the scattering process. This rules out "jitter" radiation on self-excited fluctuations with a < 1 as a source of gamma-rays, although high-energy photons might still be produced when the jitter photons are upscattered in an analog of the synchrotron self-Compton process. In fluctuations with a > 1, radiation is generated by the standard synchrotron mechanism, and the maximum photon energy rises linearly with a, until saturating at approximately 70 MeV.

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arXiv2009

Two-Dimensional particle-in-cell simulations of the nonresonant, cosmic-ray driven instability in SNR shocks

In supernova remnants, the nonlinear amplification of magnetic fields upstream of collisionless shocks is essential for the acceleration of cosmic rays to the energy of the "knee" at 10^{15.5}eV. A nonresonant instability driven by the cosmic ray current is thought to be responsible for this effect. We perform two-dimensional, particle-in-cell simulations of this instability. We observe an initial growth of circularly polarized non-propagating magnetic waves as predicted in linear theory. It is demonstrated that in some cases the magnetic energy density in the growing waves, can grow to at least 10 times its initial value. We find no evidence of competing modes, nor of significant modification by thermal effects. At late times we observe saturation of the instability in the simulation, but the mechanism responsible is an artefact of the periodic boundary conditions and has no counterpart in the supernova-shock scenario.

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arXiv2008

Prolific pair production with high-power lasers

Prolific electron-positron pair production is possible at laser intensities approaching 10^{24} W/cm^2 at a wavelength of 1 micron. An analysis of electron trajectories and interactions at the nodes (B=0) of two counter-propagating, circularly polarised laser beams shows that a cascade of gamma-rays and pairs develops. The geometry is generalised qualitatively to linear polarisation and laser beams incident on a solid target.

hep-ph↗

arXiv2008

Relativistic Shock Acceleration: A Hartree-Fock Approach

We examine the problem of particle acceleration at a relativistic shocks assuming pitch-angle scattering and using a Hartree-Fock method to approximate the associated eigenfunctions. This leads to a simple transcendental equation determining the power-law index, $s$, given the up and downstream velocities. We compare our results with accurate numerical solutions obtained using the eigenfunction method. In addition to the power-law index this method yields the angular and spatial distributions upstream of the shock.

astro-ph↗

arXiv2008

Gamma-ray polarization constraints on Planck scale violations of special relativity

Using recent polarimetric observations of the Crab Nebula in the hard X-ray band by INTEGRAL, we show that the absence of vacuum birefringence effects constrains O(E/M) Lorentz violation in QED to the level |ξ| < 9x10^{-10} at three sigma CL, tightening by more than three orders of magnitude previous constraints. We show that planned X-ray polarimeters have the potential the potential to probe |ξ|~ 10^{-16} by detecting polarization in active galaxies at red-shift ~1.

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arXiv2007

New constraints on Planck-scale Lorentz Violation in QED from the Crab Nebula

We set constraints on O(E/M) Lorentz Violation in QED in an effective field theory framework. A major consequence of such assumptions is the modification of the dispersion relations for electrons/positrons and photons, which in turn can affect the electromagnetic output of astrophysical objects. We compare the information provided by multiwavelength observations with a full and self-consistent computation of the broad-band spectrum of the Crab Nebula. We cast constraints of order 10^{-5} at 95% confidence level on the lepton Lorentz Violation parameters.

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arXiv2003

Modelling the TeV gamma-ray spectra of two low redshift AGNs: Mkn 501 and Mkn 421

We discuss the results of modelling the TeV gamma-ray spectra of two AGNs, Mkn 501 and Mkn 421 that have almost the same redshifts: z=0.031 and z=0.034, respectively. The effect of intergalactic gamma-ray absorption is treated as an uncertainty in the measurement of the intrinsic spectrum. Although the objects differ, we obtain satisfactory fits for both of them in a synchrotron self-Compton scenario. Compared to previous models, our fits are characterised by higher values of the Doppler factor (>= 50) and an electron injection spectrum extending to higher energies (Gmax = 1.5x10^5). In the case of Mkn 421, the observed difference in spectral slope in X-rays and TeV gamma-rays between the high and low states can be explained as a variation of a single parameter - the maximum energy Gmax mc^2 at which electrons are injected.

astro-ph↗

arXiv2001

Modelling Plerion Spectra and their Evolution

We review recent theoretical developments on pulsar winds, their nebulae and relativistic shock acceleration, and show how they illuminate unsolved problems in plerion spectra, in particular the multiple spectral breaks in the Crab and the low-frequency breaks of plerions such as G 21.5-0.9 and 3C 58. Recent work on Fermi acceleration theory at relativistic shocks shows that a particle spectral index of 2.2-2.3, compatible with the X-ray spectra of plerions, results under a wide variety of assumptions. If pulsar winds contain ions as well as electrons and positrons, the mechanism of Hoshino et al. (1992), which yields harder spectra, would operate at lower energies and may explain the flat radio spectral indices of plerions. This scenario implies wind parameters in the Crab compatible with the pulsar wind acceleration model of Lyubarsky & Kirk (2001). Recent hydrodynamical simulations of plerion evolution inside SNR blast waves demonstrate that the passage of the reverse shock rapidly compresses the plerion. Using a simple isobaric model, we investigate the influence of the resulting magnetic field compression and decrease in shock radius on the evolution of the plerion spectrum. We suggest that the passage of the reverse shock may explain the low-frequency breaks in 3C 58 and G 21.5-0.9, as well as the increase in 3C 58's radio flux.

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arXiv2001

Particle acceleration by ultra-relativistic shocks: theory and simulations

We consider the acceleration of charged particles near ultra-relativistic shocks, with Lorentz factor Gamma_s >> 1. We present simulations of the acceleration process and compare these with results from semi-analytical calculations. We show that the spectrum that results from acceleration near ultra-relativistic shocks is a power law, N(E) \propto E^{-s}, with a nearly universal value s \approx 2.2 - 2.3 for the slope of this power law. We confirm that the ultra-relativistic equivalent of Fermi acceleration at a shock differs from its non-relativistic counterpart by the occurence of large anisotropies in the distribution of the accelerated particles near the shock. In the rest frame of the upstream fluid, particles can only outrun the shock when their direction of motion lies within a small loss cone of opening angle theta_c \approx 1/Gamma_s around the shock normal. We also show that all physically plausible deflection or scattering mechanisms can change the upstream flight direction of relativistic particles originating from downstream by only a small amount: Delta theta ~ 1/Gamma_s. This limits the energy change per shock crossing cycle to Delta E ~ E, except for the first cycle where particles originate upstream. In that case the upstream energy is boosted by a factor ~ Gamma_s^2 for those particles that are scattered back across the shock into the upstream region.

astro-ph↗

arXiv2000

Size-Luminosity scaling and Inverse Compton Seed Photons in Blazars

We present preliminary results of our work on blazar unification. We assume that all blazars have a broad line region (BLR) and that the size of the BLR scales with the power of the source in a manner similar to that derived through reverberation mapping in radio quiet active galactic nuclei (AGNs). Using a self-consistent emission model that includes particle acceleration we show that according to this scaling, in weak sources like MKN 421, the inverse Compton (IC) scattering losses are dominated by synchrotron-self Compton scattering (SSC), while in powerful sources, like 3C 279, they are dominated by external Compton (EC) scattering of BLR photons. In agreement with other workers, we show that even in the powerful sources that are dominated by EC scattering, the hard X-ray emission is due to SSC. Finally, we show that this scaling reproduces well the observed sequence of blazar properties with luminosity.

astro-ph↗

arXiv2000

An eigenfunction method for particle acceleration at ultra-relativistic shocks

We adapt and modify the eigenfunction method of computing the power-law spectrum of particles accelerated at a relativistic shock front via the first-order Fermi process (Kirk, J.G., Schneider, P., Astrophysical Journal 315, 425 (1987)) to apply to shocks of arbitrarily high Lorentz factor. The power-law index of accelerated particles undergoing isotropic small-angle scattering at an ultrarelativistic, unmagnetized shock is found to be s=4.23 +/- 0.2 (where s=d\ln f/ d\ln p, with f the Lorentz-invariant phase-space density and p the momentum), in agreement with the results of Monte-Carlo simulations. We present results for shocks in plasmas with different equations of state and for Lorentz factors ranging from 5 to infinity.

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