Radiation force on relativistic jets in active galactic nuclei

Radiative deceleration of relativistic jets in active galactic nuclei as the result of inverse Compton scattering of soft photons from accretion discs is discussed. The Klein-Nishina (KN) cross section is used in the calculation of the radiation force due to inverse Compton scattering. Our result shows that deceleration due to scattering in the KN regime is important only for jets starting with a bulk Lorentz factor larger than 1000. When the bulk Lorentz factor satisfies this condition, particles scattering in the Thomson regime contribute positively to the radiation force (acceleration), but those particles scattering in the KN regime are dominant and the overall effect is deceleration. In the KN limit, the drag due to Compton scattering, though less severe than in the Thomson limit, strongly constrains the bulk Lorentz factor. Most of the power from the deceleration goes into radiation and hence the ability of the jet to transport significant power (in particle kinetic energy) out of the subparsec region is severely limited. The deceleration efficiency decreases significantly if the jet contains protons and the proton to electron number density ratio satisfies the condition $n_p/n_{e0}>2\gamma_{\rm min}/\mu_p$ where $\gamma_{\rm min}$ is the minimum Lorentz factor of relativistic electrons (or positrons) in the jet frame and $\mu_p$ is the proton to electron mass ratio.Comment: 10 pages including 8 figures; accepted for publication in MNRA

Resonant inverse Compton scattering above polar caps: Gap acceleration efficiency for young pulsars

It is shown that for moderately hot polar caps (with effective temperature of $\sim 10^6$ K), the efficiency of polar gap acceleration is lower compared to the case in which the polar caps are relatively cool and inverse Compton scattering plays no role in controlling the gap. For young pulsars with superstrong magnetic fields ($\geq10^9 T$) and hot polar caps (with temperature of $\ge 5\times10^6 K$), because of the energy loss of electrons or positrons due to resonant inverse Compton scattering in the vicinity of polar caps, pair cascades occur at distances further away from the polar cap, and in this case, we have a relatively high acceleration efficiency with ions carrying most of the particle luminosity.Comment: 11 pages including 4 figures; figures adde

Large-Amplitude, Pair-Creating Oscillations in Pulsar and Black Hole Magnetospheres

A time-dependent model for pair creation in a pulsar magnetosphere is developed. It is argued that the parallel electric field that develops in a charge-starved region (a gap) of a pulsar magnetosphere oscillates with large amplitude. Electrons and positrons are accelerated periodically and the amplitude of the oscillations is assumed large enough to cause creation of upgoing and downgoing pairs at different phases of the oscillation. With a charge-starved initial condition, we find that the oscillations result in bursts of pair creation in which the pair density rises exponentially with time. The pair density saturates at $N_\pm\simeq E_{0}^2/(8\pi m_ec^2\Gamma_{\rm thr})$, where $E_0$ is the parallel electric field in the charge-starved initial state, and $\Gamma_{\rm thr}$ is the Lorentz factor for effec tive pair creation. The frequency of oscillations following the pair creation burst is given roughly by $\omega_{\rm osc}=eE_0/(8m_ec\Gamma_{\rm thr})$. A positive feedback keeps the system stable, such that the average pair creation rate balances the loss rate due to pairs escaping the magnetosphere.Comment: 21 pages, 6 figures, ApJ submitte

The evolution of extragalactic radio sources

A model for the evolution of low-luminosity radio galaxies is presented. In the model, the lobes inflated by low-power jets are assumed to expand in near pressure-balance against the external medium. Both cases of constant external pressure and decreasing external pressure are considered. Evolution of an individual source is described by the power-size track. The source appears as its lobe is inflated and radio luminosity increases to above the detection level; the source then moves along the track and eventually disappears as its luminosity drops below the detection limit. The power-size tracks are calculated including the combined energy losses due to synchrotron radiation, adiabatic expansion, and inverse Compton scattering. It is shown that in general, the constant-pressure model predicts an excess number of luminous, small-size sources while underpredicting large-size sources in the power-size diagram. The predicted spectra are steep for most sources, which is inconsistent with observations. By comparison, the pressure-limiting model fits observations better. In this model, low-luminosity sources undergo substantial expansion losses in the initial phase and as a result, it predicts fewer luminous, small-size sources. The resultant spectra are flat for most sources except for the oldest ones, which seems consistent with observations. The power-size tracks, in contrast to that of high-luminosity radio galaxies, are characterized by a slow increase in luminosity for most of the source's life, followed by a rapid decline when the synchrotron or inverse Compton scattering losses set in.Comment: 13 pages, 8 figures, 2 tables, accepted for publication in Ap

Anisotropic weak turbulence of Alfven waves in collisionless astrophysical plasmas

The evolution of Alfven turbulence due to three-wave interactions is discussed using kinetic theory for a collisionless, thermal plasma. There are three low-frequency modes, analogous to the three modes of compressible MHD. When only Alfven waves are considered, the known anisotropy of turbulence in incompressible MHD theory is reproduced. Inclusion of a fast mode wave leads to separation of turbulence into two regimes: small wave numbers where three-wave processes involving a fast mode is dominant, and large wave numbers where the three Alfven wave process is dominant. Possible application of the anisotropic Alfven turbulence to the interstellar medium and dissipation of magnetic energy in magnetars is discussed.Comment: 9 pages, accepted for publication in MNRA

Coherent synchrotron emission from cosmic ray air showers

Coherent synchrotron emission by particles moving along semi-infinite tracks is discussed, with a specific application to radio emission from air showers induced by high-energy cosmic rays. It is shown that in general, radiation from a particle moving along a semi-infinite orbit consists of usual synchrotron emission and modified impulsive bremsstrahlung. The latter component is due to the instantaneous onset of the curved trajectory of the emitting particle at its creation. Inclusion of the bremsstrahlung leads to broadening of the radiation pattern and a slower decay of the spectrum at the cut-off frequency than the conventional synchrotron emission. Possible implications of these features for air shower radio emission are discussed.Comment: 8 pages, 10 figures, accepted for publication in MNRA

The induced turbulence effect on propagation of radio emission in pulsar magnetospheres

The effect of photon-beam-induced turbulence on propagation of radio emission in a pulsar magnetosphere is discussed. Beamed radio emission with a high brightness temperature can generate low-frequency plasma waves in the pulsar magnetosphere and these waves scatter the radio beam. We consider this effect on propagation of radio emission both in the open field line region and in the closed field line region. The former is applicable to most cases of pulsar radio emission where the propagation is confined to the polar region; it is shown that the induced process is not effective for radio emission of moderately high brightness temperature but can have a severe effect on giant pulses. For giant pulses not to be affected by this process, they must be emitted very close to the light cylinder. We show that the induced process is efficient in the closed field line region, inhibiting propagation of the radio emission in this region.Comment: 11 pages, 3 figures, accepted for publication in MNRA

Oscillating pulsar polar gaps

An analytical model for oscillating pair creation above the pulsar polar cap is presented in which the parallel electric field is treated as a large amplitude, superluminal, electrostatic wave. An exact formalism for such wave is derived in one-dimension and applied to both the low-density regime in which the pair plasma density is much lower than the corotating charge density and the high-density regime in which the pair plasma density is much higher than the corotating charge density. In the low-density regime, which is relevant during the phase leading to a pair cascade, a parallel electric field develops resulting in rapid acceleration of particles. The rapid acceleration leads to bursts of pair production and the system switches to the oscillatory phase, corresponding to the high density regime, in which pairs oscillate with net drift motion in the direction of wave propagation. Oscillating pairs lead to a current that oscillates with large amplitude about the Goldreich-Julian current. The drift motion can be highly relativistic if the phase speed of large amplitude waves is moderately higher than the speed of light. Thus, the model predicts a relativistic outflow of pairs, a feature that is required for avoiding overheating of the pulsar polar cap and is also needed for the pulsar wind.Comment: 13 pages, 8 figures, accepted for publication in MNRA

Saturated magnetic field amplification at supernova shocks

Cosmic-ray streaming instabilities at supernova shocks are discussed in the quasilinear diffusion formalism which takes into account the feedback effect of wave growth on the cosmic ray streaming motion. In particular, the nonresonant instability that leads to magnetic field amplification in the short wavelength regime is considered. The linear growth rate is calculated using kinetic theory for a streaming distribution. We show that the nonresonant instability is actually driven by a compensating current in the background plasma. The nonresonant instability can develop into a nonlinear regime generating turbulence. The saturation of the amplified magnetic fields due to particle diffusion in the turbulence is derived analytically. It is shown that the evolution of parallel and perpendicular cosmic-ray pressures is predominantly determined by nonresonant diffusion. However, the saturation is determined by resonant diffusion which tends to reduce the streaming motion through pitch angle scattering. The saturated level can exceed the mean background magnetic field.Comment: 8 pages. Accepted for publication in MNRA