arXiv subjects

Anatoly Spitkovsky

Publications and source records attributed to Anatoly Spitkovsky.

97 records · Page 6Linked to original sources

arXiv2004

Probing Relativistic Winds: The case of PSRJ07370-3039 A & B

We propose synchrotron absorption in a magnetosheath forming a cocoon around the magnetosphere of pulsar B to be the origin of the eclipse phenomena seen in the recently discovered double pulsar system PSRJ07370-3039 A & B. The model predicts the eclipses will clear at frequencies higher than those of the observations reported to date (nominally, above $ν\sim 5$ GHz.) The model also predicts synchrotron emission at the level of a few to 10 $μ$Jy, peaking at $ν\sim 2-5$ GHz with possible orbital modulation. We use simplified semi-analytic models and particle-in-cell simulations to elucidate the structure of the B magnetosphere, showing that B's magnetic field is confined to within a radius less than 50,000 km from B,on the side facing A. We estimate a ``propellor'' spindown torque on B, which is the dominant torque on this star, which yields a polar dipole field $\sim 7 \times 10^{11}$ Gauss (magnetic moment $μ_B \sim 3.5 \times 10^{29}$ cgs). The model can explain the known eclipses only if the A wind's density is at least 4 orders of magnitude greater than is expected from popular pair creation models, and discuss the implications of this result for our general understanding of pulsar physics. Our proposal was outlined in Kaspi et al. (2004) and Demorest et al. (2004); a similar proposal has been made by Lyutikov (2004).

astro-ph↗

arXiv2004

Time-dependence in Relativistic Collisionless Shocks: Theory of the Variable "Wisps" in the Crab Nebula

We describe results from time-dependent numerical modeling of the collisionless reverse shock terminating the pulsar wind in the Crab Nebula. We treat the upstream relativistic wind as composed of ions and electron-positron plasma embedded in a toroidal magnetic field, flowing radially outward from the pulsar in a sector around the rotational equator. The relativistic cyclotron instability of the ion gyrational orbit downstream of the leading shock in the electron-positron pairs launches outward propagating magnetosonic waves. Because of the fresh supply of ions crossing the shock, this time-dependent process achieves a limit-cycle, in which the waves are launched with periodicity on the order of the ion Larmor time. Compressions in the magnetic field and pair density associated with these waves, as well as their propagation speed, semi-quantitatively reproduce the behavior of the wisp and ring features described in recent observations obtained using the Hubble Space Telescope and the Chandra X-Ray Observatory. By selecting the parameters of the ion orbits to fit the spatial separation of the wisps, we predict the period of time variability of the wisps that is consistent with the data. When coupled with a mechanism for non-thermal acceleration of the pairs, the compressions in the magnetic field and plasma density associated with the optical wisp structure naturally account for the location of X-ray features in the Crab. We also discuss the origin of the high energy ions and their acceleration in the equatorial current sheet of the pulsar wind.

astro-ph↗

arXiv2003

Electrodynamics of pulsar magnetospheres

I review the theoretical understanding of the global structure of pulsar magnetospheres concentrating on recent progress in force-free electrodynamics and first-principles simulations of magnetospheres.

astro-ph↗

arXiv2002

Simulations of pulsar wind formation

We present initial results of the first self-consistent numerical model of the outer magnetosphere of a pulsar. By using the relativistic ``particle-in-cell'' method with special boundary conditions to represent plasma dynamics in 3D, we are able to follow magnetospheric plasma through the light cylinder into the wind zone for arbitrary magnetic inclination angles. For aligned rotators we confirm the ``disk-dome'' charge-separated structure of the magnetosphere and find that this configuration is unstable to a 3D nonaxisymmetric diocotron instability. This instability allows plasma to move across the field lines and approach the corotating Goldreich-Julian solution within several rotation periods. For oblique rotators formation of the spiral ``striped wind'' in the equatorial direction is demonstrated and the acceleration of the wind and its magnetization is discussed. We find that the wind properties vary with stellar latitude; however, whether injection conditions at the pulsar are responsible for the observed jet-equator geometry of Crab and Vela is currently under investigation. We also comment on the electrodynamics of the simulated magnetospheres, their current closure, and future simulations.

astro-ph↗

arXiv2001

Propagation of thermonuclear flames on rapidly rotating neutron stars: extreme weather during type I X-ray bursts

We analyze the global hydrodynamic flow in the ocean of an accreting, rapidly rotating, non-magnetic neutron star in an LMXB during a type I X-ray burst. Our analysis takes into account the rapid rotation of the star and the lift-up of the burning ocean during the burst. We find a new regime for spreading of a nuclear burning front, where the flame is carried along a coherent shear flow across the front. If turbulent viscosity is weak, the speed of flame propagation is ~20 km/s, while, if turbulent viscosity is dynamically important, the flame speed increases, and reaches the maximum value, ~300 km/s, when the eddy overturn frequency is comparable to the Coriolis parameter. We show that, due to rotationally reduced gravity, the thermonuclear runaway is likely to begin on the equator. The equatorial belt is ignited first, and the flame then propagates from the equator to the poles. Inhomogeneous cooling (equator first, poles second) drives strong zonal currents which may be unstable to formation of Jupiter-type vortices; we conjecture that these vortices are responsible for modulation of X-ray flux in the tails of some bursts. We consider the effect of strong zonal currents on the frequency of modulation of the X-ray flux and show that the large values of the frequency drifts observed in some bursts can be accounted for within our model combined with the model of homogeneous radial expansion. Also, if inhomogeneities are trapped in the forward zonal flows around the propagating burning front, chirps with large frequency ranges (~25-500 Hz) may be detectable during the burst rise. We argue that an MHD dynamo within the burning front can generate a small-scale magnetic field, which may enforce vertically rigid flow in the front's wake and can explain the coherence of oscillations in the burst tail.

astro-ph↗

arXiv2000

Longitudinal Laser Shaping in Laser Wakefield Accelerators

We study the energetics of wake excitation during the laser-plasma interaction in application to laser wakefield accelerators. We find that both the wake amplitude and the accelerating efficiency (transformer ratio) can be maximized by properly shaping the longitudinal profile of the driving laser pulse. The corresponding family of laser pulse shapes is derived in the nonlinear regime of laser-plasma interaction. Such shapes provide theoretical upper limit on the magnitude of the wakefield and efficiency by allowing for uniform photon deceleration inside the laser pulse. We also construct realistic optimal pulse shapes that can be produced in finite-bandwidth laser systems.

physics.acc-ph↗

arXiv2000

Laser Shaping and Optimization of the Laser-Plasma Interaction

The physics of energy transfer between the laser and the plasma in laser wakefield accelerators is studied. We find that wake excitation by arbitrary laser shapes can be parameterized using the total pulse energy and pulse depletion length. A technique for determining laser profiles that produce the required plasma excitation is developed. We show that by properly shaping the longitudinal profile of the driving laser pulse, it is possible to maximize both the transformer ratio and the wake amplitude, achieving optimal laser-plasma coupling. The corresponding family of laser pulse shapes is derived in the nonlinear regime of laser-plasma interaction. Such shapes provide theoretical upper limit on the magnitude of the wakefield and efficiency of the accelerating stage by allowing for uniform photon deceleration inside the laser pulse. We also construct realistic optimal pulse shapes that can be produced in finite-bandwidth laser systems and propose a two-pulse wake amplification scheme using the optimal solution.

physics.acc-ph↗