Yuri Lyubarsky

A giant γ-ray flare from the magnetar SGR 1806-20

Two classes of rotating neutron stars—soft γ-ray repeaters (SGRs) and anomalous X-ray pulsars—are magnetars, whose X-ray emission is powered by a very strong magnetic field (B ≈ 1015 G). SGRs occasionally become ‘active’, producing many short X-ray bursts. Extremely rarely, an SGR emits a giant flare with a total energy about a thousand times higher than in a typical burst. Here we report that SGR 1806–20 emitted a giant flare on 27 December 2004. The total (isotropic) flare energy is 2 × 1046 erg, which is about a hundred times higher than the other two previously observed giant flares. The energy release probably occurred during a catastrophic reconfiguration of the neutron stars magnetic field. If the event had occurred at a larger distance, but within 40 megaparsecs, it would have resembled a short, hard γ-ray burst, suggesting that flares from extragalactic SGRs may form a subclass of such bursts.1 Los Alamos National Laboratory, Los Alamos, NM, 87545, USA 2 NASA/Goddard Space Flight Center, Greenbelt, MD, 20771, USA 3 NASA/Marshall Space Flight Center, NSSTC, XD-12, 320 Sparkman Dr., Huntsville, AL 35805, USA 4 Department of Physics, Ben Gurion University, POB 653, Beer Sheva 84105, Israel 5 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ, Amster-

Reconnection in a striped pulsar wind

It is generally thought that most of the spin-down power of a pulsar is carried away in an MHD wind dominated by Poynting flux. In the case of an oblique rotator, a significant part of this energy can be considered to be in a low-frequency wave, consisting of stripes of a toroidal magnetic field of alternating polarity propagating in a region around the equatorial plane. Magnetic reconnection in such a structure has been proposed as a mechanism for transforming the Poynting flux into particle energy in the pulsar wind. We have reexamined this process and conclude that the wind accelerates significantly in the course of reconnection. This dilates the timescale over which the reconnection process operates so that the wind requires a much larger distance than was previously thought in order to convert the Poynting flux to particle flux. In the case of the Crab pulsar, the wind is still Poynting-dominated at the radius at which a standing shock is inferred from observation. An estimate of the radius of the termination shock for other pulsars implies that all except the millisecond pulsars have Poynting flux-dominated winds all the way out to the shock front.

On the relativistic magnetic reconnection

Reconnection of the magnetic lines of force is considered in case the magnetic energy exceeds the rest energy of the matter. It is shown that the classical Sweet-Parker and Petschek models are generalized straightforwardly to this case and the reconnection rate may be estimated by substituting the Alfven velocity in the classical formulas by the speed of light. The outflow velocity in the Sweet-Parker configuration is mildly relativistic. In the Petschek configuration, the outflow velocity is ultrarelativistic whereas the angle between the slow shocks is very small. Due to the strong compression, the plasma outflow in the Petschek configuration may become strongly magnetized if the reconnecting fields are not exactly antiparallel.

An Expanding Radio Nebula Produced by a Giant Flare from the Magnetar SGR 1806-20

Soft γ-ray repeaters (SGRs) are ‘magnetars’, a small class of slowly spinning neutron stars with extreme surface magnetic fields, B ≈ 1015 gauss (refs 1 , 2 –3). On 27 December 2004, a giant flare was detected from the magnetar SGR 1806 - 20 (ref. 2), only the third such event recorded. This burst of energy was detected by a variety of instruments and even caused an ionospheric disturbance in the Earths upper atmosphere that was recorded around the globe. Here we report the detection of a fading radio afterglow produced by this outburst, with a luminosity 500 times larger than the only other detection of a similar source. From day 6 to day 19 after the flare from SGR 1806 - 20, a resolved, linearly polarized, radio nebula was seen, expanding at approximately a quarter of the speed of light. To create this nebula, at least 4 × 1043 ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles.

Asymptotic structure of Poynting dominated jets

In relativistic, Poynting-dominated outflows, acceleration and collimation are intimately connected. An important point is that the Lorentz force is nearly compensated by the electric force; therefore the acceleration zone spans a large range of scales. We derived the asymptotic equations describing relativistic, axisymmetric magnetohydrodynamic flows far beyond the light cylinder. These equations do not contain either intrinsic small scales (like the light cylinder radius) or terms that nearly cancel each other (like the electric and magnetic forces); therefore they could be easily solved numerically. They also suit well for qualitative analysis of the flow and, in many cases, they could even be solved analytically or semianalytically. We show that there are generally two collimation regimes. In the first regime, the residual of the hoop stress and the electric force is counterbalanced by the pressure of the poloidal magnetic field so that, at any distance from the source, the structure of the flow is the same as the structure of an appropriate cylindrical equilibrium configuration. In the second regime, the pressure of the poloidal magnetic field is negligibly small so that the flow could be conceived as composed from coaxial magnetic loops. In the two collimation regimes, the flow is accelerated in different ways. We study in detail the structure of jets confined by the external pressure with a power-law profile. In particular, we obtained simple scalings for the extent of the acceleration zone, for the terminal Lorentz factor, and for the collimation angle.

A model for fast extragalactic radio bursts

Bursts of millisecond duration were recently discovered in the 1 GHz band. There is a strong evidence that they come from

CURVATURE RADIATION IN PULSAR MAGNETOSPHERIC PLASMA

\sim 1

The Theory of Pulsar Winds and Nebulae

Gpc distances, which implies extraordinary high brightness temperature. I propose that these bursts could be attributed to synchrotron maser emission from relativistic, magnetized shocks. At the onset of the magnetar flare, a strongly magnetized pulse is formed, which propagates away through the relativistic magnetar wind and eventually reaches the nebula inflated by the wind within the surrounding medium. I show that the observed radio bursts could be generated at shocks formed via the interaction of the magnetic pulse with the plasma within the nebula. The model predicts strong millisecond bursts in the TeV band, which could be observed even from distant galaxies.

Are Gamma-Ray Burst Shocks Mediated by the Weibel Instability?

We consider the curvature radiation of a pointlike charge moving relativistically along curved magnetic field lines through a pulsar magnetospheric electron-positron plasma. We demonstrate that the radiation power is largely suppressed compared with the vacuum case, but still at a considerable level, high enough to explain the observed pulsar luminosities. The outgoing waves are polarized perpendicularly to the plane of the dipolar magnetic field lines. Our results strongly support coherent curvature radiation by the spark-associated solitons as a plausible mechanism of pulsar radio emission.

PARTICLE ACCELERATION IN THE DRIVEN RELATIVISTIC RECONNECTION

Summary. We review current theoretical ideas on pulsar winds and their surrounding nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the striped wind, together with models of magnetic dissipation are discussed. It is shown that the observational signature of this dissipation is likely to be point-like, rather than extended, and that pulsed emission may be produced. The possible pulse shapes and polarisation properties are described. Particle acceleration at the termination shock of the wind is discussed, and it is argued that two distinct mechanisms must be operating, with the first-order Fermi mechanism producing the high-energy electrons (above 1TeV) and either magnetic annihilation or resonant absorption of ion cyclotron waves responsible for the 100MeV to 1TeV electrons. Finally, MHD models of the morphology of the nebula are discussed and compared with observation.