Interaction of a magnetized shell with an ambient medium: limits on impulsive magnetic acceleration

The interaction of relativistic magnetized ejecta with an ambient medium is studied for a range of structures and magnetization of the unshocked ejecta. We particularly focus on the effect of the ambient medium on the dynamics of an impulsive, high-sigma shell. It is found that for sufficiently high values of the initial magnetization $\sigma_0$ the evolution of the system is significantly altered by the ambient medium well before the shell reaches its coasting phase. The maximum Lorentz factor of the shell is limited to values well below $\sigma_0$; for a shell of initial energy $E=10^{52}E_{52}$ ergs and size $r_0=10^{12}T_{30}$ cm expelled into a medium having a uniform density $n_i$ we obtain $\Gamma_{\rm max}\simeq180(E_{52}/T_{30}^3 n_i)^{1/8}$ in the high sigma limit. The reverse shock and any internal shocks that might form if the source is fluctuating are shown to be very weak. The restriction on the Lorentz factor is more severe for shells propagating in a stellar wind. Intermittent ejection of small sub-shells doesn't seem to help, as the shells merge while still highly magnetized. Lower sigma shells start decelerating after reaching the coasting phase and spreading away. The properties of the reverse shock then depend on the density profiles of the coasting shell and the ambient medium. For a self-similar cold shell the reverse shock becomes strong as it propagates inwards, and the system eventually approaches the self-similar solution derived recently by Nakamura \& Shigeyama.Comment: 22 pages, 8 figs, post referee versio

Role of Reconnection in AGN Jets

We discuss the possible role of reconnection in electro-magnetically dominated cores of relativistic AGN jets. We suggest that reconnection may proceed in a two-fold fashion: initial explosive collapse on the Alfven time-scale of a current-carrying jet (which is of the order of the light crossing time) and subsequent slow quasi-steady reconnection. Sites of explosive collapse are associated with bright knots, while steady-state reconnection re-energizes particles in the ``bridges'' between the knots. Ohmic dissipation in reconnection layers leads to particle acceleration either by inductive electric fields or by stochastic particle acceleration in the ensuing electro-magnetic turbulence.Comment: 4 pages, Proceedings of the conference "The Physics of Relativistic Jets in the CHANDRA and XMM Era", 23-27 September 2002, Bologn

Radiation- and pair-loaded shocks

We consider the structure of mildly relativistic shocks in dense media, taking into account the radiation and pair loading, and diffusive radiation energy transfer within the flow. For increasing shock velocity (increasing post-shock temperature), the first important effect is the efficient energy redistribution by radiation within the shock that leads to the appearance of an isothermal jump, whereby the flow reaches the finial state through a discontinuous isothermal transition. The isothermal jump, on scales much smaller that the photon diffusion length, consists of a weak shock and a quick relaxation to the isothermal conditions. Highly radiation-dominated shocks do not form isothermal jump. Pair production can mildly increase the overall shock compression ratio to $\approx 10$ (from $4$ for matter-dominated shocks and $7$ of the radiation-dominated shocks)

Did Swift measure GRB prompt emission radii?

The Swift X-Ray Telescope often observes a rapidly decaying X-ray emission stretching to as long as $t \sim 10^3$ seconds after a conventional prompt phase. This component is most likely due to a prompt emission viewed at large observer angles $\theta > 1/\Gamma$, where $\theta\sim 0.1$ is a typical viewing angle of the jet and$\Gamma\geq 100$ is the Lorentz factor of the flow during the prompt phase. This can be used to estimate the prompt emission radii, $r_{em} \geq 2 t c/\theta^2 \sim 6 \times 10^{15}$ cm. These radii are much larger than is assumed within a 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. due to a random relativistic motion of ''fundamental emitters''. This may also offer a possible explanation for X-ray flares observed during early afterglows.Comment: 9 pages, 2 figure, submitted to MNRAS Letter

Orbital modulation of emission of the binary pulsar J0737-3039B

In binary radio pulsar system J0737-3039, slow pulsar B shows orbital modulations of intensity, being especially bright at two short orbital phases. We propose that these modulations are due to distortion of pulsar B magnetosphere by pulsar A wind which produces orbital phase-dependent changes of the direction along which radio waves are emitted. In our model, pulsar B is intrinsically bright at all times but its radiation beam misses the Earth at most orbital phases. We employ a simple model of distorted B magnetosphere using stretching transformations of Euler potentials of dipolar fields. To fit observations we use parameters of pulsar B derived from modeling of A eclipses (Lyutikov and Thompson 2005). The model reproduces two bright regions approximately at the observed orbital phases, explains variations of the pulse shape between them and regular timing residuals within each emission window. It also makes predictions for timing properties and secular variations of pulsar B profiles.Comment: 15 pages, 6 figure, submitted to MNRA