Relativistic blast waves and synchrotron emission

Relativistic shocks can accelerate particles by the first-order Fermi mechanism; the particles then emit synchrotron emission in the post-shock gas. This process is of particular interest in the models used for the afterglow of gamma-ray bursts. In this paper we use recent results in the theory of particle acceleration at highly relativistic shocks to model the synchrotron emission in an evolving, inhomogeneous and highly relativistic flow. We have developed a numerical code that integrates the relativistic Euler equations for fluid dynamics with a general equation of state, together with a simple transport equation for the accelerated particles. We present tests of this code and, in addition, we use it to study the gamma-ray burst afterglow predicted by the fireball model, along with the hydrodynamics of a spherically-symmetric relativistic blast wave.
We find that, while broadly speaking the behaviour of the emission is similar to that already predicted with semi-analytic approaches, the detailed behaviour is somewhat different. The 'breaks' in the synchrotron spectrum behave differently with time, and the spectrum above the final break is harder than had previously been expected. These effects are due to the incorporation of the geometry of the (spherical) blast wave, along with relativistic beaming and adiabatic cooling of the energetic particles leading to a mix, in the observed spectrum, between recently injected 'uncooled' particles and the older 'cooled' population in different parts of the evolving, inhomogeneous flow.     

Comparative hydrochemical assessment of the current state of some water bodies in the Upper Volga

Results of field studies of water chemistry in the Upper Volga and some its tributaries in the reach between the Volga source and Tver City, as well as in lakes Sterzh, Vselug, Peno, and Volgo, which are constituent parts of the Verkhnevolzhskoe Reservoir, as well as Selizharovskii Pool of Lake Seliger. The year-to-year and season-to-season dynamics of the hydrochemical regime of the examined water bodies and their variations downstream the Volga under the effect of natural and anthropogenic factors are analyzed.     

Activation of the Blandford–Znajek mechanism in collapsing stars

The collapse of massive stars may result in the formation of accreting black holes in their interiors. The accreting stellar matter may advect substantial magnetic flux on to the black hole and promote the release of its rotational energy via magnetic stresses (the Blandford–Znajek mechanism). In this paper we explore whether this process can explain the stellar explosions and relativistic jets associated with long gamma-ray bursts. In particular, we show that the Blandford–Znajek mechanism is activated when the rest mass–energy density of matter drops below the energy density of the magnetic field in the near vicinity of the black hole (within its ergosphere). We also discuss whether such a strong magnetic field is in conflict with the rapid rotation of the stellar core required in the collapsar model, and suggest that the conflict can be avoided if the progenitor star is a component of a close binary. In this case the stellar rotation can be sustained via spin-orbital interaction. In an alternative scenario the magnetic field is generated in the accretion disc, but in this case the magnetic flux through the black hole ergosphere is not expected to be sufficiently high to explain the energetics of hypernovae by the BZ mechanism alone. However, this energy deficit can be recovered via the additional power provided by the disc.     

Chaos and Wavefront Reversal for Long-Range Sound Propagation

Doklady Earth Sciences - Long-range sound propagation in the deep ocean is considered. Attention is concentrated on the procedure of acoustic wavefield refocusing by means of the wavefront...     

Numerical simulations of relativistic magnetized jets

The propagation of light highly relativistic jets carrying a toroidal magnetic field is studied numerically. The results show that jets with high Poynting flux develop the conspicuous nose cones discovered earlier in simulations of classical magnetized jets. The size of the nose cone is significantly reduced in kinetic energy-dominated jets, which develop extensive cocoons. The magnetic field nevertheless plays a significant role in the jet–cocoon dynamics by allowing self-confined flows. The results are explained in terms of the properties of perpendicular magnetohydrodynamic shocks.