Dead zone in the polar-cap accelerator of pulsars

We study plasma flows above pulsar polar caps using time-dependent simulations of plasma particles in the self-consistent electric field. The flow behavior is controlled by the dimensionless parameter alpha=(j/c rho_GJ) where j is the electric current density and rho_GJ is the Goldreich-Julian charge density. The region of the polar cap where 0<alpha<1 is a "dead zone" --- in this zone particle acceleration is inefficient and pair creation is not expected even for young, rapidly rotating pulsars. Pulsars with polar caps near the rotation axis are predicted to have a hollow-cone structure of radio emission, as the dead zone occupies the central part of the polar cap. Our results apply to charge-separated flows of electrons (j0). In the latter case, we consider the possibility of a mixed flow consisting of different ion species, and observe the development of two-stream instability. The dead zone at the polar cap is essential for the development of an outer gap near the null surface rho_GJ=0.Comment: 10 pages, 11 figures, accepted to Ap

Neutron-loaded outflows in gamma-ray bursts

Relativistic neutron-loaded outflows in gamma-ray bursts are studied at their early stages, before deceleration by a surrounding medium. The outflow has four components: radiation, electrons, protons and neutrons. The components interact with each other and exchange energy as the outflow expands. The presence of neutrons significantly changes the outflow evolution. Before neutrons decouple from protons, friction between the two components increases their temperatures by many orders of magnitude. After the decoupling, the gradual neutron decay inside the outflow has a drag effect on the protons and reduces their final Lorentz factor.Comment: 11 pages, 9 figures, submitted to MNRAS EMR current affiliation: JILA, UC at Boulde

Early Stages of the GRB Explosion

Physics of GRB blast waves is discussed with a focus on two effects: (1) pair creation in the external medium by the gamma-ray front and (2) decay of neutrons ahead of the decelerating blast wave. Both effects impact the afterglow mechanism at radii up to 10^{17}cm.Comment: 5 pages, to appear in the proceedings of the 2003 GRB Conference, Santa Fe, Sep 8-1

On the Efficiency of Internal Shocks in Gamma-Ray Bursts

The fraction of a fireball kinetic kinetic energy that can be radiated away by internal shocks is sensitive to the amplitude of initial fluctuations in the fireball. We give a simple analytical description for dissipation of modest-amplitude fluctuations and confirm it with direct numerical simulations. At high amplitudes, the dissipation occurs in a non-linear regime with efficiency approaching 100 %. Most of the explosion energy is then radiated away by the prompt GRB and only a fraction remains to be radiated by the afterglow.Comment: submitted to ApJ Letter

Super-Eddington accretion disc around a Kerr black hole

We calculate the structure of accretion disc around a rapidly rotating black hole with a super-Eddington accretion rate. The luminosity and height of the disc are reduced by the advection effect. In the case of a large viscosity parameter, alpha > 0.03, the accretion flow strongly deviates from thermodynamic equilibrium and overheats in the central region. With increasing accretion rate, the flow temperature steeply increases, reaches a maximum, and then falls off. The maximum is achieved in the advection dominated regime of accretion. The maximum temperature in the disc around a massive black hole, M=10^8 M_sun, with alpha=0.3 is of order 3 x 10^8 K. Discs with large accretion rates can emit X-rays in quasars as well as in galactic black hole candidates.Comment: accepted for publication in MNRAS, 8 page