Omer Bromberg

RECOLLIMATION AND RADIATIVE FOCUSING OF RELATIVISTIC JETS: APPLICATIONS TO BLAZARS AND M87

Recent observations of M87 and some blazars reveal violent activity in small regions located at relatively large distances from the central engine. Motivated by these considerations, we study the hydrodynamic collimation of a relativistic cooling outflow using a semianalytical model developed earlier. We first demonstrate that radiative cooling of the shocked outflow layer can lead to a focusing of the outflow and its reconfinement in a region having a very small cross-sectional radius. Such a configuration can produce rapid variability at large distances from the central engine via reflections of the converging recollimation shock. Possible applications of this model to TeV blazars are discussed. We then apply our model to M87. The low radiative efficiency of the M87 jet renders focusing unlikely. However, the shallow profile of the ambient medium pressure inferred from observations results in extremely good collimation that can explain the reported variability of the X-ray flux emitted from the HST-1 knot.

Hydrodynamic Collimation of Relativistic Outflows: Semianalytic Solutions and Application to Gamma-Ray Bursts

A model is developed for the confinement and collimation of a baryon-poor outflow by its surrounding medium. Both confinement by kinetic pressure of a static corona and confinement by the ram pressure of a supersonic wind emanating from a disk surrounding the inner source are considered. Solutions are presented for the structure of the shocked layers of a deflected baryon-poor jet (BPJ) and exterior wind. The dependence of the opening angle of the BPJ on the parameters of the confining medium are carefully examined. It is found that the BPJ shock may either converge to the symmetry axis or diverge away from it, depending on the opening angle of the BPJ injection cone. In the latter case, the inner flow exhibits a nonuniform structure, consisting of an ultrarelativistic core containing the unshocked BPJ enveloped by the slower, shocked BPJ layer. The implications of our results to the prompt GRB emission are briefly discussed.

Relativistic Photon Mediated Shocks

A system of equations governing the structure of a steady, relativistic radiation-dominated shock is derived, starting from the general form of the transfer equation obeyed by the photon distribution function. Closure is obtained by truncating the system of moment equations at some order. The anisotropy of the photon distribution function inside the shock is shown to increase with increasing shock velocity, approaching nearly perfect beaming at upstream Lorentz factors Gamma(-) >>1. Solutions of the shock equations are presented for some range of upstream conditions. These solutions are shown to converge as the truncation order is increased.

An observational limit on the earliest gamma-ray bursts

We predict the redshift of the first observable (i.e. in our past light cone) gamma-ray burst (GRB) and calculate the GRB rate redshift distribution of the Population III stars at very early times (z = 20-60). Using the last two year of data from Swift, we place an upper limit on the efficiency (nGRB) of GRB production per solar mass from the first generation of stars. We find that the first observable GRB is most likely to have formed at redshift 60. The observed rate of extremely high redshift GRBs (XRGs) is a subset of a group of 15 long GRBs per year, with no associated redshift and no optical afterglow counterparts, detected by Swift. Taking this maximal rate, we get that nGRB < 1.1 x 10 -3 GRBs per solar mass in stars. A more realistic evaluation, for example, taking a subgroup of 5 per cent of the total sample of Swift gives an upper limit of n GRB <3.2 x 10 -4 GRBS per solar mass.

The

We present the first relativistic MHD numerical simulation of a magnetic jet that propagates through and emerges from the dynamical ejecta of a binary neutron star merger. Generated by the magnetized rotation of the merger remnant, the jet propagates through the ejecta and produces an energetic cocoon that expands at mildly relativistic velocities and breaks out of the ejecta. We show that if the ejecta has a low-mass (

\gamma

\sim10^{-7} M_\odot

-rays that accompanied GW170817 and the observational signature of a magnetic jet breaking out of NS merger ejecta

) high-velocity (

COLLIMATION AND RADIATIVE DECELERATION OF JETS IN TEV AGNs

v\sim0.85

Focusing of relativistic cooling jets

c) tail, the cocoon shock breakout will generate

The gravitational-wave spectrum of a non-axisymmetric torus around a rapidly spinning black hole

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