Abstract:
The prompt emissions of gamma-ray bursts are seeded by radiating ultrarelativistic electrons. Internal shocks propagating through a jet launched by a stellar implosion, are expected to amplify the magnetic field & accelerate electrons. We explore the effects of density asymmetry & a quasi-parallel magnetic field on the collision of plasma clouds. A 2D relativistic PIC simulation models the collision of two plasma clouds, in the presence of a quasi-parallel magnetic field. The cloud density ratio is 10. The densities of ions & electrons & the temperature of 131 keV are equal in each cloud. The mass ratio is 250. The peak Lorentz factor of the electrons is determined, along with the orientation & strength of the magnetic field at the cloud collision boundary. The magnetic field component orthogonal to the initial plasma flow direction is amplified to values that exceed those expected from shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification, caused by a current sheet which develops in response to the differing deflection of the incoming upstream electrons & ions. The electron deflection implies a charge separation of the upstream electrons & ions; the resulting electric field drags the electrons through the magnetic field, whereupon they acquire a relativistic mass comparable to the ions. We demonstrate how a magnetic field structure resembling the cross section of a flux tube grows in the current sheet of the shock transition layer. Plasma filamentation develops, as well as signatures of orthogonal magnetic field striping. Localized magnetic bubbles form. Energy equipartition between the ion, electron & magnetic energy is obtained at the shock transition layer. The electronic radiation can provide a seed photon population that can be energized by secondary processes (e.g. inverse Compton).

Abstract:
The energetic electromagnetic eruptions observed during the prompt phase of gamma-ray bursts are attributed to synchrotron emissions. The internal shocks moving through the ultrarelativistic jet, which is ejected by an imploding supermassive star, are the likely source of this radiation. Synchrotron emissions at the observed strength require the simultaneous presence of powerful magnetic fields and highly relativistic electrons. We explore with one and three-dimensional relativistic particle-in-cell simulations the transition layer of a shock, that evolves out of the collision of two plasma clouds at a speed 0.9c and in the presence of a quasi-parallel magnetic field. The cloud densities vary by a factor of 10. The number densities of ions and electrons in each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor of the electrons is determined in the 1D simulation, as well as the orientation and the strength of the magnetic field at the boundary of the two colliding clouds. The relativistic masses of the electrons and ions close to the shock transition layer are comparable as in previous work. The 3D simulation shows rapid and strong plasma filamentation behind the transient precursor. The magnetic field component orthogonal to the initial field direction is amplified in both simulations to values that exceed those expected from the shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification. The simultaneous presence of highly relativistic electrons and strong magnetic fields will give rise to significant synchrotron emissions.

Abstract:
We consider the stochastic acceleration of particles which results from resonant interactions with plasma waves in black hole magnetospheres. We calculate acceleration rates and escape time scales for protons and electrons resonating with Alfv\'en waves, and for electrons resonating with whistlers. Assuming either a Kolmogorov or Kraichnan wave spectrum, accretion at the Eddington limit, magnetic field strengths near equipartition, and turbulence energy densities $\sim 10\%$ of the total magnetic field energy density, we find that Alfv\'en waves accelerate protons to Lorentz factors $\lte 10^4$--$10^6$ before they escape from the system. Acceleration of electrons by fast mode and whistler waves can produce a nonthermal population of relativistic electons whose maximum energy is determined by a competition with radiation losses. Particle energization and outflow is not possible at lower accretion rates, magnetic field strengths, or turbulence levels due to dominant Coulomb losses. Increases in the accretion luminosity relative to the Eddington luminosity can trigger particle acceleration out of the thermal background, and this mechanism could account for the differences between radio-quiet and radio-loud active galactic nuclei. Observations of outflowing radio-emitting components following transient X-ray events in galactic X-ray novae and gamma-ray flares in blazars are in accord with this scenario.

Abstract:
We explore the Reissner-Nordstr\"{o}m naked singularities with a charge $Q$ larger than its mass $M$ from the perspective of the particle acceleration. We first consider a collision between two test particles following the radial geodesics in the Reissner-Nordstr\"{o}m naked singular geometry. An initially radially ingoing particle turns back due to the repulsive effect of gravity in the vicinity of naked singularity. Such a particle then collides with an another radially ingoing particle. We show that the center of mass energy of collision taking place at $r \approx M$ is unbound, in the limit where the charge transcends the mass by arbitrarily small amount $0<1-M/Q\ll1$.The acceleration process we described avoids fine tuning of the parameters of the particle geodesics for the unbound center of mass energy of collisions and the proper time required for the process is also finite. We show that the coordinate time required for the trans-Plankian collision to occur around one solar mass naked singularity is around million years while it is many orders of magnitude larger than Hubble time in the black hole case. We then study the collision of the neutral spherically symmetric shells made up of dust particles. In this case, it is possible to treat the situation by exactly taking into account the gravity due to the shells using Israel`s thin shell formalism, and thus this treatment allows us to go beyond the test particle approximation. The center of mass energy of collision of the shells is then calculated in a situation analogous to the test particle case and is shown to be bounded above. However, we find thatthe energy of a collision between two of constituent particles of the shells at the center of mass frame can exceed the Planck energy.

Abstract:
Icosahedral shells undergo a buckling transition as the ratio of Young's modulus to bending stiffness increases. Strong bending stiffness favors smooth, nearly spherical shapes, while weak bending stiffness leads to a sharply faceted icosahedral shape. Based on the phonon spectrum of a simplified mass-and-spring model of the shell, we interpret the transition from smooth to faceted as a soft-mode transition. In contrast to the case of a disclinated planar network where the transition is sharply defined, the mean curvature of the sphere smooths the transitition. We define elastic susceptibilities as the response to forces applied at vertices, edges and faces of an icosahedron. At the soft-mode transition the vertex susceptibility is the largest, but as the shell becomes more faceted the edge and face susceptibilities greatly exceed the vertex susceptibility. Limiting behaviors of the susceptibilities are analyzed and related to the ridge-scaling behavior of elastic sheets. Our results apply to virus capsids, liposomes with crystalline order and other shell-like structures with icosahedral symmetry.

Abstract:
The two-fluid effects on the radial outflow of relativistic electron-positron plasma are considered. It is shown that for large enough Michel magnetization parameter (1969) and multiplication parameter, the one-fluid MHD approximation remains correct in the whole region where |E| < |B|. In the case in which the longitudinal electric current is smaller than the Goldreich-Julian one, the acceleration of particles is determined near the light surface where |E| = |B|. It is shown that, as in the previously considered cylindrical geometry (Beskin, Gurevich & Istomin 1983), almost all electromagnetic energy is transformed into the energy of particles in the narrow boundary layer. The relative thickness of this layer is shown to be of the order of the inverse of multiplication parameter.

Abstract:
Fermi-LAT analyses show that the gamma-ray photon spectral indices Gamma_gamma of a large sample of blazars correlate with the vFv peak synchrotron frequency v_s according to the relation Gamma_gamma = d - k log v_s. The same function, with different constants d and k, also describes the relationship between Gamma_gamma and peak Compton frequency v_C. This behavior is derived analytically using an equipartition blazar model with a log-parabola description of the electron energy distribution (EED). In the Thomson regime, k = k_EC = 3b/4 for external Compton processes and k = k_SSC = 9b/16 for synchrotron self-Compton (SSC) processes, where b is the log-parabola width parameter of the EED. The BL Lac object Mrk 501 is fit with a synchrotron/SSC model given by the log-parabola EED, and is best fit away from equipartition. Corrections are made to the spectral-index diagrams for a low-energy power-law EED and departures from equipartition, as constrained by absolute jet power. Analytic expressions are compared with numerical values derived from self-Compton and external Compton scattered gamma-ray spectra from Ly alpha broad-line region and IR target photons. The Gamma_gamma vs. v_s behavior in the model depends strongly on b, with progressively and predictably weaker dependences on gamma-ray detection range, variability time, and isotropic gamma-ray luminosity. Implications for blazar unification and blazars as ultra-high energy cosmic-ray sources are presented. Arguments by Ghisellini et al. (2014) that the jet power exceeds the accretion luminosity depend on the doubtful assumption that we are viewing at the Doppler angle.

Abstract:
Observations performed with the Fermi-LAT telescope have revealed the presence of a spectral break in the GeV spectrum of flat-spectrum radio quasars (FSRQs) and other low- and intermediate-synchrotron peaked blazars. We propose that this feature can be explained by Compton scattering of broad-line region (BLR) photons by a non-thermal population of electrons described by a log-parabolic function. We consider in particular a scenario in which the energy densities of particles, magnetic field, and soft photons in the emitting region are close to equipartition. We show that this model can satisfactorily account for the overall spectral energy distribution of the FSRQ 3C 454.3, reproducing the GeV spectal cutoff due to Klein-Nishina effects and a curving electron distribution.

Abstract:
The problem of mutual equilibration between two finite, identical quantum systems, A and B, prepared initially at different temperatures is elucidated. We show that the process of energy exchange between the two systems leads to accurate equipartition within energy shells in the Hilbert space of the total non-interacting, composite system, A \otimes B. This scenario occurs under the general condition of a weak interaction between the systems. We predict that the sole hypothesis of such equipartition is sufficient to obtain a relaxation of the peers, A and B, towards a common thermal-like state. This conjecture is fully corroborated by an exact diagonalization of several quantum models.

Abstract:
As part of a survey of HI 21-cm emission in the Southern Milky Way, we have detected two large shells in the interstellar neutral hydrogen near l=279 deg. The center velocities are +36 and +59 km/s, which puts the shells at kinematic distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and very empty, with density contrast of at least 15 between the middle and the shell walls. It has expansion velocity of about 20 km/s and swept up mass of several million solar masses. The energy indicated by the expansion may be as high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s and swept up mass of about 10^6 solar masses. Morphologically both regions appear to be shells, with high density regions mostly surrounding the voids, although the first appears to have channels of low density which connect with the halo above and below the HI layer. They lie on the edge of the Carina arm, which suggests that they may be expanding horizontally into the interarm region as well as vertically out of the disk. If this interpretation is correct, this is the first detection of an HI chimney which has blown out of both sides of the disk.