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 Physics , 2013, DOI: 10.1103/PhysRevLett.110.051101 Abstract: We propose the particle acceleration model coupled with multiple plasmoid ejections in a solar flare. Unsteady reconnection produces plasmoids in a current sheet and ejects them out to the fast shocks, where particles in a plasmoid are reflected upstream the shock front by magnetic mirror effect. As the plasmoid passes through the shock front, the reflection distance becomes shorter and shorter driving Fermi acceleration, until it becomes proton Larmor radius. The fractal distribution of plasmoids may also have a role in naturally explaining the power-law spectrum in nonthermal emissions.
 Physics , 2015, Abstract: We investigate the distribution of particle acceleration sites, independently of the actual acceleration mechanism, during plasmoid-dominated, relativistic collisionless magnetic reconnection by analyzing the results of a particle-in-cell numerical simulation. The simulation is initiated with Harris-type current layers in pair plasma with no guide magnetic field, negligible radiative losses, no initial perturbation, and using periodic boundary conditions. We find that the plasmoids develop a robust internal structure, with colder dense cores and hotter outer shells, that is recovered after each plasmoid merger on a dynamical time scale. We use spacetime diagrams of the reconnection layers to probe the evolution of plasmoids, and in this context we investigate the individual particle histories for a representative sample of energetic electrons. We distinguish three classes of particle acceleration sites associated with (1) magnetic X-points, (2) regions between merging plasmoids, and (3) the trailing edges of accelerating plasmoids. We evaluate the contribution of each class of acceleration sites to the final energy distribution of energetic electrons -- magnetic X-points dominate at moderate energies, and the regions between merging plasmoids dominate at higher energies. We also identify the dominant acceleration scenarios, in order of decreasing importance -- (1) single acceleration between merging plasmoids, (2) single acceleration at a magnetic X-point, and (3) acceleration at a magnetic X-point followed by acceleration in a plasmoid. Particle acceleration is absent only in the vicinity of stationary plasmoids. The effect of magnetic mirrors due to plasmoid contraction does not appear to be significant in relativistic reconnection.
 Physics , 1993, DOI: 10.1103/PhysRevD.49.2909 Abstract: We consider dilaton gravity theories in four spacetime dimensions parametrised by a constant $a$, which controls the dilaton coupling, and construct new exact solutions. We first generalise the C-metric of Einstein-Maxwell theory ($a=0$) to solutions corresponding to oppositely charged dilaton black holes undergoing uniform acceleration for general $a$. We next develop a solution generating technique which allows us to embed" the dilaton C-metrics in magnetic dilaton Melvin backgrounds, thus generalising the Ernst metric of Einstein-Maxwell theory. By adjusting the parameters appropriately, it is possible to eliminate the nodal singularities of the dilaton C-metrics. For $a<1$ (but not for $a\ge 1$), it is possible to further restrict the parameters so that the dilaton Ernst solutions have a smooth euclidean section with topology $S^2\times S^2-{\rm\{pt\}}$, corresponding to instantons describing the pair production of dilaton black holes in a magnetic field. A different restriction on the parameters leads to smooth instantons for all values of $a$ with topology $S^2\times \R^2$.
 Oscar J. C. Dias Physics , 2004, DOI: 10.1103/PhysRevD.70.024007 Abstract: We analyze the quantum process in which a cosmic string breaks in an anti-de Sitter (AdS) background, and a pair of charged or neutral black holes is produced at the ends of the strings. The energy to materialize and accelerate the pair comes from the strings tension. In an AdS background this is the only study done in the process of production of a pair of correlated black holes with spherical topology. The acceleration $A$ of the produced black holes is necessarily greater than (|L|/3)^(1/2), where L<0 is the cosmological constant. Only in this case the virtual pair of black holes can overcome the attractive background AdS potential well and become real. The instantons that describe this process are constructed through the analytical continuation of the AdS C-metric. Then, we explicitly compute the pair creation rate of the process, and we verify that (as occurs with pair creation in other backgrounds) the pair production of nonextreme black holes is enhanced relative to the pair creation of extreme black holes by a factor of exp(Area/4), where Area is the black hole horizon area. We also conclude that the general behavior of the pair creation rate with the mass and acceleration of the black holes is similar in the AdS, flat and de Sitter cases, and our AdS results reduce to the ones of the flat case when L=0.
 Physics , 2000, Abstract: The proposed linear accelerator ("scanator") consists of a terawatt table-top laser and a set of passive elements - beam splitters, dispersion elements for stretching of the laser pulse and chirping of the splitted beams, and dispersion elements for angle scanning of crossed frequency-modulated laser beams. Ions are trapped and accelerated in RF wells by the electron component of plasmoids in the intersection zone of the scanning laser beams. Computational studies give encouraging results. A proof-of-principle experiment on the base of a table-top laser is outlined.
 Jerome P. Gauntlett Physics , 1994, Abstract: This article is based on a talk given at the IInd International Colloquium on Modern Quantum Field Theory, Bombay 1994. The Ernst solution of dilaton gravity describes charged black holes undergoing uniform accleration in a background magnetic field. By analytically continuing the Ernst solution one obtains instantons that describe the pair production of black holes in the background field. We review various aspects of these solutions paying special attention to the Einstein-Maxwell, low-energy string and $d=5$ Kaluza-Klein theories. It is based on work done in collaboration with {}F. Dowker, S. Giddings, G. Horowitz, D. Kastor and J. Traschen \refs{\DGKT,\DGGH}.
 Physics , 2013, Abstract: We present an extension of the BELM code (Belmont et al 2008) to investigate the microphysics of particle acceleration in black holes accretion disc corona. The updated version of the code accounts for the dynamics of resonant slab waves as well as their interaction with both leptons or protons. It is found that the proton temperature is an important regulating effect of the stochastic particle acceleration process in accretion disk corona. We present a preliminary fit of the high soft spectral state of Cygnus X-1.
 Physics , 2012, DOI: 10.1088/0264-9381/30/4/045011 Abstract: Tidal effects have long ago locked the Moon in synchronous rotation with the Earth and progressively increase the Earth-Moon distance. This "tidal acceleration" hinges on dissipation. Binaries containing black holes may also be tidally accelerated, dissipation being caused by the event horizon - a flexible, viscous one-way membrane. In fact, this process is known for many years under a different guise: superradiance. In General Relativity, tidal acceleration is obscured by gravitational-wave emission. However, when coupling to light scalar degrees of freedom is allowed, an induced dipole moment produces a "polarization acceleration", which might be orders of magnitude stronger than tidal quadrupolar effects. Consequences for optical and gravitational-wave observations are intriguing and it is not impossible that imprints of such mechanism have already been observed.
 Physics , 2004, DOI: 10.1016/j.newar.2004.11.001 Abstract: We review recently developed models of galactic discrete sources of high energy neutrinos. Some of them are based on a simple rescaling of the TeV $\gamma$-ray fluxes from recently detected galactic sources, such as, shell-type supernova remnants or pulsar wind nebulae. Others present detailed and originally performed modeling of processes occurring close to compact objects, i.e. neutron stars and low mass black holes, which are supposed to accelerate hadrons close to dense matter and radiation fields. Most of the models considered in this review optimistically assume that the energy content in relativistic hadrons is equal to a significant part of the maximum observable power output in specific sources, i.e. typically $\sim 10%$. This may give a large overestimation of the neutrino fluxes. This is the case of models which postulate neutrino production in hadron-photon collisions already at the acceleration place, due to the likely $e^\pm$ pair plasma domination. Models postulating neutrino production in hadron-hadron collisions avoid such problems and therefore seem to be more promising. The neutrino telescopes currently taking data have not detected any excess from discrete sources yet, although some models could already be constrained by the limits they are providing.
 O. B. Zaslavskii Physics , 2012, DOI: 10.1142/S0218271813500284 Abstract: If two particles collide near the black hole horizon, the energy in the centre of mass (CM) frame can grow indefinitely (the so-called the BSW effect). This requires fine-tuning the parameters (the energy, angular momentum or electric charge) of one particle. We show that the CM energy can be unbound also for collisions in the space-time of quasiblack holes - QBHs (the objects on the threshold of forming the horizon which do not collapse). It does not require special fine-tuning of parameters and occurs when any particle inside a QBH having a finite energy collides with the particle that entered a QBH from the outside region.
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