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Relativistic shocks propagating into a medium with low magnetization are generated and sustained by small-scale but very strong magnetic field turbulence. This so-called "microturbulence" modifies the typical shock acceleration process, and in particular that of electrons. In this work we perform Monte Carlo (MC) simulations of electrons encountering shocks with microturbulent fields. The simulations cover a three-dimensional parameter space in shock speed, acceleration efficiency, and peak magnetic field strength. From these, a Markov Chain Monte Carlo (MCMC) method was employed to estimate the maximum electron momentum from the MC-simulated electron spectra. Having estimated this quantity at many points well-distributed over an astrophysically relevant parameter space, an MCMC method was again used to estimate the parameters of an empirical formula that computes the maximum momentum of a Fermi-accelerated electron population anywhere in this parameter space. The maximum energy is well-approximated as a broken power law in shock speed, with the break occurring when the shock decelerates to the point where electrons can begin to escape upstream from the shock. [ABSTRACT FROM AUTHOR]

Some fast-moving pulsars, such as the Guitar and the Lighthouse, exhibit asymmetric non-thermal emission features that extend well beyond their ram pressure confined pulsar wind nebulae (PWNe). Based on our 3D relativistic simulations, we analytically explain these features as kinetically streaming pulsar wind particles that escaped into the interstellar medium (ISM) due to reconnection between the PWN and ISM magnetic fields. The structure of the reconnecting magnetic fields at the incoming and outgoing regions produces highly asymmetric magnetic bottles therefore and result in asymmetric extended features. For the features to become visible, the ISM magnetic field should be sufficiently high, B ISM > 10  |$\mu$| G. We also discuss archival observations of PWNe displaying evidence of kinetic jets: the Dragonfly PWN (PSR J2021 + 3651), G327.1–1.1, and MSH 11–62, the latter two of which exhibit symmetric 'snail eyes' morphologies. We suggest that in those cases the pulsar is moving along the ambient magnetic field in a frisbee-type configuration. [ABSTRACT FROM AUTHOR]

The stability of current sheets in collisionless relativistic pair plasma was studied via twodimensional two-fluid relativistic magnetohydrodynamic simulations with vanishing internal friction between fluids. In particular, we investigated the linear growth of the tearing and drift-kink modes in the current sheets both with and without the guide field and obtained the growth rates which are very similar to what has been found in the corresponding particle in cell (PIC) simulations. This suggests that the two-fluid simulations can be useful in studying the large-scale dynamics of astrophysical relativistic plasmas in problems involving magnetic reconnection. [ABSTRACT FROM AUTHOR]

Context. Extragalactic jets are formed close to supermassive black-holes in the center of galaxies. Large amounts of gas, dust, and stars cluster in the galaxy nucleus, and interactions between this ambient material and the jet base should be frequent, having dynamical as well as radiative consequences. Aims. This work studies the dynamical interaction of an obstacle, a clump of matter or the atmosphere of an evolved star, with the innermost region of an extragalactic jet. Jet mass-loading and the high-energy outcome of this interaction are briefly discussed. Methods. Relativistic hydrodynamical simulations with axial symmetry have been carried out for homogeneous and inhomogeneous obstacles inside a relativistic jet. These obstacles may represent a medium inhomogeneity or the disrupted atmosphere of a red giant star. Results. Once inside the jet, an homogeneous obstacle expands and gets disrupted after few dynamical timescales, whereas in the inhomogeneous case, a solid core can smoothen the process, with the obstacle mass-loss dominated by a dense and narrow tail pointing in the direction of the jet. In either case, matter is expected to accelerate and eventually get incorporated to the jet. Particles can be accelerated in the interaction region, and produce variable gamma-rays in the ambient matter, magnetic and photon fields. Conclusions. The presence of matter clumps or red giants into the base of an extragalactic jet likely implies significant jet massloading and slowing down. Fast flare-like gamma-ray events, and some level of persistent emission, are expected due to these interactions. [ABSTRACT FROM AUTHOR]

The very first stars in the Universe can be very massive, up to . If born in large numbers, such massive stars can have a strong impact on the subsequent star formation, producing strong ionizing radiation and contaminating the primordial gas with heavy elements. They would leave behind massive black holes that could act as seeds for growing supermassive black holes of active galactic nuclei. Given the anticipated fast rotation, such stars would end their life as supermassive collapsars and drive powerful magnetically dominated jets. In this Letter, we investigate the possibility of observing the bursts of high-energy emission similar to the long gamma-ray bursts associated with normal collapsars. We show that during the collapse of supercollapsars, the Blandford–Znajek mechanism can produce jets as powerful as few erg s−1 and release up to 1054 erg of the black hole rotational energy. Due to the higher intrinsic time-scale and higher redshift, the initial bright phase of the burst can last for about 104 s, whereas the central engine would remain active for about 1 d. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 20–30 keV. The peak total flux density is relatively low, , but not prohibitive. If one supercollapsar is produced per every minihalo of dark matter arising from the 3σ cosmological fluctuations, then the whole sky frequency of such bursts could reach several tens per year. [ABSTRACT FROM AUTHOR]

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the long-duration gamma-ray bursts (LGRBs) with duration in excess of 100 s as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 104 s. These data suggest that some other, most likely magnetic mechanisms have to be considered. Since the efficiency of magnetic mechanisms does not depend that much on the mass accretion rate, the magnetic models do not require the development of accretion disc within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova (SN) state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion discs in the collapsar model and investigate the available range of mass accretion rates, black hole (BH) masses and spins. We find that the BH mass can be much higher than , usually assumed in the collapsar model, and normally exceeds half of the pre-SN mass. The BH spin is rather moderate, , but still high enough for the Blandford–Znajek mechanism to remain efficient provided the magnetic field is sufficiently strong. Our numerical simulations confirm the possibility of magnetically driven stellar explosions, in agreement with previous studies, but point towards the required magnetic flux on the BH horizon in excess of . At present, we cannot answer with certainty whether such a strong magnetic field can be generated in the stellar interior. Perhaps, the SN explosions associated with LGRBs are still neutrino-driven and their gamma-ray signature is the precursors. The SN blast clears up escape channels for the magnetically driven gamma-ray burst (GRB) jets, which may produce the main pulse. In this scenario, the requirements on the magnetic field strength can be lowered. A particularly interesting version of the binary progenitor involves merger of a Wolf–Rayet star with an ultracompact companion, neutron star or BH. In this case, we expect the formation of very long-lived accretion discs, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation. [ABSTRACT FROM AUTHOR]

We present numerical simulations of cold, axisymmetric, magnetically driven relativistic outflows. The outflows are initially sub-Alfvénic and Poynting-flux dominated, with total–to–rest-mass energy flux ratio up to μ ~ 620. To study the magnetic acceleration of jets we simulate flows confined within a funnel with a rigid wall of prescribed shape, which we take to be z ∝ ra (in cylindrical coordinates, with a ranging from 1 to 2). This allows us to eliminate the numerical dissipative effects induced by a free boundary with an ambient medium. We find that in all cases they converge to a steady state characterized by a spatially extended acceleration region. For the jet solutions the acceleration process is very efficient — on the outermost scale of the simulation more than half of the Poynting flux has been converted into kinetic energy flux, and the terminal Lorentz factor approached its maximum possible value (Γ∞ ≃ μ). The acceleration is accompanied by the collimation of magnetic field lines in excess of that dictated by the funnel shape. The numerical solutions are generally consistent with the semi-analytic self-similar jets solutions and the spatially extended acceleration observed in some astrophysical relativistic jets. In agreement with previous studies, we also find that the acceleration is significantly less effective for wind solutions suggesting that pulsar winds may remain Poynting dominated when they reach the termination shock. [ABSTRACT FROM AUTHOR]

In this paper, we report on the early evolution of a core-collapse supernova explosion following the birth of a magnetar with the dipolar magnetic field of and the rotational period of 2 ms, which was studied by means of axisymmetric general relativistic magnetohydrodynamic simulations. In this study, we use realistic equation of state and take into account the cooling and heating associated with emission, absorption, annihilation, and scattering of neutrinos; the neutrino transport is treated in the optically-thin regime. The supernova explosion is initiated via introducing into the initial solution the ‘radiation bubble’, whose total thermal energy is comparable with the typical energy of supernova ejecta. The numerical models exhibit highly collimated magnetically driven jets very early on. The jets are super-Alfvénic but remain subfast until the end of the simulations . The power released in the jets is about which implies the spin-down time of ≃37 s. The total rotational energy of the magnetar, , is sufficient to drive a hypernova but it is not clear as to how large a fraction of this energy can be transferred to the stellar ejecta. Given the observed propagation speed of the jets, , they are expected to traverse the progenitor in few seconds and after this most of the released rotational energy would be simply carried away by these jets into the surrounding space. Three-dimensional effects such as the kink mode instability may reduce the jet propagation speed and increase the amount of energy transferred by the jets to the supernova ejecta. Our results provide the first more or less self-consistent numerical model of a central engine capable of producing, in the supernova setting and on a long-term basis, collimated jets with sufficient power to explain long-duration gamma-ray bursts (GRBs) and their afterglows. Although the flow speed of our jets is relatively low, only , the cooling of protoneutron star will eventually result in much higher magnetization of its magnetosphere and ultrarelativistic asymptotic speeds of the jets. Given the relatively long cooling time-scale, we still expect the jets to be only weakly relativistic by the time of break-out. This leads to a model of GRB jets with a systematic longitudinal variation in the Lorentz factor which may have specific observational signatures both in the prompt and in the afterglow emission. The simulations also reveal quasi-periodic ejection of plasma clouds into the jet on a time-scale of 20 ms related to the large-scale global oscillation of magnetar's magnetosphere caused by the opening–closing of the dead zone field lines. These kinds of central engine variability may be partly responsible for the internal shocks of GRB jets and the short-time variability of their gamma-ray emission. [ABSTRACT FROM AUTHOR]