Abstract
Multi-wave band synchrotron linear polarization of gamma-ray burst (GRB) afterglows is studied under the assumption of an anisotropic turbulent magnetic field with a coherence length of the ...plasma skin-depth scale in the downstream of forward shocks. We find that for typical GRBs, in comparison to optical polarization, the degree of radio polarization shows a similar temporal evolution but a significantly smaller peak value. This results from differences in observed intensity image shapes between the radio and optical bands. We also show that the degree of the polarization spectrum undergoes a gradual variation from the low- to the high-polarization regime above the intensity of the spectral peak frequency, and that the difference in polarization angles in the two regimes is zero or 90°. Thus, simultaneous multi-wave band polarimetric observations of GRB afterglows would be a new determinative test of the plasma-scale magnetic field model. We also discuss theoretical implications from the recent detection of radio linear polarization in GRB 171205A with the Atacama Large Millimeter/submillimeter Array and other models of magnetic field configuration.
Spark gaps are likely the source of plasma in active black hole (BH) magnetospheres. In this paper, we present results of 1D general relativistic particle-in-cell simulations of a starved BH ...magnetosphere with a realistic treatment of inverse-Compton scattering and pair production, for a broad range of conditions, run times longer than in previous studies, and different setups. We find that following the initial discharge, the system undergoes gradual evolution over prolonged time until either restoring the vacuum state or reaching a state of quasiperiodic oscillations, depending on the spectral shape and luminosity of the ambient radiation field. The oscillations occur near the null charge surface in cases where the global magnetospheric current is in the direction defined by the product of the asymptotic Goldreich-Julian charge density and the radial velocity, while they occur near the boundary of the simulation box when it is the opposite direction (return current). Their amplitude and the resultant luminosity of TeV photons emitted from the gap depend sensitively on the conditions; for the cases studied here the ratio of TeV luminosity to the Blandford-Znajek power ranges from 10−5 to 10−2, suggesting that strong flares may be generated by moderate changes in disk emission. We also examined the dependence of the solution on the initial number of particles per cell (PPC) and found convergence for PPC of about 50 for the cases examined. At lower PPC values the pair multiplicity is found to be artificially high, affecting the solution considerably.
Abstract
We study the response of a starved Kerr black hole magnetosphere to abrupt changes in the intensity of disk emission and in the global magnetospheric current, by means of one-dimensional ...general relativistic particle-in-cell simulations. Such changes likely arise from the intermittency of the accretion process. We find that in cases where the pair-production opacity contributed by the soft disk photons is modest, as in, e.g., M87, such changes can give rise to delayed, strong teraelectronvolt (TeV) flares, dominated by curvature emission of particles accelerated in the gap. The flare rise time, and the delay between the external variation and the onset of the flare emitted from the outer gap boundary, are of the order of the light-crossing time of the gap. The rapid, large-amplitude TeV flares observed in M87, and perhaps, other active galactic nuclei may be produced by such a mechanism.
We propose a novel interpretation that gamma rays from nearby radio galaxies are hadronic emission from magnetically arrested disks (MADs) around central black holes (BHs). The magnetic energy in ...MADs is higher than the thermal energy of the accreting plasma, where the magnetic reconnection or turbulence may efficiently accelerate nonthermal protons. They emit gamma rays via hadronic processes, which can account for the observed gamma rays for M87 and NGC 315. Nonthermal electrons are also accelerated with protons and produce MeV gamma rays, which is useful to test our model by proposed MeV satellites. The hadronic emission from the MADs may significantly contribute to the GeV gamma-ray background and produce the multi-PeV neutrino background detectable by IceCube-Gen2. In addition, gamma rays from MADs provide electron-positron pairs through two-photon pair production at the BH magnetosphere. These pairs can screen the vacuum gap, which affects high-energy emission and jet-launching mechanisms in radio galaxies.
Major Atmospheric Gamma Imaging Cerenkov Telescopes (MAGIC) detected the gamma-ray afterglow of GRB 190114C, which can constrain microscopic parameters of the shock-heated plasma emitting non-thermal ...emission. Focusing on the early afterglow of this event, we numerically simulate the spectrum and multi-wavelength light curves with constant and wind-like circumstellar medium using a time-dependent code. Our results show that the electron acceleration timescale at the highest energies is likely shorter than 20 times the gyroperiod to reproduce the GeV gamma-ray flux and its spectral index reported by Fermi. This gives an interesting constraint on the acceleration efficiency for Weibel-mediated shocks. We also constrain the number fraction of non-thermal electrons fe, and the temperature of the thermal electrons. The early optical emission can be explained by the thermal synchrotron emission with fe 0.01. On the other hand, the X-ray light curves restrict efficient energy transfer from protons to the thermal electrons, and fe ∼ 1 is required if the energy fraction of the thermal electrons is larger than ∼10%. The parameter constraints obtained in this work give important clues to probing plasma physics with relativistic shocks.
One of the mechanisms widely considered for driving relativistic jets in active galactic nuclei, Galactic microquasars and gamma-ray bursts is the electromagnetic extraction of the rotational energy ...of a central black hole, i.e. the Blandford–Znajek process, although the origin of the electromotive force in this process is still under debate. We study this process as the steady unipolar induction in the Kerr black hole magnetosphere filled with a collisionless plasma screening the electric field (the
D
field) along the magnetic field (the
B
field), i.e.
D
·
B
= 0. We extend the formulations and arguments made by Komissarov, and generally show that the origin of the electromotive force is ascribed to the ergosphere. It is explicitly shown that open magnetic field lines penetrating the ergosphere have a region where the
D
field is stronger than the
B
field in the ergosphere, and it keeps driving the poloidal currents and generating the electromotive force and the outward Poynting flux. The range of the possible value of the so-called angular velocity of the magnetic field line ΩF is deduced for the field lines threading the equatorial plane in the ergosphere. We briefly discuss the relation between our conclusion and the ideal magnetohydrodynamic condition.
This paper theoretically investigates the relations between the structure of relativistic jets and produced synchrotron images, by using a steady, axisymmetric force-free jet model. We especially ...focus on the limb-brightened jets that are largely symmetric to the jet axes and observed in some active galactic nuclei, such as M87, Mrk 501, Cyg A, and 3C84. We find that symmetrically limb-brightened images can be produced when magnetic field lines of the jet penetrate a fast-spinning black hole (BH), as motivated by the Blandford-Znajek mechanism. On the other hand, jets with magnetic field lines that pass through a slowly spinning BH or the Keplerian accretion disk produce highly asymmetric radio images. In addition, the edge of a counterjet tends to be luminous in the accretion-disk model even for rather small viewing angles, which may be problematic for some observed jets. We also suggest that the site of particle accelerations in relativistic jets can be constrained by fitting the radio images to observations. This kind of study focusing on the jet images far away from the central engine is complementary to those concentrating directly on the innermost region with upcoming data from the Event Horizon Telescope.
Abstract
High-resolution very long baseline interferometry (VLBI) radio observations have resolved the detailed emission structures of active galactic nucleus jets. General relativistic ...magnetohydrodynamic (GRMHD) simulations have improved the understanding of jet production physics, although theoretical studies still have difficulty constraining the origin and distribution of jetted matter. We construct a new steady, axisymmetric GRMHD jet model to obtain approximate solutions of black hole (BH) magnetospheres, and examine the matter density distribution of jets. By assuming fixed poloidal magnetic field shapes that mimic force-free analytic solutions and GRMHD simulation results and assuming constant poloidal velocity at the separation surface, which divides the inflow and outflow, we numerically solve the force balance between the field lines at the separation surface and analytically solve the distributions of matter velocity and density along the field lines. We find that the densities at the separation surface in our parabolic field models roughly follow
in the far zone from the BH, where
r
ss
is the radius of the separation surface. When the BH spin is larger or the velocity at the separation surface is smaller, the density at the separation surface becomes concentrated closer to the jet edge. Our semianalytic model, combined with radiative transfer calculations, may help us interpret the high-resolution VLBI observations and understand the origin of jetted matter.
Recent radio very long baseline interferometry observations of the relativistic jet in the M87 radio galaxy have shown a triple-ridge structure that consists of the conventional limb-brightened ...feature and a central narrow ridge. Motivated by these observations, we examine a steady axisymmetric force-free model of a jet driven by the central black hole (BH) with its electromagnetic structure being consistent with general relativistic magnetohydrodynamic simulations, and find that it can produce triple-ridge images even if we assume a simple Gaussian distribution of emitting electrons at the base of the jet. We show that the fluid velocity field associated with the electromagnetic field produces the central ridge component due to the relativistic beaming effect, while the limb-brightened feature arises due to a strong magnetic field around the jet edge that also induces the electrons to be dense there. We also show that the computed image strongly depends on the electromagnetic field structure, viewing angle, and parameters related to the electrons' spatial distribution at the jet base. This study will help constrain the non-thermal electron injection mechanism of BH jets and be complementary to theoretical analyses of the upcoming data of the Event Horizon Telescope.
ABSTRACT
Diverging supersonic flows are accelerating, as in the case of a de Laval nozzle, and the same concept has been applied for acceleration of magnetohydrodynamic flows in the universe. Here, ...we study the dynamics of ‘non-diverging’ cylindrical supersonic flows and show that they can be accelerated by effects of radiative cooling and the tangled magnetic field. In addition to radiative cooling of the jet materials (cooling effect), conversion of the ordered magnetic field into the turbulent one (conversion effect) and dissipation of the turbulent magnetic field (dissipation effect) are formulated according to our study on pulsar wind nebulae. Although each of the cooling and conversion effects is an ineffective acceleration process, the terminal velocity of magnetized cylindrical jets attains about half of the maximum possible value when the cooling, conversion, and dissipation effects work simultaneously. The radiation efficiency is also about half of the total luminosity of the jet in the case of maximal acceleration. The concept for flow acceleration by the non-ideal magnetohydrodynamic effects may be useful for studying relativistic jets in active galactic nuclei, in which the region near the jet axis is expected to be cylindrical and kink unstable.
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