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We point out the dominant importance of plasma injection effects for relativistic winds from pulsars and black holes. We demonstrate that outside the light cylinder the magnetically dominated outflows while sliding along the helical magnetic field move in fact nearly radially with very large Lorentz factors $\gamma_0 \gg 1 $, imprinted into the flow during pair production within the gaps. Only at larger distances, $r \geq \gamma_0 (c/\Omega)$, the MHD acceleration $\Gamma \propto r$ takes over. As a result, Blandford-Znajek (BZ) driven outflows would produce spine-brightened images, contrary to observations of the edge-brightened jet in M87. We conclude that M87 jet is not BZ-driven. Other implications include: (i) variability time scale of emission even from non-blazar AGNe like M87 and Cen A can be much shorter than the BH spin period; (ii) the model explains blazar-like phenomena in clearly non-aligned AGNe; (iii) in PIC simulations of pulsars' and black holes' magnetospheres with pair production, the pairs should be injected with large Lorentz factors - this would drastically change the resulting magnetospheric and inner-wind structure, if compared with pair injection at rest; (iv) the model resolves the problem of cyclotron absorption in the Crab pulsar wind.

We study dynamics of relativistic Coronal Mass Ejections (CMEs), from launching by shearing of foot-points (either slowly - the ``Solar flare'' paradigm, or suddenly - the ``star quake" paradigm), to propagation in the preceding magnetar wind. For slow shear, most of the energy injected into the CME is first spent on the work done on breaking through the over-laying magnetic field. At later stages, sufficiently powerful CMEs may experience ``detonation" and lead to opening of the magnetosphere beyond some equipartition radius $r_{eq}$, where the energy of the CME becomes larger than the decreasing external magnetospheric energy. Post-CME magnetosphere relaxes via formation of a plasmoid-mediated current sheet, initially at $\sim r_{eq}$ and slowly reaching the light cylinder (this transient stage has much higher spindown rate and may produce an ``anti-glitch''). Both the location of the foot-point shear and the global magnetospheric configuration affect the frequent-and-weak versus rare-and-powerful CME dichotomy - to produce powerful flares the slow shear should be limited to field lines that close near the star. After the creation of a topologically disconnected flux tube, the tube quickly (at $\sim$ the light cylinder) comes into force-balance with the preceding wind, and is passively advected/frozen in the wind afterward. For fast shear (a local rotational glitch), the resulting large amplitude Alfven waves lead to opening of the magnetosphere (which later recovers similarly to the slow shear case). At distances much larger than the light cylinder, the resulting shear Alfven waves propagate through the wind non-dissipatively. Implications to Fast Radio Bursts are discussed.

We performed fully 3D relativistic magnetohydrodynamical simulation of "stellar wind"-"pulsar wind" interaction in massive binary system, taking into account various possible pulsar geometries ("Frisbees", "Cartwheels" and "Bullets" - a reference to the direction of the pulsar's spin, plane of the orbit and the direction of motion), and various wind trust ratios. The resulting intrinsic morphologies, and different lines of sight, lead to significantly different orbital-phase dependent flow shapes. For the case of companion-dominated wind in the "Bullets-Cartwheel" configuration, the tails length - region of unshocked pulsar wind - can change by an order of magnitude over quarter of the orbit., 11 pages, 13 figures

Rich observational phenomenology associated with Pulsar B in PSR J0737$-$3039A/B system resembles in many respects phenomena observed in the Earth and Jupiter magnetospheres, originating due to the wind-magnetosphere interaction. We consider particle dynamics in the fast corotating magnetosphere of Pulsar B, when the spin period is shorter than the third adiabatic period. We demonstrate that trapped particles occasionally experience large radial variations of the L-parameter (effective radial distance) due to the parametric interaction of the gyration motion with the large scale electric fields induced by the deformations of the magnetosphere, in what could be called a betatron-induced diffusion. The dynamics of particles from the wind of Pulsar A trapped inside Pulsar B magnetosphere is governed by Mathieu's equation, so that the parametrically unstable orbits are occasionally activated; particle dynamics is not diffusive per se. The model explains the high plasma density on the closed field lines of Pulsar B, and the fact that the observed eclipsing region is several times smaller than predicted by the hydrodynamic models.

We discuss dissipative processes occurring during production and escape of Fast Radio Bursts (FRBs) from magnetars' magnetospheres, the presumed loci of FRBs. High magnetic fields are required in the emission region, both to account for the overall energetics of FRBs, and in order to suppress ``normal'' (non-coherent) radiative losses of radio emitting particles; this limits the emission radii to $\leq {\rm few} \times 10 R_{NS}$. Radiative losses by particles in the strong FRB pulse may occur in the outer regions of the magnetosphere for longer rotation periods, $P\geq 1$ second. These losses are suppressed by several effects: (i) the ponderomotive pre-acceleration of background plasma along the direction of wave propagation (losses reduced approximately as $\gamma_\parallel^{3}$: smaller frequency, $ \propto \gamma_\parallel^2$ in power, and times scales stretched, $ \propto \gamma_\parallel$); this acceleration is non-dissipative and is reversed on the declining part of the pulse; (ii) Landau-Pomeranchuk-Migdal effects (long radiation formation length and ensuing destructive interference of scattered waves). In some cases an FRB pulse may be dissipated on external perturbations (e.g., an incoming pulse of Alfven waves): this may produce a pulse of UV/soft X-rays, a swan song of an FRB, possibly detectable by Chandra.

We discuss a novel mechanism of axion production during scattering of Alfven waves by a fast moving Schwarzschild black holes. The process couples classical macroscopic objects, and effectively large amplitude electromagnetic (EM) waves, to microscopic axions. The key ingredient is that the motion of a black hole (BH) across magnetic field creates classical non-zero second Poincare invariant, the electromagnetic anomaly (Lyutikov 2011). In the case of magnetized plasma supporting Alfven wave, it is the fluctuating component of the magnetic field that contributes to the anomaly: for sufficiency small BH moving with the super-Alfvenic velocity the plasma does not have enough time to screen the parallel electric field. This creates time-dependent $ {\bf E} \cdot {\bf B} \neq 0$, and production of axions via the axion-EM coupling.

We consider magnetospheric structure of rotating neutron stars with internally twisted axisymmetric magnetic fields. The twist-induced and rotation-induced toroidal magnetic fields align/counter-align in different hemispheres. Using analytical and numerical calculations (with PHAEDRA code) we show that as a result the North-South symmetry is broken: the magnetosphere and the wind become "angled", of conical shape. Angling of the magnetosphere affects the spindown (making it smaller for mild twists), makes the return current split unequally at the Y-point, produces anisotropic wind and linear acceleration that may dominate over gravitational acceleration in the Galactic potential and give a total kick up to $\sim 100$ km/s. We also consider analytically the structure of the Y-point in the twisted magnetosphere, and provide estimate of the internal twist beyond which no stable solutions exist: over-twisted magnetospheres must produce plasma ejection events., Comment: Further analysis indicted that angling is a transient effect

The radiation formation length for relativistic particles, $l_c \sim \gamma^2 \lambda$ ($\gamma$ is the Lorentz factor, $\lambda$ is the emitted wavelength), is much lager than the inter-particle distances in many astrophysical applications. This leads to the importance of plasma effects even for the high energy emission. The consequences are nontrivial: (i) averaging of the phases of the emitting particles reduces the power (a.k.a., a circle current does not emit); (ii) density fluctuations may lead to the sporadic production of coherent emission; (iii) plasma effects during assembly of a photon may lead to the suppression of the emission (Razin-Tsytovich effect for the superluminal modes), or, in the opposite limit of subluminous normal modes, to the newly discussed synchrotron super-radiance. For synchrotron emission the radiation formation length is the same for all emitted waves, $\sim c/\omega_B$ (non-relativistic Larmor length); for curvature emission it is $R/\gamma$ - macroscopically long in pulsar magnetospheres (e.g., kilometers for radio). The popular model of coherent curvature emission by bunches", with kilometers-long radiation formation length, particles swinging-out in a rotating magnetospheres before they finish emitting a wave, extreme requirements on the momentum spreads, and demands on the electric energy needed to keep the electrostatically repulsing charges together, all make that model internally inconsistent. Long radiation formation lengths affect how emission from PIC simulations should be interpreted: phases of the emitted wave should be added over the radiation formation length, not just the powers from the instantaneous acceleration of each particle.

Afterglows of gamma-ray bursts often show flares, plateaus, and sudden intensity drops: these temporal features are difficult to explain as coming from the forward shock. We calculate radiative properties of early GRB afterglows with the dominant contribution from the reverse shock (RS) propagating in an ultra-relativistic (pulsar-like) wind produced by the long-lasting central engine. RS emission occurs in the fast cooling regime -- this ensures high radiative efficiency and allows fast intensity variations. We demonstrate that: (i) mild wind power, of the order of $\sim 10^{46}$ erg s$^{-1}$, can reproduce the afterglows' plateau phase; (ii) termination of the wind can produce sudden steep decays; (iii) mild variations in the wind luminosity can produce short-duration afterglow flares., Comment: arXiv admin note: substantial text overlap with arXiv:2004.13600

Intense radiation field can modify plasma properties, the corresponding refractive index, and lead to such nonlinear propagation effects as self-focusing. We estimate the corresponding effects in pair plasma, both in unmagnetized and strongly magnetically dominated case. First, in the unmagnetized pair plasma the ponderomotive force does not lead to charge separation, but to density depletion. Second, for astrophysically relevant plasmas of pulsar magnetospheres, (and possible loci of Fast Radio Bursts), where cyclotron frequency $\omega_B$ dominates over plasma frequency $\omega_p$ and the frequency of the electromagnetic wave, $\omega_B \gg \omega_p,\, \omega$, we show that (i) there is virtually no nonlinearity due to changing effective mass in the field of the wave; (ii) ponderomotive force is $F_p^{(B)} =- {m_e c^2}/({4 B_0^2}) \nabla E^2$; it is reduced by a factor $(\omega/\omega_B)^2$ if compared to the unmagnetized case ($B_0$ is the external magnetic field and $E$ is the electric field of the wave); (iii) for radiation beam propagating along constant magnetic field in pair plasma with density $n_\pm$, the ponderomotive force leads to appearance of circular currents that lead to the decrease of the field within the beam by a factor $\Delta B/B_0 = 2\pi n_\pm m_e c^2 {E^2}/{B_0^4}$. Applications to the physics of FRBs are discussed; we conclude that for parameters of FRB's the dominant magnetic field completely suppresses nonlinear radiation effects.

We develop a model of the white dwarf (WD) - red dwarf (RD) binaries AR Sco and AE Aqr as systems in a transient propeller stage of highly asynchronous intermediate polars. The WDs are relatively weakly magnetized with magnetic field of $\sim 10^6$ G. We explain the salient observed features of the systems due to the magnetospheric interaction of two stars. Currently, the WD's spin-down is determined by the mass loading of the WD's magnetosphere from the RD's at a mild rate of $\dot{M}_{WD} \sim 10^{-11} M_\odot $/yr. Typical loading distance is determined by the ionization of the RD's wind by the WD's UV flux. The WD was previously spun up by a period of high accretion rate from the RD via Roch lobe overflow with $\dot{M} \sim 10^{-9} M_\odot $/yr, acting for as short a period as tens of thousands of years. The non-thermal X-ray and optical synchrotron emitting particles originate in reconnection events in the magnetosphere of the WD due to the interaction with the flow from the RD. In the case of AR Sco, the reconnection events produce signals at the WD's rotation and beat periods - this modulation is due to the changing relative orientation of the companions' magnetic moments and resulting variable reconnection conditions. Radio emission is produced in the magnetosphere of the RD, we hypothesize, in a way that it is physically similar to the Io-induced Jovian decametric radiation.

Lyutikov (2002) predicted "radio emission from soft gamma-ray repeaters (SGRs) during their bursting activity". Detection of a Mega-Jansky radio burst in temporal coincidence with high energy bursts from a Galactic magnetar SGR 1935+2154 confirms that prediction. Similarity of this radio event with Fast Radio Bursts (FRBs) suggests that FRBs are produced within magnetar magnetospheres. We demonstrate that SGR 1935+2154 satisfies the previously derived constraints on the physical parameters at the FRBs' loci. Coherent radio emission is generated in the inner parts of the magnetosphere at $r< 100 R_{\rm NS}$. The radio emission is produced by the yet unidentified plasma emission process, occurring during the initial stages of reconnection events.

We discuss coherent free electron laser (FEL) operating during explosive reconnection events in magnetized pair plasma of magnetar magnetospheres. The model explains many salient features of Fast Radio Bursts/magnetars' radio emission: temporal coincidence of radio and high energy bursts, high efficiency of conversion of plasma kinetic energy into coherent radiation, presence of variable, narrow-band emission features drifting down in frequency, high degree of linear polarization. The model relies on magnetar-specific drifting $e^\pm$ plasma components (which generate wiggler field due to the development of the firehose instability) and the presence of reconnection-generated particle beam with mild Lorentz factor of $\gamma_b \sim$ few hundred.

We interpret recent observations of high-to-low frequency drifting features in the spectra of the repeating FRBs as evidence of sharply changing plasma properties in the emission region, presumably the neutron stars magnetospheres. The drifts are then FRBs' analogues of radius-to-frequency mapping in pulsars and Solar type-III radio burst (but not in a sense of a particular emission mechanism). The drifts rates of $\sim 100$ MHz ms$^{-1}$ at frequencies $\sim$ GHz translate to physical size of $\sim {c \omega}/{\dot{\omega}} \sim$ few $\times 10^8$ cm, matching the hypothesis of the FRB origin in the magnetospheres of neutron stars. We suggest that reconnection events result in generation of upward propagating plasma beams that produce radio emission with frequency related to the decreasing local magnetic field and plasma density., Comment: arXiv admin note: substantial text overlap with arXiv:1901.03260

We discuss physical constrains that observations of high brightness temperature coherent radio emission, with brightness temperatures as high as $T_b \sim 10^{35}$ K, impose on the plasma parameters at relativistically moving astrophysical sources. High brightness temperatures imply a minimal plasma energy density at the source. Additional important constraints come from the fact that resonantly emitting particles lose most of their energy to non-resonant inverse Compton and synchrotron processes. We also interpret recent observations of high-to-low frequency drifting features in the spectra of repeating FRBs as analogues of type-III Solar radio bursts produced by reconnection plasma beams within magnetospheres of highly magnetized neutron stars.

We consider conditions for jet break-out through ejecta following mergers of neutron stars and provide simple relations for the break out conditions. We demonstrate that: (i) break-out requires that the isotropic-equivalent jet energy $E_j$ exceeds the ejecta energy $E_{ej}$ by $E_j \geq E_{ej}/ \beta_0$, where $ \beta_0 = V_{ej}/c$, $V_{ej}$ is the maximum velocity of the ejecta. If the central engine terminates before the break out, the shock approaches the edge of the ejecta slowly $\propto 1/t$; late break out occurs only if at the termination moment the head of the jet was relatively close to the edge. (ii) If there is a substantial delay between the ejecta's and the jet's launching, the requirement on the jet power increases. (iii) The forward shock driven by the jet is mildly strong, with Mach number $M\approx 5/4$ (increasing with time delay $t_d$); (iii) the delay time $t_d$ between the ejecta and the jet's launching is important for $t_d > t_0= ({3 }/{16} ) {c M_{ej} V_{ej}}/{L_j} = 1.01 {\rm sec} M_{ej, -2} L_{j, 51} ^{-1} \left ( {\beta_{ej}} /{0.3} \right)$, where $M_{ej}$ is ejecta mass, $L_j$ is the jet luminosity (isotropic equivalent). For small delays, $t_0 $ is also an estimate of the break-out time.

Mergers of white dwarfs (WDs) may lead to a variety of transient astrophysical events, SNIa being one possible outcome. Lyutikov & Toonen (2017, 2019) argued that mergers of WDs result, under various parameter regimes, in unusual central engine-powered supernova and a type of short Gamma Ray Bursts that show extended emission tails. Observations by Gvaramadze et al. (2019) of the central star and the nebula J005311 match to the details the model of Lyutikov & Toonen (2017, 2019) for the immediate product of a merger of a heavy ONeMg WD with CO WD (age, luminosity, stellar size, hydrogen deficiency and chemical composition)., Contribution to "COMPACT WHITE DWARF BINARIES", Yerevan, Armenia

We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class AT2018cow result from an electron-capture collapse to a \NS\ following a merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less massive WD during the merger and the formation of a shell burning non-degenerate star incorporating the ONeMg core. During the shell burning stage a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after $\sim 10^2-10^4$ years. During the formation of a neutron star as little as $\sim 10^{-2} M_\odot $ of the material is ejected at the bounce-off with mildly relativistic velocities and total energy $\sim$ few $ 10^{50}$ ergs. This ejecta becomes optically thin on a time scale of days - this is the FBOT. During the collapse, the neutron star is spun up and magnetic field is amplified. The ensuing fast magnetically-dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high energy emission in a manner similar to Pulsar Wind Nebulae. If the secondary WD was of the DA type, the wind will likely have $\sim 10^{-4} M_\odot$ of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high energy emission of similar luminosity to the optical, and late infra-red features., arXiv admin note: text overlap with arXiv:1709.02221

We reconsider the structure of thermally driven rotating Parker wind. Rotation, without \Bf, changes qualitatively the structure of the subsonic region: solutions become non-monotonic and do not extend to the origin. For small angular velocities solutions have two critical points - X-point and O-points, which merge at the critical angular velocity of the central star $\Omega_{crit} = G M_\ast/(2 \sqrt{2} c_s R_{b}^2)$ (where $M_\ast$ and $R_{b}$ are mass and radius of the central star, $c_s$ is the sound speed in the wind). For larger spins there is no critical points in the solution. For disk winds (when the base of the wind rotates with Keplerian velocity) launched equatorially the coronal sound speed should be smaller than $\approx 0.22 v_K$ in order to connect to the critical curve ($v_K$ is the Keplerian velocity at a given location on the disk).

We consider electromagnetic interaction and precursor emission of merging neutron stars. Orbital motion of the magnetized neutron stars may revive pair production within the common magnetosphere years before the merger, igniting pulsar-like magnetospheric dynamics. We identify two basic scenarios: (i) only one star is magnetized (1M-DNS scenario) and (ii) both stars are magnetized (2M-DNS scenario). Inductively created electric fields can have component along the total magnetic field (gaps) and/or the electric field may exceed the value of the local magnetic field. The key to the detection is orbital modulation of the emission. If only one star is magnetized (1M-DNS scenario) the emission is likely to be produced along the direction of the magnetic field at the location of the secondary; then, if the magnetic axis is misaligned with the orbital spin, this direction is modulated on the orbital period. For the 2M-DNS scenario, the structure of the common magnetosphere of the non-rotating neutron stars is complicated, with gaps, but no $E>B$ regions; there is strong orbital variations for the case of misaligned magnetic moments. For the same parameters of neutron stars the 2M-DNS scenario has intrinsically higher potential than the 1M-DNS one. The overall powers are not very high, $\leq 10^{45} $ erg s$^{-1}$; the best chance to detect electromagnetic precursors to the merging neutron stars is if the interaction of their magnetospheres leads to the production of pulsar-like coherent radio emission modulated at the orbital period, with luminosity of up to $\sim 1$ Jankys at the time the merger., This ArXive version corrects a sign error in the published paper in Eq. (15). Equations (15)-(18) and figures 2 and 3 are affected. Overall results and conclusions remain the same

We perform spectral analyses of four bright $\gamma$-ray pulsars: PSR J0007+7303, Vela, Geminga and PSR J2021+3651 concentrating on the high-energy tails, defined as emission above 10\,GeV. The two competing models of pulsar $\gamma$-ray emission predict qualitatively different spectra well above the break energy: curvature emission predicts an exponential cut-off in the spectra, while the inverse-Compton scattering mechanism favors a power-law. We perform fits to the phase-averaged spectral energy distributions for each of the four pulsars. We find that in all cases both models fit the data equally well---the present data set does not allow any firm claim to be made about the shape of the spectra above 10\,GeV. In no case is the power-law fit or exponential cut-off fit significantly preferred over the other. The Crab pulsar remains the only known pulsar in which the power-law fit is clearly preferred over the exponential cut-off.

We performed calculations of the late radio and X-ray afterglow of GRB/GW170817 in the cocoon-jet paradigm, predicting the appearance of a second peak in the afterglow light curve ~ one-three years after the explosion. The model assumes that the prompt emission and early afterglows originate from a cocoon generated during break-out of the delayed magnetically powered jet. As the jet breaks out from the torus-generated wind, a nearly isotropic mildly relativistic outflow is generated; at the same time the primary jet accelerates to high Lorentz factors and avoids detection. As the fast jet slows down, it should become visible to the off-axis observer. Thus, the model has a clear prediction: the X-ray and radio afterglows should first experience a decay, as the cocoon slows down, followed by a rebrightening when the primary jet starts emitting toward an observer., Comment: 6 pages, 4 figures, submitted to MNRAS

We point out that for sufficiently strong shocks, with Mach number $ M_1 > \sqrt{\frac{3 \gamma -1}{(3-\gamma ) \gamma }}= 1.34$ ($\gamma=5/3$), the solutions for cosmic ray-modified shocks experiences a bifurcation. As a result, for super-critical flows an isothermal jump forms (which is not a shock). The isothermal jump forms due to the energy diffusion of fast, but energetically subdominant cosmic rays. For super-critical flows the isothermal jump appears regardless of a particular feed-back mechanism from the CRs. The compression ratio at the isothermal jump is $2/ (\gamma-1)=3$, so that in the test particle regime the expected spectrum of low energy CRs experiencing first-order Fermi process is $p = 2\gamma/(3- \gamma) = 5/2$, steeper than conventional $p=2$.

We consider structure and emission properties of a pair plasma fireball that cools due to radiation. At temperatures $T \geq 0.5 m_e c^2$ the cooling takes a form of clearly defined cooling wave, whereby the temperature and pair density experience a sharp drop within a narrow region. The surface temperature, corresponding to the location where the optical depth to infinity reaches unity, never falls much below $0.1 m_e c^2 \approx 50$ keV. The propagation velocity of the cooling wave is much smaller than the speed of light and decreases with increasing bulk temperature.

We discuss a scenario of short Gamma-Ray Bursts (GRBs) following a merger of a massive ONeMg white dwarf (WD) with a CO WD, and an ensuing accretion induced collapse (AIC). An initial system with the primary mass $M_1 \sim 6-10 \, M_\odot$ and the secondary mass $M_2 \sim 3-6\, M_\odot$ forms, via two distinct evolutionary channels, a double degenerate CO-ONeMg WD system. For sufficiently large mass ratio $q \equiv M_2/M_1 > q_{crit} \sim 0.25$ the ensuing gravitational wave-driven mass transfer is unstable, whereby the less massive CO WD is disrupted and transfers its mass to the primary ONeMg WD on a few orbital time scales. The merger product ignites shell CO burning, adding mass to the degenerate core; at the same time mass and angular momentum is lost due to powerful winds. For an ONeMg WD sufficiently close to the Chandrasekhar mass an electron-capture accretion induced collapse (AIC) follows $\sim 10^4$ years afterwards. We associate the prompt short GRB emission with a direct collapse of an ONeMg WD to a neutron star, without formation of an accretion disk. After the collapse the accretion of the unburnt part of the shell onto the newly formed NS powers the extended emission (EE). During the collapse the neutron star is spun to millisecond periods and produce long lasting relativistic winds that shock against the material lost during the shell-burning stage, and produce afterglow emission from the wind termination shock., Submitted to MNRAS on July 17th 2017

In our prior papers, we considered the non-relativistic linear stability analysis of magnetized jets that do not have current sheet at the boundary. In this paper, we extend our analysis to relativistic jets. In order to find the unstable modes of current sheet-free, magnetized relativistic jets, we linearize full relativistic magnetohydrodynamics equations and solve them numerically. We find the dispersion relation of the pinch and kink mode instabilities. By comparing the dispersion relations of mildly relativistic jet (Lorentz factor 2) with moderately relativistic jet (Lorentz factor 10), we find that the jet with higher Lorentz factor is significantly more stable in both pinch and kink modes. We show that inclusion of the current sheet-free magnetic field in the jet further enhances the stability. Both pinch and kink mode instabilities become progressively more stable with increasing magnetization. We also show a scaling relation between the maximum temporal growth rate of the unstable mode and the Lorentz factor of the jet. The maximum temporal growth rates of the unstable modes are inversely proportion to the Lorentz factors for most of the modes that we study. However, for the fundamental pinch mode it is inversely proportional to the square of the Lorentz factor. This very beneficial scaling relation holds regardless of the presence of a magnetic field., Comment: 41 pages, 12 figures, Accepted for publication in MNRAS

We can probe observationally and reproduce theoretically intricate properties of the Crab Nebula nearest to the pulsar - The Inner Knot. The tiny knot is indeed a bright spot on the surface of a quasi-stationary magnetic relativistic shock that accelerates particles. It is required that the part of the wind that produces the Inner Knot has low magnetization; thus, it is not a site of gamma-ray flares. We develop a model of particle acceleration during explosive reconnection events in relativistic highly magnetized plasma and apply the model to explain the Crab gamma-ray flares. Particles are efficiently accelerated by charge-starved DC-type electric fields during initial stages of magnetic flux merges. By implication, the magnetic reconnection is an important, and possibly dominant process of particle acceleration in high energy astrophysical sources., Proceedings, Shklovsky 100

We consider the double-shock structure of ultra-relativistic flows produced by the interaction of magnetized wind with magnetized external medium. The contact discontinuity (CD) is a special point in the flow - density, kinetic pressure and magnetic field experience a jump or are non-analytic on the CD. To connect dynamically the outside region (the forward shock flow) with the inside region (the reverse shock flow) requires resolving flow singularities at the contact discontinuity. On the CD the pressure is communicated exclusively by the magnetic field on both sides - the CD becomes an Alfven tangential discontinuity. Thus, the dynamic amplification of the magnetic field leads to a formation of a narrow magnetosheath. We discussed a possibility that particles emitting early $X$-ray afterglows, as well as Fermi GeV photons, are accelerated via magnetic reconnection processes in the post-reverse shock region, and in the magnetosheath in particular.

The direction of polarization produced by a moving source rotates with the respect to the rest frame. We show that this effect, induced by pulsar rotation, leads to an important correction to polarization swings within the framework of rotating vector model (RVM); this effect has been missed by previous works. We construct relativistic RVM taking into account finite heights of the emission region that lead to aberration, time-of-travel effects and relativistic rotation of polarization. Polarizations swings at different frequencies can be used, within the assumption of the radius-to-frequency mapping, to infer emission radii and geometry of pulsars.

We argue that the physical constraints required by the association of the Fermi GBM signal contemporaneous with GW150914 - radiative power of $10^{49} $ erg s$^{-1}$, and corresponding magnetic fields on the black hole of the order of $10^{12}$ Gauss - are astrophysical highly implausible. Combined with the relatively high random probability of coincidence of 0.22 percents, we conclude that the electromagnetic signal is likely unrelated to the BH merger.

We discuss a novel instability in inertia-less electron magneto-hydrodynamics (EMHD), which arises from a combination of electron velocity shear and electron density gradients. The unstable modes have a lengthscale longer than the transverse density scale, and a growth-rate of the order of the inverse Hall timescale. We suggest that this density-shear instability may be of importance in magnetic reconnection regions on scales smaller than the ion skin depth, and in neutron star crusts. We demonstrate that the so-called Hall drift instability, previously argued to be relevant in neutron star crusts, is a resistive tearing instability rather than an instability of the Hall term itself. We argue that the density-shear instability is of greater significance in neutron stars than the tearing instability, because it generally has a faster growth-rate and is less sensitive to geometry and boundary conditions. We prove that, for uniform electron density, EMHD is "at least as stable" as regular, incompressible MHD, in the sense that any field configuration that is stable in MHD is also stable in EMHD. We present a connection between the density-shear instability in EMHD and the magneto-buoyancy instability in anelastic MHD., Comment: 1 Figure

(Abridged) We consider dynamics and turbulent interaction of whistler modes within the framework of inertialess electron MHD (EMHD). We argue there is no energy principle in EMHD: any stationary closed configuration is neutrally stable. We consider the turbulent cascade of whistler modes. We show that (i) harmonic whistlers are exact non-linear solutions; (ii) co-linear whistlers do not interact (including counter-propagating); (iii) waves with the same value of the wave vector, $k_1=k_2$, do not interact; (iv) whistler modes have a dispersion that allows a three-wave decay, including into a zero frequency mode; (v) the three-wave interaction effectively couples modes with highly different wave numbers and propagation angles. In addition, linear interaction of a whistler with a single zero-mode can lead to spatially divergent structures via parametric instability. All these properties are drastically different from MHD, so that the qualitative properties of the Alfven turbulence cannot be transferred to the EMHD turbulence. We derive the Hamiltonian formulation of EMHD, and using Bogolyubov transformation reduce it to the canonical form; we calculate the matrix elements for the three-wave interaction of whistlers. We solve numerically the kinetic equation and show that, generally, the EMHD cascade develops within a broad range of angles, while transiently it may show anisotropic, nearly two dimensional structures. Development of a cascade depends on the forcing (non-universal) and often fails to reach a steady state. Analytical estimates predict the spectrum of magnetic fluctuations for the quasi-isotropic cascade $\propto k^{-2}$. The cascade remains weak (not critically-balanced). The cascade is UV-local, while the infrared locality is weakly (logarithmically) violated.

We discuss three topics: (i) the dynamics of afterglow jet breaks; (ii) the origin of Fermi-LAT photons; (iii) the electromagnetic model of short GRBs, Comment: Proceedings, GRB 2013, Huntsville Gamma-Ray Burst Symposium

We attribute the rapid spindown of magnetar 1E 2259+586 observed by Archibald et al. (2013), termed the "anti-glitch", to partial opening of the magnetosphere during the X-ray burst, followed by changes of the structure of the closed field line region. To account for the observed spin decrease during the X- ray flare all that is needed is the transient opening, for just one period, of a relatively small fraction of the magnetosphere, of the order of only few percent. More generally, we argue that in magnetars all timing irregularities have magnetospheric origin and are induced either by (i) the fluctuations in the current structure of the magnetosphere (similar to the long term torque variations in the rotationally powered pulsars); or, specifically to magnetars, by (ii) opening of a fraction of the magnetosphere during bursts and flares - the latter events are always accompanied by rapid spindown, an "anti-glitch". Slow rotational motion of the neutron star crust, driven by crustal magnetic fields, leads beyond some twist limit to explosive instability of the external magnetic fields and transient opening of a large magnetic flux in a CME-type event, the post-flares increase of magnetospheric currents accompanied by enhanced X-luminosity and spindown rate, changing profiles, as well as spectral hardening, all in agreement with the magnetospheric model of torque fluctuations.

I. There is growing evidence that pulsars' high energy emission is generated via Inverse Compton mechanism. II. The particles producing Crab flares, and possibly most of the Crab Nebula's high energy emission, are accelerated via reconnection events, and not at shock via Fermi mechanisms., Proceedings, XMM-Newton Science Workshop "Fast and Furious", 6 pages

We present several models of the magnetic structure of solar coronal mass ejections (CMEs). First, we model CMEs as expanding force-free magnetic structures. While keeping the internal magnetic field structure of the stationary solutions, expansion leads to complicated internal velocities and rotation, while the field structures remain force-free. Second, expansion of a CME can drive resistive dissipation within the CME changing the ionization states of different ions. We fit in situ measurements of ion charge states to the resistive spheromak solutions. Finally, we consider magnetic field structures of fully confined stable magnetic clouds containing both toroidal and poloidal magnetic fields and having no surface current sheets. Expansion of such clouds may lead to sudden onset of reconnection events., proceedings, XIIth Hvar Astrophys. Colloquium

The unusually short durations, high luminosities, and high photon energies of the Crab Nebula gamma-ray flares require relativistic bulk motion of the emitting plasma. We explain the Crab flares as the result of randomly oriented relativistic "minijets" originating from reconnection events in a magnetically dominated plasma. We develop a statistical model of the emission from Doppler boosted reconnection minijets and find analytical expressions for the moments of the resulting nebula light curve (e.g. time average, variance, skewness). The light curve has a flat power spectrum that transitions at short timescales to a decreasing power-law of index 2. The flux distribution from minijets follows a decreasing power-law of index ~ 1, implying the average flux from flares is dominated by bright rare events. The predictions for the flares' statistics can be tested against forthcoming observations. We find the observed flare spectral energy distributions (SEDs) have several notable features: A hard power-law index of p ~< 1 for accelerated particles that is expected in various reconnection models, including some evidence of a pile-up near the radiation reaction limit. Also, the photon energy at which the SED peaks is higher than that implied by the synchrotron radiation reaction limit, indicating the flare emission regions' Doppler factors are ~> few. We conclude that magnetic reconnection can be an important, if not dominant, mechanism of particle acceleration within the nebula., Accepted by MNRAS

We consider the dynamics of a mechanical failure induced by a shear stress in a strongly magnetized neutron-star crust. We show that even if the elastic properties of the crust allow the creation of a shear crack, the strongly sheared magnetic field around the crack leads to a back-reaction from the Lorentz force which does not allow large relative displacement of the crack surfaces. Instead, the global evolution of the crack proceeds on a slow resistive time scale, and is unable to release any substantial mechanical energy. Our calculations demostrate that for {\it some} magnetic-field configurations, the magnetic forces cause, effectively, a plastic deformation of the crust when the resulting elastic shear stress exceeds the critical value for mechanical failure., 9 pages, 1 figure, submitted to MNRAS

The "no hair" theorem is not applicable to black holes formed from collapse of a rotating neutron star. Rotating neutron stars can self-produce particles via vacuum breakdown forming a highly conducting plasma magnetosphere such that magnetic field lines are effectively "frozen-in" the star both before and during collapse. In the limit of no resistivity, this introduces a topological constraint which prohibits the magnetic field from sliding off the newly-formed event horizon. As a result, during collapse of a neutron star into a black hole, the latter conserves the number of magnetic flux tubes N_B = e \Phi_\infty /(\pi c \hbar), where \Phi_\infty is the initial magnetic flux through the hemispheres of the progenitor and out to infinity. The black hole's magnetosphere subsequently relaxes to the split monopole magnetic field geometry with self-generated currents outside the event horizon. The dissipation of the resulting equatorial current sheet leads to a slow loss of the anchored flux tubes, a process that balds the black hole on long resistive time scales rather than the short light-crossing time scales expected from the vacuum "no-hair" theorem., Comment: Proceedings, "Relativity and Gravitation, 100 Years after Einstein in Prague"

Very high energy gamma-ray flares from the Crab nebular detected by AGILE and Fermi satellites challenge our understanding of the pulsar wind nebulae. The short duration of the flares, only few days, is particularly puzzling since it is much shorter than the dynamical times scale of the nebular. In this work we investigate analytically and via numerical simulations the electromagnetic signatures expected from the large amplitude low frequency magnetosonic waves generated within the Crab nebular which induce the corrugation perturbations of the termination shock. As a result, the oblique termination shock produces time-dependent, mildly relativistic post-shock flow. Using the relativistic MHD version of the RIEMANN code, we simulate the interaction of the termination shock with downstream perturbations. We demonstrate that mild Doppler boosting of the synchrotron emission in the post-shock flow can produce bright, short time scale flares.

Expansion of non-spherical relativistic blast waves is considered in the Kompaneets (the thin shell) approximation. We find that the relativistic motion effectively "freezes out" the lateral dynamics of the shock front: only extremely strongly collimated shocks, with the opening angles $\Delta \theta \leq 1/\Gamma^2$, show appreciable modification of profiles due to sideways expansion. For less collimated profiles the propagation is nearly ballistic; the sideways expansion of relativistic shock becomes important only when they become mildly relativistic.

We consider dynamical scales in magnetized GRB outflows, using the solutions to the Riemann problem of expanding arbitrarily magnetized outflows (Lyutikov 2010). For high ejecta magnetization, the behavior of the forward shock closely resembles the so-called thick shell regime of the hydrodynamical expansion. The exception is at small radii, where the motion of the forward shock is determined by the dynamics of subsonic relativistic outflows. The behaviors of the reverse shock is different in fluid and magnetized cases: in the latter case, even for medium magnetization, sigma ~ 1, the reverse shock forms at fairly large distances, and may never form in a wind-type external density profile.

Observations of clusters of galaxies show ubiquitous presence of X-ray cavities, presumably blown by the AGN jets. We consider magnetic field structures of these cavities. Stability requires that they contain both toroidal and poloidal magnetic fields, while realistic configurations should have vanishing magnetic field on the boundary. For axisymmetric configurations embedded in unmagnetized plasma, the continuity of poloidal and toroidal magnetic field components on the surface of the bubble then requires solving the elliptical Grad-Shafranov equation with both Dirichlet and Neumann boundary conditions. This leads to a double eigenvalue problem, relating the pressure gradients and the toroidal magnetic field to the radius of the bubble. We have found fully analytical stable solutions. This result is confirmed by numerical simulation. We present synthetic X-ray images and synchrotron emission profiles and evaluate the rotation measure for radiation traversing the bubble., Comment: 10 pages, 13 figures, accepted by MNRAS

Progenitors of long GRBs, and core-collapse supernovae in general, may have two separate mechanisms driving the outflows: quasi-isotropic neutrino-driven supernova explosions followed by a highly collimated relativistic outflow driven by the GRB central engine, a black hole or a magnetar. We consider the dynamics of the second GRB-driven explosion propagating through expanding envelope generated by the passage of the primary supernova shock. Beyond the central core, in the region of steep density gradient created by the SN shock breakout, the accelerating secondary quasi-spherical GRB shock become unstable to corrugation and under certain conditions may form a highly collimated jet, a "chimney", when a flow expands almost exclusively along a nearly cylindrically collimated channel. Thus, weakly non-spherical driving and/or non-spherical initial conditions of the wind cavity may produce highly non-spherical, jetted outflows. For a constant luminosity GRB central engine, this occurs for density gradient in the envelope \rho ~ r^{-\omega} steeper than \omega >4.

We calculate the evolution of the prompt intrinsic Poynting power generated by merging black holes. Orbiting black holes induce rotation of the space-time. In a presence of magnetic field supported by an accretion disk outside of the orbit, this results in a generation of an electromagnetic outflow via the Blandford-Znajek-type process with luminosity $L_{EM} \sim G^3 M^3 B^2/(c^5 R_{orb}) $ and reaching a fairly low maximum values of $L_{EM} = 10^{37}-10^{39} m_6 $ erg s$^{-1}$ ($m_6$ is the masses of black holes in millions of Solar mass) at the time of the merger. Dissipation of the wind power may produce two types of observed signatures: a highly variable collimated emission coming from the internal dissipation within the jets and a broad-band near-isotropic emission generated at the termination shocks.

Magnetically-driven non-stationary acceleration of jets in AGNs results in the leading parts of the flow been accelerated to much higher Lorentz factors than in the case of steady state acceleration with the same parameters. The higher Doppler-boosted parts of the flow may dominate the high energy emission of blazar jets. We suggest that highly variable GeV and TeV emission in blazars is produced by the faster moving leading edges of highly magnetized non-stationary ejection blobs, while the radio data trace the slower-moving bulk flow. Model predictions compare favorably with the latest Fermi gamma-ray and MOJAVE radio VLBI results.

The basic principles of General Theory of Relativity historically have been tested in gedanken experiments in rotating frame of references. One of the key issues, which still evokes a lot of controversy, is the centrifugal acceleration. Machabeli & Rogava (1994) argued that centrifugal acceleration reverse direction for particles moving radially with relativistic velocities within a "bead on a wire" approximation. We show that this result is frame-dependent and reflects a special relativistic dilution of time (as correctly argued by de Felice (1995)) and is analogous to freezing of motion on the black hole horizon as seen by a remote observer. It is a reversal of coordinate acceleration; there is no such effect as measured by a defined set of observers, e.g., proper and/or comoving. Frame-independent velocity of a "bead" with respect to stationary rotating observers increases and formally reaches the speed of light on the light cylinder. In general relativity, centrifugal force does reverse its direction at photon circular orbit, r=3M in Schwarzschild metric, as argued by Abramowicz (1990)., rejected by American Journal of Physics on the grounds that "metric, relativistic Hamilton-Jacobi theory, and Killing vectors [...] are not accessible to physicists from outside the specific subdiscipline"

(Abridged) We develop a theoretical framework to construct axisymmetric magnetic equilibria in stars, consisting of both poloidal and toroidal magnetic field components. In a stationary axisymmetric configuration, the poloidal current is a function of the poloidal magnetic flux only, and thus should vanish on field lines extending outside of the star. Non-zero poloidal current is limited to a set of toroid-shape flux surfaces fully enclosed inside the star. If we demand that there are no current sheets, then on the separatrix delineating the regions of zero and finite toroidal magnetic field both the poloidal flux function and its derivative should match. Thus, for a given magnetic field in the bulk of the star, the elliptical Grad-Shafranov equation that describes magnetic field structure inside the toroid is an ill-posed problem, with both Dirichlet and Newman boundary conditions and {\it a priori} unknown distribution of toroidal and poloidal electric currents. We discuss a procedure which allows to solve this ill-posed problem by adjusting the unknown current functions. We find a poloidal current-carrying solution that leaves the shape of the flux function and, correspondingly, the toroidal component of the electric current the same as in the case of no poloidal current. The equilibria discussed in this paper may have arbitrary large toroidal magnetic field, and may include a set of stable equilibria. The method developed here can also be applied to magnetic structure of differentially rotating stars, as well as to calculate velocity field in incompressible isolated fluid vortex with a swirl., 17 pages, 7 figures

Kinetic diffusion of cosmic rays ahead of perpendicular shocks induces charge non-neutrality, which is mostly, yet not completely, screened by the bulk plasma via polarization drift current. Hydrodynamic shear instabilities as well as modified Buneman instability of the polarization current generate the turbulence necessary for a Fermi-type acceleration. Thus, similar to the case of parallel shocks, in perpendicular shocks the diffusing cosmic rays generate unstable plasma currents that in turn excite turbulence. This allows a self-consistent evolution of a shock-cosmic rays system. In the kinetic regime of the modified Buneman instability, electrons may be heated in the cosmic ray precursor., Comment: 12 pages, 1 Fig

Pulsars and central engines of long gamma ray burst -- collapsars -- may produce highly magnetized (Poynting flux dominated) outflows expanding in a dense surrounding (interstellar medium or stellar material). For certain injection conditions, the magnetic flux of the wind cannot be accommodated within the cavity. In this case, ideal (non-dissipative) MHD models, similar to the Kennel and Coroniti (1984) model of the Crab nebular, break down (the so-called sigma problem). This is typically taken to imply that the wind should become particle-dominated on scales much smaller than the size of the cavity. The wind is then slowed down by a fluid-type (low magnetization) reverse shock. Recent Fermi results, indicating that synchrotron spectrum of the Crab nebula extends well beyond the upper limit of the most efficient radiation reaction-limited acceleration, contradict the presence of a low sigma reverse shock. We propose an alternative possibility, that the excessive magnetic flux is destroyed in a reconnection-like process in two regions: near the rotational axis and near the equator. We construct an example of such highly magnetized wind having two distinct reconnection regions and suggest that these reconnection cites are observed as tori and jets in pulsar wind nebulae. The model reproduces, qualitatively, the observed morphology of the Crab nebula. In parts of the nebular the dissipation occurs in a relativistically moving wind, alleviating the requirements on the acceleration rate., Comment: 18 pages, 2 figures, submitted to MNRAS