arXiv subjects

Maxim Lyutikov

Publications and source records attributed to Maxim Lyutikov.

At least 55 records · Page 3Linked to original sources

arXiv2021

Rotating neutron stars without light cylinders

We find a class of twisted and differentially rotating neutron star magnetospheres that do not have a light cylinder, generate no wind and thus do not spin-down. The magnetosphere is composed of embedded differentially rotating flux surfaces, with the angular velocity decreasing as $Ω\propto 1/r$ (equivalently, becoming smaller at the foot-points closer to the axis of rotation). For each given North-South self-similar twist profile there is a set of self-similar angular velocity profiles (limited from above) with a "smooth", dipolar-like magnetic field structure extending to infinity. For spin parameters larger than some critical value, the light cylinder appears, magnetosphere opens up, and the wind is generated.

astro-ph.HE↗

arXiv2021

Resolving the emission regions of the Crab pulsar's giant pulses

The Crab pulsar has striking radio emission properties, with the two dominant pulse components -- the main pulse and the interpulse -- consisting entirely of giant pulses. The emission is scattered in both the Crab nebula and the interstellar medium, causing multi-path propagation and thus scintillation. We study the scintillation of the Crab's giant pulses using phased Westerbork Synthesis Radio Telescope data at 1668\,MHz. We find that giant pulse spectra correlate at only $\sim 2 \%$, much lower than the $1/3$ correlation expected from a randomized signal imparted with the same impulse response function. In addition, we find that the main pulse and the interpulse appear to scintillate differently; the 2D cross-correlation of scintillation between the interpulse and main pulse has a lower amplitude, and is wider in time and frequency delay than the 2D autocorrelation of main pulses. These lines of evidence suggest that the giant pulse emission regions are extended, and that the main pulse and interpulse arise in physically distinct regions which are resolved by the scattering screen. Assuming the scattering takes place in the nebular filaments, the emission regions are of order a light cylinder radius, as projected on the sky. With further VLBI and multi-frequency data, it may be possible to measure the distance to the scattering screens, the size of giant pulse emission regions, and the physical separation between the pulse components.

astro-ph.HE↗

arXiv2021

Radiation formation length in astrophysical high brightness sources

The radiation formation length for relativistic particles, $l_c \sim γ^2 λ$ ($γ$ is the Lorentz factor, $λ$ 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, $\sim c/ω_B$ (non-relativistic Larmor length); for curvature emission it is $R/γ$ - 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.

astro-ph.HE↗

arXiv2021

Escape of Fast Radio Bursts from magnetars' magnetospheres

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 $γ_\parallel^{3}$: smaller frequency, $ \propto γ_\parallel^2$ in power, and times scales stretched, $ \propto γ_\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.

astro-ph.HE↗

arXiv2021

Magnetic topology in coupled binaries, spin-orbital resonances, and flares

We consider topological configurations of the magnetically coupled spinning stellar binaries (e.g., merging neutron stars or interacting star-planet systems). We discuss conditions when the stellar spins and the orbital motion nearly `compensate' each other, leading to very {\it slow} overall winding of the coupled magnetic fields; slowly winding configurations allow gradual accumulation of magnetic energy, that is eventually released in a flare when the instability threshold is reached. We find that this slow winding can be global and/or local. We describe the topology of the relevant space $\mathbb{F}=T^1S^2$ as the unit tangent bundle of the two-sphere and find conditions for slowly winding configurations in terms of magnetic moments, spins and orbital momentum. These conditions become ambiguous near the topological bifurcation points; in certain cases they also depend on the relative phases of the spin and orbital motions. In the case of merging magnetized neutron stars, if one of the stars is a millisecond pulsar, spinning at $\sim$ 10 msec, the global resonance $ω_1+ω_2= 2 Ω$ (spin-plus beat is two times the orbital period) occurs approximately a second before the merger; the total energy of the flare can be as large as $10\%$ of the total magnetic energy, producing bursts of luminosity $\sim 10^{44}$ erg s$^{-1}$. Higher order local resonances may have similar powers, since the amount of involved magnetic flux tubes may be comparable to the total connected flux.

astro-ph.HE↗

arXiv2021

Magnetic loading of magnetars' flares

Magnetars, the likely sources of Fast Radio Bursts (FRBs), produce both steady highly relativistic magnetized winds, and occasional ejection events. We demonstrate that the requirement of conservation of the magnetic flux dominates the overall dynamics of magnetic explosions. This is missed in conventional hydrodynamic models of the ejections as expanding shell with parametrically added magnetic field, as well as one-dimensional models of magnetic disturbances. Most of the initial free energy of an explosion is actually spent on stretching its own internal magnetic field, while doing minimal $pdV$ work against the surrounding. Magnetic explosions from magnetars come into force balance with the pre-flares wind close to the light cylinder. They are then advected quietly with the wind, or propagate as electromagnetic disturbances. No powerful shock waves are generated in the wind.

astro-ph.HE↗

arXiv2021

Production of axions during scattering of Alfven waves by fast-moving Schwarzschild black holes

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.

hep-th↗

arXiv2021

Radio afterglow of magnetars' giant flares

We develop a model for the radio afterglow of the giant flare of SGR 1806-20 arising due to the interaction of magnetically-dominated cloud, an analogue of Solar Coronal Mass Ejections (CMEs), with the interstellar medium (ISM). The CME is modeled as a spheromak-like configuration. The CME is first advected with the magnetar's wind and later interacts with the ISM, creating a strong forward shock and complicated backwards exhaust flow. Using three-dimensional magnetohydrodynamic simulations, we study various relative configurations of the magnetic field of the CME with respect to the ISM's magnetic field. We show that the dynamics of the forward shock mostly follows the Sedov-Taylor blastwave, while the internal structure of the shocked medium is considerably modified by the back flow, creating a multiple shock configuration. We calculate synthetic synchrotron emissivity maps and light curves using two assumptions: (i) magnetic field compression; (ii) amplification of the magnetic field at the shock.We find that models with magnetic field amplification account better for the observed radio emission.

astro-ph.HE↗

arXiv2021

Brightness temperature constraints on coherent processes in magnetospheres of neutron stars

We discuss constraints that the observed brightness temperatures impose on coherent processes in pulsars and Fast Radio Bursts (FRBs), and in particular on the hypothesis of coherent curvature emission by bunches. We estimate the peak brightness temperature that a bunch of charge $Ze$ can produce via synchrotron and/or curvature emission as $k_B T \sim (Z e)^2/λ$, where $λ$ is the typical emitted wavelength. We demonstrate that the bunch's electrostatic energy required to produce observed brightness temperature is prohibitively high, of the order of the total {\it bulk } energy. We compare corresponding requirements for the Free Electron Laser mechanism (Lyutikov 2021) and find that in that case the constraints are much easier satisfied.

astro-ph.HE↗

arXiv2021

Angled pulsar magnetospheres

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.

astro-ph.HE↗

arXiv2021

Dynamics and emission of wind-powered afterglows of gamma-ray bursts: flares, plateaus and steep decays

We develop a model of early X-ray afterglows of gamma-ray bursts originating from the reverse shock (RS) propagating through ultra-relativistic, highly magnetized pulsar-like winds produced by long-lasting central engines. We first perform fluid and MHD numerical simulations of relativistic double explosions. We demonstrate that even for constant properties of the wind a variety of temporal behaviors can be produced, depending on the energy of the initial explosion and the wind power, the delay time for the switch-on of the wind, and magnetization of the wind. X-ray emission of the highly magnetized RS occurs in the fast cooling regime - this ensures high radiative efficiency and allows fast intensity variations. We demonstrate that: (i) RS emission naturally produces light curves showing power-law temporal evolution with various temporal indices; (ii) mild wind power, of the order of $\sim 10^{46}$ erg s$^{-1}$ (equivalent isotropic), can reproduce the afterglows' plateau phase; (iii) termination of the wind can produce sudden steep decays; (iv) short-duration afterglow flares are due to mild variations in the wind luminosity, with small total injected energy.

astro-ph.HE↗

arXiv2020

Peeking Between the Pulses: The Far-UV Spectrum of the Previously Unseen White Dwarf in AR Scorpii

The compact object in the interacting binary AR Sco has widely been presumed to be a rapidly rotating, magnetized white dwarf (WD), but it has never been detected directly. Isolating its spectrum has proven difficult because the spin-down of the WD generates pulsed synchrotron radiation that far outshines the WD's photosphere. As a result, a previous study of AR Sco was unable to detect the WD in the averaged far-ultraviolet spectrum from a Hubble Space Telescope (HST) observation. In an effort to unveil the WD's spectrum, we reanalyze these HST observations by calculating the average spectrum in the troughs between synchrotron pulses. We identify weak spectral features from the previously unseen WD and estimate its surface temperature to be 11500$\pm$500K. Additionally, during the synchrotron pulses, we detect broad Lyman-$α$ absorption consistent with hot WD spectral models. We infer the presence of a pair of hotspots, with temperatures between 23000K and 28000K, near the magnetic poles of the WD. As the WD is not expected to be accreting from its companion, we describe two possible mechanisms for heating the magnetic poles. The Lyman-$α$ absorption of the hotspots appears relatively undistorted by Zeeman splitting, constraining the WD's field strength to be 100 MG, but the data are insufficient to search for the subtle Zeeman splits expected at lower field strengths.

astro-ph.SR↗

arXiv2020

Tilting Instability of Magnetically Confined Spheromaks

We consider the tilting instability of a magnetically confined spheromak using 3D MHD and relativistic PIC calculations with an application to astrophysical plasmas, specifically those occurring in magnetar magnetospheres. The instability is driven by the counter alignment of the spheromak's intrinsic magnetic dipole with the external magnetic field. Initially the spheromak rotates - tilts - trying to lower its magnetic potential energy. As a result a current sheet forms between the internal magnetic field of a spheromak and the confining field. Magnetic reconnection sets in; this leads to the annihilation of the newly counter-aligned magnetic flux of the spheromak. This occurs on few Alfvén time scales. In the case of higher order (second order) spheromak, the internal core is first pushed out of the envelope, resulting in formation of two nearly independent tilting spheromaks. Thus, the magnetically twisted outer shell cannot stabilize the inner core. During dissipation, helicity of the initial spheromak is carried away by torsional Alfvén waves, violating the assumptions of the Taylor relaxation theorem. In applications to magnetars' giant flares, fast development of tilting instabilities, and no stabilization of the higher order spheromaks, make it unlikely that trapped spheromaks are responsible for the tail emission lasting hundreds of seconds.

physics.plasm-ph↗

arXiv2020

Wind-powered afterglows of gamma-ray bursts: flares, plateaus and steep decays

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.

astro-ph.HE↗

arXiv2020

Free electron laser in magnetars/Fast Radio Bursts

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 $γ_b \sim$ few hundred.

astro-ph.HE↗

arXiv2020

Fast moving pulsars as probes of interstellar medium

Pulsars moving through ISM produce bow shocks detected in hydrogen H$α$ line emission. The morphology of the bow shock nebulae allows one to probe the properties of ISM on scales $\sim 0.01$ pc and smaller. We performed 2D RMHD modeling of the pulsar bow shock and simulated the corresponding H$α$ emission morphology. We find that even a mild spatial inhomogeneity of ISM density, $δρ/ρ\sim 1$, leads to significant variations of the shape of the shock seen in H$α$ line emission. We successfully reproduce the morphology of the Guitar Nebula. We infer quasi-periodic density variations in the warm component of ISM with a characteristic length of $\sim0.1$~pc. Structures of this scale might be also responsible for the formation of the fine features seen at the forward shock of Tycho SNR in X-rays. Formation of such short periodic density structures in the warm component of ISM is puzzling, and bow-shock nebulae provide unique probes to study this phenomenon.

astro-ph.HE↗

arXiv2020

Turbulent model of Crab nebula radiation

We construct a turbulent model of the Crab Nebula's non-thermal emission. The present model resolves a number of long-standing problems of the Kennel-Coroniti (1984) model: (i) the sigma problem; (ii) the hard spectrum of radio electrons; (iii) the high peak energy of gamma-ray flares; (iv) and the spacial evolution of the infrared (IR) emission. The Nebula contains two populations of injected particles: Component-I accelerated at the wind termination shock via Fermi-I mechanism, and Component-II accelerated in reconnecting turbulence in highly magnetized ($σ$ $\gg 1$) plasma in the central part of the Crab Nebula. The reconnecting turbulence Component-II extends from radio to gamma rays: it accelerate radio electrons with a hard spectrum, destroy the large scale magnetic flux (and thus resolves the sigma-problem), and occasionally produces gamma-ray flares (from the largest scale reconnection events). The model reproduces the broad-band spectrum of the Crab Nebula, from low-frequency synchrotron emission in radio to inverse-Compton emission at TeV energies, as well as spatially resolved evolution of the spectral indices in IR and optical bands.

astro-ph.HE↗

arXiv2020

Fast Radio Bursts from reconnection events in magnetar magnetospheres

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.

astro-ph.HE↗