arXiv subjects

Maxim Lyutikov

Publications and source records attributed to Maxim Lyutikov.

At least 73 records · Page 4Linked to original sources

arXiv2020

Magnetospheric interaction in white dwarf binaries AR Sco and AE Aqr

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.

astro-ph.HE↗

arXiv2020

Coherence constraints on physical parameters at bright radio sources and FRB emission mechanism

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.

astro-ph.HE↗

arXiv2020

Nonlinear optics in strongly magnetized pair plasma, with applications to FRBs

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 $ω_B$ dominates over plasma frequency $ω_p$ and the frequency of the electromagnetic wave, $ω_B \gg ω_p,\, ω$, 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 $(ω/ω_B)^2$ 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 $ΔB/B_0 = 2πn_\pm m_e c^2 {E^2}/{B_0^4}$. 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.

astro-ph.HE↗

arXiv2019

Electrodynamics of double neutron star mergers

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.

astro-ph.HE↗

arXiv2019

Nonlinear force-free configurations in cylindrical geometry

We find a new family of solutions for force-free magnetic structures in cylindrical geometry. These solutions have radial power-law dependance and are periodic but non-harmonic in azimuthal direction; they generalize the vacuum $z$-independent potential fields to current-carrying configurations.

physics.plasm-ph↗

arXiv2019

Conditions for jet break-out in NS mergers

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 $E_j \geq E_{ej}/ β_0$, where $ β_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 $t_d > t_0= ({3 }/{16} ) {c M_{ej} V_{ej}}/{L_j} = 1.01 {\rm sec} M_{ej, -2} L_{j, 51} ^{-1} \left ( {β_{ej}} /{0.3} \right)$, 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.

astro-ph.HE↗

arXiv2019

Transients following white dwarfs mergers

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).

astro-ph.HE↗

arXiv2019

Frequency drifts in FRBs due to radius-to-frequency mapping in magnetospheres of neutron stars

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 $\sim {c ω}/{\dotω} \sim$ 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.

astro-ph.HE↗

arXiv2019

Interpreting Crab Nebula synchrotron spectrum: two acceleration mechanisms

We outline a model of the Crab Pulsar Wind Nebula with two different populations of synchrotron emitting particles, arising from two different acceleration mechanisms: (i) Component-I due to Fermi-I acceleration at the equatorial portion of the termination shock, with particle spectral index $p_I \approx 2.2$ above the injection break corresponding to $γ_{wind} σ_{wind} \sim 10^5$, peaking in the UV ($γ_{wind} \sim 10^2$ is the bulk Lorentz factor of the wind, $σ_{wind} \sim 10^3$ is wind magnetization); (ii) Component-II due to acceleration at reconnection layers in the bulk of the turbulent Nebula, with particle index $p_{II} \approx 1.6$. The model requires relatively slow but highly magnetized wind. For both components the overall cooling break is in the infra-red at $\sim 0.01$ eV, so that the Component-I is in the fast cooling regime (cooling frequency below the peak frequency). In the optical band Component-I produces emission with the cooling spectral index of $α_o \approx 0.5$, softening towards the edges due to radiative losses. Above the cooling break, in the optical, UV and X-rays, Component-I mostly overwhelms Component-II. We hypothesize that acceleration at large-scale current sheets in the turbulent nebula (Component-II) extends to the synchrotron burn-off limit of $ε_s \approx 100$ MeV. Thus in our model acceleration in turbulent reconnection (Component-II) can produce both hard radio spectra and occasional gamma-ray flares. This model may be applicable to a broader class of high energy astrophysical objects, like AGNe and GRB jets, where often radio electrons form a different population from the high energy electrons.

astro-ph.HE↗

arXiv2019

FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs

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.

astro-ph.HE↗

arXiv2019

On the nature of radio filaments near the Galactic Center

We suggest that narrow, long radio filaments near the Galactic Center arise as kinetic jets - streams of high energy particles escaping from ram-pressure confined pulsar wind nebulae (PWNe). The reconnection between the PWN and interstellar magnetic field allows pulsar wind particles to escape, creating long narrow features. They are the low frequency analogs of kinetic jets seen around some fast-moving pulsars, such as The Guitar and The Lighthouse PWNe. The radio filaments trace a population of pulsars also responsible for the Fermi GeV excess produced by the Inverse Compton scattering by the pulsar wind particles. The magnetic flux tubes are stretched radially by the large scale Galactic winds. In addition to PWNe accelerated particles can be injected at supernovae remnants. The model predicts variations of the structure of the largest filaments on scales of $\sim$ dozens of years - smaller variations can occur on shorter time scales. We also encourage targeted observations of the brightest sections of the filaments and of the related unresolved point sources in search of the powering PWNe and pulsars.

astro-ph.HE↗

arXiv2019

Rotating Parker wind

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 $Ω_{crit} = G M_\ast/(2 \sqrt{2} c_s R_{b}^2)$ (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).

astro-ph.SR↗

arXiv2019

3D dynamics and morphology of bow-shock Pulsar Wind Nebulae

Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight (magnetic inclination angle may also have a certain influence on the morphology). We identify three basic types of bow-shock nebulae: (i) a "Rifle Bullet" (pulsar spin axis and proper velocity are aligned); (ii) a "Frisbee" (pulsar spin axis and proper velocity are orthogonal with the spin axis lying in the plane of the sky), and (iii) a Cart Wheel" (like frisbee but the spin axis is perpendicular to the plane of the sky). Using 3D RMHD simulations, as well as analytical calculations, we reproduce the key morphological features of the bow-shock PWNe, as well as variations, are seen across different systems. magnetic stresses within the shocked pulsar wind affect the overall structure strongly, producing "whiskers", "tails", "filled-in" and "mushroom" shapes, as well as non-symmetric morphologies. On the other hand, the interstellar medium inhomogeneities and the anisotropy of the energy flux in the pulsar wind have only a mild impact of the PWN morphology. In a few cases, when we clearly identify specific morphological structures, our results do not favor alignment of the pulsar spin axis and proper velocity. Our calculations of the underlying emission processes explain the low synchrotron X-ray efficiency (in terms of the spin-down luminosity) and imply an energetical subdominant contribution of the inverse Compton process.

astro-ph.HE↗

arXiv2019

Driving the Beat: Time-Resolved Spectra of the White Dwarf Pulsar AR Scorpii

We obtained high temporal resolution spectroscopy of the unusual binary system AR Sco covering nearly an orbit. The H$α$ emission shows a complex line structure similar to that seen in some polars during quiescence. Such emission is thought to be due to long-lived prominences originating on the red dwarf. A difference between AR Sco and these other systems is that the white dwarf in AR Sco is rapidly spinning relative to the orbital period. "Slingshot" prominences stable at 3 to 5 stellar radii require surface magnetic fields between 100 and 500 G. This is comparable to the estimated WD magnetic field strength near the surface of the secondary. Our time-resolved spectra also show emission fluxes, line equivalent widths, and continuum color varying over the orbit and the beat/spin periods of the system. During much of the orbit, the optical spectral variations are consistent with synchrotron emission with the highest energy electrons cooling between pulses. On the time-scale of the beat/spin period we detect red and blue-shifted H$α$ emission flashes that reach velocities of 700 km/s. Red-shifted Balmer emission flashes are correlated with the bright phases of the continuum beat pulses while blue-shifted flashes appear to prefer the time of minimum in the beat light curve. We propose that much of the energy generated in AR Sco comes from fast magnetic reconnection events occurring near the inward face of the secondary and we show that the energy generated by magnetic reconnection can account for the observed excess luminosity from the system.

astro-ph.SR↗

arXiv2018

Prediction of the second peak in the afterglow of GW170817

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.

astro-ph.HE↗

arXiv2018

Particle acceleration in relativistic magnetic flux-merging events

Using analytical and numerical methods (fluid and particle-in-cell simulations) we study a number of model problems involving merger of magnetic flux tubes in relativistic magnetically-dominated plasma. Mergers of current-carrying flux tubes (exemplified by the two dimensional `ABC' structures) and zero total current magnetic flux tubes are considered. In all cases regimes of spontaneous and driven evolution are investigated. We identify two stages of particle acceleration during flux mergers: (i) fast explosive prompt X-point collapse and (ii) ensuing island merger. The fastest acceleration occurs during the initial catastrophic X-point collapse, with the reconnection electric field of the order of the magnetic field. During the X-point collapse particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization $σ$. For plasma magnetization $σ\leq 10^2$ the spectrum power law index is $p> 2$; in this case the maximal energy depends linearly on the size of the reconnecting islands. For higher magnetization, $σ\geq 10^2$, the spectra are hard, $p< 2$, yet the maximal energy $γ_{max}$ can still exceed the average magnetic energy per particle, $ \sim σ$, by orders of magnitude (if $p$ is not too close to unity). The X-point collapse stage is followed by magnetic island merger that dissipates a large fraction of the initial magnetic energy in a regime of forced magnetic reconnection, further accelerating the particles, but proceeds at a slower reconnection rate.

astro-ph.HE↗

arXiv2018

Explosive X-point collapse in relativistic magnetically-dominated plasma

The extreme properties of the gamma ray flares in the Crab Nebula present a clear challenge to our ideas on the nature of particle acceleration in relativistic astrophysical plasma. It seems highly unlikely that standard mechanisms of stochastic type are at work here and hence the attention of theorists has switched to linear acceleration in magnetic reconnection events. In this series of papers, we attempt to develop a theory of explosive magnetic reconnection in highly-magnetized relativistic plasma which can explain the extreme parameters of the Crab flares. In the first paper, we focus on the properties of the X-point collapse. Using analytical and numerical methods (fluid and particle-in-cell simulations) we extend Syrovatsky's classical model of such collapse to the relativistic regime. We find that the collapse can lead to the reconnection rate approaching the speed of light on macroscopic scales. During the collapse, the plasma particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization $σ$. For sufficiently high magnetizations and vanishing guide field, the non-thermal particle spectrum consists of two components: a low-energy population with soft spectrum, that dominates the number census; and a high-energy population with hard spectrum, that possesses all the properties needed to explain the Crab flares.

astro-ph.HE↗

arXiv2018

Kinetic "jets" from fast moving pulsars

Some fast-moving pulsars, like the Guitar and the Lighthouse, exhibit asymmetric non-thermal emission features that extend well beyond their ram pressure confined pulsar wind nebulae (PWNe). Using 3D relativistic simulations we 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 produce highly asymmetric magnetic bottles, and result in asymmetric extended features. For the features to become visible, the ISM magnetic field should be sufficiently high, $B_{\rm ISM}>10$~$μ$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 "snail eyes" morphologies. We suggest that in those cases the pulsar is moving along the ambient magnetic field in a frisbee-type configuration.

astro-ph.HE↗