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Maxim Lyutikov

Publications and source records attributed to Maxim Lyutikov.

At least 91 records · Page 5Linked to original sources
arXiv2018

Radiation- and pair-loaded shocks

We consider the structure of mildly relativistic shocks in dense media, taking into account the radiation and pair loading, and diffusive radiation energy transfer within the flow. For increasing shock velocity (increasing post-shock temperature), the first important effect is the efficient energy redistribution by radiation within the shock that leads to the appearance of an isothermal jump, whereby the flow reaches the finial state through a discontinuous isothermal transition. The isothermal jump, on scales much smaller that the photon diffusion length, consists of a weak shock and a quick relaxation to the isothermal conditions. Highly radiation-dominated shocks do not form isothermal jump. Pair production can mildly increase the overall shock compression ratio to $\approx 10$ (from $4$ for matter-dominated shocks and $7$ of the radiation-dominated shocks).

astro-ph.HE↗
arXiv2018

GRB170817A associated with GW170817: multifrequency observations and modeling of prompt gamma-ray emission

We present our observations of electromagnetic transients associated with GW170817/GRB 170817A using optical telescopes of Chilescope observatory and Big Scanning Antenna (BSA) of Pushchino Radio Astronomy Observatory at 110MHz. The Chilescope observatory detected an optical transient of $\sim19^m$ on the third day in the outskirts of the galaxy NGC 4993; we continued observations following its rapid decrease. We put an upper limit of $1.5\times10^{4}$ Jy on any radio source with a duration of 10-60 s which may be associated with GW170817/GRB 170817A. The prompt gamma-ray emission consists of two distinctive components - a hard short pulse delayed by $\sim2$ seconds with respect to the LIGO signal and softer thermal pulse with $T\sim 10 $ keV lasting for another $\sim2$ seconds. The appearance of a thermal component at the end of the burst is unusual for short GRBs. Both the hard and the soft components do not satisfy the Amati relation, making GRB 170817A distinctively different from other short GRBs. Based on gamma-ray and optical observations, we develop a model for the prompt high-energy emission associated with GRB 170817A. The merger of two neutron stars creates an accretion torus of $\sim10^{-2} M_\odot$, which supplies the black hole with magnetic flux and confines the Blandford-Znajek-powered jet. We associate the hard prompt spike with the quasispherical breakout of the jet from the disk wind. As the jet plows through the wind with subrelativistic velocity, it creates a radiation-dominated shock that heats the wind material to tens of kiloelectron volts, producing the soft thermal component.

astro-ph.HE↗
arXiv2018

High energy tails of pulsar gamma-ray emission

We perform spectral analyses of four bright $γ$-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 $γ$-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.

astro-ph.HE↗
arXiv2017

Cosmic ray-modified shocks: appearance of an isothermal jump

We point out that for sufficiently strong shocks, with Mach number $ M_1 > \sqrt{\frac{3 γ-1}{(3-γ) γ}}= 1.34$ ($γ=5/3$), 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 $2/ (γ-1)=3$, so that in the test particle regime the expected spectrum of low energy CRs experiencing first-order Fermi process is $p = 2γ/(3- γ) = 5/2$, steeper than conventional $p=2$.

astro-ph.HE↗
arXiv2017

Cooling waves in pair plasma

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.

astro-ph.HE↗
arXiv2017

Short Gamma-Ray Bursts following mergers of an ONeMg with a CO white dwarf

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.

astro-ph.HE↗
arXiv2017

Self-similar structure of ultra-relativistic magnetized termination shocks and the role of reconnection in long-lasting GRB ouflows

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.

astro-ph.HE↗
arXiv2017

Rotation of polarization by a moving gravitational lens

We present a simple prescription for the rotation of polarization produced by a relativistically moving gravitational lens, applicable to arbitrary deflection angle and arbitrary velocity of the lens. When geometric optics is applicable, two independent components contribute to the total rotation of polarization: (i) in the frame of the lens the polarization vector experiences minimal rotation defined by the deflection angle (as measured by a set of remote observers, or no rotation if defined in terms of parallel-propagated tetrad); (ii) the effect of the motion of the lens on the polarization can be taken into account exactly using special relativistic Lorentz transformation of polarization. The effects of the gravitational lensing are thus parametrized by the deflection angle of the null geodesics (not necessarily small) and the motion of the lens (not necessarily with velocities much smaller than that of light).

gr-qc↗
arXiv2017

On the Linear Stability of Magnetized Jets Without Current Sheets - Relativistic Case

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.

astro-ph.HE↗
arXiv2017

Polarization swings in blazars

We present a model of blazar variability that can both reproduce smooth large polarization angle swings, and at the same time allow for the seemingly random behaviour of synchrotron fluxes, polarization fraction and, occasionally, $π$/2 polarization jumps. We associate blazar flaring activity with a jet carrying helical magnetic fields and propagating along a variable direction (and possibly with a changing bulk Lorentz factor). The model predicts that for various jet trajectories (i) EVPA can experience large smooth temporal variations while at the same time polarization fraction ($Π$) can be highly variable; (ii) $Π\sim 0$ near sudden EVPA jumps of 90$^{\circ}$, but can also remain constant for large, smoother EVPA swings; (iii) the total angle of EVPA rotation can be arbitrary large; (iv) intensity I is usually maximal at points of fastest EVPA changes, but can have a minimum. Thus, even for a regular, deterministic motion of a steadily emitting jet the observed properties can vary in a non-monotonic and/or seemingly stochastic way. Intrinsic fluctuations of the emissivity will further complicated the intensity profiles, but are expected to preserve the polarization structure.

astro-ph.HE↗
arXiv2017

What we recently learnt about Crab: structure of the wind, the shock, flares and reconnection

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.

astro-ph.HE↗
arXiv2017

Ultra-relativistic double explosions

We consider fluid dynamics of relativistic double explosion - when a point explosion with energy $E_1$ is followed by a second explosion with energy $E_2$ after time $t_d$ (the second explosion could be in a form of a long lasting wind). The primary explosion creates a self-similar relativistic blast wave propagating with \Lf\ $Γ_1(t)$. A sufficiently strong second explosion, with total energy $E_2 \geq 10^{-2} E_1$, creates a fast second shock in the external fluid previously shocked by the primary shock. At times longer than the interval between the explosions $t_d$, yet short compared with the time when the second shock catches up the primary shock at $\sim t_d Γ_1^2$, the structure of the second shock is approximately self-similar. Self-similar structure of the second shock exist for the case of constant external density (in this case $Γ_2 \propto t^{-7/3}$), but not for the wind environment. At early times the \Lf\ of the second shock may exceed that of the primary shock and may boost the synchrotron emission of locally accelerated electrons into the Fermi LAT range.

astro-ph.HE↗
arXiv2017

Early GRB afterglows from reverse shocks in ultra-relativistic long-lasting winds

We develop a model of early GRB afterglows with the dominant $X$-ray contribution from the reverse shock (RS) propagating in highly relativistic (Lorentz factor $γ_w \sim 10^6$) magnetized wind of a long-lasting central engine. The model reproduces, in a fairly natural way, the overall trends and yet allows for variations in the temporal and spectral evolution of early optical and $X$-ray afterglows. The high energy and the optical synchrotron emission from the RS particles occurs in the fast cooling regime; the resulting synchrotron power $L_s$ is a large fraction of the wind luminosity, $L_s \approx L_w/\sqrt{1+σ_w}$ ($L_w$ and $σ_w$ are wind power and magnetization). Thus, plateaus - parts of afterglow light curves that show slowly decreasing spectral power - are a natural consequence of the RS emission. Contribution from the forward shock (FS) is negligible in the $X$-rays, but in the optical both FS and RS contribute similarly: the FS optical emission is in the slow cooling regime, producing smooth components, while the RS optical emission is in the fast cooling regime, and thus can both produce optical plateaus and account for fast optical variability correlated with the $X$-rays, e.g., due to changes in the wind properties. We discuss how the RS emission in the $X$-rays and combined FS and RS emission in the optical can explain many of puzzling properties of early GRB afterglows.

astro-ph.HE↗
arXiv2016

Emission knots and polarization swings of swinging jets

Knots (emission features in jets of active galactic nuclei) often show non-ballistic dynamics and variable emission/polarization properties. We model these features as emission pattern propagating in a jet that carries helical magnetic field and is launched along a changing direction. The model can reproduce a wide range of phenomena observed in the motion of knots: non-ballistic motion (both smooth and occasional sudden change of direction, and/or oscillatory behavior), variable brightness, confinement of knots' motion within an overlaying envelope. The model also reproduces smooth large polarization angle swings, and at the same time allows for the seemingly random behavior of synchrotron fluxes, polarization fraction and occasional $π/2$ polarization jumps.

astro-ph.HE↗
arXiv2016

Relativistic Rotating Vector Model

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.

astro-ph.HE↗
arXiv2016

How else can we detect Fast Radio Bursts?

We discuss possible electromagnetic signals accompanying Fast Radio Bursts (FRBs) that are expected in the scenario where FRBs originate in neutron star magnetospheres. For models involving Crab-like giant pulses, no appreciable contemporaneous emission is expected at other wavelengths. Magnetar giant flares, driven by the reconfiguration of the magnetosphere, however, can produce both contemporaneous bursts at other wavelengths as well as afterglow-like emission. We conclude that the best chances are: (i) prompt short GRB-like emission; (ii) a contemporaneous optical flash that can reach naked eye peak luminosity (but only for a few milliseconds); (iii) a high energy afterglow emission. Case (i) could be tested by coordinated radio and high-energy experiments. Case (ii) could be seen in a coordinated radio-optical surveys, \eg\ by the Palomar Transient Factory in a 60-second frame as a transient object of $m=15-20$ magnitude with an expected optical detection rate of about 0.1~hr$^{-1}$, an order of magnitude higher than in radio. Shallow, but large-area sky surveys such as ASAS-SN and EVRYSCOPE could also detect prompt optical flashes from the more powerful Lorimer-burst clones. The best constraints on the optical-to-radio power for this kind of emission could be provided by future observations with facilities like LSST. Case (iii) might be seen in relatively rare cases that the relativistically ejected magnetic blob is moving along the line of sight.

astro-ph.HE↗
arXiv2016

Particle acceleration in explosive relativistic reconnection events and Crab Nebula gamma-ray flares

We develop a model of particle acceleration in explosive reconnection events in relativistic magnetically-dominated plasmas and apply it to explain gamma-ray flares from the Crab Nebula. The model relies on development of current-driven instabilities on macroscopic scales (not related to plasma skin depths). Using analytical and numerical methods (fluid and particle-in-cell simulations), we study a number of model problems in relativistic magnetically-dominated plasma: (i) we extend Syrovatsky's classical model of explosive X-point collapse to magnetically-dominated plasmas; (ii) we consider instability of two-dimensional force-free system of magnetic flux tubes; (iii) we consider merger of two zero total poloidal current magnetic flux tubes. In all cases regimes of spontaneous and driven evolution are investigated. We identify two stages of particle acceleration: (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. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization. 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 reconnection, further accelerating the particles, but proceeds at a slower reconnection rate. Crab flares result from the initial explosive stages of magnetic island mergers of magnetic flux tubes produced in the bulk of nebula at intermediate polar regions. The post-termination shock plasma flow in the wind sectors with mild magnetization naturally generates large-scale highly magnetized structures. Internal kink-like instabilities lead to the formation of macroscopic current-carrying magnetic flux tubes that merge explosively.

astro-ph.HE↗