arXiv subjects

Maxim Lyutikov

Publications and source records attributed to Maxim Lyutikov.

At least 181 records · Page 10Linked to original sources

arXiv2004

Ain't No Crab, PWN Got A Brand New Bag: Correlated radio and X-ray Structures in Pulsar Wind Nebulae

The traditional view of radio pulsar wind nebulae (PWN), encouraged by the Crab nebula's X-ray and radio morphologies, is that they are a result of the integrated history of their pulsars' wind. The radio emission should therefore be largely unaffected by recent pulsar activity, and hence minimally correlated with structures in the X-ray nebulae. Observations of several PWN, both stationary (sPWN) and rapidly moving (rPWN), now show clear morphological relationships between structures in the radio and X-ray with radio intensity variations on the order of unity. We present high-resolution X-ray and radio images of several PWN of both types and discuss the morphological relationships between the two wavebands.

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arXiv2004

Polarization and structure of relativistic parsec-scale AGN jets

(Abridged) We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron--positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical, and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factor $Γ$. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. Since the typical degrees of polarization are $\leq 15%$, the emitting parts of the jets must have comparable rest-frame toroidal and poloidal fields. In this case, most relativistic jets are strongly dominated by the toroidal magnetic field component in the observer's frame, $B_ϕ/B_z \sim Γ$. We also discuss the possibility that relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In this case, dissipation of the toroidal magnetic field (and not fluid shocks) may be responsible for particle acceleration.

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arXiv2004

On the nature of eclipses in binary pulsar J0737-3039

We consider magnetohydrodynamical interaction between relativistic pulsar wind and static magnetosphere in binary pulsar system PSR J0737-3039. We construct semi-analytical model describing the form of the interface separating the two pulsars. An assumption of vacuum dipole spin down for Pulsar B leads to eclipse duration ten times longer than observed. We discuss possible Pulsar B torque modification and magnetic field estimates due to the interaction with Pulsar A wind. Unless the orbital inclination is $\leq 86 ^\circ$, the duration of eclipses is typically shorter than the one implied by the size of the eclipsing region. We propose that eclipses occur due to synchrotron absorption by mildly relativistic particles in the shocked Pulsar A wind. The corresponding optical depth may be high enough if Pulsar A wind density is at the upper allowed limits. We derive jump conditions at oblique, relativistic, magnetohydrodynamical shocks and discuss the structure of the shocked Pulsar A wind. Finally, we speculate on a possible mechanism of orbital modulation of Pulsar B radio emission.

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arXiv2004

The origin of non-thermal X-ray filaments and TeV emission in young SNRs

At the early, ejecta dominated, stage of supernova remnant (SNR) expansion a fraction of the swept-up circumstellar magnetic field is dynamically compressed to approximate equipartition at the contact discontinuity separating the SN progenitor's wind (or the ISM) and the ejecta. We propose that the thin non-thermal X-ray filaments observed by the Chandra satellite in several young SNRs are associated with such ``pile-up'' of the magnetic field. We use a one-dimensional diffusion-convection transport equation to describe the propagation of non-thermal electrons near the contact discontinuity of a young SNR and to calculate spatially resolved emission spectra in the X-ray and TeV bands. The results suggest that the high-energy electrons are possibly accelerated at the forward shock, and emitting efficiently only when they diffuse into regions of high magnetic field near the contact discontinuity. Much more likely, though, is that they are locally accelerated at the contact discontinuity, in which case the acceleration cannot be related to Fermi-type processes and should occur due to other plasma mechanisms. As a consequence, the forward shock in young SNRs is inconspicuous and often unobservable, similar to that in the Crab nebular.

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arXiv2003

Gamma Ray Bursts as Electromagnetic Outflows

(Abridged) We interpret gamma ray bursts as relativistic, electromagnetic explosions. Specifically, we propose that they are created when a rotating, relativistic, stellar-mass progenitor loses much of its rotational energy in the form of a Poynting flux during an active period lasting $\sim 100$ s. Initially, a non-spherically symmetric, electromagnetically-dominated bubble expands non-relativistically inside the star, most rapidly along the rotational axis of the progenitor. After the bubble breaks out from the stellar surface and most of the electron-positron pairs annihilate, the bubble expansion becomes highly relativistic. After the end of the source activity most of the electromagnetic energy is concentrated in a thin shell inside the contact discontinuity between the ejecta and the shocked circumstellar material. This electromagnetic shell pushes a relativistic blast wave into the circumstellar medium. Current-driven instabilities develop in this shell at a radius $\sim3\times10^{16}$ cm and lead to dissipation of magnetic field and acceleration of pairs which are responsible for the $γ$-ray burst. At larger radii, the energy contained in the electromagnetic shell is mostly transferred to the preceding blast wave. Particles accelerated at the forward shock may combine with electromagnetic field from the electromagnetic shell to produce the afterglow emission.

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arXiv2003

Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow

Observations by the {\RHESSI} satellite of large polarization of the prompt $γ$-ray emission from the Gamma Ray Burst GRB021206 \citep{coburn03} imply that the magnetic field coherence scale is larger than the size of the visible emitting region, $\sim R/Γ$, where $R$ is the radius of the flow, $Γ$ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs \citep{lyutikov02}, in which large scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive Stokes parameters of synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse averaged polarization fraction of the emission from a relativistically expanding shell carrying global toroidal magnetic field. For viewing angles larger than $1/Γ$ the observed patch of the emitting shell has almost homogeneous magnetic field, producing a large fractional polarization ($56% $ for a power-law energy distribution of relativistic particles $dn/dε\propto ε^{-3}$). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in uniform magnetic field due to relativistic kinematic effects.

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arXiv2003

Electromagnetic (versus fireball) model of GRBs

We briefly review the electromagnetic model of Gamma Ray Bursts and then discuss how various models account for high prompt polarization. We argue that if polarization is confirmed at a level $Π\geq 10%$ the internal shock model is excluded.

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arXiv2003

Explosive reconnection in magnetars

X-ray activity of Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters may result from the heating of their magnetic corona by direct currents dissipated by magnetic reconnection. We investigate the possibility that X-ray flares and bursts observed from AXPs and SGRs result from magnetospheric reconnection events initiated by development of tearing mode in magnetically-dominated relativistic plasma. We formulate equations of resistive force-free electrodynamics, discuss its relation to ideal electrodynamics, and give examples of both ideal and resistive equilibria. Resistive force-free current layers are unstable toward the development of small-scale current sheets where resistive effects become important. Thin current sheets are found to be unstable due to the development of resistive force-free tearing mode. The growth rate of tearing mode is intermediate between the short \Alfven time scale $ τ_A$ and a long resistive time scale $τ_R$: $Γ\sim 1/(τ_R τ_A)^{1/2}$, similar to the case of non-relativistic non-force-free plasma. We propose that growth of tearing mode is related to the typical rise time of flares, $\sim 10$ msec. Finally, we discuss how reconnection may explain other magnetar phenomena and ways to test the model.

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arXiv2002

Role of Reconnection in AGN Jets

We discuss the possible role of reconnection in electro-magnetically dominated cores of relativistic AGN jets. We suggest that reconnection may proceed in a two-fold fashion: initial explosive collapse on the Alfven time-scale of a current-carrying jet (which is of the order of the light crossing time) and subsequent slow quasi-steady reconnection. Sites of explosive collapse are associated with bright knots, while steady-state reconnection re-energizes particles in the ``bridges'' between the knots. Ohmic dissipation in reconnection layers leads to particle acceleration either by inductive electric fields or by stochastic particle acceleration in the ensuing electro-magnetic turbulence.

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arXiv2002

Electromagnetic Outflows and GRBs

We study the dynamics of relativistic electromagnetic explosions as a possible mechanism for the production of Gamma-Ray Bursts. We propose that a rotating relativistic stellar-mass progenitor loses much of its spin energy in the form of an electromagnetically-dominated outflow. After the flow becomes optically thin, it forms a relativistically expanding, non-spherically symmetric magnetic bubble - a ''cold fireball''. We analyze the structure and dynamics of such a cavity in the force-free approximation. During relativistic expansion, most of the magnetic energy in the bubble is concentrated in a thin shell near its surface (contact discontinuity). We suggest that either the polar current or the shell currents become unstable to electromagnetic instabilities at a radius $\sim10^{16}$ cm. This leads to acceleration of pairs and causes the $γ$-ray emission. At a radius $\sim10^{17}$ cm, the momentum contained in the electromagnetic shell will have been largely transferred to the surrounding blast wave propagating into the circumstellar medium. Particles accelerated at the fluid shock may combine with electromagnetic field from the electromagnetic shell to produce the afterglow emission.

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arXiv2002

Rates of reconnection in pulsar winds

Using the formulation of relativistic reconnection by Lyutikov & Uzdensky (2002) we estimate the upper possible rates of reconnection in pulsar winds using Bohm prescription for resistivity. We find that the velocity of plasma inflow into the reconnection layer may be relativistic, of the order of the speed of light in the plasma rest-frame. This in principle may allow efficient dissipation of the magnetic field energy in the wind and/or destruction of the toroidal magnetic flux. The efficiency of reconnection realized in pulsar winds remains an open question: it should depend both on the microphysical properties of plasma and on the three-dimensional structure of the reconnection flow.

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arXiv2002

Dynamics of relativistic reconnection

The dynamics of the steady-state Sweet--Parker-type reconnection is analyzed in relativistic regime when energy density in the inflowing region is dominated by magnetic field. The structure of reconnection layer (its thickness, inflow and outflow velocities) depends on the ratio of two large dimensionless parameters of the problem - magnetization parameter $σ\gg 1$ (the ratio of the magnetic to particle energy-densities in the inflowing region) and the Lundquist number $S$. The inflow velocity may be relativistic (for $ S< σ$) or non-relativistic (for $ S > σ$), while the outflowing plasma is moving always relativisticly. For extremely magnetized plasmas with $σ\geq S^2$, the inflow four-velocity becomes of the order of the \Alfven four-velocity.

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arXiv2002

The Pulsar Wind Nebula in G11.2-0.3

We present an X-ray and radio study of the wind nebula surrounding the central pulsar PSR J1811-1925 in the supernova remnant G11.2-0.3. Using high resolution data obtained with the Chandra X-ray observatory and with the VLA radio telescope we show the X-ray and radio emission is asymmetric around the pulsar, despite the latter's central position in the very circular shell. The new X-ray data allow us to separate the synchrotron emission of the pulsar wind nebula from the surrounding thermal emission and that from the pulsar itself. Based on X-ray data from two epochs, we observe temporal variation of the location of X-ray hot spots near the pulsar, indicating relativistic motion. We compare thermal emission observed within the shell, which may be associated with the forward shock of the pulsar wind nebula, to thermal emission from a nearby portion of the remnant shell, the temperature of which implies an expansion velocity consistent with the identification of the remnant with the historical event of 386 A.D. The measured X-ray and radio spectral indices of the nebula synchrotron emission are found to be consistent with a single synchrotron cooling break. The magnetic field implied by the break frequency is anomalously large given the apparent size and age of the nebula if a spherical morphology is assumed, but is consistent with a bipolar morphology.

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arXiv2002

Radio Emission from Magnetars

We discuss properties of the expected radio emission from Soft Gamma-ray Repeaters (SGRs) during their bursting activity in the framework of the model of Thompson, Lyutikov and Kulkarni (2002), in which the high energy emission is powered by the dissipation of super-strong magnetic fields in the magnetospheres through reconnection-type events. Drawing on analogies with Solar flares we predict that coherent radio emission resembling solar type-III radio bursts may be emitted in SGRs during X-ray bursts. The radio emission should have correlated pulse profiles with X-rays, narrow-band-type radio spectrum with $Δν\leq ν$ with the typical frequency $ ν\geq 1 $ GHz, and, possibly, a drifting central frequency. We encourage sensitive radio observations of SGRs during the bursting activity.

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arXiv2002

Mass loading of pulsar winds

The dynamics of relativistic magnetized mass loaded outflows carrying toroidal magnetic field is analyzed in the context of Pulsar Wind Nebulae (PWNs). Mass loading is very efficient in slowing down super-relativistic magnetized flows and weakening of relativistic shocks. We suggest that weakening of relativistic reverse shocks by mass loading in PWNs is responsible for the low radiative efficiencies of the majority of the PWNs. Mass loading may also result in a shock transition near the fast magnetosonic point; this is unlikely to happen in majority of PWNs. The evolution of magnetized mass loaded flows beyond the reverse shock is complicated: after initial deceleration to the minimal velocity required to transport the magnetic flux, the mass loaded flows have to {\it accelerate}. In order to be able to expand to infinity, magnetized flows should either become time dependent or destroy the toroidal magnetic flux by developing internal instabilities. Destruction of the magnetic flux initiated by mass loading may allow for the flow to slow down to sub-relativistic velocities and resolve the $σ$ paradox of the pulsar wind nebula.

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arXiv2001

Dynamics of Relativistic Magnetized Blast Waves

The dynamics of a relativistic blast wave propagating through magnetized medium is considered taking into account possible inhomogeneities of density and magnetic field and additional energy supply. Under the simplifying assumption of a spherically symmetric explosion in a medium with toroidal magnetic field self-similar solutions for the internal dynamics of the flow are derived. In the weakly magnetized case, when the bulk of the flow may be described by the unmagnetized solutions, there is a strongly magnetized sheath near the contact discontinuity (when it exists). Self-similar solutions inside the sheath are investigated. In the opposite limit of strongly magnetized upstream plasma new analytical self-similar solutions are found. Possible application to the physics of Gamma-Ray Bursts is discussed.

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arXiv2000

Precursors of gamma-ray bursts: a clue to the burster's nature

In relativistic strongly magnetized winds outflowing from the fast-rotating compact progenitors of gamma-ray bursts (GRBs) there are three regions where powerful high-frequency emission may be generated: (i) the thermal photosphere, (ii) the region of the internal wind instability and (iii) the region of the wind interaction with an ambient gas. This results in a multicomponent structure of GRBs. The emission from the thermal photosphere may be observed as a weak precursor to the main burst. The precursor should have a blackbody-like spectrum with the mean energy of photons of $\sim 1$ MeV, and its intensity should be tens to hundreds of times smaller than that of the main GRB emission. Observations of such precursors with future $γ$-ray missions like GLAST can clarify the nature of bursters.

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arXiv2000

Looking into Pulsar Magnetospheres

Diffractive and refractive magnetospheric scintillations may allow a direct testing of the plasma inside the light cylinder. Unusual electrodynamics of the strongly magnetized electron-positron plasma allow separation of the magnetospheric and interstellar scattering. The most distinctive feature of the magnetospheric scintillations is their independence on frequency. Diffractive scattering due to small scale inhomogeneities produce a scattering angle that may be as large as 0.1 radians, and a typical decorrelation time of $10^{-8}$ seconds. Refractive scattering due to large scale inhomogeneities is also possible, with a typical angle of $10^{-3}$ radians and a correlation time of the order of $10^{-4}$ seconds. Some of the magnetospheric propagation effects may have already been observed.

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