arXiv subjects

A. N. Timokhin

Publications and source records attributed to A. N. Timokhin.

18 recordsLinked to original sources

arXiv2018

On the maximum pair multiplicity of pulsar cascades

We study electron-positron pair production in polar caps of energetic pulsars to determine the maximum multiplicity of pair plasma a pulsar can produce under the most favorable conditions. This paper complements and updates our study of pair cascades presented in Timokhin & Harding (2015) with more accurate treatment of the effects of ultra strong B>3x10^{12}G magnetic fields and emission processes of primary and secondary particles. We include pairs produced by curvature and synchrotron radiation photons as well as resonant Compton scattered photons. We develop a semi-analytical model of electron-positrons cascades which can efficiently simulate pair cascades with an arbitrary number of microphysical processes and use it to explore cascade properties for a wide range of pulsar parameters. We argue that the maximum cascade multiplicity can not exceed ~ a few x 10^5 and the multiplicity has a rather weak dependence on pulsar period on pulsar period. The highest multiplicity is achieved in pulsars with magnetic field 4x10^{12} 10^6K. We also derive analytical expressions for several physical quantities relevant for electromagnetic cascade in pulsars which may be useful in future works on pulsar cascades, including the upper limit on cascade multiplicity and various approximations for the parameter χ, the exponential factor in the expression for photon attenuation in strong magnetic field.

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arXiv2018

On the polar cap cascade pair multiplicity of young pulsars

We study the efficiency of pair production in polar caps of young pulsars under a variety of conditions to estimate the maximum possible multiplicity of pair plasma in pulsar magnetospheres. We develop a semi-analytic model for calculation of cascade multiplicity which allows efficient exploration of the parameter space and corroborate it with direct numerical simulations. Pair creation processes are considered separately from particle acceleration in order to assess different factors affecting cascade efficiency, with acceleration of primary particles described by recent self-consistent non-stationary model of pair cascades. We argue that the most efficient cascades operate in the curvature radiation/synchrotron regime, the maximum multiplicity of pair plasma in pulsar magnetospheres is ~few x 10^5. The multiplicity of pair plasma in magnetospheres of young energetic pulsars weakly depends on the strength of the magnetic field and the radius of curvature of magnetic field lines and has a stronger dependence on pulsar inclination angle. This result questions assumptions about very high pair plasma multiplicity in theories of pulsar wind nebulae.

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arXiv2012

Current Flow and Pair Creation at Low Altitude in Rotation Powered Pulsars' Force-Free Magnetospheres: Space-Charge Limited Flow

(shortened) We report the results of an investigation of particle acceleration and electron-positron plasma generation at low altitude in the polar magnetic flux tubes of Rotation Powered Pulsars, when the stellar surface is free to emit whatever charges and currents are demanded by the force-free magnetosphere. We observe novel behavior. a) When the current density is less than the Goldreich-Julian (GJ) value (0 1), the system develops high voltage drops, causing emission of gamma rays and intense bursts of pair creation. The bursts exhibit limit cycle behavior, with characteristic time scales somewhat longer than the relativistic fly-by time over distances comparable to the polar cap diameter (microseconds). c) In return current regions, where j/j_{GJ}<0, the system develops similar bursts of pair creation. In cases b) and c), the intermittently generated pairs allow the system to simultaneously carry the magnetospherically prescribed currents and adjust the charge density and average electric field to force-free conditions. We also elucidate the conditions for pair creating beam flow to be steady, finding that such steady flows can occupy only a small fraction of the current density parameter space of the force-free magnetospheric model. The generic polar flow dynamics and pair creation is strongly time dependent. The model has an essential difference from almost all previous quantitative studies, in that we sought the accelerating voltage as a function of the applied current.

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arXiv2012

The complex charm of the pulsar magnetosphere

I give a brief overview of recent results from self-consistent modeling of electron-positrons cascades in pulsar polar caps. These results strongly suggest that the pulsar magnetosphere is a more complex system than was assumed before.

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arXiv2012

On plasma rotation and drifting subpulses in pulsars; using aligned pulsar B0826-34 as a voltmeter

We derive the exact drift velocity of plasma in the pulsar polar cap, in contrast to the order-of-magnitude expressions presented by Ruderman & Sutherland (1975) and generally used throughout the literature. We emphasize that the drift velocity depends not on the absolute value, as is generally used, but on the variation of the accelerating potential across the polar cap. If we assume that drifting subpulses in pulsars are indeed due to this plasma drift, several observed subpulse-drift phenomena that are incompatible with the Ruderman & Sutherland family of models can now be explained: we show that variations of drift rate, outright drift reversals, and the connection between drift rates and mode changes have natural explanations within the frame of the "standard" pulsar model, when derived exactly. We apply this model for drifting subpulses to the case of PSR B0826-34, an aligned pulsar with two separate subpulse-drift regions emitted at two different colatitudes. Careful measurement of the changing and reversing drift rate in each band independently sets limits on the variation of the accelerating potential drop. The derived variation is small, ~10^{-3} times the vacuum potential drop voltage. We discuss the implications of this result for pulsar modeling.

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arXiv2010

Time-dependent pair cascades in magnetospheres of neutron stars I. Dynamics of the polar cap cascade with no particle supply from the neutron star surface

I argue that the problem of electromagnetically driven electron-positron cascades in magnetospheres of neutron stars must be addressed starting from first principles. I describe a general numerical algorithm for doing self-consistent kinetic simulations of electron-positron cascades - wherein particle acceleration, pair creation and screening of the electric field are calculated simultaneously - and apply it to model the Ruderman and Sutherland cascade in one dimension. I find that pair creation is quite regular and quasi-periodic. In each cycle a blob of ultrarelativistic electron-positron plasma is generated, it propagates into the magnetosphere leaving a tail of less relativistic plasma behind, and the next discharge occurs when this mildly relativistic plasma leaves the polar cap. A short burst of pair formation is followed by a longer quiet phase when accelerating electric field is screened and no pairs are produced. Some of freshly injected electron-positrons pairs get trapped in plasma oscillations creating a population of low energy particles. The cascade easily adjusts to the current density required by the pulsar magnetosphere by reversing some of the low energy particles. Each discharge generates a strong coherent superluminal electrostatic wave, which may be relevant for the problem of pulsar radioemission.

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arXiv2010

A model for nulling and mode changing in pulsars

We propose that in some pulsars the magnetosphere has different states with different geometries or/and different distributions of currents, it occasionally switches between them. These states have different spindown rates and emission beams, in some of the states no radioemission is produced at all. Switching into a different state manifests as a mode change when we see different parts of the emission beam or the beams in different states have significantly different geometries, it manifests as nulling when we either miss the new beam or no radioemission is generated in the new state. We show that modest variations in the beam shape can be accompanied by large variations in the pulsar spindown rate W - the dependence of W on the opening angle of the emission beam $α$ can be as strong as W\proptoα^4. We speculate about physical mechanisms which may cause reconfiguration of the magnetosphere.

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arXiv2009

Self-consistent modeling of pair cascades in the polar cap of a pulsar

Here we briefly report on first results of self-consistent simulation of non-stationary electron-positron cascades in the polar cap of pulsar using specially developed hybrid PIC/Monte-Carlo numerical code. We consider the case of Ruderman-Sutherland cascade -- when particles cannot be extracted from the surface of the neutrons star.

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arXiv2007

On the nature of QPO in the tail of SGR giant flares

A model is presented for the quasiperiodic component of magnetar emission during the tail phase of giant flares. The model invokes modulation of the particle number density in the magnetosphere. The magnetospheric currents are modulated by torsional motion of the surface and we calculate that the amplitude of neutron star surface oscillation should be ~1% of the NS radius in order to produce the observed features in the power spectrum. Using an axisymmetric analytical model for structure of the magnetosphere of an oscillating NS, we calculate the angular distribution of the optical depth to the resonant Compton scattering. The anisotropy of the optical depth may be why QPO are observed only at particular rotational phases.

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arXiv2006

INTEGRAL Observations of SS433: Analysis of Precessional and Orbital X-ray Periodicities

Hard X-ray INT observations of SS 433 carried out during 2003-2005 years with an analysis of precessional and orbital variability is presented. The width of X-ray eclipse in the 25-50 keV range at the precessional phase $ψ=0.1$ (accretion disk is open to observer) is higher than that in the Ginga 18.4-27.6 keV range. This fact suggests existance the presence of hot extended corona around the supercritical accretion disk. Spectrum of hard X-rays in the range 10-200 keV does not change with the precessional phase which also suggests that hard X-ray flux is generated in the hot extended corona around the accretion disk. The parameters of this hot corona are: kT=23-25 keV, τ= 1.8-2.8. Mass ratio estimated from the analysis of the ingress part of the eclipse light curve is in the range q=m_x/m_v=0.3-0.5.

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arXiv2006

On the force-free magnetosphere of aligned rotator

We investigate in details properties of stationary force-free magnetosphere of aligned rotator assuming the last closed field line lying in equatorial plane at large distances from pulsar. The pulsar equations is solved numerically using multigrid code with high numerical resolution, physical properties of the magnetosphere are obtained with high accuracy. We found a set of solutions with different sizes of the closed magnetic field line zone and verify the applicability of the force-free approximation. We discuss the role of electron-positron cascades in supporting of the force-free magnetosphere and argue that the closed field line zone should grow with time slower than the light cylinder. This yield the pulsar breaking index less than 3. It is shown, that models of aligned rotator magnetosphere with widely accepted configuration of magnetic field, like one considered in this paper, have serious difficulties. We discuss solutions of this problem and argue that in any case pulsar energy losses should evolve with time differently than predicted by the magnetodipolar formula.

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arXiv2005

INTEGRAL observations of SS433: Results of coordinated campaign

Results of simultaneous INTEGRAL and optical observations of galactic microquasar SS433 in May 2003 and INTEGRAL/RXTE observations in March 2004 are presented. Persistent precessional variability with a maximum to minimum uneclipsed hard X-ray flux ratio of \sim 4 is discovered. The 18-60 keV X-ray eclipse is found to be in phase with optical and near infrared eclipses. The orbital eclipse observed by INTEGRAL in May 2003 is at least two times deeper and apparently wider than in soft X-ray band. The broadband X-ray spectrum 2-100 keV simultaneously detected by RXTE/INTEGRAL in March 2004 can be described by bremsstrahlung emission from optically thin thermal plasma with kT\sim 30 keV. The optical spectroscopy with the 6-m SAO BTA telescope confirmed the optical companion to be an A5-A7 supergiant. For the first time, spectorscopic indications of a strong heating effect in the optical star atmosphere are found. The measurements of absorption lines which are presumably formed in the non-illuminated side of the supergiant yield its radial velocity semi-amplitude K_v=132\pm 9 km/s. The analysis of the observed hard X-ray light curve and the eclipse duration, combined with spectroscopically found optical star radial velocity corrected for the strong heating effect, allows us to model SS433 as a massive X-ray binary. Assuming that the hard X-ray source in SS433 is eclipsed by the donor star that exactly fills its Roche lobe, the masses of the optical and compact components in SS433 are suggested to be M_v\approx 30 M_\odot and M_x\approx 9M_\odot, respectively. This provides further evidence that SS433 is a massive binary system with supercritical accretion onto a black hole.

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arXiv2004

Oscillating neutron star as a pulsar

Pulsar "standard model" of rotating magnetized conducting sphere surrounded by plasma is generalized in its essential parts for the case of oscillating star. Goldreich-Julian charge density, electromagnetic energy losses as well as polar cap scenario of particle accelerations are considered. Despite similarities, there are substantial differences between magnetospheres of rotating and oscillating stars. Distortion of particle acceleration mechanism in radiopulsars by neutron star oscillations, excited by a strong glitch, for example, is discussed.

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arXiv2004

Coordinated INTEGRAL and optical observations of SS433

Results of simultaneous {\it INTEGRAL} and optical observations of galactic microquasar SS433 in May 2003 are presented. The analysis of the X-ray and optical eclipse duration and hard X-ray spectra obtained by {\it INTEGRAL} together with optical spectroscopy obtained on the 6-m telescope allows us to construct a model of SS433 as a massive X-ray binary. X-ray eclipse in hard X-rays has a depth of $\sim 80%$ and extended wings. The optical spectroscopy allows us to identify the optical companion as a A5-A7 supergiant and to measure its radial velocity semi-amplitude $K_v=132$ km/s. A strong heating effect in the optical star atmosphere is discovered spectroscopically. The observed broadband X-ray spectrum 2-100 keV can be described by emission from optically thin thermal plasma with $kT\sim 15-20 keV$

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arXiv2003

On the non-thermal high energy radiation of galaxy clusters

The origin of the nonthermal EUV and hard X-ray emission "excess" reported from some galaxy clusters has been intensively debated over last several years. The most favored models which refer this excess to relativistic electrons upscattering the 2.7 K CMBR generally requires very low magnetic field, significantly below the estimates derived from the Faraday Rotation Measurements, unless one invokes rather nonstandard assumptions concerning the energy and pitch angle distributions of nonthermal electrons. In this paper we suggest a new model assuming that the "nonthermal" excess is due to synchrotron radiation of ultrarelativistic (multi-TeV) electrons of "photonic" origin. These electrons are continuously introduced throughout the entire intracluster medium by very high energy (hypothetical) gamma-rays through interactions with the diffuse extragalactic radiation fields. We present numerical calculations for the Coma cluster, and briefly discuss implications of the model for other galaxy clusters both in the X- and gamma-ray energy domains.

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arXiv2002

On the origin of highest energy gamma-rays from Mkn 501

The spectra of very high energy gamma-radiation from distant extragalactic objects suffer significant deformations during the passage of primary gamma-rays through the intergalactic medium. The recently reported fluxes of diffuse infrared background radiation indicate that we detect, most probably, heavily absorbed TeV radiation from Mkn 421 and Mkn 501. This implies that the absorption-corrected spectrum of Mkn 501 may contain a sharp pile-up which contradicts to the predictions of the conventional models of TeV blazars, and thus may leads to the so-called "IR background-TeV gamma-ray crisis". To overcome this difficulty, in this paper we propose two independent hypotheses assuming that (i) the TeV radiation from Mkn 501 has a secondary origin, i.e. it is formed during the development of electron-photon cascades in the intergalactic medium initiated by primary gamma-rays; (ii) the pile-up in the source spectrum is a result of comptonization (in deep Klein-Nishina regime) of ambient optical radiation by an ultrarelativistic conical cold outflow (jet) with bulk motion Lorentz factor Gamma_0 >= 3 10^7. Within the uncertainties caused by the limited energy resolution of spectral measurements, the observed TeV radiation of Mkn 501 formally can be explained by the intergalactic cascade gamma-rays, assuming however an extremely low intergalactic magnetic field in the direction to the source at the level of < 10^{-18} G. We also demonstrate that the "bulk motion comptonization" scenario can quite naturally reproduce the unusual spectral features in the absorption-corrected TeV spectrum of Mkn 501, and briefly discuss the astrophysical implications of this hypothesis.

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arXiv2000

Magnetosphere of Oscillating Neutron Star. Nonvacuum Treatment

We generalize a formula for the Goldreich-Julian charge density (ρ), originally derived for rotating neutron star, for arbitrary oscillations of a neutron star with arbitrary magnetic field configuration under assumption of low current density in the inner parts of the magnetosphere. As an application we consider toroidal oscillation of a neutron star with dipole magnetic field and calculate energy losses. For some oscillation modes the longitudinal electric field can not be canceled by putting charged particles in the magnetosphere without a presence of strong electric current j = (c/(ωr))ρc. It is shown that the energy losses are strongly affected by plasma in the magnetosphere, and cannot be described by vacuum formulas.

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arXiv1999

Optical counterparts of cosmological GRBs due to heating of ISM in the parent galaxy

We investigated influence of cosmological GRB on the surrounding interstellar medium. It was shown that γ-radiation from the burst heats interstellar gas to the temperatures > 10^4 K up to the distance \sim 10 pc. For high density ISM optical and UV radiation of the heated gas can be observed on the Eath several years as a GRB`s counterpart

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