arXiv subjects

George Melikidze

Publications and source records attributed to George Melikidze.

12 recordsLinked to original sources

arXiv2018

Relativistic charge solitons created due to nonlinear Landau damping: A candidate for explaining coherent radio emission in pulsars

A potential resolution for the generation of coherent radio emission in pulsar plasma is the existence of relativistic charge solitons, which are solutions of nonlinear Schrödinger equation (NLSE). In an earlier study, Melikidze et al. (2000) investigated the nature of these charge solitons; however, their analysis ignored the effect of nonlinear Landau damping, which is inherent in the derivation of the NLSE in the pulsar pair plasma. In this paper we include the effect of nonlinear Landau damping and obtain solutions of the NLSE by applying a suitable numerical scheme. We find that for reasonable parameters of the cubic nonlinearity and nonlinear Landau damping, soliton-like intense pulses emerge from an initial disordered state of Langmuir waves and subsequently propagate stably over sufficiently long times, during which they are capable of exciting the coherent curvature radiation in pulsars. We emphasize that this emergence of {\em stable} intense solitons from a disordered state does not occur in a purely cubic NLSE; thus, it is {\em caused} by the nonlinear Landau damping.

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arXiv2014

Two Modes of Partially Screened Gap

The analysis of X-ray observations suggest an ultrastrong ($B\gtrsim 10^{14} \,{\rm G}$) surface magnetic field at the polar cap of pulsars (Szary, 2013). On the other hand, the temperature of the polar caps is about a few millions Kelvin. Based on these two facts we use the Partially Screened Gap (PSG) model to describe the Inner Acceleration Region (IAR). The PSG model assumes that the temperature of the actual polar cap is equal to the so-called critical value, i.e. the temperature at which the outflow of thermal ions from the surface screens the gap completely. We have found that, depending on the conditions above the polar cap, the generation of high energetic photons in IAR can be caused either by Curvature Radiation (CR) or by Inverse Compton Scattering (ICS). Completely different properties of both processes result in two different scenarios of breaking the acceleration gap: the so-called PSG-off mode for the gap dominated by CR and the PSG-on mode for the gap dominated by ICS. The existence of two different mechanisms of gap breakdown naturally explains the mode-changing phenomenon. Different characteristics of plasma generated in the acceleration region for both modes also explain the pulse nulling phenomenon.

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arXiv2014

Radio efficiency of pulsars

We investigate radio emission efficiency $ξ$ of pulsars and report a near linear inverse correlation between $ξ$ and the spindown power $\dot E$, as well as a near linear correlation between $ξ$ and pulsar age $τ$. This is a consequence of very weak, if any, dependences of radio luminosity $L$ on pulsar period $P$ and period derivative $\dot{P}$, in contrast to X-ray or $γ$-ray emission luminosities. The analysis of radio fluxes suggests that these correlations are not due to a selection effect, but are intrinsic to the pulsar radio emission physics. We have found that, although with a large variance, the radio luminosity of pulsars is $\left \approx 10^{29} \,{\rm erg/s}$, regardless of the position in the $P-\dot P$ diagram. Within such a picture, a model-independent statement can be made that the death line of radio pulsars corresponds to an upper limit in the efficiency of radio emission. If we introduce the maximum value for a radio efficiency into Monte Carlo-based population syntheses we can reproduce the observed sample using the random luminosity model. The Kolmogorov-Smirnov test on a synthetic flux distribution shows high probability of reproducing the observed distribution. Our results suggests that the plasma responsible for generating radio emission is produced under similar conditions regardless of pulsar age, dipolar magnetic field strength, and spin-down rate. The magnetic fields near the pulsar surface are likely dominated by crust-anchored magnetic anomalies, which do not significantly differ among pulsars, leading to similar conditions for generating electron-positron pairs necessary to power radio emission.

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arXiv2012

Observational consequences of the Partially Screened Gap

Observations of the thermal X-ray emission from old radio pulsars implicate that the size of hot spots is much smaller then the size of the polar cap that follows from the purely dipolar geometry of pulsar magnetic field. Plausible explanation of this phenomena is an assumption that the magnetic field at the stellar surface differs essentially from the purely dipolar field. Using the conservation of the magnetic flux through the area bounded by open magnetic field lines we can estimate the surface magnetic field as of the order of $10^{14}$G. Based on observations that the hot spot temperature is about a few million Kelvins the Partially Screened Gap (PSG) model was proposed which assumes that the temperature of the actual polar cap equals to the so called critical temperature. We discuss correlation between the temperature and corresponding area of the thermal X-ray emission for a number of pulsars. We have found that depending on the conditions in a polar cap region the gap breakdown can be caused either by the Curvature Radiation (CR) or by the Inverse Compton Scattering (ICS). When the gap is dominated by ICS the density of secondary plasma with Lorentz factors $10^{2}-10^{3}$ is at least an order of magnitude higher then in a CR scenario. We believe that two different gap breakdown scenarios can explain the mode-changing phenomenon and in particular the pulse nulling. Measurements of the characteristic spacing between sub-pulses ($P_{2}$) and the period at which a pattern of pulses crosses the pulse window ($P_{3}$) allowed us to determine more strict conditions for avalanche pair production in the PSG.

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arXiv2006

Thermal radiation from hot polar cap in pulsars

Thermal radiation from hot polar caps is examined in radio pulsars with drifting subpulses. It is argued that if these subpulses correspond to sparking discharges of the inner acceleration region right above the polar cap surface then a simple relationship between the observed subpulse drift rate in radio and thermal X-ray luminosity from the polar cap heated by sparks should exist. This relationship is derived and tested in pulsars for which an appropriate good quality data is available.

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arXiv2006

Interrelation between radio and X-ray signatures of drifting subpulses in pulsars

We examined a model of partially screened gap region above the polar cap, in which the electron-positron plasma generated by sparking discharges coexists with thermionic flow ejected by the bombardment of the surface beneath these sparks. Our special interest was the polar cap heating rate and the subpulse drifting rate, both phenomena presumably associated with sparks operating at the polar cap. We investigated correlation between the intrinsic drift rate and polar cap heating rate and found that they are coupled to each other in such a way that the thermal X-ray luminosity $L_x$ from heated polar cap depends only on the observational tertiary subpulse drift periodicity $\hat{P}_3$ (polar cap carousel time). Within our model of partially screened gap we derived the simple formula relating $L_x$ and $\hat{P}_3$, and showed that it holds for PSRs B0943$+$10 and B1133+16, which are the only two pulsars in which both $L_x$ and $\hat{P}_3$ are presently known.

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arXiv2006

Formation of a Partially-Screened Inner Acceleration Region in Radio Pulsars: Drifting Subpulses and Thermal X-Ray Emission from Polar Cap Surface

The subpulse drifting phenomenon in pulsar radio emission is considered within the partially screened inner gap model, in which the sub-Goldreich-Julian thermionic flow of iron ions or electrons coexists with the spark-associated electron-positron plasma flow. We derive a simple formula that relates the thermal X-ray luminosity $L_{\rm x}$ from the spark-heated polar cap and the \EB subpulse periodicity $\hat{P}_3$ (polar cap carousel time). For PSRs B0943+10 and B1133+16, the only two pulsars for which both $\hat{P}_3$ and $L_{\rm x}$ are known observationally, this formula holds well. For a few other pulsars, for which only one quantity is measured observationally, we predict the value of the other quantity and propose relevant observations that can confirm or discard the model. Then we further study the detailed physical conditions that allow such partially screened inner gap to form. By means of the condition $T_{\rm c}/T_{\rm s}>1$ (where $T_{\rm c}$ is the critical temperature above which the surface delivers a thermal flow to adequately supply the corotation charge density, and $T_{\rm s}$ is the actual surface temperature), it is found that a partially-screened gap (PSG) can be formed given that the near surface magnetic fields are very strong and curved. We consider both curvature radiation (CR) and resonant inverse Compton scattering (ICS) to produce seed photons for pair production, and find that the former is the main agency to produce gamma-rays to discharge PSG.

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arXiv2001

Extraordinary features of pair plasma and \\quasilinear theory of Cherenkov-drift instability

We examine linear and quasi-liner stages of Cherenkov-drift instability in the relativistic magnetized electron-positron plasma. The external magnetic field lines are assumed to be slightly curved. In this case the curvature drift of relativistic beam particles plays decisive role in the development of the instability. Quasi-linear relaxation of the relativistic beam leads to diffusion of the resonant particles in the momenta space. The expressions for diffusion coefficients of Cherenkov-drift instability are obtained.

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arXiv2000

The spark-associated soliton model for pulsar radio emission

We propose a new, self-consistent theory of coherent pulsar radio emission based on the non-stationary sparking model of Ruderman & Sutherland (1975), modified by Gil & Sendyk (2000) in the accompanying Paper I. According to these authors, the polar cap is populated as densely as possible by a number of sparks with a characteristic perpendicular dimension D approximately equal to the polar gap height scale h, separated from each other also by about h. Each spark reappears in approximately the same place on the polar cap for a time scale much longer than its life-time and delivers to the open magnetosphere a sequence of electron-positron clouds which flow orderly along a flux tube of dipolar magnetic field lines. The overlapping of particles with different momenta from consecutive clouds leads to effective two-stream instability, which triggers electrostatic Langmuir waves at the altitudes of about 50 stellar radii. The electrostatic oscillations are modulationally unstable and their nonlinear evolution results in formation of ``bunch-like'' charged solitons. A characteristic soliton length along magnetic field lines is about 30 cm, so they are capable of emitting coherent curvature radiation at radio wavelengths. The net soliton charge is about 10^21 fundamental charges, contained within a volume of about 10^14 cm^3. For a typical pulsar, there are about 10^5 solitons associated with each of about 25 sparks operating on the polar cap at any instant. One soliton moving relativisticaly along dipolar field lines with a Lorentz factor of the order of 100 generates a power of about 10^21 erg/s by means of curvature radiation. Then the total power of a typical radio pulsar can be estimated as being about 10^(27-28) erg/s.

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arXiv2000

On the optical pulsations from the Geminga pulsar

We present a model for generation mechanisms of the optical pulsations recently detected from the Geminga pulsar. We argue that this is just a synchrotron radiation emitted along open magnetic field lines at altitudes of a few light cylinder radii (which requires that Geminga is an almost aligned rotator), where charged particles acquire non-zero pitch-angles as a result of the cyclotron absorption of radio waves in the magnetized pair plasma. This explains self-consistently both the lack of apparent radio emission, at least at frequencies higher than about 100 MHz, and the optical pulsations from the Geminga pulsar. From our model it follows that the synchrotron radiation is a maximum in the infrared band, which suggests that Geminga should also be a source of a pulsed infrared emission.

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arXiv2000

Nature of microstructure in pulsar radio emission

We present a model for microstructure in pulsar radio emission. We propose that micropulses result from the alteration of the radio wave generation region by nearly transverse drift waves propagating across the pulsar magnetic field and encircling the bundle of the open magnetic field lines. It is demonstrated that such waves can modify significantly curvature of these dipolar field lines. This in turn affects strongly fulfillment of the resonance conditions necessary for the excitation of radio waves. The time-scale of micropulses is therefore determined by the wavelength of drift waves. Main features of the microstructure are naturally explained in the frame of this model.

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arXiv2000

Nature of eclipsing pulsars

We present a model for pulsar radio eclipses in some binary systems, and test this model for PSRs B1957+20 and J2051-0827. We suggest that in these binaries the companion stars are degenerate dwarfs with strong surface magnetic fields. The magnetospheres of these stars are permanently infused by the relativistic particles of the pulsar wind. We argue that the radio waves emitted by the pulsar split into the eigenmodes of the electron-positron plasma as they enter the companion's magnetosphere and are then strongly damped due to cyclotron resonance with the ambient plasma particles. Our model explains in a natural way the anomalous duration and behavior of radio eclipses observed in such systems. In particular, it provides stable, continuous, and frequency-dependent eclipses, in agreement with the observations. We predict a significant variation of linear polarization both at eclipse ingress and egress. In this paper we also suggest several possible mechanisms of generation of the optical and $X$-ray emission observed from these binary systems.

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