George I. Melikidze - J.Kepler Institute of Astronomy, University of Zielona Góra, Poland

George I. Melikidze
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Name
George I. Melikidze
Affiliation
J.Kepler Institute of Astronomy, University of Zielona Góra, Poland
City
Zielona Gora
Country
Poland

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Astrophysics (15)
 
High Energy Astrophysical Phenomena (11)
 
Astrophysics of Galaxies (3)
 
Solar and Stellar Astrophysics (3)

Publications Authored By George I. Melikidze

We constrain the X-ray properties of the nearby $(360\,{\rm pc})$, old ($5\,{\rm Myr}$) pulsar B1133+16 with $\sim 100\,{\rm ks}$ effective exposure time by {\it XMM-Newton}. The observed pulsar flux in the 0.2-3 keV energy range is $\sim 10^{-14} \, {\rm erg \, cm}^{-2} \, {\rm s}^{-1}$, which results in the recording of $\sim 600$ source counts with the EPIC pn and MOS detectors. Read More

We have conducted the Meterwavelength Single-pulse Polarimetric Emission Survey to study the radio emission properties of normal pulsars. A total of 123 pulsars with periods between 0.1 seconds and 8. Read More

A large sample of pulsars was observed as part of the Meterwavelength Single-pulse Polarimetric Emission Survey. We carried out a detailed fluctuation spectral analysis which revealed periodic features in 46% pulsars including 22 pulsars where drifting characteristics were reported for the first time. The pulsar population can be categorized into three distinct groups, pulsars which show systematic drift motion within the pulse window, the pulsars showing no systematic drift but periodic amplitude fluctuation and pulsars with no periodic variations. Read More

We present a model that explains the observed deviation of the spectra of some pulsars and magnetars from the power-law spectra which are seen in the bulk of the pulsar population. Our model is based on the assumption that the observed variety of pulsar spectra can be naturally explained by the thermal free-free absorption that takes place in the surroundings of the pulsars. In this context, the variety of the pulsar spectra can be explained according to the shape, density and temperature of the absorbing media and the optical path of the line-of-sight across that. Read More

Magnetars are the most magnetized objects in the known universe. Powered by the magnetic energy, and not by the rotational energy as in the case of radio pulsars, they have long been regarded as a completely different class of neutron stars. The discovery of pulsed radio emission from a few magnetars weakened the idea of a clean separation between magnetars and normal pulsars. Read More

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). Read More

The pulsar radio emission is generated in the near magnetosphere of the neutron star and it has to propagate through the rest of it to emerge into the interstellar medium. An important issue is whether this propagation affects the planes of polarization of the generated radiation. Observationally, there is a sufficient evidence that the emerging radiation is polarized parallel or perpendicular to the magnetic field line planes that should be associated with the ordinary O and extraordinary X plasma modes respectively, excited by some radiative process. Read More

We investigate radio emission efficiency $\xi$ of pulsars and report a near linear inverse correlation between $\xi$ and the spindown power $\dot E$, as well as a near linear correlation between $\xi$ and pulsar age $\tau$. 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 $\gamma$-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. Read More

We have examined the cyclotron resonance instability developing in the relativistic out-flowing plasma in the pulsar magnetosphere. The instability condition leads to radio emission in the sub-GHz frequency regime which is likely to be seen as off-pulse emission. Recent studies have shown the presence of off-pulse emission in long period pulsars, and we demonstrate this plasma process to be an energetically viable mechanism. Read More

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. Read More

2011Aug
Affiliations: 1J.Kepler Institute of Astronomy, University of Zielona Góra, Poland, 2J.Kepler Institute of Astronomy, University of Zielona Góra, Poland, 3J.Kepler Institute of Astronomy, University of Zielona Góra, Poland

Observations of the thermal X-ray emission from 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. Most plausible explanation of this phenomena is an assumption that the magnetic field at the stellar surface differs essentially from the purely dipolar field. We can determine magnetic field at the surface by the conservation of the magnetic flux through the area bounded by open magnetic field lines. Read More

An interesting paper has recently been published claiming that the long-sought Rosetta Stone needed to decipher the nature of pulsar radio emission has been finally identified as the double features in averaged pulsar profiles. The authors argue that highly symmetric bifurcated features are produced by a split-fan beams of extraordinary-mode curvature radiation emitted by thin microscopic streams of magnetospheric plasma conducted by a very narrow bundle of magnetic field lines. We examined arguments leading to these intriguing conclusions and found a number of flaws. Read More

Forty years have passed since the discovery of pulsars, yet the physical mechanism of their coherent radio emission is a mystery. Recent observational and theoretical studies strongly suggest that the radiation outcoming from the pulsar magnetosphere consists mainly of extraordinary waves polarized perpendicular to the planes of pulsar dipolar magnetic field. However, the fundamental question whether these waves are excited by maser or coherent curvature radiation, remains open. Read More

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. Read More

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. Read More

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). Read More

It is argued that the nanosecond giant subpulses detected recently in the Crab pulsar are generated by means of the coherent curvature radiation of charged relativistic solitons associated with sparking discharges of the inner gap potential drop above the polar cap. Read More

We consider the curvature radiation of the point-like charge moving relativistically along curved magnetic field lines through a pulsar magnetospheric electron-positron plasma. We demonstrate that the radiation power is largely suppressed as compared with the vacuum case, but still at a considerable level, high enough to explain the observed pulsar luminosities. The emitted radiation is polarized perpendicularly to the plane of the curved magnetic filed lines coincides with $ which can freely escape from the magnetospheric plasma. Read More

The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated subbeams of subpulse emission circulate around the magnetic axis due to the EB drift, provides a natural and plausible physical mechanism of the subpulse drift phenomenon. Recent progress in the analysis of drifting subpulses in pulsars has provided a strong support to this model by revealing a number of subbeams circulating around the magnetic axis in a manner compatible with theoretical predictions. However, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations. Read More

2002Sep
Affiliations: 1University of Zielona Gora, 2University of Zielona Gora
Category: Astrophysics

The problem of formation of the inner vacuum gap in neutron stars with ${\bf\Omega}\cdot{\bf B}<0$ is considered. It is argued by means of the condition $T_i/T_s>1$, where $T_i$ is the melting temperature of surface $^{56}_{26}$Fe ions and $T_s$ is the actual temperature of the polar cap surface, that the inner vacuum gap can form, provided that the actual surface magnetic field is extremaly strong ($B_s\gtrsim 10^{13}$ G) and curved (${\cal R}<10^6$ cm), irrespective of the value of dipolar component measured from the pulsar spin down rate. The calculations are carried out for pulsars with drifting subpulses and/or periodic intensity modulations, in which the existence of the quasi steady vacuum gap discharging via ${\bf E}\times{\bf B}$ drifting sparks is almost unavoidable. Read More

The problem of formation of the Ruderman-Sutherland type inner vacuum gap in neutron stars with ${\bf\Omega}\cdot{\bf B}<0$ is considered. It is argued by means of the condition $T_i/T_s>1$ (where $T_i$ is the critical temperature above which $^{56}_{26}$Fe ions will not be bound at the surface and $T_s$ is the actual temperature of the polar cap surface heated by the back-flow of relativistic electrons) that the inner vacuum gap can form, provided that the actual surface magnetic field is extremaly strong ($B_s\gtrsim 10^{13}$ G) and curved (${\cal R}<10^6$ cm), irrespective of the value of dipolar component measured from the pulsar spin down rate. Calculations are carried out for pulsars with drifting subpulses and/or periodic intensity modulations, in which the existence of the quasi steady vacuum gap discharging via ${\bf E}\times{\bf B}$ drifting sparks is almost unavoidable. Read More

PSR B0943+10 is known to show remarkably stable drifting subpulses, which can be interpreted in terms of a circumferential motion of 20 sparks, each completing one circulation around the periphery of the polar cap in 37 pulsar periods. We use this observational constraint and argue that the vacuum gap model can adequately describe the observed drift patterns. Further we demonstrate that {\em only} the presence of strong non-dipolar surface magnetic field can favor such vacuum gap formation. Read More

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. Read More

2001Nov
Affiliations: 1Institute of Astronomy, University of Zielona Gora, Poland, 2Institute of Astronomy, University of Zielona Gora, Poland, 3Max-Planck Institute for Radioastronomy, Bonn, Germany
Category: Astrophysics

We propose a vacuum gap (VG) model which can be applied uniformly for normal and high magnetic field pulsars. The model requires strong and non-dipolar surface magnetic field near the pulsar polar cap. We assume that the actual surface magnetic field in pulsars results from a superposition of global dipole field and crust-anchored small scale magnetic anomaly. Read More

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. Read More

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. Read More

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. Read More

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. Read More