arXiv subjects

George I. Melikidze

Publications and source records attributed to George I. Melikidze.

At least 19 recordsLinked to original sources

arXiv2025

Characterizing the Nature of Periodic Amplitude Modulation in Pulsars

In recent years periodic amplitude modulation has emerged as a unique emission feature in the single pulse sequence of pulsars alongside periodic nulling and subpulse drifting. Despite ample evidence for the uniqueness of this phenomenon, the periodic modulation in several pulsars are often confused with subpulse drifting, primarily due to lack of clear characterisation of the emission features from a representative sample of pulsars. In this work we present a detailed analysis of the single pulse behaviour from seventeen pulsars exhibiting periodic amplitude modulation, six of them being new detections. The pulsar switches between different intensity states as a result of periodic amplitude modulation and we propose a novel statistical scheme to identify these emission states. The periodic modulation can be divided into three broad categories, phase stationary modulation, modulations with phase shift and intermittent periodic modulations. The phase stationary behaviour is seen when the emission intensity across a major part of the pulse window changes periodically. The phase shifts are associated with intensity changes at specific locations within the emission window in a periodic manner; while in some pulsars the periodic modulations become more prominent only at specific intervals resulting in intermittent behaviour.

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arXiv2025

Polarization Properties of Energetic Pulsars at Meterwavelengths

Polarization behaviour shows a transition in the pulsar population, where energetic sources with higher spin-down energy loss, $\dot{E} > 10^{34}$ erg~s$^{-1}$, often have fractional linear polarisation ($L/I$) close to 100\%, while below this range $L/I$ is usually lower than 50\%. The polarisation behaviour has been primarily studied at higher frequencies above 1 GHz, and in this work we explore the single pulse polarisation behaviour in pulsars with $\dot{E} > 5\times10^{33}$ erg~s$^{-1}$ at a lower frequency range of 300-750 MHz. The polarisation behaviour can be divided into two categories, the first with $L/I>$ 70\% where the polarisation position angle (PPA) follows a single track, and a second group with $L/I <$ 70\% and scattered PPA behaviour with or without orthogonal modes. However, there are some single pulses in the first category that also have lower $L/I$ and exhibit the presence of two polarisation modes along orthogonal tracks. The radio emission in pulsars arises due to coherent curvature radiation (CCR) from charge bunches, which develops due to non-linear instabilities in the pulsar plasma forming charge separated envelope solitons. The CCR excites orthogonally polarised X and O modes oriented perpendicular and parallel to the magnetic field line planes, that detach in the plasma and propagate independently. The O-mode is seven times stronger than the X-mode but gets damped in the medium. We show that incoherent mixing of the X and O modes with different levels of damping can reproduce the observed polarisation features in the energetic pulsar population.

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arXiv2025

Pulsar Coherent Radio Emission from Solitons : Average Emission Properties

Observations have established that coherent radio emission from pulsars arise at few hundred kilometers above stellar surface. Recent polarization studies have further demonstrated that plasma instabilities are necessary for charge bunching that gives rise to coherent emission. The formation of charged solitons in the electron-positron plasma is the only known bunching mechanism that can be realised at these heights. More than five decades of observations have revealed a number of emission features that should emerge from any valid radio emission mechanism. We have carried out numerical calculations to find the features of average emission from curvature radiation due to charged solitons. The characteristic curvature radiation spectrum has been updated from the well known one-dimensional dependence into a general two-dimensional form, and contribution from each soliton along observer's line of sight (LOS) has been added to reproduce the pulsar emission. The outflowing plasma is formed by sparking discharges above the stellar surface that are located within concentric rings resembling the core-cone emission beam, and uniform distribution of solitons along any LOS has been assumed. The observed effects of radius to frequency mapping, where the lower frequency emission originates from higher altitudes, is seen in this setup. The power law spectrum and relative steepening of the core spectra with respect to the cones also emerges. The estimated polarization position angle reflects the geometrical configuration of pulsars as expected. These studies demonstrate the efficacy of coherent curvature radiation from charged solitons to reproduce the average observational features of pulsars.

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arXiv2024

On the flux density spectral property of high linearly polarized signal from Pulsar J0332+5434

The polarization position angles (PPA) of time samples with high linear polarization often show two parallel tracks across the pulsar profile that follow the rotating vector model (RVM). This feature support coherent curvature radiation (CCR) as the underlying mechanism of radio emission from pulsars, where the parallel tracks of the PPA represent the orthogonal extraordinary X and ordinary O eigen modes of strongly magnetized pair plasma. However, the frequency evolution of these high linearly polarized signals remains unexplored. In this work we explore the flux density spectral nature of high linearly polarized signals by studying the emission from PSR J0332+5434 over a frequency range between 300 MHz and 750 MHz, using the Giant Metrewave Radio Telescope. The pulsar average profile comprises of a central core and a pair of conal components. We find the high linearly polarized time samples to be broadband in nature and in many cases they resemble a narrow spiky feature in the conal regions. These spiky features are localised within a narrow pulse longitude, over the entire frequency range, and their spectral shapes sometimes resemble an inverted parabolic shape. In all such cases the PPA are exclusively along one of the orthogonal RVM tracks, likely corresponding to the X-mode. The inverted spectral shape can in principle be explained if the high linearly polarized emission in these time samples are formed due to incoherent addition of CCR from a large number of charged solitons (charge bunches) exciting the X-mode.

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arXiv2023

Mode changing in PSR B0844-35 and PSR B1758-29 with enhanced emission at the profile centers

We have studied the single pulse emission from two pulsars, PSR B0844-35 and PSR B1758-29, over a wide frequency range of 300-750 MHz using the uGMRT. The two pulsars have relatively wide profiles with multiple components, that are a result of the line of sight traversing near center of the emission beam. In both pulsars the single pulse sequences show the presence of two distinct emission states, where the profiles become much brighter at the center with prominent core components during one of the modes, while in the other mode the single pulses show odd-even subpulse drifting with periodicity around 2$P$, $P$ being the rotation period of the pulsar. The centrally bright mode was seen for 10 percent of the observing duration in PSR B0844-35, which usually lasted for short durations of around 10 pulses, but had two longer sequences of around 100 pulses. On the contrary the centrally bright mode was dominant in PSR B1758-29 and was seen for around 60 percent of the observing duration. PSR B1758-29 also showed period amplitude modulations of 60-70$P$ in both modes. The mode changing in these two pulsars facilitates investigation of the sparking process in the inner acceleration region, dominated by non-dipolar magnetic fields. The change in the surface magnetic field configurations likely results in the emission mode change.

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arXiv2023

Meterwavelength Single-pulse Polarimetric Emission Survey. VI. Towards understanding the Phenomenon of Pulsar Polarization in Partially Screened Vacuum Gap model

We have observed 123 pulsars with periods longer than 0.1 seconds in the Meterwavelength Single-pulse Polarimetric Emission Survey. In this work a detailed study of polarization behaviour of these pulsars have been carried out. We were able to fit the rotating vector model to the polarization position angle sweeps in 68 pulsars, and in 34 pulsars the emission heights could be measured. In all cases the radio emission was constrained to arise below 10\% of the light cylinder radius. In pulsars with low spindown energy loss, $\dot{E}<10^{34}$ ergs s$^{-1}$, we found the mean fractional linear polarization of the individual times samples in single pulses to be around 0.57 (57\%) which is significantly larger than the fractional linear polarization of 0.29 (29\%) obtained from the average profiles. On the other hand the mean fractional circular polarization of the individual time samples in single pulses is around 0.08 (8\%), similar to the measurements from the average profiles. To explain the observed polarization features, we invoke the partially screened vacuum gap model of pulsars, where dense spark associated plasma clouds exist with high pair plasma multiplicity, with significant decrease of density in the regions between the clouds, that are dominated by iron ions. The coherent radio emission is excited by curvature radiation from charge bunches in these dense plasma clouds and escape as linearly polarized waves near cloud boundaries. We suggest that the circular polarization arises due to propagation of waves in the low pair multiplicity, ion dominated inter-cloud regions.

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arXiv2023

Estimating the evolution of Sparks in Partially Screened Gap of Pulsars from Subpulse Drifting

A novel scheme has been developed to show that the observed phase behaviour associated with subpulse drifting from two pulsars, J1034$-$3224 and J1720$-$2933, can be used to obtain the magnetic field configuration in the partially screened gap (PSG). The outflowing plasma along the open magnetic field line region of pulsars is generated due to spark discharges in an inner acceleration region (IAR) above the polar cap. The IAR has been modelled as a partially screened gap (PSG) with a steady supply of positively charged ions emitted from the heated polar cap surface dominated by strong non-dipolar magnetic fields. In a PSG the sparks are tightly packed and constrained to be present along the polar cap boundary. The sparks lag behind the rotation of the star during their lifetimes. As a result the sparking pattern evolves along two different directions in the clockwise and counter-clockwise manner around a stationary central spark, and can be associated with the observed phenomenon of subpulse drifting. PSR J1034$-$3224 has four prominent components and exhibit bi-drifting where alternate components show opposite sense of drifting, while PSR J1720$-$2933 has a single component profile and shows systematic coherent drift bands. We show that the differences in their drifting behaviour can be directly linked to different natures of the non-dipolar surface magnetic field configurations.

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arXiv2023

Evidence for Coherent Curvature Radiation in PSR J1645$-$0317 with Disordered Distribution of Polarization Position Angle

The diverse polarization properties in pulsars are in conflict with applying a unique emission mechanism to the population. The polarization position angle (PPA) traverse in most pulsars shows a S-shaped curve that can be interpreted using the rotating vector model (RVM) as the radio emission being directed either parallel or perpendicular to the divergent magnetic field lines and argues for a coherent curvature radiation mechanism from charge bunches in a strongly magnetized pair plasma. However, in a subset of pulsars the radio emission is significantly depolarized and the PPA shows a complex pattern which cannot be explained using RVM. We propose that even in such cases the highly polarized time samples in the single pulses should follow the RVM with possibly two parallel tracks separated by 90\degr. We have investigated PSR J1645$-$0317, with complex PPA traverse, and demonstrated for the first time that considering only the highly polarized time samples in the single pulses, the PPA distribution clearly follows the RVM. We conclude that this strongly favour the coherent curvature radiation mechanism to be universally applicable in the pulsar population.

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arXiv2022

Externally-driven plasma models as candidates for pulsar radio emission

Coherent radio emission from pulsars originates from excited plasma waves in an ultra-relativistic and strongly magnetized electron-positron pair plasma streaming along the open magnetic field lines of the pulsar. Traditional coherent radio emission models have relied on instabilities in this pair plasma. Recently alternative models have been suggested. These models appeal to direct coupling of the external electromagnetic field to the superluminal O-mode ($lt_2$ mode) during the time-dependent pair cascade process at the polar gap. The objective of this work is to provide generic constraints on plasma models based on $lt_2$ mode using realistic pulsar parameters. We find that the very short timescale associated with pair cascades does not allow $lt_{2}$ mode to be excited at radio frequencies and the impulsive energy transfer can only increase the kinetic spread ("temperature") of the pair plasma particles. Moreover, under homogeneous plasma conditions, plasma waves on both branches of O-mode (i.e. superluminal $lt_2$ and subluminal $lt_1$) cannot escape the plasma. In the strongly magnetized pair plasma, only the extraordinary mode ($t$ mode) can escape freely. We show that any generic fictitious mechanisms does not result in the wave electric field of $t$ mode to have predominant orientation either parallel or perpendicular to the magnetic field plane as observed. Such fictitious mechanisms will inevitably lead to depolarization of signals and cannot account for the highly polarized single pulses observed in pulsars. We suggest coherent curvature radiation as a promising candidate for pulsar radio emission mechanism.

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arXiv2022

Pulsar radio emission mechanism II. On the origin of relativistic Langmuir solitons in pulsar plasma

Observations suggest that coherent radio emission from pulsars is excited in a dense pulsar plasma by curvature radiation from charge bunches. Numerous studies propose that these charge bunches are relativistic charge solitons which are solutions of the non-linear Schrödinger equation (NLSE) with a group velocity dispersion ($G$), cubic-nonlinearity($q$) and non-linear Landau damping ($s$). The formation of stable solitons crucially depends on the parameters $G, q$ and $s$ and the particle distribution function. In this work, we use realistic pulsar plasma parameters obtained from observational constraints to explore the parameter space of NLSE for two representative distribution functions (DF) of particles' momenta: Lorentzian (long-tailed) and Gaussian (short-tailed). The choice of DF critically affects the value of $|s/q|$, which, in turn, determines whether solitons can form. Numerical simulations show that well-formed solitons are obtained only for small values of $|s/q| \lesssim 0.1$ while for moderate and higher values of $|s/q| \gtrsim 0.5$ soliton formation is suppressed. Small values for $|s/q| \sim 0.1$ are readily obtained for long-tailed DF for a wide range of plasma temperatures. On the other hand, short-tailed DF provides these values only for some narrow range of plasma parameters. Thus, the presence of a prominent high-energy tail in the particle DF favours soliton formation for a wide range of plasma parameters. Besides pair plasma, we also include an iron ion component and find that they make a negligible contribution in either modifying the NLSE coefficients or contributing to charge separation.

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arXiv2022

Two Dimensional Configuration and Temporal Evolution of Sparking discharges in Pulsars

We have investigated the evolution of a system of sparking discharges in the inner acceleration region (IAR) above the pulsar polar cap. The surface of the polar cap is heated to temperatures around $10^6$ K and forms a partially screened gap (PSG) due to thermionic emission of positively charged ions from the stellar surface. The sparks lag behind the co-rotation speed during their lifetimes due to variable $E$x$B$ drift. In a PSG the sparking discharges arise in locations where the surface temperatures go below the critical level ($T_i$) for ions to freely flow from the surface. The sparking commences due to the large potential drop developing along the magnetic field lines in these lower temperature regions and subsequently the back streaming particles heat the surface to $T_i$. The temperature regulation requires the polar cap to be tightly filled with sparks and a continuous presence of sparks is required around its boundary since no heating is possible from the closed field line region. We have estimated the time evolution of the sparking system in the IAR which shows a gradual shift in the spark formation along two distinct directions resembling clockwise and anti-clockwise motion in two halves of the polar cap. Due to the differential shift of the sparking pattern in the two halves, a central spark develops representing the core emission. The temporal evolution of the sparking process was simulated for different orientations of the non-dipolar polar cap and reproduced the diverse observational features associated with subpulse drifting.

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arXiv2022

Spectral variation across Pulsar Profile due to Coherent Curvature Radiation

The pulsar profile is characterised by two distinct emission components, the core and the cone. The standard model of a pulsar radio emission beam originating from dipolar magnetic fields, places the core at the centre surrounded by concentric layers of inner and outer conal components. The core emission is expected to have steeper spectra compared to the cones. We present a detailed analysis of the relative differences in spectra between the core and conal emission from a large sample of 53 pulsars over a wide frequency range between 100 MHz and 10 GHz. The core was seen to be much steeper than the cones particularly between 100 MHz and 1 GHz with a relative difference between the spectral index $Δα_{core/cone}\sim$ -1.0. In addition we also found the spectra of the outer conal components to be steeper than the inner cone with relative difference in the spectral index $Δα_{in/out}\sim$ +0.5. The flattening of the spectra from the magnetic axis towards the edge of the open field line region with increasing curvature of the field lines is a natural outcome of the coherent curvature radiation from charged soliton bunches, and explains the difference in spectra between the core and the cones. In addition due to the relativistic beaming effect, the radiation is only visible when it is directed towards the observer over a narrow angle $θ\leq 1/γ$, where $γ$ is the Lorentz factor of the outflowing plasma clouds. This restricts the emission particularly from outer cones, that are associated with field lines with larger curvature thereby making the spectra steeper than the inner cones.

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arXiv2021

Meterwavelength Single-pulse Polarimetric Emission Survey. V. Flux density, component spectral variation and emission states

We present the flux density measurements of the pulsars observed in the Meterwavelength single-pulse polarimetric emission survey. The average flux densities were estimated in 113 pulsars at two frequencies of 325 and 610 MHz using interferometric imaging. The average profile and single pulse emission in each pulsar were calibrated using the estimated flux density. We have used the flux calibrated average profile to study the variation of the spectral index across the emission beam in 21 pulsars where the core, inner cone and the outer conal components could be clearly identified. The central core component showed a steeper increase in emission at the lower frequency compared with conal emission, with an average difference in spectral index $δα_{core-cone}\sim-0.7$ between the core and the conal components in this frequency range. In contrast the inner conal components had positive difference in their spectral index compared to the outer cones with average difference $δα_{in-out}\sim+0.3$. The variation in the spectral index across the pulse window should provide valuable inputs for constraining the radio emission processes. The single pulse emission showed the presence of emission mode changing in 12 pulsars with 3 cases where the phenomenon is being reported for the first time. In addition we have also detected enhanced emission for short durations or flaring, in parts or across the entire emission window in 14 pulsars. The sudden changes in the emission during mode changing as well as these bursting states are unrelated to the emission mechanism and suggest the presence of rapid and repetitive changes during the plasma generation process.

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arXiv2020

Mode changing, subpulse drifting and nulling in four component conal pulsar PSR J2321+6024

In this study, we report on a detailed single pulse polarimetric analysis of the radio emission from the pulsar J2321+6024 (B2319+60) observed with the Giant Metrewave Radio Telescope, over wide frequencies ranging between 300 to 500 MHz and widely separated observing sessions. The pulsar profile shows the presence of four distinct conal components and belongs to a small group of pulsars classified as a conal quadrupole profile type. The single pulse sequence reveals the presence of three distinct emission modes, A, B, and ABN showing subpulse drifting. Besides, there were sequences when the pulsar did not show any drifting behaviour suggesting the possibility of a new emission state, which we have termed as mode C. The evolution of the mode changing behavior was seen during the different observing sessions with different abundance as well as the average duration of the modes seen on each date. The drifting periodicities were 7.8$\pm$0.3 $P$, 4.3$\pm$0.4 $P$, and 3.1$\pm$0.2 $P$ in the modes A, B and ABN respectively, and showed large phase variations within the mode profile. The pulsar also showed the presence of orthogonal polarization modes, particularly in the leading and trailing components, which has different characteristics for the stronger and weaker pulses. However, no correlation was found between the emission modes and their polarization behavior, with the estimated emission heights remaining roughly constant throughout. We have used the Partially Screened Gap model to understand the connection between drifting, mode changing, and nulling.

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arXiv2020

Search for Off-pulse emission in Long Period Pulsars

We have revisited the problem of off-pulse emission in pulsars, where detailed search for the presence of low level radio emission outside the pulse window is carried out. The presence of off-pulse emission was earlier reported in two long period pulsars, PSR B0525+21 and B2046-16 at frequencies below 1 GHz using the Giant Meterwave Radio Telescope (GMRT). However, subsequent studies did not detect off-pulse emission from these pulsars at higher radio frequencies (> 1 GHz). We have carefully inspected the analysis scheme used in the earlier detections and found an anomaly with data editing routines used, which resulted in leakage of signal from the on-pulse to the off-pulse region. We show that the earlier detections from PSR B0525+21 and B2046-16 were a result of this leakage. The above analysis scheme has been modified and offline-gating has been used to search for off-pulse emission in 21 long period pulsars (P > 1.2 sec) at different observing frequencies of GMRT. The presence of low level off-pulse emission of peak flux 0.5 mJy was detected in the brightest pulsar in this list PSR 0B0628-28, with off-pulse to average pulsar flux ratio of 0.25%. We suggest that coherent radio emission resulting due to cyclotron resonance near the light cylinder can be a possible source for the off-pulse emission in this pulsar.

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arXiv2020

Pulsar radio emission mechanism I : On the amplification of Langmuir waves in the linear regime

Observations suggest that in normal period radio pulsars, coherent curvature radiation is excited within 10$\%$ of the light cylinder. The coherence is attributed to Langmuir mode instability in a relativistically streaming one-dimensional plasma flow along the open magnetic field lines. In this work, we use a hot plasma treatment to solve the hydrodynamic dispersion relation of Langmuir mode for realistic pulsar parameters. The solution involves three scenarios of two-stream instability viz., driven by high energy beams, due to longitudinal drift that leads to a separation of electron-positron distribution functions in the secondary plasma and due to cloud-cloud interaction causing spatial overlap of two successive secondary plasma clouds. We find that sufficient amplification can be obtained only for the latter two scenarios. Our analysis shows that longitudinal drift is characterized by high growth rates only for certain multi-polar surface field geometry. For these configurations, very high growth rates are obtained starting from a few tens of km from the neutron star surface, which then falls monotonically with increasing distance. For cloud-cloud overlap, growth rates become high starting only after a few hundred km from the surface, which first increases and then decreases with increasing distance. A spatial window of up to around a 1000 km above the neutron star surface has been found where large amplitude Langmuir waves can be excited while the pair plasma is dense enough to account for high brightness temperature.

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arXiv2020

A Mechanism of Spark Motion in Inner Acceleration Region to Investigate Subpulse Drifting in Pulsars

Coherent radio emission in pulsars is excited due to instabilities in a relativistically streaming non-stationary plasma flow, which is generated from sparking discharges in the inner acceleration region (IAR) near the stellar surface. A number of detailed works have shown the IAR to be a partially screened gap (PSG) dominated by non-dipolar magnetic fields with continuous outflow of ions from the surface. The phenomenon of subpulse drifting is expected to originate due to variable $\mathbf{E}\times\mathbf{B}$ drift of the sparks in PSG, where the sparks lag behind corotation velocity of the pulsar. Detailed observations show a wide variety of subpulse drifting behaviour where subpulses in different components of the profile have different phase trajectories. But the drifting periodicity is seen to be constant, within measurement errors, across all components of the profile. Using the concept of sparks lagging behind corotation speed in PSG as well as the different orientations of the surface non-dipolar magnetic fields we have simulated the expected single pulse behaviour in a representative sample of pulsars. Our results show that the different types of drifting phase behaviour can be reproduced using these simple assumptions of spark dynamics in a non-dipolar IAR.

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arXiv2019

A Single spark model for PSR J2144$-$3933

The partially screened vacuum gap model (PSG) for the inner acceleration region in normal radio pulsars, a variant of the pure vacuum gap model, attempts to account for the observed thermal X-ray emission from polar caps and the subpulse drifting timescales. We have used this model to explain the presence of death lines, and extreme location of PSR J2144$-$3933 in the $P-\dot{P}$ diagram. This model requires maintaining the polar cap near a critical temperature and the presence of non-dipolar surface magnetic field to form the inner acceleration region. In the PSG model, thermostatic regulation is achieved by sparking discharges which are a feature of all vacuum gap models. We demonstrate that non-dipolar surface magnetic field reduces polar cap area in PSR J2144$-$3933 such that only one spark can be produced and is sufficient to sustain the critical temperature. This pulsar has a single component profile over a wide frequency range. Single-pulse polarimetric observations and the rotating vector model confirm that the observer's line-of-sight traverses the emission beam centrally. These observations are consistent with a single spark operating within framework of the PSG model leading to single-component emission. Additionally, single-pulse modulations of this pulsar, including lack of subpulse drifting, presence of single-period nulls and microstructure, are compatible with a single spark either in PSG or in general vacuum gap models.

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