Rahul Basu

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 characterization of the emission features from a representative sample of pulsars. In this work, we present a detailed analysis of the single-pulse behavior from 17 pulsars exhibiting periodic amplitude modulation, 6 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 behavior 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 behavior.

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.

Polarization behavior shows a transition in the pulsar population, where energetic sources with higher spin-down energy loss, E ̇ > 1 0 34 erg s⁻¹, often have fractional linear polarization (L/I) close to 100%, while below this range L/I is usually lower than 50%. The polarization behavior has been primarily studied at higher frequencies above 1 GHz, and in this work, we explore the single-pulse polarization behavior in pulsars with E ̇ > 5 × 1 0 33 erg s⁻¹ at a lower frequency range of 300–750 MHz. The polarization behavior can be divided into two categories, the first with L/I > 70% where the polarization position angle (PPA) follows a single track, and a second group with L/I < 70% and scattered PPA behavior 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 polarization modes along orthogonal tracks. The radio emission in pulsars arises due to coherent curvature radiation (CCR) from charge bunches, which develops due to nonlinear instabilities in the pulsar plasma forming charge separated envelope solitons. The CCR excites orthogonally polarized X and O modes oriented perpendicular and parallel to the magnetic field line planes, which detach in the plasma and propagate independently. The O mode is 7 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 polarization features in the energetic pulsar population.

Observations have established that coherent radio emission from pulsars arises at a few hundred kilometers above the 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 realized 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 to a general two-dimensional form, and the contribution from each soliton along the 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, are seen in this setup. The power-law spectrum and relative steepening of the core spectra with respect to the cones also emerge. 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.

We report the measurements of low radio frequency spectra of 14 gigahertz-peaked spectra (GPS) pulsar candidates, between 300 and 700 MHz, using the upgraded Giant Metrewave Radio Telescope. Combining newly collected measurements with archival results, the spectral nature of each pulsar was examined using four different physical models: simple power law, broken power law, low-frequency turnover power law, and free–free thermal absorption. Based on this analysis, we confirm the GPS nature of five pulsars, three of them being new detections. In addition, one pulsar can be classified as having a broken power law spectrum, and we found the typical power-law spectra in four other cases. In the remaining four pulsars, the spectra showed tendencies of low-frequency turnover that require further investigations at lower frequency ranges. These results demonstrate the effectiveness of wideband measurements at low frequencies, below 1 GHz, in characterizing the spectral nature in pulsars. Our results also underline the need for more systematic theoretical studies to refine existing models and better interpret pulsar 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.

We report the measurements of low radio frequency spectra of fourteen gigahertz-peaked spectra (GPS) pulsar candidates, between 300~MHz and 700~MHz, using the upgraded Giant Meterwave Radio Telescope. Combining newly collected measurements with archival results the spectral nature of each pulsar was examined using four different physical models: simple power law, broken power law, low-frequency turn-over power law and free-free thermal absorption. Based on this analysis, we confirm the GPS nature of five pulsars, three of them being new detections. In addition, one pulsar can be classified as having a broken power law spectrum, and we found the typical power law spectra in four other cases. In the remaining four pulsars the spectra showed tendencies of low frequency turn-over that require further investigations at lower frequency ranges. These results demonstrate the effectiveness of wideband measurements at low frequencies, below 1 GHz, in characterizing the spectral nature in pulsars. Our results also underline the need for more systematic theoretical studies to refine existing models and better interpret pulsar emission properties.

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 supports 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) eigenmodes 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 and 750 MHz, using the Giant Metrewave Radio Telescope. The pulsar average profile comprises 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 localized 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 is 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 is formed due to the incoherent addition of CCR from a large number of charged solitons (charge bunches) exciting the X-mode.