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Register a new userMeterwavelength Single-pulse Polarimetric Emission Survey IV: The Period Dependence of Component Widths of Pulsars
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The core component width in normal pulsars, with periods ($P$) $>$ 0.1 seconds, measured at the half-power point at 1 GHz has a lower boundary line (LBL) which closely follows the $P^{-0.5}$ scaling relation. This result is of fundamental importance for understanding the emission process and requires extended studies over a wider frequency range. In this paper we have carried out a detailed study of the profile component widths of 123 normal pulsars observed in the Meterwavelength Single-pulse Polarimetric Emission Survey at 333 and 618 MHz. The components in the pulse profile were separated into core and conal classes. We found that at both frequencies the core as well as the conal component widths versus period had a LBL which followed the $P^{-0.5}$ relation with a similar lower boundary. The radio emission in normal pulsars have been observationally shown to arise from a narrow range of heights around a few hundred kilometers above the stellar surface. In the past the $P^{-0.5}$ relation has been considered as evidence for emission arising from last open dipolar magnetic field lines. We show that the $P^{-0.5}$ dependence only holds if the trailing and leading half-power points of the component are associated with the last open field line. In such a scenario we do not find any physical motivation which can explain the $P^{-0.5}$ dependence for both core and conal components as evidence for dipolar geometry in normal pulsars. We believe the period dependence is a result of an yet unexplained physical phenomenon.
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A detailed analysis of nulling was conducted for the pulsars studied in the Meterwavelength Single-pulse Polarimetric Emission Survey. We characterized nulling in 36 pulsars including 17 pulsars where the phenomena were reported for the first time. The most dominant nulls lasted for short durations, less than five periods. The longer duration nulls extending to hundreds of periods were also seen in some cases. A careful analysis showed the presence of periodicities in the transition from the null to the burst states in 11 pulsars. In our earlier work fluctuation spectrum analysis showed multiple periodicities in 6 of these 11 pulsars. We demonstrate that the longer periodicity in each case was associated with nulling. The shorter periodicities usually originate due to subpulse drifting. The nulling periodicities were more aligned with the periodic amplitude modulation indicating a possible common origin for both. Most prevalent nulling lasts for a single period and can be potentially explained using random variations affecting the plasma processes in the pulsar magnetosphere. On the other hand, the longer duration nulls require changes in the pair production processes that need an external triggering mechanism for the change. The presence of periodic nulling puts an added constrain on the triggering mechanism which also needs to be periodic.
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 $deltaalpha_{core-cone}sim-0.7$ 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 $deltaalpha_{in-out}sim+0.3$. 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.
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. We discovered the dependence of the drifting phenomenon on the spin down energy loss ($dot{E}$), with the three categories occupying distinctly different regions along the $dot{E}$ axis. The estimation of the drift periodicity ($P_3$) from the peak frequency in the fluctuation spectra is ambiguous due to the aliasing effect. However, using basic physical arguments we were able to determine $P_3$ in pulsars showing systematic drift motion. The estimated $P_3$ values in these pulsars were anti-correlated with $dot{E}$ which favoured the Partially Screened Gap model of Inner Acceleration Region in pulsars.
We report on an Arecibo 4.5-GHz polarimetric single-pulse survey of the brightest pulsars at high frequency within its sky. The high frequency profiles are accompanied by a collection of both previously published and unpublished high quality 1.4- and 0.33-GHz observations. Here our analyses and discussion primarily involve the average and statistical properties of the 46 pulsars polarimetric pulse sequences, profile classification and frequency evolution, and polarimetric profiles and peak-occurrence histograms. In most cases both the fractional linear polarization and profile widths decrease with frequency as expected, but there are some exceptions. Similarly, we were able to review and/or extend the profile classifications for this population of pulsars and work out their beaming characteristics quantitatively showing that almost all show properties compatible with the core/double-cone emission beam model. The entirety of these observations average profiles are accessible for download.
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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