arXiv subjects

Dipanjan Mitra

Publications and source records attributed to Dipanjan Mitra.

At least 55 records · Page 3Linked to original sources

arXiv2019

Investigation of the Mode-Switching Phenomenon in Pulsar B0329+54 Through Polarimetric Analysis

The phenomenon of profile mode switching in pulsars, where the stable average pulse profile changes to another stable state on the timescale of a pulsar's period, remains poorly understood. We sought to understand how pulsars undergo profile mode switching through a comparative analysis of the polarization and geometry of the two different profile modes of PSR B0329+54. The polarization behavior and fitted parameters of the rotation-vector model remain constant between modes, and the emission height remains constant as well. These similarities lend support to a model of pair production in the surface plasma that would cause a change in the available electrons and therefore the differential emission intensity.

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arXiv2018

Classification of Subpulse Drifting in Pulsars

In this study we propose a classification scheme for the phenomenon of subpulse drifting in pulsars. We have assembled an exhaustive list of pulsars which exhibit subpulse drifting from previously published results as well as recent observations using the Giant Meterwave Radio Telescope. We have estimated detailed phase variations corresponding to the drifting features. Based on phase behaviour the drifting population was classified into four groups : coherent phase-modulated drifting, switching phase-modulated drifting, diffuse phase-modulated drifting and low-mixed phase-modulated drifting. We have re-established the previous assertion that the subpulse drifting is primarily associated with the conal components in pulsar profile. The core components generally do not show the drifting phenomenon. However, in core emission of certain pulsars longer periodic fluctuations are seen, which are similar to periodic nulling, and likely arise due to a different physical phenomenon. In general the nature of the phase variations of the drifting features across the pulsar profile appears to be associated with specific pulsar profile classes, but we also find several examples that show departures from this trend. It has also been claimed in previous works that the spin-down energy loss is anti-correlated with the drifting periodicity. We have verified this dependence using a larger sample of drifting measurements.

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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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arXiv2018

Subpulse Drifting, Nulling and Mode changing in PSR J1822$-$2256

We report a detailed observational study of the single pulses from the pulsar J1822$-$2256. The pulsar shows the presence of subpulse drifting, nulling as well as multiple emission modes. During these observations the pulsar existed primarily in two modes; mode A with prominent drift bands and mode B which was more disorderly without any clear subpulse drifting. A third mode C was also seen for a short duration with a different drifting periodicity compared to mode A. The nulls were present throughout the observations but were more frequent during the disorderly B mode. The nulling also exhibited periodicity with a clear peak in the fluctuation spectra. Before the transition from mode A to nulling the pulsar switched to a third drifting state with periodicity different from both mode A and C. The diversity seen in the single pulse behaviour of the pulsar J1822$-$2256 provides an unique window into the emission physics.

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arXiv2018

Characterising the nature of Subpulse Drifting in Pulsars

We report a detailed study of subpulse drifting in four long period pulsars. These pulsars were observed in the Meterwavelength Single-pulse Polarimetric Emission Survey and the presence of phase modulated subpulse drifting was reported in each case. We have carried out longer duration and more sensitive observations lasting 7000-12000 periods, between frequency range of 306 and 339 MHz. The drifting features were characterised in great detail including the phase variations across the pulse window. In two pulsars J0820$-$1350 and J1720$-$2933 the phases changed steadily across the pulse window. The pulsar J1034$-$3224 has five components. The leading component was very weak and was barely detectable in our observations. The four trailing components showed the presence of subpulse drifting. The phase variations changed in alternate components with a reversal in the sign of the gradient. This phenomenon is known as bi-drifting. The pulsar J1555$-$3134 showed the presence of two distinct peak frequencies of comparable strengths in the fluctuation spectrum. The two peaks did not appear to be harmonically related and were most likely a result of different physical processes. Additionally, the long observations enabled us to explore the temporal variations of the drifting features. The subpulse drifting was largely constant with time but small fluctuations around a mean value was seen.

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arXiv2018

Meterwavelength Single-pulse Polarimetric Emission Survey IV: The Period Dependence of Component Widths of Pulsars

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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arXiv2017

Toward an Empirical Theory of Pulsar Emission XII: Exploring the Physical Conditions in Millisecond Pulsar Emission Regions

The five-component profile of the 2.7-ms pulsar J0337+1715 appears to exhibit the best example to date of a core/double-cone emission-beam structure in a millisecond pulsar (MSP). Moreover, three other MSPs, the Binary Pulsar B1913+16, B1953+29 and J1022+1001, seem to exhibit core/single-cone profiles. These configurations are remarkable and important because it has not been clear whether MSPs and slow pulsars exhibit similar emission-beam configurations, given that they have considerably smaller magnetospheric sizes and magnetic field strengths. MSPs thus provide an extreme context for studying pulsar radio emission. Particle currents along the magnetic polar flux tube connect processes just above the polar cap through the radio-emission region to the light-cylinder and the external environment. In slow pulsars radio-emission heights are typically about 500 km around where the magnetic field is nearly dipolar, and estimates of the physical conditions there point to radiation below the plasma frequency and emission from charged solitons by the curvature process. We are able to estimate emission heights for the four MSPs and carry out a similar estimation of physical conditions in their much lower emission regions. We find strong evidence that MSPs also radiate by curvature emission from charged solitons.

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arXiv2017

The Nature of Coherent Radio Emission from Pulsars

The pulsar radio emission originates from regions below 10% of the light cylinder radius. This requires a mechanism where coherent emission is excited in relativistic pair plasma with frequency $ν_{cr}$ which is below the plasma frequency $ν_{\circ}$ i.e. $ν_{cr} < ν_{\circ}$. A possible model for the emission mechanism is charged bunches (charged solitons) moving relativistically along the curved open dipolar magnetic field lines capable of exciting coherent curvature radio emission. In this article we review the results from high quality observations in conjunction with theoretical models to unravel the nature of coherent curvature radio emission in pulsars.

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arXiv2017

Meterwavelength Single-pulse Polarimetric Emission Survey III: The Phenomenon of Nulling in Pulsars

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.

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arXiv2017

Periodic Longitude-Stationary Non-Drift Emission in Core-Single Radio Pulsar B1946+35

Radio pulsar PSR B1946+35 is a classical example of a core/cone triple pulsar where the observer's line-of-sight cuts the emission beam centrally. In this paper we perform a detailed single-pulse polarimetric analysis of B1946+35 using sensitive Arecibo archival and new observations at 1.4 and 4.6 GHz to re-establish the pulsar's classification wherein a pair of inner conal "outriders" surround a central core component. The new 1.4 GHz observation consisted of a long single pulse sequence of 6678 pulses, and its fluctuation spectral analysis revealed that the pulsar shows a time-varying amplitude modulation, where for a thousand periods or so the spectra have a broad low frequency "red" excess and then at intervals they suddenly exhibit highly periodic longitude-stationary modulation of both the core and conal components for several hundred periods. The fluctuations of the leading conal and the core components are in phase, while those in the trailing conal component in counterphase. These fluctuation properties are consistant with shorter pulse sequence analyses reported in an earlier study by Weltevrede et al. (2006, 2007) as well as in our shorter pulse sequence data sets. We argue that this dual modulation of core and conal emission cannot be understood by a model where subpulse modulation is associated with the plasma {\bf E}$\times${\bf B} drift phenomenon. Rather the effect appears to represent a kind of periodic emission-pattern change over timescales of $\sim$18 s (or 25 pulsar periods), which has not been reported previously for any other pulsar.

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arXiv2017

XMM-Newton observation of the nearby pulsar B1133+16

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. The X-ray radiation is dominated by nonthermal radiation and is well described by both a single power-law model (PL) and a sum of blackbody and power-law emission (BB+PL). The BB+PL model results in a spectral photon index $Γ=2.4^{+0.4}_{-0.3}$ and a nonthermal flux in the 0.2-3 keV energy range of $(7\pm 2) \times 10^{-15}\, {\rm erg \, cm}^{-2} \, {\rm s}^{-1}$. The thermal emission is consistent with the blackbody emission from a small hot spot with a radius of $R_{\rm pc} \approx 14^{+7}_{-5} \, {\rm m}$ and a temperature of $T_{\rm s} = 2.9^{+0.6}_{-0.4} \, {\rm MK}$. Assuming that the hot spot corresponds to the polar cap of the pulsar, we can use the magnetic flux conservation law to estimate the magnetic field at the surface $B_{\rm s} \approx 3.9 \times 10^{14} \, {\rm G}$. The observations are in good agreement with the predictions of the partially screened gap model, which assumes the existence of small-scale surface magnetic field structures in the polar cap region.

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arXiv2016

Meterwavelength Single-pulse Polarimetric Emission Survey

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 8.5 seconds were observed in the survey at two different frequencies, 105 profiles at 333 MHz, 118 profiles at 618 MHz and 100 pulsars at both. In this work we concentrate primarily on the time-averaged properties of the pulsar emission. The measured widths of the pulsar profiles in our sample usually exhibit the radius to frequency mapping. We validate the existence of lower bounds for the distribution of profile widths with pulsar period ($P$), which is seen for multiple definitions of the width, viz. a lower boundary line (LBL) at $2.7^{\circ} P^{-0.5}$ with width measured at 50% level of profile peak, a LBL at $5.7^{\circ} P^{-0.5}$ for 10% level of peak and LBL at $6.3^{\circ} P^{-0.5}$ for width defined as 5$σ$ above the baseline level. In addition we have measured the degree of linear polarization in the average profile of pulsars and confirmed their dependence on pulsar spindown energy loss ($\dot{E}$). The single pulse polarization data show interesting trends with the polarization position angle (PPA) distribution exhibiting the simple rotating vector model for high $\dot{E}$ pulsars while the PPA becomes more complex for medium and low $\dot{E}$ pulsars. The single pulse total intensity data is useful for studying a number of emission properties from pulsars like subpulse drifting, nulling and mode changing which is being explored in separate works.

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arXiv2016

Meterwavelength Single-pulse Polarimetric Emission Survey II: The phenomenon of Drifting Subpulses

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.

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arXiv2016

Core and Conal Component Analysis of Pulsar B1933+16 --- Investigation of the Segregated Modes

Radio pulsar B1933+16 is brightest core-radiation dominated pulsar in the Arecibo sky, and here we carry out a comprehensive high resolution polarimetric study of its radiation at both 1.5 and 4.6 GHz. At 1.5 GHz, the polarization is largely compatible with a rotating-vector model with $α$ and $β$ values of 125 and --1.2$^{\circ}$, such that the core and conal regions can be identified with the primary and secondary polarization modes and plausibly with the extraordinary and ordinary propagation modes. Polarization modal segregation of profiles shows that the core is comprised of two parts which we associate with later X-mode and earlier O-mode emission. Analysis of the broad microstructures under the core shows that they have similar timescales to those of the largely conal radiation of other pulsars studied earlier. Aberration/retardation analysis was here possible for both the conal and core radiation and showed average physical emission heights of about 200 km for each. Comparison with other core-cone pulsars suggests that the core and conal emission arises from similar heights. Assuming the inner vacuum gap model, we note that at these emission altitudes the frequency of the observed radiation $ν_{obs}$ is less than the plasma frequency $ν_p$. We then conclude that the radio emission properties are consistent with the theory of coherent curvature radiation by charged solitons where the condition $ν_{obs} < ν_{p}$ is satisfied. However, the differences that exist between core and conal emission in their geometric locations within a pulse, polarization and modulation properties are yet to be understood.

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arXiv2015

Coherent Radio Emission from Pulsars

We review a physical model where the high brightness temperature of 10$^{25}-10^{30}$ K observed in pulsar radio emission is explained by coherent curvature radiation excited in the relativistic electron-positron plasma in the pulsar magnetosphere.

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arXiv2015

Polarized quasiperiodic structures in pulsar radio emission reflect temporal modulations of non-stationary plasma flow

Bright single pulses of many radio pulsars show rapid intensity fluctuations (called microstructure) when observed with time resolutions of tens of microseconds. Here, we report an analysis of Arecibo 59.5 $μ$sec-resolution polarimetric observations of 11 P-band and 32 L-band pulsars with periods ranging from 150 msec to 3.7 sec. These higher frequency observations forms the most reliable basis for detailed microstructure studies. Close inspection of individual pulses reveals that most pulses exhibit quasiperiodicities with a well-defined periodicity timescale ($P_μ$). While we find some pulses with deeply modulating microstructure, most pulses show low-amplitude modulations on top of broad smooth subpulses features, thereby making it difficult to infer periodicities. We have developed a method for such low-amplitude fluctuations wherein a smooth subpulse envelope is subtracted from each de-noised subpulse; the fluctuating portion of each subpulse is then used to estimate $P_μ$ via autocorrelation analysis. We find that the microstructure timescale $P_μ$ is common across all Stokes parameters of polarized pulsar signals. Moreover, no clear signature of curvature radiation in vacuum in highly resolved microstructures was found. Our analysis further shows strong correlation between $P_μ$ and the pulsar period $P$. We discuss implications of this result in terms of a coherent radiation model wherein radio emission arises due to formation and acceleration of electron-positron pairs in an inner vacuum gap over magnetic polar cap, and a subpulse corresponds to a series of non-stationary sparking discharges. We argue that in this model, $P_μ$ reflects the temporal modulation of non-stationary plasma flow.

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arXiv2015

Scatter broadening measurements of 124 pulsars at 327 MHz

We present the measurements of scatter broadening time-scales ($τ_{sc}$) for 124 pulsars at 327 MHz, using the upgraded Ooty Radio Telescope (ORT). These pulsars lie in the dispersion measure range of 37 $-$ 503 pc cm$^{-3}$ and declination ($δ$) range of $-$57$^{\circ} < δ< 60^{\circ}$. New $τ_{sc}$ estimates for 58 pulsars are presented, increasing the sample of all such measurements by about 40% at 327 MHz. Using all available $τ_{sc}$ measurements in the literature, we investigate the dependence of $τ_{sc}$ on dispersion measure. Our measurements, together with previously reported values for $τ_{sc}$, affirm that the ionized interstellar medium upto 3 kpc is consistent with Kolmogorov spectrum, while it deviates significantly beyond this distance.

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arXiv2014

Toward an Empirical Theory of Pulsar Emission. X. On the Precursor and Postcursor Emission

Precursors and postcursors (PPCs) are rare emission components detected in a handful of pulsars that appear beyond the main pulse emission, in some cases far away from it. In this paper we attempt to characterize the PPC emission in relation to the pulsar main pulse geometry. In our analysis we find that PPC components have properties very different from that of outer conal emission. The separation of the PPC components from the main pulse center remains constant with frequency. In addition the beam opening angles corresponding to the separation of PPC components from the pulsar center are much larger than the largest encountered in conal emission. Pulsar radio emission is believed to originate within the magnetic polar flux tubes due to the growth of instabilities in the outflowing relativistic plasma. Observationally, there is strong evidence that the main pulse emission originates at altitudes of about 50 neutron star radii for a canonical pulsar. Currently, the most plausible radio emission model that can explain main pulse emission is the coherent curvature radiation mechanism, wherein relativistic charged solitons are formed in a non-stationary electron-positron-pair plasma. The wider beam opening angles of PPC require the emission to emanate from larger altitudes as compared to the main pulse, if both these components originate by the same emission mechanism. We explore this possibility and find that this emission mechanism is probably inapplicable at the height of the PPC emission. We propose that the PPC emission represents a new type of radiation from pulsars with a mechanism different from that of the main pulse.

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