arXiv subjects

Joanna M. Rankin

Publications and source records attributed to Joanna M. Rankin.

At least 19 recordsLinked to original sources

arXiv2025

Pulsar B1237+25 Aberration/Retardation Analysis from Decimeter to Decameter Wavelength: Challenge to "Radius-to-Frequency Mapping"

PSR B1237+25 is perhaps the canonical example of a pulsar with a core/double cone profile. Moreover, it is bright with little spectral turnover, and its profile perhaps uniquely remains undistorted by scattering far into the decametric band. Here we assemble more than a dozen of the highest quality profiles (30 MHz to 5 GHz) from half a dozen observatories, where possible polarimetric. The pulsar's 2.6$^{\circ}$ core component marks the magnetic axis longitude, and we confirm that this point coincides both with the linear polarization angle inflection point and the zero-crossing of its antisymmetric circular signature -- thus providing the possibility to estimate emission heights over a very broad band using aberration/retardation (A/R). We then carefully fit the profile components with Gaussians to identify and study the subtle asymmetries produced by A/R. We find a consistent A/R in the pulsar's profiles of some 0.5$^{\circ}$ longitude or 2 ms -- corresponding to a putative conal emission height of 200-400 km -- with a formal error of about 100 km. Our analysis finds no evidence whatsoever for an emission height increase with wavelength, the so-called ``radius-to-frequency mapping''. Nor do we find any significant difference in A/R effect between the outer and inner cones.

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arXiv2024

Determining the Magnetic Field in the Galactic Plane from New Arecibo Pulsar Faraday Rotation Measurements

We develop a new method for studying the Galactic magnetic field along the spiral arms using pulsar Faraday rotation measures (RMs). Our new technique accounts for the dot-product nature of Faraday rotation and also splits the associated path integral into segments corresponding to particular zones along the LOS. We apply this geometrically-corrected, arm-by-arm technique to the low-latitude portion of a recently published set of Arecibo Faraday RMs for 313 pulsars along with previously obtained RMs in the same regions. We find disparities $>1σ$ between the magnitude of the field above and below the plane in the Local Arm, Sagittarius Arm, Sagittarius-to-Scutum Interarm, the Scutum Arm, and Perseus Arm. We find evidence for a single field reversal near the Local/Sagittarius arm boundary. Interestingly, our results suggest that this field reversal is dependent on latitude, occurring inside the Sagittarius arm at negative Galactic latitudes and at the Local-to-Sagittarius Arm boundary at positive Galactic latitudes. We discuss all of our results in the context of different models and other observational Galactic magnetic field analyses.

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arXiv2022

Radio Pulsar Beam Geometry at Lower Frequencies: Bright Sources Outside the Arecibo Sky

We present pulsar emission beam analyses and models in an effort to examine pulsar geometry and physics at the lowest frequencies scattering permits. We consider two populations of well-studied pulsars that lie outside the Arecibo sky, the first drawing on the Jodrell Bank Gould & Lyne survey down to -35° declination and a second using Parkes surveys in the far south. These assemble the full sky population of 487 pulsars known before the late 1990s which conveniently all have "B" names. We make full use of the core/double-cone emission beam model to assess its efficacy at lower frequencies, and we outline how different pair plasma sources probably underlie its validity. The analysis shows that with a very few exceptions pulsar radio emission beams can be modeled quantitatively with two concentric conal beams and a core beam of regular angular dimensions at 1 GHz. Further, the beamforms at lower frequencies change progressively in size but not in configuration. Pulsar emission-beam properties divide strongly depending on whether the plasma excitation is central within the polar fluxtube producing a core beam or peripheral along the edges generating conal beams, and this seems largely determined by whether their spindown energy is greater or less than about 10$^{32.5}$ ergs/s. Core emission dominated pulsars tend concentrate closely along the Galactic plane and in the direction of the Galactic center; whereas conal pulsars are somewhat more uniformly distributed both in Galactic longitude and latitude. Core dominated pulsars also tend to be more distant and particularly so in the inner Galaxy region.

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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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arXiv2016

Quasi-Periodicities in the Anomalous Emission Events in Pulsars B1859+07 and B0919+06

A quasi-periodicity has been identified in the strange emission shifts in pulsar B1859+07 and possibly B0919+06. These events, first investigated by Rankin, Rodriguez & Wright in 2006, originally appeared disordered or random, but further mapping as well as Fourier analysis has revealed that they occur on a fairly regular basis of approximately 150 rotation periods in B1859+07 and perhaps some 700 in B0919+06. The events-which we now refer to as "swooshes"-are not the result of any known type of mode-changing, but rather we find that they are a uniquely different effect, produced by some mechanism other than any known pulse-modulation phenomenon. Given that we have yet to find another explanation for the swooshes, we have appealed to a last resort for periodicities in astrophysics: orbital dynamics in a binary system. Such putative "companions" would then have semi-major axes comparable to the light cylinder radius for both pulsars. However, in order to resist tidal disruption their densities must be at least some 10$^5$ grams/cm$^3$-therefore white-dwarf cores or something even denser might be indicated.

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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

Toward an Empirical Theory of Pulsar Emission XI. Understanding the Orientations of Pulsar Radiation and Supernova "Kicks"

Two entwined problems have remained unresolved since pulsars were discovered nearly 50 years ago: the orientation of their polarized emission relative to the emitting magnetic field and the direction of putative supernova ``kicks' relative to their rotation axes. The rotational orientation of most pulsars can be inferred only from the (``fiducial') polarization angle of their radiation, when their beam points directly at the Earth and the emitting polar fluxtube field is $\parallel$ to the rotation axis. Earlier studies have been unrevealing owing to the admixture of different types of radiation (core and conal, two polarization modes), producing both $\parallel$ or $\perp$ alignments. In this paper we analyze the some 50 pulsars having three characteristics: core radiation beams, reliable absolute polarimetry, and accurate proper motions. The ``fiducial' polarization angle of the core emission, we then find, is usually oriented $\perp$ to the proper-motion direction on the sky. As the primary core emission is polarized $\perp$ to the projected magnetic field in Vela and other pulsars where X-ray imaging reveals the orientation, this shows that the proper motions usually lie $\parallel$ to the rotation axes on the sky. Two key physical consequences then follow: first, to the extent that supernova ``kicks' are responsible for pulsar proper motions, they are mostly $\parallel$ to the rotation axis; and second that most pulsar radiation is heavily processed by the magnetospheric plasma such that the lowest altitude ``parent' core emission is polarized $\perp$ to the emitting field, propagating as the extraordinary (X) mode.

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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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arXiv2013

Drifting, moding, and nulling: another look at pulsar B1918+19

Arecibo observations of the conal triple pulsar B1918+19 at 0.327- and 1.4-GHz are used to analyse its subpulse behaviour in detail. We confirm the presence of three distinct drift modes (A,B,C) plus a disordered mode (N) and show that they follow one another in specific cycles. Interpreting the pulsar's profile as resulting from a sightline traverse which cuts across an outer cone and tangentially grazes an inner cone, we demonstrate that the phase modulation of the inner cone is locked to the amplitude modulation of the outer cone in all the drift modes. The 9% nulls are found to be largely confined to the dominant B and N modes, and, in the N mode, create alternating bunches of nulls and emission in a quasi-periodic manner with an averaged fluctuation rate of about 12 rotation periods ($P_1$). We explore the assumption that the apparent drift is the first alias of a faster drift of subbeams equally spaced around the cones. This is shown to imply that all modes A, B and C have a common circulation time of 12 $P_1$ and differ only in the number of subbeams. This timescale is on the same order as predicted by the classic {\bf E}$\times${\bf B} drift and also coincides with the N-mode modulation. We therefore arrive at a picture where the circulation speed remains roughly invariant while the subbeams progressively diminish in number from modes A to B to C, and are then re-established during the N mode. We suggest that aliasing combined with subbeam loss may be responsible for apparently dramatic changes in drift rates in other pulsars.

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arXiv2010

On the long and short nulls, modes and interpulse emission of radio pulsar B1944+17

We present a single pulse study of pulsar B1944+17, whose non-random nulls dominate nearly 70% of its pulses and usually occur at mode boundaries. When not in the null state, this pulsar displays four bright modes of emission, three of which exhibit drifting subpulses. B1944+17 displays a weak interpulse whose position relative to the main pulse we find to be frequency independent. Its emission is nearly 100% polarized, its polarization-angle traverse is very shallow and opposite in direction to that of the main pulse, and it nulls approximately two-thirds of the time. Geometric modeling indicates that this pulsar is a nearly aligned rotator whose alpha value is hardly 2 degrees--i.e., its magnetic axis is so closely aligned with its rotation axis that its sightline orbit remains within its conal beam. The star's nulls appear to be of two distinct types: those with lengths less than about 8 rotation periods appear to be pseudonulls--that is, produced by "empty" sightline traverses through the conal beam system; whereas the longer nulls appear to represent actual cessations of the pulsar's emission engine.

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arXiv2009

The beam topology and dynamic emission properties of pulsar B0943+10 -- VI. Discovery of a 'Q'-mode precursor and comparison with pulsar B1822-09

This paper reports new observations of pulsars B0943+10 and B1822--09 carried out with the Arecibo Observatory (AO) and the Giant Metrewave Radio Telescope (GMRT), respectively. Both stars exhibit two stable emission modes. We report the discovery in B0943+10 of a highly linearly polarized precursor component that occurs primarily in only one mode. This emission feature closely resembles B1822-09's precursor which also occurs brightly in only one mode. B0943+10's other mode is well known for its highly regular drifting subpulses that are apparently produced by a rotating carousel system of 20 beamlets. Similary, B1822-09 exhibits subpulse-modulation behavior only in the mode where its precursor is absent. We survey our 18 hours of B0943+10 observations and find that the sideband-modulation features, from which the carousel-rotation time can be directly determined, occur rarely--less than 5% of the time--but always indicating 20 beamlets. We present an analysis of B1822-09's modal modulation characteristics at 325-MHz and compare them in detail with B0943+10. The pulsar never seems to null, and we find a 43-rotation-period feature in the star's Q mode that modulates the interpulse as well as the conal features in the main pulse. We conclude that B1822-09 must have a nearly orthogonal geometry and that its carousel circulation time is long compared to the modal sub-sequences available in our observations, and the mainpulse/interpulse separation is almost exactly 180 degrees. We conclude the precursors for both stars are incompatible with core-cone emission. We assess the interesting suggestion by Dyks et al. that downward-going radiation produces B1822-09's precursor emission.

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arXiv2008

Arecibo Multi-frequency Time-Aligned Pulsar Average-Profile and Polarization Database

We present Arecibo time-aligned, total intensity profiles for 46 pulsars over an unusually wide range of radio frequencies and multi-frequency, polarization-angle density diagrams and/or polarization profiles for 58 pulsars pulsars at some or all of the frequencies 50, 111/130, 430 and 1400 MHz. The frequency-dependent dispersion delay has been removed In order to align the profiles for study of their spectral evolution and wherever possible the profiles of each pulsar are displayed on the same longitude scale. Most of the pulsars within Arecibo's declination range that are sufficiently bright for such spectral or single pulse analysis are included in this survey--and the calibrated pulse sequences are available by web download for further study.

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arXiv2008

The `Periodic Nulls' of Radio Pulsar J1819+1305

We present a single-pulse study of the four-component pulsar J1819+1305, whose ``null'' pulses bunch at periodic intervals of around 57 times the rotation period. The emission bursts between the null bunches exhibit characteristic modulations at two shorter periodicities of approximately 6.2 and 3 times the rotation period, the former found largely in the two outer components, and the latter only in the first component. Many bursts commence with bright emission in second component, exhibit positive six-period drift across the full profile width, and end with 3-period modulation in the leading component. The 57-period cycle can be modelled geometrically as a sparsely filled subbeam carousel with nulls appearing whenever our line of sight intersects a circulating empty region. This interpretation is compatible with other recent evidence for periodic, carousel-related nulling and appears to support the physics of a polar-gap emission model for ``drifting'' subpulses, but the subtle structure of the emission bursts defies an easy explanation.

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arXiv2007

Absolute Broadband Polarization Behaviour of PSR B0329+54: A Glimpse of the Core Emission Process

In this paper we report multifrequency single pulse polarization observations of the PSR B0329+54 normal mode using the Giant Meterwave Radio Telescope at 325 and 610 MHz and the Effelsberg Observatory at 2695 MHz. Our observations show that towards the central part of the polarization position-angle traverse there is an unusual ``arc''-like structure, which is comprised of a broadband ``kink'' and a frequency-dependent ``spot.'' The features are intimately connected with the intensity dependence of the core component: the stronger emission arrives earlier and its linear polarization is displaced farther along the ``kink''. Moreover, at high intensities, the circular polarization is --/+ antisymmetric; the nearly complete positive circular is characteristic of the weaker, later core subpulses. We find that the ``kink'' emission is associated with the extraordinary (X) propagation mode, and hence propagation effects do not appear capable of producing the core component's broadband, intensity-dependent emission. Rather, the overall evidence points to a largely geometric interpretation in which the ``kink'' provides a rare glimpse of the accelerating cascade or height-dependent amplifier responsible for the core radiation.

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arXiv2007

A model for double notches and bifurcated components in radio profiles of pulsars and magnetars - Evidence for the parallel acceleration maser in pulsar magnetosphere

Averaged pulse profiles of three nearby pulsars: B1929+10, J0437-4715 and B0950+08 exhibit unusual `double notches'. These W-like looking features consist of two adjacent V-shaped dips that approach each other at increasing observation frequency nuobs roughly at a rate sep \propto nuobs^{-1/2}, where sep is the separation between the notches' minima. We show that basic properties of the notches, namely their W-like look and the rate of their converging can be understood within a narrow class of models of coherent radio emission from pulsars: the free electron maser models based on coherent inverse Compton scattering of parallel oscillations of ambient electric field. The observed properties of the pulsars imply that the Fourier spectrum of the wiggler-like oscillations is narrow and that the broad-band character of the radio emission reflects the width of the electron energy distribution. Such a model provides a natural explanation for the frequency-independent separation between the main pulse and interpulse of B0950+08 as well as for the lack of radius to frequency mapping in the conal-like emission of J0437-4715. The frequency behaviour of the main pulse in the profile of the first radio magnetar XTE J1810-197 can also be explained within this model.

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arXiv2006

The Topology and Polarisation of Subbeams Associated with the `Drifting' Subpulse Emission of Pulsar B0943+10 -- IV. Q-to-B-Mode Recovery Dynamics

Pulsar B0943+10 is well known for its `B' (burst) mode, characterized by accurately drifting subpulses, in contrast to its chaotic `Q' (quiet) mode. Six new Arecibo observations at 327 MHz with durations of 2+ hours each have shed considerable light on the modal dynamics of this pulsar. Of these, three were found to be exclusively `B' mode, and three were discovered to exhibit transitions from the `Q' to the `B' mode. One of these observations has permitted us to determine the circulation time of the subbeam carousel in the `Q' mode for the first time, at some 36.4$\pm$0.9 stellar rotation periods. The onset of the `B' mode is then observed to commence similarly in all three observations. The initial circulation time is about 36 periods and relaxes to nearly 38 periods in a roughly exponential fashion with a characteristic time of some 1.2 hours. This is the longest characteristic time ever found in a mode-switching pulsar. Moreover, just after the `B'-mode onset the pulsar exhibits a symmetrical resolved-double profile form with a somewhat stronger trailing component, but this second component slowly dies away leaving the usual single `B'-mode profile with the longitude of the magnetic axis falling at about its trailing half power point. Thus it would appear that Q-to-B- and B-to-Q-transitions have different characteristic times. Some speculations are given on the nature of this slow modal alternation.

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arXiv2006

Bistable Profile Illumination in Pulsars B0919+06 & B1859+07

A new single pulse behaviour has been identified in two pulsars, B0919+06 and B1859+07. Normally both starts emit bright subpulses in a region near the trailing edge of heir profile. However, occasionally these stars exhibit "events" wherein the emission longitude gradually decreases by about their profile width, remains in this position for typically tens of pulses and then gradually returns over a few pulses to the usual longitude. The effect bears some resemblance to a profile "mode change", but here the effect is gradual and episodic. When the separated profiles of the normal and "event" emission, they reveal a broad and complex profile structure in each pulsar - but one which can probably be understood correctly in terms of a single conical beam. Possibly the effect entails an extreme example of "absorption"-induced profile asymmetry, as suspected in other pulsars. Alternatively, shifting sources of illumination within the pulsar beam may be responsible.

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arXiv2005

Core and Conal Component Analysis of Pulsar B1237+25

The paper provides a new analysis of this famous five-component ({\bf M}) pulsar. In addition to the star's core-active ``abnormal'' mode, we find two distinct behaviors within its ``normal'' mode, a ``quiet-normal'' mode with regular 2.8-period subpulse modulation and little or no core activity, and a ``flare-normal'' mode, where the core is regularly bright and a nearly 4-period modulation is maintained. The ``flare-normal'' mode appears to be an intermediate state between the ``quiet normal'' and ``abnormal'' behaviors. Short 5--15-pulse ``flare-normal''-mode ``bursts'' and ``quiet normal'' intervals alternate with each other quasi-periodically, making a cycle some 60--80 pulses in duration. ``Abnormal''-mode intervals are interspersed within this overall cycle, usually persisting for only a few pulses, but occasionally lasting for scores or even many hundreds of pulses. Within subsequences where the core is exceptionally quiet, the pulsar provides a nearly ``textbook'' example of a central PA sightline traverse--with a PA rate measured to be at least some 180$°/°$. On this basis it is shown that the sightline impact angle $β$ must be about 0.25\degr or some 5% of the outer conal beam radius. The star's core component is found to be incomplete, despite the fact that the core's full antisymmetric circularly polarized signature is present. We find that the star's PA traverse is disrupted through the action of orthogonally polarized linear power in the longitude range of the core component. This behavior is modeled to show its effect. Finally, the star provides two well defined fiducial points from which emission-height estimates can be computed, the respective centers of both the linear PA traverse and the zero-crossing point of the circular polarization signature.

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