ABSTRACT Two entwined problems have remained unresolved since pulsars were discovered nearly 50 yr 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 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 or alignments. In this paper we analyze some 50 pulsars having three characteristics: core radiation beams, reliable absolute polarimetry, and accurate proper motions (PMs). The "fiducial" polarization angle of the core emission, we then find, is usually oriented to the PM direction on the sky. The primary core emission is polarized to the projected magnetic field in Vela and other pulsars where X-ray imaging reveals the orientation. This shows that the PMs usually lie to the rotation axes on the sky. Two key physical consequences then follow: first, to the extent that supernova "kicks" are responsible for pulsar PMs, they are mostly to the rotation axis; and, second, most pulsar radiation is heavily processed by the magnetospheric plasma such that the lowest altitude "parent" core emission is polarized to the emitting field, propagating as the extraordinary (X) mode.
ABSTRACT 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 s resolution polarimetric observations of 11 P-band and 32 L-band pulsars with periods ranging from 150 ms to 3.7 s. These higher-frequency observations form the most reliable basis for detailed microstructure studies. Close inspection of individual pulses reveals that most pulses exhibit quasi-periodicities with a well-defined periodicity timescale ( P ). While we find some pulses with deeply modulating microstructure, most pulses show low-amplitude modulations on top of broad smooth subpulse 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 via autocorrelation analysis. We find that the microstructure timescale P 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 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 reflects the temporal modulation of non-stationary plasma flow.
Precursors and postcursors (PPCs) are rare emission components, which appear beyond the main pulse emission, in some cases far away from it, and are detected in a handful of pulsar. 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.
ABSTRACT
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.
ABSTRACT
We report a study of the transitional intervals between pulsar B0943+10’s two primary Q and B emission modes using Arecibo 327-MHz observations. The goal of this study was to detect signs of ...a ‘transitional’ mode at 327 MHz, discovered recently at lower frequencies. We have found subpulse drift and profile form patterns at 327 MHz similar to those identified at lower frequencies in the Q-to-B mode transition process. Pulse fading during about 15 stellar rotations preceding the appearance of subpulse drift was observed as well. Another part of the work is devoted to a detailed study of the pulse polarization variations in the main modes. A complex behaviour of the linear polarization percentage (LPP) of the dominant first component of the average profile with B-mode age has been found: during the first 4 h, the LPP continuously increases from 5 to 40 per cent, and over the next 1.5 h gradually decreases down to 30 per cent until the subsequent onset of the Q mode. In contrast, the LPP of the second component does not change over the B-mode lifetime, remaining at the level of 22 per cent. A non-instantaneous decrease in the LPP was detected at Q-mode onset. No systematic change of the LPP of the averaged Q-mode pulses over several hours of age was found. The results are discussed within the framework of the core–cone beam model and orthogonal polarization modes.
The recently discovered 100X weaker quiescent (Q) mode in pulsar B0823+26 is X-ray quiet, unlike its usual bright (B) mode. Arecibo polarimetric observations were conducted to confirm the pulsar's ...orthogonal geometry and investigate the emission associated with its main pulse (MP), interpulse (IP), and postcursor (PC) components. Main results: (1) the pulsar's MP, PC, and IP are present in both modes and exhibit a two-pole orthogonal geometry. (2) The B-mode MP is dominated by core emission with weak conal outriders, whereas, the Q-mode double profile shows mainly residual conal emission with little core. The IP is conal in both modes. (3) Sporadic intrapulse emission trailing the PC is detected in the Q mode. (4) B0823+26 falls close to an boundary of 1032.5 erg s−1 (or B12/P2 ∼ 2.5) between core- and conal-dominated profiles-which also represents a boundary between pair-plasma source configurations above the polar cap. For larger energies, the pair-formation front is central, flat, and generates backflow heating, whereas for smaller energies it is peripheral, lower, and produces little heating. (5) Apparently, the pulsar is able to assume both core- and conal-dominated "states" corresponding to its bright and weak modes. These circumstances appear to explain B0823+26's B-mode X-ray bright/core-dominated radio emission or Q-mode X-ray faint/conal radio emission-and why the IP is X-ray quiet in both modes. (6) These same considerations applied to B0943+10 may explain why its brighter radio mode was conal and X-ray quiet, while the weaker one was X-ray bright-because its peripheral sightline would miss most core radiation.
Radio pulsar B2034
+
19 exhibits all three ‘canonical’ pulse-sequence phenomena—that is, pulse nulling, two distinct profile modes and subpulses with periodic modulation. Indeed, the bursts and nulls ...in the pulsar are short at several score pulses and quasi-periodic such that about 1/3 of the pulses are nulls. The pulsar’s two modes have very different characteristics, the first shows emission almost completely confined to the leading half of the profile and highly modulated in a 2-period odd–even manner; whereas the second mode illuminates both the leading and trailing parts of the star’s profile about equally with the appearance of drift bands at about a 3-period separation. The second mode occurs much less frequently than the first (about 15% of the time) and thus the leading part of the profile has a much larger average intensity than the trailing part. B2034
+
19 represents an interesting example of a pulsar with emission primarily in the leading part of its profile window with only occasional illumination in the trailing part. This suggests that there are pulsars that perhaps never emit in a part of their profile window, connecting with earlier work on pulsars with apparent ‘partial cone’ profiles.
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 semimajor 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 super( 5) g cm super( -3) -- therefore, white-dwarf cores or something even denser might be indicated.
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 per cent 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-order alias of a faster drift of subbeams equally spaced around the cones. This is shown to imply that the drift modes A, B and C have a common circulation time of 12P
1 and differ only in the number of subbeams. This time-scale is on the same order as predicted by the classic
E
×
B
drift model of Ruderman & Sutherland 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 the apparently dramatic changes in drift rates in other pulsars.
Lyne & Manchester identified a group of some 50 pulsars they called 'partial cones' which they found difficult to classify and interpret. They were notable for their asymmetric average profiles and ...asymmetric polarization position angle (PPA) traverses, wherein the steepest gradient (SG) point fell toward one edge of the total intensity profile. Over the last two decades, this population of pulsars has raised cautions regarding the core/cone model of the radio pulsar emission beam which implies a high degree of order, symmetry, and geometric regularity. In this paper, we reinvestigate this population 'partial cone' pulsars on the basis of new single pulse polarimetric observations of 39 of them, observed with the Giant Meterwave Radio Telescope in India and the Arecibo Observatory in Puerto Rico. These highly sensitive observations help us to establish that most of these 'partial cones' exhibit a core/cone structure just as did the 'normal' pulsars studied in the earlier papers of this series. In short, we find that many of these 'partial cones' are partial in the sense that the emission above different areas of their polar caps can be (highly) asymmetric. However, when studied closely we find that their emission geometries are overall identical to a core/double cone structure encountered earlier--that is, with specific conal dimensions scaling as the polar cap size. Further, the 'partial cone' population includes a number of stars with conal single profiles that are asymmetric at meter wavelengths for unknown reasons (e.g., like those of B0809+74 or B0943+10). We find that aberration-retardation appears to play a role in distorting the core/cone emission-beam structure in rapidly rotating pulsars. We also find several additional examples of highly polarized pre- and postcursor features that do not appear to be generated at low altitude but rather at high altitude, far from the usual polar flux tube emission sites of the core and conal radiation.
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