skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. X. ON THE PRECURSOR AND POSTCURSOR EMISSION

Abstract

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 altitudesmore » 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.« less

Authors:
;  [1];  [2]
  1. National Centre for Radio Astrophysics, P.O. Bag 3, Pune University Campus, Pune 411 007 (India)
  2. Department of Physics, University of Vermont, Burlington, VT 05401 (United States)
Publication Date:
OSTI Identifier:
22364628
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 798; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; ELECTRONS; MAGNETIC FLUX; NEUTRON STARS; PHOTON EMISSION; POSITRONS; PRECURSOR; PULSARS; PULSES; RELATIVISTIC PLASMA; RELATIVISTIC RANGE; SOLITONS

Citation Formats

Basu, Rahul, Mitra, Dipanjan, and Rankin, Joanna M., E-mail: rahul@astro.ia.uz.zgora.pl. TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. X. ON THE PRECURSOR AND POSTCURSOR EMISSION. United States: N. p., 2015. Web. doi:10.1088/0004-637X/798/2/105.
Basu, Rahul, Mitra, Dipanjan, & Rankin, Joanna M., E-mail: rahul@astro.ia.uz.zgora.pl. TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. X. ON THE PRECURSOR AND POSTCURSOR EMISSION. United States. doi:10.1088/0004-637X/798/2/105.
Basu, Rahul, Mitra, Dipanjan, and Rankin, Joanna M., E-mail: rahul@astro.ia.uz.zgora.pl. Sat . "TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. X. ON THE PRECURSOR AND POSTCURSOR EMISSION". United States. doi:10.1088/0004-637X/798/2/105.
@article{osti_22364628,
title = {TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. X. ON THE PRECURSOR AND POSTCURSOR EMISSION},
author = {Basu, Rahul and Mitra, Dipanjan and Rankin, Joanna M., E-mail: rahul@astro.ia.uz.zgora.pl},
abstractNote = {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.},
doi = {10.1088/0004-637X/798/2/105},
journal = {Astrophysical Journal},
number = 2,
volume = 798,
place = {United States},
year = {Sat Jan 10 00:00:00 EST 2015},
month = {Sat Jan 10 00:00:00 EST 2015}
}
  • 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 pulsars 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 themore » 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.« less
  • The circular polarization associated with pulsar emission is examined phenomenologically. Virtually all circular polarization is observed in core components - that is, in core-single profiles and in the central components of triple and five-component profiles. Two extreme types of circular signature are identified in the observations: (1) an antisymmetric type wherein the circular polarization changes sense in midpulse, and (2) a symmetric type wherein it is predominantly of one sense. It is found that circular polarization of the antisymmetric type is strongly correlated with the sense of rotation of the linear position angle. Transitions from positive (LH) to negative (RH)more » are found to accompany negative (clockwise) rotations of the position angle and vice versa. The correlation requires that the antisymmetric circular polarization is also a purely geometric property of the emission process. Curvature radiation will have significant net circular polarization if there are gradients in the emissivity over angular scales comparable with the emission cone of a single charge. Furthermore, no net circular polarization is produced if the emissivity is circularly symmetric about the magnetic dipole axis. 37 refs.« less
  • 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 ∥ ormore » ⊥ 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.« less
  • The core-component widths of pulsars with interpulses are studied in an effort to define the geometric properties of the core emission region. The results are then applied to a large population of core-single, triple, and five-component pulsars which all have core components. Core-component widths are intimately related to the polar-cap geometry at the stellar surface. A simple mathematical expression, established through the study of two-pole interpulsars, indicates that the core-component widths depend only upon the pulsar period and alpha, the angle between the rotation and magnetic axes of the star. The relationship can then be used to estimate alpha inmore » any pulsar with a core component. Values of alpha are estimated for about 110 core single (St), triple (T), and five-component (M) pulsars. These results have important implications for the nature of the core radiation process. Core emission appears to come essentially from the stellar surface, filling the entire polar-cap 'gap' region where particle acceleration is thought to take place. 70 refs.« less
  • Lyne and 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 newmore » 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.« less