The nature of pulsar radio emission

📝 Abstract

High-quality averaged radio profiles of some pulsars exhibit double, highly symmetric features both in emission and absorption. It is shown that both types of features are produced by a split-fan beam of extraordinary-mode curvature radiation (CR) that is emitted/absorbed by radially-extended streams of magnetospheric plasma. With no emissivity in the plane of the stream, such a beam produces bifurcated emission components (BFCs) when our line of sight passes through the plane. A distinct example of double component created in that way is present in averaged profile of the 5 ms pulsar J1012+5307. We show that the component can indeed be very well fitted by the textbook formula for the non-coherent beam of curvature radiation in the polarisation state that is orthogonal to the plane of electron trajectory. The observed width of the BFC decreases with increasing frequency at the rate that confirms the curvature origin. Likewise, the double absorption features (double notches) are produced by the same beam of the extraordinary-mode CR, when it is eclipsed by thin plasma streams. The intrinsic property of CR to create bifurcated fan beams explains the double features in terms of very natural geometry and implies the curvature origin of pulsar radio emission. (abbreviated)

💡 Analysis

High-quality averaged radio profiles of some pulsars exhibit double, highly symmetric features both in emission and absorption. It is shown that both types of features are produced by a split-fan beam of extraordinary-mode curvature radiation (CR) that is emitted/absorbed by radially-extended streams of magnetospheric plasma. With no emissivity in the plane of the stream, such a beam produces bifurcated emission components (BFCs) when our line of sight passes through the plane. A distinct example of double component created in that way is present in averaged profile of the 5 ms pulsar J1012+5307. We show that the component can indeed be very well fitted by the textbook formula for the non-coherent beam of curvature radiation in the polarisation state that is orthogonal to the plane of electron trajectory. The observed width of the BFC decreases with increasing frequency at the rate that confirms the curvature origin. Likewise, the double absorption features (double notches) are produced by the same beam of the extraordinary-mode CR, when it is eclipsed by thin plasma streams. The intrinsic property of CR to create bifurcated fan beams explains the double features in terms of very natural geometry and implies the curvature origin of pulsar radio emission. (abbreviated)

📄 Content

arXiv:0908.1359v2 [astro-ph.SR] 23 Nov 2009 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 30 October 2018 (MN LATEX style file v2.2) The nature of pulsar radio emission J. Dyks1, B. Rudak1, and P. Demorest2 1Nicolaus Copernicus Astronomical Center, Toru´n, Poland 2Department of Astronomy, University of California, Berkeley, CA 94720-3411 Accepted …. Received 2009 August 10; in original form 2009 July 14 ABSTRACT High-quality averaged radio profiles of some pulsars exhibit double, highly symmetric features both in emission and absorption. It is shown that both types of features are produced by a split-fan beam of extraordinary-mode curvature radiation (CR) that is emitted/absorbed by radially-extended streams of magnetospheric plasma. With no emissivity in the plane of the stream, such a beam produces bifurcated emission components (BFCs) when our line of sight passes through the plane. A distinct ex- ample of double component created in that way is present in averaged profile of a 5 ms pulsar J1012+5307. We show that the component can indeed be very well fitted by the textbook formula for the non-coherent beam of curvature radiation in the po- larisation state that is orthogonal to the plane of electron trajectory. The observed width of the BFC decreases with increasing frequency at the rate that confirms the curvature origin. Likewise, the double absorption features (double notches) are pro- duced by the same beam of the extraordinary-mode CR, when it is eclipsed by thin plasma streams. The intrinsic property of CR to create bifurcated fan beams explains the double features in terms of very natural geometry and implies the curvature origin of pulsar radio emission. Similarly, “double conal” profiles of class D are due to a cut through a wider stream with finite extent in magnetic azimuth. Therefore, their width reacts very slowly to changes of viewing geometry due to the geodetic precession. The stream-cut interpretation implies highly nonorthodox origin of both the famous S-swing of polarisation angle, and the low-frequency pulse broadening in D profiles. Azimuthal structure of polarisation modes in the CR beam allows us to understand the polarised ‘multiple imaging’ and the edge depolarisation of pulsar profiles. Key words: pulsars: general – pulsars: individual: J1012+5307 – J0437-4715 – B0525+21 – B1913+16 – Radiation mechanisms: non-thermal. 1 INTRODUCTION Double ‘absorption’ features in radio-pulse profiles were first identified in radio data independently by Rankin & Rath- nasree (1997; B1929+10) and Navarro, Manchester, Sandhu, et al. (1997; J0437−4715). McLaughlin & Rankin (2004) dis- covered the double notches in the leading wing of the main pulse of B0950+08. Navarro et al. have noticed that the feature must be ‘intrinsic to the emission mechanism’ because it becomes narrower at larger observation frequency νobs. However, the initial interpretive efforts of theorists did not follow that sug- gestion. Wright (2004) interpreted the features in terms of altitude-dependent special-relativistic effects, but was forced to assume unlikely emission geometry and postulated an opaque absorber of unknown origin that corotates at/near the light-cylinder. This work, however, pioneered the im- portant idea that a single entity must be responsible for both notches. Dyks, Fr¸ackowiak, S lowikowska, et al. (2005) considered the neutron star itself (embedded in an opaque plasma cloud) as the absorber/eclipser and their emission region neatly followed the geometry of magnetic field lines. However, to make this far more natural geometry really work, the radio emission had to be directed inward, toward the neutron star. The model has achieved some agreement with the data, but the symmetry of double notches, as well as their frequency evolution remained unsolved. Considerable progress in the ability to interpret the data was done by Dyks, Rudak & Rankin (2007; hereafter DRR), who interpreted the notches as a direct imprint of the hollow beam intrinsic to the radiation emitted by elec- trons accelerated parallel to their velocity. This work was the first to interpret the double features in terms of micro- physical beam intrinsic to a specific radiation mechanism. The model managed to ensure symmetric and double shape of the notches whenever they were not washed out by spa- tial extent of the emitter/absorber. The separation ∆n of the notches (as observed at that time) was consistent with c⃝0000 RAS 2 J. Dyks, B. Rudak, and P. Demorest Figure 1. Pulse profiles of PSR J0437−4715 at three frequencies. The double features at φ ≃51◦and 70◦make it possible to align the profiles in absolute way. Note the lack of “radius-to- frequency” mapping. Zero levels and flux units are arbitrary and different for each profile. Data courtesy: R. Manchester. the inverse-Compton-like version of the parallel acceleration maser: νobs ∼γ2νpl ∝∆−2 n . Since the mechanism is intrin- sically broad-band, one could naturally understood the lack of radius-to

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