Rotating Vector Model predictions in millisecond pulsar magnetospheres with dipole and quadrupole field components
Abstract
The rotating vector model (RVM) is widely used to interpret pulsar radio polarization under the assumption of a dipolar magnetic field. Motivated by tentative evidence for non-dipolar magnetic structure in millisecond pulsars provided by recent modeling of surface thermal X-ray pulsations, we examine how a global quadrupole component modifies position-angle (PA) behavior within the classical RVM framework. We numerically integrate magnetic field lines in static vacuum magnetospheres composed of a centered dipole component that is inclined with respect to the spin axis, and a centered quadrupole component aligned with it. We then compute the open field line regions (OFLRs), emission skymaps, and PA curves for a range of dipole inclinations, relative quadrupole strengths, and light cylinder radii corresponding to two trial spin periods of 1\,ms and 5\,ms. We find that quadrupole-modified magnetospheres produce a wide variety of PA behaviors and substantially alter the morphology and total area of OFLRs, with the OFLR area varying non-monotonically with quadrupole strength and, in some cases, exceeding that of the corresponding pure dipole. Depending sensitively on the field configuration and viewing geometry, three broad PA regimes can be distinguished: a) nearly dipolar PA swings that are largely insensitive to emission altitude; b) moderately distorted, altitude-dependent PA tracks that can bias dipolar RVM fits; and c) strongly dispersed PA behavior associated with emission from different OFLRs, which can remain incompatible with a single dipolar PA track even within narrow emission altitude ranges. These results demonstrate that dipole-like PA curves do not uniquely imply a purely dipolar magnetic field, while global quadrupole components alone are insufficient to reproduce the full complexity of observed millisecond pulsar PA profiles.
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BibTeX
@misc{bilous2026rotating,
title = {{Rotating Vector Model predictions in millisecond pulsar magnetospheres with dipole and quadrupole field components}},
author = {Anna V. Bilous},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.15779},
eprint = {2610.15779},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.15779},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Rotating Vector Model predictions in millisecond pulsar magnetospheres with dipole and quadrupole field components
AU - Anna V. Bilous
PY - 2026
DA - 2026/10/11
PB - arXiv
DO - 10.48550/arxiv.2610.15779
UR - https://arxiv.org/abs/2610.15779
AB - The rotating vector model (RVM) is widely used to interpret pulsar radio polarization under the assumption of a dipolar magnetic field. Motivated by tentative evidence for non-dipolar magnetic structure in millisecond pulsars provided by recent modeling of surface thermal X-ray pulsations, we examine how a global quadrupole component modifies position-angle (PA) behavior within the classical RVM framework. We numerically integrate magnetic field lines in static vacuum magnetospheres composed of a centered dipole component that is inclined with respect to the spin axis, and a centered quadrupole component aligned with it. We then compute the open field line regions (OFLRs), emission skymaps, and PA curves for a range of dipole inclinations, relative quadrupole strengths, and light cylinder radii corresponding to two trial spin periods of 1\,ms and 5\,ms. We find that quadrupole-modified magnetospheres produce a wide variety of PA behaviors and substantially alter the morphology and total area of OFLRs, with the OFLR area varying non-monotonically with quadrupole strength and, in some cases, exceeding that of the corresponding pure dipole. Depending sensitively on the field configuration and viewing geometry, three broad PA regimes can be distinguished: a) nearly dipolar PA swings that are largely insensitive to emission altitude; b) moderately distorted, altitude-dependent PA tracks that can bias dipolar RVM fits; and c) strongly dispersed PA behavior associated with emission from different OFLRs, which can remain incompatible with a single dipolar PA track even within narrow emission altitude ranges. These results demonstrate that dipole-like PA curves do not uniquely imply a purely dipolar magnetic field, while global quadrupole components alone are insufficient to reproduce the full complexity of observed millisecond pulsar PA profiles.
ER -