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Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431

Doroshenko, V; Poutanen, J; Heyl, J; Tsygankov, SS; Caiazzo, I; Turolla, R; Veledina, A; ... Swartz, DA; + view all (2023) Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431. Astronomy & Astrophysics , 677 , Article A57. 10.1051/0004-6361/202347088. Green open access

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Abstract

We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January- February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar s geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind.

Type: Article
Title: Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431
Open access status: An open access version is available from UCL Discovery
DOI: 10.1051/0004-6361/202347088
Publisher version: https://doi.org/10.1051/0004-6361/202347088
Language: English
Additional information: © The Authors 2023. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0).
Keywords: accretion, accretion disks, magnetic fields, pulsars: individual: RX J0440.9+4431, stars: neutron, X-rays: binaries
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/10177597
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