Abstract
Pulsars are rotating neutron stars emitting lighthouse-like beams. Owing to their unique properties, pulsars are a unique astrophysical tool to test general relativity, inform on matter in extreme conditions, and probe galactic magnetic fields. Understanding pulsar physics and emission mechanisms is critical to these applications. Here we show that mechanical-optical rotation in the pulsar magnetosphere affects polarisation in a way which is indiscernible from Faraday rotation in the interstellar medium for typical GHz observations frequency, but which can be distinguished in the sub-GHz band. Besides being essential to correct for possible systematic errors in interstellar magnetic field estimates, this result offers a unique means to determine the rotation direction of pulsars, providing additional constraints on magnetospheric physics. With the ongoing development of sub-GHz observation capabilities, our finding promises discoveries, such as the spatial distribution of pulsars rotation directions, which could exhibit potentially interesting, but presently invisible, correlations or features.
Highlights
Pulsars are rotating neutron stars emitting lighthouse-like beams
Pulsars’ highly polarised emission and compactness make them unmatched sources to probe the magnetic fields through Faraday rotation[9], and pulsars have been instrumental in mapping magnetic field properties in the interstellar medium (ISM) of the Milky Way[10,11,12]
Pulsars are surrounded by a magnetosphere. With it the mechanism responsible for pulsars’ emission, remains largely uncertain[13,14], it is widely accepted that the magnetosphere is populated by relativistic electron–positron (e–p) pairs, and that it, or at least its inner region, co-rotates with the neutron star
Summary
Pulsars are rotating neutron stars emitting lighthouse-like beams. Owing to their unique properties, pulsars are a unique astrophysical tool to test general relativity, inform on matter in extreme conditions, and probe galactic magnetic fields. Pulsars’ highly polarised emission and compactness make them unmatched sources to probe the magnetic fields through Faraday rotation[9], and pulsars have been instrumental in mapping magnetic field properties in the interstellar medium (ISM) of the Milky Way[10,11,12]. These studies often rely on the assumption that polarisation rotation Δφ results only from the Faraday effect experienced in the magnetised plasma between the polarised point source and the observer. To our knowledge, the gyrotropy (i.e. circular birefringence) that results from mechanical rotation, even in a symmetrical e–p plasma, and the associated polarisation rotation effect have not yet been considered
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