Probing the nonthermal emission geometry of AR Sco via optical phaseresolved polarimetry
Abstract
AR Sco is a binary system that contains a white and red dwarf. The rotation rate of the white dwarf (WD) has been observed to slow down, analogous to rotationpowered radio pulsars; it has thus been dubbed a 'white dwarf pulsar'. We previously fit the traditional radio pulsar rotating vector model to the linearly polarized optical data from this source, constraining the system geometry as well as the WD mass. Using a much more extensive data set, we now explore the application of the same model to binary phaseresolved optical polarimetric data, thought to be the result of nonthermal synchrotron radiation, and derive the magnetic inclination angle α and the observer angle ζ at different orbital phases. We obtain an ~10° variation in α and ~30° variation in ζ over the orbital period. The variation patterns in these two parameters is robust, regardless of the binning and epoch of data used. We speculate that the observer is detecting radiation from an asymmetric emission region that is a stable structure over several orbital periods. The success of this simple model lastly implies that the pitch angles of the particles are small and the pulsed, nonthermal emission originates relatively close to the WD surface.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 February 2022
 DOI:
 10.1093/mnras/stab3595
 arXiv:
 arXiv:2112.08708
 Bibcode:
 2022MNRAS.510.2998D
 Keywords:

 polarization;
 radiation mechanisms: nonthermal;
 methods: data analysis;
 techniques: polarimetric;
 stars: white dwarfs;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 13 pages, 14 figures, accepted by MNRAS