The X-ray polarimetry view of the accreting pulsar Cen X-3

Sergey S. Tsygankov, Victor Doroshenko, Juri Poutanen, Jeremy Heyl, Alexander A. Mushtukov, Ilaria Caiazzo, Alessandro Di Marco, Sofia V. Forsblom, Denis González-Caniulef, Moritz Klawin, Fabio La Monaca, Christian Malacaria, Herman L. Marshall, Fabio Muleri, Mason Ng, Valery F. Suleimanov, Rashid A. Sunyaev, Roberto Turolla, Iván Agudo, Lucio A. Antonelli, Matteo Bachetti, Luca Baldini, Wayne H. Baumgartner, Ronaldo Bellazzini, Stefano Bianchi, Stephen D. Bongiorno, Raffaella Bonino, Alessandro Brez, Niccolò Bucciantini, Fiamma Capitanio, Simone Castellano, Elisabetta Cavazzuti, Stefano Ciprini, Enrico Costa, Alessandra De Rosa, Ettore Del Monte, Laura Di Gesu, Niccolò Di Lalla, Immacolata Donnarumma, Michal Dovčiak, Steven R. Ehlert, Teruaki Enoto, Yuri Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. Garcia, Shuichi Gunji, Kiyoshi Hayashida, Wataru Iwakiri et al.

Submitted on 6 September 2022


Cen X-3 is the first X-ray pulsar discovered 50 years ago. Radiation from such objects is expected to be highly polarized due to birefringence of plasma and vacuum associated with propagation of photons in presence of the strong magnetic field. Here we present results of the observations of Cen X-3 performed with the Imaging X-ray Polarimetry Explorer. The source exhibited significant flux variability and was observed in two states different by a factor of ~20 in flux. In the low-luminosity state no significant polarization was found either in pulse phase-averaged (with the 3σ upper limit of 12%) or phase-resolved data (the 3σ upper limits are 20-30%). In the bright state the polarization degree of 5.8±0.3% and polarization angle of 49.6deg±1.5deg with significance of about 20σ was measured from the spectro-polarimetric analysis of the phase-averaged data. The phase-resolved analysis showed a significant anti-correlation between the flux and the polarization degree as well as strong variations of the polarization angle. The fit with the rotating vector model indicates a position angle of the pulsar spin axis of about 49deg and a magnetic obliquity of 17deg. The detected relatively low polarization can be explained if the upper layers of the neutron star surface are overheated by the accreted matter and the conversion of the polarization modes occurs within the transition region between the upper hot layer and a cooler underlying atmosphere. A fraction of polarization signal can also be produced by reflection of radiation from the neutron star surface and the accretion curtain.


Comment: Submitted to ApJL. 18 pages, 12 figures

Subject: Astrophysics - High Energy Astrophysical Phenomena