Determination of X-ray pulsar geometry with IXPE polarimetry

Victor Doroshenko , Juri Poutanen , Sergey S. Tsygankov , Valery F. Suleimanov , Matteo Bachetti , Ilaria Caiazzo , Enrico Costa , Alessandro Di Marco , Jeremy Heyl , Fabio La Monaca , Fabio Muleri , Alexander A. Mushtukov , George G. Pavlov , Brian D. Ramsey , John Rankin , Andrea Santangelo , Paolo Soffitta , Rüdiger Staubert , Martin C. Weisskopf , Silvia Zane , Iván Agudo , Lucio A. Antonelli , 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 , 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 , Svetlana G. Jorstad , Vladimir Karas , Takao Kitaguchi , Jeffery J. Kolodziejczak , Henric Krawczynski , Luca Latronico , Ioannis Liodakis , Simone Maldera , Alberto Manfreda , Frédéric Marin (1) , Andrea Marinucci , Alan P. Marscher , Herman L. Marshall , Giorgio Matt , Ikuyuki Mitsuishi , Tsunefumi Mizuno , Chi-Yung Ng , Stephen L. O'Dell , Nicola Omodei , Chiara Oppedisano , Alessandro Papitto , Abel L. Peirson , Matteo Perri , Melissa Pesce-Rollins , Maura Pilia , Andrea Possenti , Simonetta Puccetti , Ajay Ratheesh , Roger W. Romani , Carmelo Sgrò , Patrick Slane , Gloria Spandre , Rashid A. Sunyaev , Toru Tamagawa , Fabrizio Tavecchio , Roberto Taverna , Yuzuru Tawara , Allyn F. Tennant , Nicolas E. Thomas , Francesco Tombesi , Alessio Trois , Roberto Turolla , Jacco Vink , Kinwah Wu , Fei Xie

1 ObAS - Observatoire astronomique de Strasbourg

Using observations of X-ray pulsar Hercules X-1 by the Imaging X-ray Polarimetry Explorer we report a highly significant (>17σ) detection of the polarization signal from an accreting neutron star. The observed degree of linear polarization of ~10% is far below theoretical expectations for this object, and stays low throughout the spin cycle of the pulsar. Both the degree and angle of polarization exhibit variability with the pulse phase, allowing us to measure the pulsar spin position angle 57(2) deg and the magnetic obliquity 12(4) deg, which is an essential step towards detailed modelling of the intrinsic emission of X-ray pulsars. Combining our results with the optical polarimetric data, we find that the spin axis of the neutron star and the angular momentum of the binary orbit are misaligned by at least ~20 deg, which is a strong argument in support of the models explaining the stability of the observed superorbital variability with the precession of the neutron star.

Keywords: Astrophysics - High Energy Astrophysical Phenomena

Domains: Sciences of the Universe [physics]

Victor Doroshenko, Juri Poutanen, Sergey S. Tsygankov, Valery F. Suleimanov, Matteo Bachetti, et al.. Determination of X-ray pulsar geometry with IXPE polarimetry. Nature Astronomy, 2022, ⟨10.1038/s41550-022-01799-5⟩. ⟨insu-03836995⟩