Discovery of strongly variable X-ray polarization in the neutron star low-mass X-ray binary transient XTE J1701−462
Authors: Cocchi Massimo , Gnarini Andrea , Fabiani Sergio , Ursini Francesco , Poutanen Juri , Capitanio Fiamma , Bobrikova Anna , Farinelli Ruben , Paizis Adamantia , Sidoli Lara , Veledina Alexandra , Bianchi Stefano , Di Marco Alessandro , Ingram Adam , Kajava Jari J. E. , La Monaca Fabio , Matt Giorgio , Malacaria Christian , Mikušincová Romana , Rankin John , Zane Silvia , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Di Lalla Niccolò , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Kaaret Philip , Karas Vladimir , Kislat Fabian , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marscher Alan P. , Marshall Herman L. , Massaro Francesco , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , Negro Michela , Ng Chi-Yung , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Puccetti Simonetta , Ramsey Brian D. , Ratheesh Ajay , Roberts Oliver J. , Romani Roger W. , Sgrò Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei 
Published: June 2023

Context. After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701−462 turned on again in September 2022, allowing for the first study of its X-ray polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE).
Aims. Polarimetric studies of XTE J1701−462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object.
Methods. The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its Z-track of the color–color diagram.
Results. During the first pointing on 2022 September 29, an average 2–8 keV polarization degree of (4.6 ± 0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a ∼0.6% average degree was obtained during the second pointing ten days later.
Conclusions. The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3–4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701−462, which is the strongest when the source was in the horizontal branch of its Z-track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the Z-sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources.