Discovery of a strong rotation of the X-ray polarization angle in the galactic burster GX 13+1
Done, Chris; Kaaret, Philip; Wu, Kinwah; Enoto, Teruaki; Turolla, Roberto; Pavlov, George G.; Weisskopf, Martin C.; Tennant, Allyn F.; Swartz, Douglas A.; Salmi, Tuomo; Slane, Patrick; Papitto, Alessandro; Hayashida, Kiyoshi; Vink, Jacco; Bianchi, Stefano; Matt, Giorgio; Kitaguchi, Takao; García, Javier A.; Doroshenko, Victor; Tsygankov, Sergey S.; Poutanen, Juri; Kajava, Jari J. E.; Tombesi, Francesco; Mitsuishi, Ikuyuki; Mizuno, Tsunefumi; Tamagawa, Toru; Tawara, Yuzuru; Zane, Silvia; Dovčiak, Michal; Iwakiri, Wataru; Bachetti, Matteo; Tavecchio, Fabrizio; Thomas, Nicholas E.; Pesce-Rollins, Melissa; Marshall, Herman L.; Heyl, Jeremy; Ratheesh, Ajay; Marinucci, Andrea; De Rosa, Alessandra; Marin, Frédéric; Petrucci, Pierre-Olivier; Ursini, Francesco; Baumgartner, Wayne H.; Negro, Michela; Perri, Matteo; Taverna, Roberto; Ng, Mason; Muleri, Fabio; Di Marco, Alessandro; Krawczynski, Henric; Possenti, Andrea; Baldini, Luca; Agudo, Iván; Antonelli, Lucio A.; Bellazzini, Ronaldo; Bongiorno, Stephen D.; Bonino, Raffaella; Brez, Alessandro; Bucciantini, Niccolò; Capitanio, Fiamma; Castellano, Simone; Cavazzuti, Elisabetta; Ciprini, Stefano; Costa, Enrico; Del Monte, Ettore; Di Gesu, Laura; Di Lalla, Niccolò; Donnarumma, Immacolata; Ehlert, Steven R.; Evangelista, Yuri; Fabiani, Sergio; Ferrazzoli, Riccardo; Gunji, Shuichi; Jorstad, Svetlana G.; Karas, Vladimir; Kislat, Fabian; Kolodziejczak, Jeffery J.; La Monaca, Fabio; Latronico, Luca; Liodakis, Ioannis; Maldera, Simone; Manfreda, Alberto; Marscher, Alan P.; Massaro, Francesco; O'Dell, Stephen L.; Omodei, Nicola; Oppedisano, Chiara; Peirson, Abel L.; Pilia, Maura; Puccetti, Simonetta; Ramsey, Brian D.; Rankin, John; Roberts, Oliver J.; Romani, Roger W.; Soffitta, Paolo; Spandre, Gloria; Trois, Alessio; Xie, Fei; Bobrikova, Anna; Forsblom, Sofia V.; Ravi, Swati; Loktev, Vladislav; Veledina, Alexandra; Rogantini, Daniele; Gnarini, Andrea; Nitindala, Anagha P.; Chen, Chien-Ting; Ng, Chi-Yung; Sgrò, Carmelo
Finland, Italy, United States, Spain, Sweden, Netherlands, United Kingdom, Japan, Canada, Germany, Czech Republic, Russia, France, Hong Kong SAR, China
Abstract
Weakly magnetized neutron stars in X-ray binaries show a complex phenomenology with several spectral components that can be associated with the accretion disk, the boundary, and/or a spreading layer, a corona, and a wind. Spectroscopic information alone, however, is not enough to distinguish these components. The analysis of the timing data revealed that most of the variability, and in particular, kilohertz quasi-period oscillations, are associated with the high-energy component that corresponds to the boundary and/or spreading layer. Additional information about the nature of the spectral components, and in particular, about the geometry of the emission region, can be provided by X-ray polarimetry. One of the objects of the class, a bright, persistent, and rather peculiar galactic Type I X-ray burster GX 13+1, was observed with the Imaging X-ray Polarimetry Explorer (IXPE) and the XMM-Newton. Using the XMM-Newton data, we obtained the best-fit values for the continuum spectral parameters and detected strong absorption lines associated with the accretion disk wind. IXPE data showed the source to be significantly polarized in the 2-8 keV energy band, with an overall polarization degree (PD) of 1.4%±0.3% at a polarization angle (PA) of −2° ±6° (errors at the 68% confidence level). During the two-day long observation, we detected rotation of the PA by about 70° with the corresponding changes in the PD from 2% to nondetectable and then up to 5%. These variations in polarization properties are not accompanied by visible spectral state changes of the source. The energy-resolved polarimetric analysis showed a significant change in polarization, from being strongly dependent on energy at the beginning of the observation to being almost constant with energy in the later parts of the observation. As a possible interpretation, we suggest a constant polarization component, strong wind scattering, or a different polarization of the two main spectral components with an individually peculiar behavior. The rotation of the PA suggests a misalignment of the neutron star spin from the orbital axis.