IXPE observations of the quintessential wind-accreting X-ray pulsar Vela X-1

Full list of authors: Forsblom, Sofia; Poutanen, Juri S.; Tsygankov, Sergey; Bachetti, Matteo; Marco, Alessandro Di; Doroshenko, Victor; Heyl, Jeremy; Monaca, Fabio La; Malacaria, Christian L.; Marshall, Herman; Muleri, Fabio A.; Mushtukov, Alexander; Pilia, Maura; Rogantini, Daniele F.; Suleimanov, Valery; Taverna, Roberto; Xie, Fei; Agudo, Ivan A.; Antonelli, Lucio; Baldini, Luca H.; Baumgartner, Wayne; Bellazzini, Ronaldo; Bianchi, Stefano D.; Bongiorno, Stephen; Bonino, Raffaella; Brez, Alessandro; Bucciantini, Niccolo; Capitanio, Fiamma; Castellano, Simone; Cavazzuti, Elisabetta; Chen, Chien-Ting; Ciprini, Stefano; Costa, Enrico; De Rosa, Alessandra; Del Monte, Ettore; Di Gesu, Laura; Di Lalla, Niccolo; Donnarumma, Immacolata; Dovciak, Michal R.; Ehlert, Steven; Enoto, Teruaki; Evangelista, Yuri; Fabiani, Sergio; Ferrazzoli, Riccardo A.; Garcia, Javier; Gunji, Shuichi; Hayashida, Kiyoshi; Iwakiri, Wataru G.; Jorstad, Svetlana; Kaaret, Philip; Karas, Vladimir; Kitaguchi, Takao J.; Kolodziejczak, Jeffery; Krawczynski, Henric; Latronico, Luca; Liodakis, Ioannis; Maldera, Simone; Manfreda, Alberto; Marin, Frederic; Marinucci, Andrea P.; Marscher, Alan; Matt, Giorgio; Mitsuishi, Ikuyuki; Mizuno, Tsunefumi; Negro, Michela; Ng, Chi-Yung L.; O'Dell, Stephen; Omodei, Nicola; Oppedisano, Chiara; Papitto, Alessandro G.; Pavlov, George L.; Peirson, Abel; Perri, Matteo; Pesce-Rollins, Melissa; Petrucci, Pierre-Olivier; Possenti, Andrea; Puccetti, Simonetta D.; Ramsey, Brian; Rankin, John; Ratheesh, Ajay J.; Roberts, Oliver W.; Romani, Roger; Sgro, Carmelo; Slane, Patrick; Soffitta, Paolo; Spandre, Gloria A.; Sunyaev, Rashid A.; Swartz, Douglas; Tamagawa, Toru; Tavecchio, Fabrizio; Tawara, Yuzuru F.; Tennant, Allyn E.; Thomas, Nicholas; Tombesi, Francesco; Trois, Alessio; Turolla, Roberto; Vink, Jacco C.; Weisskopf, Martin; Wu, Kinwah; Zane, Silvia; IXPE Collaboration.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
The radiation from accreting X-ray pulsars was expected to be highly polarized, with some estimates for the polarization degree of up to 80%. However, phase-resolved and energy-resolved polarimetry of X-ray pulsars is required in order to test different models and to shed light on the emission processes and the geometry of the emission region. Here we present the first results of the observations of the accreting X-ray pulsar Vela X-1 performed with the Imaging X-ray Polarimetry Explorer. Vela X-1 is considered to be the archetypal example of a wind-accreting, high-mass X-ray binary system, consisting of a highly magnetized neutron star accreting matter from its supergiant stellar companion. The spectropolarimetric analysis of the phase-averaged data for Vela X-1 reveals a polarization degree (PD) of 2.3% ± 0.4% at the polarization angle (PA) of −47fdg3 ± 5fdg4. A low PD is consistent with the results obtained for other X-ray pulsars and is likely related to the inverse temperature structure of the neutron star atmosphere. The energy-resolved analysis shows the PD above 5 keV reaching 6%–10% and a ∼90° difference in the PA compared to the data in the 2–3 keV range. The phase-resolved spectropolarimetric analysis finds a PD in the range 0%–9% with the PA varying between −80° and 40°. © 2023. The Author(s). Published by the American Astronomical Society.
The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). We acknowledge support from the RSF grant 19-12-00423 (SST), the Academy of Finland grants 333112, 349144, 349373, and 349906 (JP, SST), the German Academic Exchange Service (DAAD) travel grant 57525212 (VD, VFS), and the German Research Foundation (DFG) grant WE 1312/53-1 (VFS).
With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).