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Published April 20, 2023 | Published
Journal Article Open

IXPE Observations of the Quintessential Wind-accreting X-Ray Pulsar Vela X-1

Forsblom, Sofia V. ORCID icon
Poutanen, Juri ORCID icon
Tsygankov, Sergey S. ORCID icon
Bachetti, Matteo ORCID icon
Di Marco, Alessandro ORCID icon
Doroshenko, Victor ORCID icon
Heyl, Jeremy ORCID icon
La Monaca, Fabio ORCID icon
Malacaria, Christian ORCID icon
Marshall, Herman L. ORCID icon
Muleri, Fabio ORCID icon
Mushtukov, Alexander A. ORCID icon
Pilia, Maura ORCID icon
Rogantini, Daniele ORCID icon
Suleimanov, Valery F. ORCID icon
Taverna, Roberto ORCID icon
Xie, Fei ORCID icon
Agudo, Iván ORCID icon
Antonelli, Lucio A. ORCID icon
Baldini, Luca ORCID icon
Baumgartner, Wayne H. ORCID icon
Bellazzini, Ronaldo ORCID icon
Bianchi, Stefano ORCID icon
Bongiorno, Stephen D. ORCID icon
Bonino, Raffaella ORCID icon
Brez, Alessandro ORCID icon
Bucciantini, Niccolò ORCID icon
Capitanio, Fiamma ORCID icon
Castellano, Simone ORCID icon
Cavazzuti, Elisabetta ORCID icon
Chen, Chien-Ting ORCID icon
Ciprini, Stefano ORCID icon
Costa, Enrico ORCID icon
De Rosa, Alessandra ORCID icon
Del Monte, Ettore ORCID icon
Di Gesu, Laura ORCID icon
Di Lalla, Niccolò ORCID icon
Donnarumma, Immacolata ORCID icon
Dovčiak, Michal ORCID icon
Ehlert, Steven R. ORCID icon
Enoto, Teruaki ORCID icon
Evangelista, Yuri ORCID icon
Fabiani, Sergio ORCID icon
Ferrazzoli, Riccardo ORCID icon
García, Javier A. ORCID icon
Gunji, Shuichi ORCID icon
Hayashida, Kiyoshi ORCID icon
Iwakiri, Wataru ORCID icon
Jorstad, Svetlana G. ORCID icon
Kaaret, Philip ORCID icon
Karas, Vladimir ORCID icon
Kitaguchi, Takao
Kolodziejczak, Jeffery J. ORCID icon
Krawczynski, Henric ORCID icon
Latronico, Luca ORCID icon
Liodakis, Ioannis ORCID icon
Maldera, Simone ORCID icon
Manfreda, Alberto ORCID icon
Marin, Frédéric ORCID icon
Marinucci, Andrea ORCID icon
Marscher, Alan P. ORCID icon
Matt, Giorgio ORCID icon
Mitsuishi, Ikuyuki
Mizuno, Tsunefumi ORCID icon
Negro, Michela ORCID icon
Ng, Chi-Yung ORCID icon
O'Dell, Stephen L. ORCID icon
Omodei, Nicola ORCID icon
Oppedisano, Chiara ORCID icon
Papitto, Alessandro ORCID icon
Pavlov, George G. ORCID icon
Peirson, Abel L. ORCID icon
Perri, Matteo ORCID icon
Pesce-Rollins, Melissa ORCID icon
Petrucci, Pierre-Olivier ORCID icon
Possenti, Andrea ORCID icon
Puccetti, Simonetta ORCID icon
Ramsey, Brian D. ORCID icon
Rankin, John ORCID icon
Ratheesh, Ajay ORCID icon
Roberts, Oliver J. ORCID icon
Romani, Roger W. ORCID icon
Sgrò, Carmelo ORCID icon
Slane, Patrick ORCID icon
Soffitta, Paolo ORCID icon
Spandre, Gloria ORCID icon
Sunyaev, Rashid A. ORCID icon
Swartz, Douglas A. ORCID icon
Tamagawa, Toru ORCID icon
Tavecchio, Fabrizio ORCID icon
Tawara, Yuzuru
Tennant, Allyn F. ORCID icon
Thomas, Nicholas E. ORCID icon
Tombesi, Francesco ORCID icon
Trois, Alessio ORCID icon
Turolla, Roberto ORCID icon
Vink, Jacco ORCID icon
Weisskopf, Martin C. ORCID icon
Wu, Kinwah ORCID icon
Zane, Silvia ORCID icon
IXPE Collaboration


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 −47.°3 ± 5.°4. 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°.

Additional Information

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 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). Facilities: IXPE. - Software: astropy (Astropy Collaboration et al. 2013, 2018), xspec (Arnaud 1996), ixpeobssim (Baldini et al. 2022).

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August 22, 2023
October 20, 2023