A1 Refereed original research article in a scientific journal
X-Ray Dips and Polarization Angle Swings in GX 13+1
Authors: Di Marco, Alessandro; La Monaca, Fabio; Bobrikova, Anna; Stella, Luigi; Papitto, Alessandro; Poutanen, Juri; Baglio, Maria Cristina; Bachetti, Matteo; Loktev, Vladislav; Pilia, Maura; Rogantini, Daniele
Publisher: American Astronomical Society
Publishing place: BRISTOL
Publication year: 2025
Journal: Astrophysical Journal Letters
Journal name in source: The Astrophysical Journal Letters
Journal acronym: ASTROPHYS J LETT
Article number: L47
Volume: 979
Issue: 2
Number of pages: 12
ISSN: 2041-8205
eISSN: 2041-8213
DOI: https://doi.org/10.3847/2041-8213/ada7f8
Web address : https://doi.org/10.3847/2041-8213/ada7f8
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/485045887
We present the result from the 2024 April observation of the low-mass X-ray binary GX 13+1 with the Imaging X-ray Polarimetry Explorer (IXPE), together with Neutron Star Interior Composition Explorer and Swift- X-Ray Telescope coordinated observations. Two light-curve dips were observed; during them, the harder Comptonized spectral component was dominant and the polarization degree (PD) higher than in the softer, off-dip intervals. Through a joint analysis of the three IXPE observations, which also included the dip from the first observation, we demonstrate that the polarization properties varied in response to the intensity and spectral hardness changes associated with the dips. The PD attained values up to similar to 4%. The polarization angle (PA) showed a swing of similar to 70 degrees across the dip and off-dip states, comparable to the continuous rotation seen during the first IXPE observation. We discuss these results in the context of models for polarized emission from the accretion disk and the boundary/spreading layer on the neutron star surface. We also draw attention to the role that an extended accretion disk corona or disk wind can play in generating high PDs and, possibly, swings of the PA.
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Funding information in the publication:
The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2022-13-I.0, agreements ASI-INAF-2022-19-HH.0 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 thank Swift Project Scientists for approving our DDT request to observe GX 13+1. The authors acknowledge the NICER team for the simultaneous observations. A.D.M. and F.L.M. are partially supported by MAECI with grant CN24GR08 "GRBAXP: Guangxi-Rome Bilateral Agreement for X-ray Polarimetry in Astrophysics". This research has been supported by the Finnish Cultural Foundation grant 00240328 (A.B.) and the Academy of Finland grant 333112 (A.B., J.P.).
