A1 Refereed original research article in a scientific journal
Constraining the geometry of the dipping atoll 4U 1624–49 with X-ray spectroscopy and polarimetry
Authors: Gnarini, Andrea; Saade, M. Lynne; Ursini, Francesco; Bianchi, Stefano; Capitanio, Fiamma; Kaaret, Philip; Matt, Giorgio; Poutanen, Juri; Zhang, Wenda
Publisher: EDP Sciences
Publishing place: LES ULIS CEDEX A
Publication year: 2024
Journal: Astronomy and Astrophysics
Journal name in source: Astronomy & Astrophysics
Journal acronym: ASTRON ASTROPHYS
Article number: A230
Volume: 690
Number of pages: 11
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202450716
Web address : https://doi.org/10.1051/0004-6361/202450716
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/470973652
We present the spectro-polarimetric results obtained from simultaneous X-ray observations with IXPE, NuSTAR, and NICER of the dipping neutron star X-ray binary 4U 1624-49. This source is the most polarized Atoll source so far observed with IXPE, with a polarization degree of 2.7%+/- 0.9% in the 2-8 keV band during the nondip phase and marginal evidence of an increasing trend with energy. The higher polarization degree compared to other Atolls can be explained by the high inclination of the system (i approximate to 60 degrees). The spectra are well described by the combination of soft thermal emission, a Comptonized component, and reflection of soft photons off the accretion disk. During the dips, the hydrogen column density of the highly ionized absorber increases while the ionization state decreases. The Comptonized radiation seems to be the dominant contribution to the polarized signal, with additional reflected photons that contribute significantly even though their fraction in the total flux is not high.
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Funding information in the publication:
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). AG, SB, FC, GM and FU acknowledge financial support by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-INAF-2022-19-HH.0. This research was also supported by the Istituto Nazionale di Astrofisica (INAF) grant 1.05.23.05.06: “Spin and Geometry in accreting X-ray binaries: The first multi frequency spectro-polarimetric campaign”. JP thanks the Academy of Finland grant 333112 for support. WZ acknowledges the support by the Strategic Pioneer Program on Space Science, Chinese Academy of Sciences through grants XDA15310000, by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB0550200, and by the National Natural Science Foundation of China (grant 12333004). 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). This work was supported in part by NASA through the NICER mission and the Astrophysics Explorers Program, together with the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. The NuSTAR Data Analysis Software (NUSTARDAS), jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA), has also been used in this project. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC.
