A1 Refereed original research article in a scientific journal
X-Ray Polarization from the Atoll 4U 1735-44 Suggests a Low Inclination; 
Authors: Díaz Teodori, María Alejandra; Bobrikova, Anna; Gnarini, Andrea; Ursini, Francesco; Forsblom, Sofia V.; Poutanen, Juri; Salganik, Alexander; Bianchi, Stefano; Capitanio, Fiamma; Cocchi, Massimo; Fabiani, Sergio; Farinelli, Ruben; Kaaret, Philip; Kajava, Jari J. E.; Matt, Giorgio; Ng, Mason; Ravi, Swati; Soffitta, Paolo; Tarana, Antonella; Zane, Silvia
Publisher: Institute of Physics Publishing
Publication year: 2026
Journal: Astrophysical Journal
Article number: 143
Volume: 1002
Issue: 2
ISSN: 0004-637X
eISSN: 1538-4357
DOI: https://doi.org/10.3847/1538-4357/ae5a8e
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://iopscience.iop.org/article/10.3847/1538-4357/ae5a8e
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/526556836
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
X-ray polarimetry is a new tool capable of probing the geometry of accretion onto weakly magnetized neutron stars. Here we present the first X-ray spectropolarimetric results from coordinated observations of the atoll source 4U 1735−44, conducted with the Imaging X-ray Polarimetry Explorer (IXPE), Neutron Star Interior Composition Explorer, and Nuclear Spectroscopic Telescope Array. Over the 2–8 keV energy range, we obtained a marginal detection of polarization with the polarization degree of 1.4% ± 0.7% and polarization angle of −29° ± 14°, corresponding to a 3σ upper limit on the polarization degree of 3.5%. The best-fit model to describe the spectrum comprises a thermal component associated with the accretion disk, a Comptonized blackbody component, and a relativistic reflection component. From the reflection model, we infer a disk inclination of ∼40°. The spectroscopic and polarimetric properties of 4U 1735−44 are consistent with those observed in other atoll sources studied by IXPE, with its low polarization likely due to its low inclination.
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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). 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 partially supported by the Research Council of Finland Centre of Excellence in Neutron-Star Physics (grant 374064). M.A.D.T. acknowledges support from the EDUFI Fellowship and the Johannes Andersen Student Programme at the Nordic Optical Telescope. A.B. was supported by the Finnish Cultural Foundation (grants 002200175 and 00240328). A.G., F.U., S.B., F.C., S.F., G.M., and A.T. acknowledge financial support from the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through the contract ASI-INAF-2022-19-HH.0 and by the Istituto Nazionale di Astrofisica (INAF) through the grant 1.05.23.05.06: “Spin and Geometry in accreting X-ray binaries: The first multi frequency spectro-polarimetric campaign.” The Italian contribution was also supported by ASI through contract ASI-OHBI-2022-13-I.0, agreement 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 Fisica Nucleare (INFN) in Italy. A.G. was supported by an appointment to the NASA Postdoctoral Program at the Marshall Space Flight Center (MSFC), administered by Oak Ridge Associated Universities under contract with NASA. M.N. is a Fonds de Recherche du Quebec—Nature et Technologies (FRQNT) postdoctoral fellow. A.S. acknowledges the support of the Jenny and Antti Wihuri Foundation (grant No. 00240331).
