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arXiv · 2604.18048

X-Ray Polarization from the Atoll 4U 1735-44 Suggests a Low Inclination

M. A. Díaz Teodori·A. Bobrikova·A. Gnarini·F. Ursini·S. V. Forsblom·J. Poutanen·A. Salganik·S. Bianchi·F. Capitanio·M. Cocchi·S. Fabiani·R. Farinelli·P. Kaaret·J. J. E. Kajava·G. Matt·M. Ng·S. Ravi·P. Soffitta·A. Tarana·S. Zane
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Abstract

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), NICER, and NuSTAR. Over the 2-8 keV energy range, we obtained a marginal detection of polarization with the polarization degree of $1.4\%\pm0.7\%$ and polarization angle of $-29^\circ\pm14^\circ$, corresponding to a $3\sigma$ 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 $\sim 40^\circ$. 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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M. A. Díaz Teodori, A. Bobrikova, A. Gnarini, F. Ursini, S. V. Forsblom, J. Poutanen, A. Salganik, S. Bianchi, F. Capitanio, M. Cocchi, S. Fabiani, R. Farinelli, P. Kaaret, J. J. E. Kajava, G. Matt, M. Ng, S. Ravi, P. Soffitta, A. Tarana, S. Zane. 2026-04-20. X-Ray Polarization from the Atoll 4U 1735-44 Suggests a Low Inclination. https://doi.org/10.3847/1538-4357%2Fae5a8e

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