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%±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.
@article{arxiv.2604.18048,
title = {X-Ray Polarization from the Atoll 4U 1735-44 Suggests a Low Inclination},
author = {M. A. Díaz Teodori and A. Bobrikova and A. Gnarini and F. Ursini and S. V. Forsblom and J. Poutanen and A. Salganik and S. Bianchi and F. Capitanio and M. Cocchi and S. Fabiani and R. Farinelli and P. Kaaret and J. J. E. Kajava and G. Matt and M. Ng and S. Ravi and P. Soffitta and A. Tarana and S. Zane},
journal= {arXiv preprint arXiv:2604.18048},
year = {2026}
}