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

TL;DR

This study presents the first X-ray spectropolarimetric observations of the atoll source 4U 1735-44, revealing low polarization consistent with a low inclination angle of the accretion disk.

Contribution

First X-ray spectropolarimetric results of 4U 1735-44 using IXPE, NICER, and NuSTAR, providing insights into its disk inclination and polarization properties.

Findings

Detected marginal polarization of 1.4% with a 3σ upper limit of 3.5%.

Inferred a disk inclination of approximately 40° from reflection modeling.

Polarimetric properties are consistent with other atoll sources and suggest a low inclination angle.

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.