X-ray spectropolarimetric characterization of GX 340+0 in the horizontal branch: A highly inclined source?

¹ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
² Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁴ Department of Physics and Astronomy, Wayne State University, 666 W. Hancock St., 48201 Detroit (MI), USA
⁵ Department of Physics and Astronomy, 20014 University of Turku, Finland
⁶ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China

^⋆ Corresponding author; fabio.lamonaca@inaf.it

Received: 23 August 2024
Accepted: 2 October 2024

Abstract

We report the first detection of X-ray polarization in the horizontal branch for GX 340+0 as obtained by the Imaging X-ray Polarimetry Explorer (IXPE). A polarization degree of 4.3% ± 0.4% at a confidence level of 68% is obtained. This value agrees with the previous polarization measurements of Z-sources in the horizontal branch. The spectropolarimetric analysis, performed using a broadband spectral model obtained by NICER and NuSTAR quasi-simultaneous observations, allowed us to constrain the polarization for the soft and hard spectral components that are typical of these sources. The polarization angle for the two components differs by ∼40°. This result can be explained by a misalignment of the NS rotation axis with respect to the accretion disk axis. We compared the results with the polarization that is expected in different models. Theoretical expectations for the polarization of the disk and the Comptonized components favor a higher orbital inclination for GX 340+0 than 60°, as expected for Cyg-like sources. This is in contrast with the results we report for the reflection component based on the broadband spectrum.