X-Ray and Radio Campaign of the Z-source GX 340+0. II. The X-Ray Polarization in the Normal Branch

Journal article

We present the first X-ray polarization measurement of the neutron star low-mass X-ray binary and Z-source, GX 340+0, in the normal branch (NB) using a 200 ks observation with the Imaging X-ray Polarimetric Explorer (IXPE). This observation was performed in 2024 August. Along with IXPE, we also conducted simultaneous observations with NICER, AstroSat, Insight-HXMT, the Australia Telescope Compact Array (ATCA), and the Giant Metrewave Radio Telescope (GMRT) to investigate the broadband spectral and timing properties in the X-ray and radio wavelengths. During the campaign, the source traced a complete Z-track during the IXPE observation but spent most of the time in the NB. We measure X-ray polarization degree (PD) of 1.3% ± 0.3% in the 2–8 keV energy band with a polarization angle (PA) of 38° ± 6°. The PD in the NB is observed to be weaker than in the horizontal branch (HB) but aligned in the same direction. The PD of the source exhibits a marginal increase with energy, while the PA shows no energy dependence. The joint spectro-polarimetric modeling is consistent with the observed X-ray polarization originating from a single spectral component from the blackbody, the Comptonized emission, or reflection feature, while the disk emission does not contribute toward the X-ray polarization. GMRT observations at 1.26 GHz during HB had a tentative detection at 4.5 ± 0.7 mJy, while ATCA observations a day later during the NB detected the source at 0.70 ± 0.05 mJy and 0.59 ± 0.05 mJy in the 5.5 and 9 GHz bands, respectively, suggesting an evolving jet structure depending on the Z-track position.

Authors: Bhargava, Y; Russell, TD; Ng, M; Balasubramanian, A; Zhang, L

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 1003
• Issue: 1
• Pages: 11
• Article number: 11
• Publication date: 2026-05-11
• Acceptance date: 2025-05-16
• DOI: 10.3847/1538-4357/ae5a27
• EISSN: 1538-4357
• ISSN: 0004637X, 0004-637X
• Language: English
• Keywords: X-ray astronomy; X-ray binary stars; Polarimetry; Accretion; Stellar accretion disks