X-ray and Radio Campaign of the Z-source GX 340+0 II: the X-ray polarization in the normal branch

TL;DR

This study reports the first X-ray polarization measurement of the neutron star binary GX 340+0 in the normal branch, revealing a polarization degree of about 1.2% and insights into the emission mechanisms and jet evolution.

Contribution

First X-ray polarization measurement of GX 340+0 in the normal branch, combined with broadband spectral and radio observations, providing new insights into emission processes and jet behavior.

Findings

X-ray polarization degree of 1.22% in 2-8 keV band

Polarization weaker in the normal branch than in the horizontal branch

Radio observations suggest evolving jet structure with Z-track position

Abstract

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, ATCA, and 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 $1.22\pm0.25\%$ in the 2-8 keV energy band with a polarization angle (PA) of $38\pm6^\circ$. 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 or the Comptonized emission while the disk emission does not contribute towards the X-ray polarization. GMRT observations at 1.26 GHz during HB had a tentative detection at 4.5$\pm$0.7 mJy while ATCA observations a day later during the NB detected the source at 0.70$\pm$0.05 mJy and 0.59$\pm$0.05 mJy in the 5.5 & 9 GHz bands, respectively, suggesting an evolving jet structure depending on the Z-track position.