Decoding the X-ray Polarization in the Z-source GX 340+0 : Spectral Component Origins and Radio Constraints

Neutron star low-mass X-ray binaries (NS LMXBs) provide a unique laboratory to study the physics of accretion, jet formation, and strong gravity near compact objects. GX 340+0 is a well-known high-luminosity LMXB that contains a weakly magnetized NS. It exhibits complex spectro-timing behavior along its characteristic Z-track, providing valuable clues about accretion processes and emission mechanisms. In this talk, I will discuss the discovery of the X-ray polarization in GX 340+0, using an observation with the Imaging X-ray Polarimetry Explorer (IXPE) held in March 2024 (arXiv:2405.19324). A coordinated X-ray (NICER, NuSTAR, AstroSat, HXMT) and radio (GMRT, ATCA) monitoring campaigns also complimented the IXPE observation to characterize the source properties. Throughout the X-ray observational campaign, we observed that GX 340+0 appearing to be on the horizontal branch (HB) and showing a strong evolution of the spectro-timing properties as a function of the Z-track position. With the IXPE observation, we measured a significant X-ray polarization in 2.0-8.0 keV energy band with polarization degree (PD) of 4.02 +- 0.35% and polarization angle (PA) of 37.5 +- 2.5 degree with a confidence level of 68%. The spectro-polarimetric analysis from simultaneous IXPE and AstroSat observations revealed distinct polarization contributions from different spectral components, with a variation in polarization angle between the accretion disk and Comptonized emission. Additionally, the blackbody emission is consistently unpolarized, possibly originating from the boundary layer or neutron star surface. The findings from the coordinated radio campaign provided complementary constraints on the source’s jet activity. In conclusion, these results provided key insights into the emission geometry, polarization properties, and jet activity of GX 340+0. In my talk, I will discuss the origin of the X-ray polarization and its implications on the geometry of the spectral components in NS LMXBs systems. Authors: Yash Bhargava, Mason Ng, Liang Zhang, Arvind Balasubramanian, Thomas D. Russell, Aman Kaushik, Vishal Jadoliya, Swati Ravi, Sudip Bhattacharyya, Mayukh Pahari, Jeroen Homan, Herman L. Marshall, Deepto Chakrabarty, and Francesco Carotenuto.