Bhargava et al. present results from simultaneous observations of the neutron star low-mass X-ray binary and Z-source, GX 340+0, with X-ray satellites IXPE, NICER, AstroSat, and Insight-HXMT, and radio telescopes ATCA, and GMRT. Z-sources are a unique subclass of accreting neutron star (NS) low mass X-ray binaries (LMXBs) that trace a characteristic Z-shaped track on their X-ray Hardness–Intensity diagrams or color-color diagrams. The Z-track can be divided into three main branches: the horizontal branch (HB), the normal branch (NB) and the flaring branch (FB). During the four-day campaign, the source traced a complete Z-track during the IXPE observation but spent most of the time in the NB. 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. The IXPE observations resulted in the first X-ray polarization measurement of GX 340+0 in the normal branch, with an X-ray polarization degree (PD) of 1.22±0.25% in the 2-8 keV energy band.

The figure above shows the evolution of GX 340+0 as seen by IXPE with black diamonds. The IXPE observation was supplemented with joint observations with NICER (orange circles), AstroSat (green crosses), and Insight-HXMT (purple squares). Results from the radio observations conducted during the X-ray campaign are shown as the magenta and red points (where the radio flux density can be read on the right y-axis). The intervals for the FB/EFB and the HB are indicated by the cyan and grey shaded regions, respectively. The source was in the NB at all other times.