Over the past week, NuSTAR has conducted an intensive observing campaign on the neutron star X-ray binary GX 340+0, in coordination with the Australian Telescope Compact Array (ATCA, radio) and the X-ray Telescope aboard NASA’s Gehrels-Swift observatory. This well-known system belongs to the so-called "Z-source" family – a group of bright, accreting neutron stars that persistently trace a distinctive "Z-shaped" pattern in diagrams of their X-ray "hardness" (or color) plotted against their X-ray intensity. As these systems move back and forth along this Z-shaped track, typically over a timescale of about a week, their X-ray spectral and timing properties undergo a clear evolution. However, this evolution is significantly less pronounced compared to most other X-ray binaries, whether they contain neutron stars or black holes as the central accreting object. Interestingly, despite the relatively subtle changes in their X-ray properties, the radio emission from Z-sources has been observed to vary dramatically depending on their position along the Z-track. This suggests that the physical properties of their radio jets evolve far more than the underlying accretion flow. This behavior is unexpected, as such dramatic jet evolution is typically accompanied by equally dramatic changes in X-ray spectral-timing properties – especially in black hole systems. This discrepancy raises the possibility of different jet-launching mechanisms, distinct physical conditions, and/or unique correlations between jets and accretion flows in neutron star versus black hole X-ray binaries. To investigate this intriguing scenario, six simultaneous NuSTAR and ATCA observations were coordinated with a near-daily cadence over the course of a week. This observing strategy will, for the first time, track the co-evolution of the jet (radio) and accretion flow (X-rays) in this system, providing unique and unprecedented insights into the factors that govern jet evolution in Z-sources.

Authors: Alessio Marino (Postdoctoral fellow, ICE-CSIC, Spain)