We report on quasi-simultaneous observations from radio to X-ray frequencies of the neutron star X-ray binary Aql X–1 over accretion state transitions during its 2016 outburst .
All the observations show radio to millimetre spectra consistent with emission from a jet , with a spectral break from optically thick to optically thin synchrotron emission that decreases from \sim 100 GHz to < 5.5 GHz during the transition from a hard to a soft accretion state .
The 5.5 GHz radio flux density as the source reaches the soft state , 0.82 \pm 0.03 mJy , is the highest recorded to date for this source .
During the decay of the outburst , the jet spectral break is detected again at a frequency of \sim 30–100 GHz .
The flux density is 0.75 \pm 0.03 mJy at 97.5 GHz at this stage .
This is the first time that a change in the frequency of the jet break of a neutron star X-ray binary has been measured , indicating that the processes at play in black holes are also present in neutron stars , thus supporting the idea that the internal properties of the jet rely most critically on the conditions of the accretion disc and corona around the compact object , rather than the black hole mass or spin or the neutron star surface or magnetic field .