2018A&A...616A..23D

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 ∼100GHz to <5.5GHz during the transition from a hard to a soft accretion state. The 5.5GHz radio flux density as the source reaches the soft state, 0.82±0.03mJy, 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 ∼30-100GHz. The flux density is 0.75±0.03mJy at 97.5GHz 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, 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.