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We report on quasi-simultaneous observations from radio to X-ray frequenciesof the neutron star X-ray binary Aql X-1 over accretion state transitionsduring its 2016 outburst. All the observations show radio to millimetre spectraconsistent with emission from a jet, with a spectral break from optically thickto optically thin synchrotron emission that decreases from ~100 GHz to <5.5 GHzduring the transition from a hard to a soft accretion state. The 5.5 GHz radioflux density as the source reaches the soft state, 0.82$\pm$0.03 mJy, is thehighest recorded to date for this source. During the decay of the outburst, thejet spectral break is detected again at a frequency of ~30-100 GHz. The fluxdensity is 0.75$\pm$0.03 mJy at 97.5 GHz at this stage. This is the first timethat a change in the frequency of the jet break of a neutron star X-ray binaryhas been measured, indicating that the processes at play in black holes arealso present in neutron stars, thus supporting the idea that the internalproperties of the jet rely most critically on the conditions of the accretiondisc and corona around the compact object, rather than the black hole mass orspin or the neutron star surface or magnetic field.

The evolving jet spectrum of the neutron star X-ray binary Aql X-1 in transitional states during its 2016 outburst