In section 7. Stellar structure and evolution
The evolving jet spectrum of the neutron star X-ray binary Aql X-1 in transitional states during its 2016 outburst
Aql X-1 is the most prolific neutron star X-ray binary transient. Despite there being more than 30 outbursts observed so far, the simultaneous coverage in X-rays and radio band is poor. The 2016 outburst from Aql X-1 was observed seven times in the radio (ATCA) and mm (ALMA) bands, together with near-daily observations at X-ray and optical frequencies by Diaz Trigo et al. (2017). For the first time it was possible to see the rise to the outburst peak and the decay after at all frequencies. Radio and mm observations are consistent with the presence of a jet. The synchrotron emission peak shifts from ~100 GHz to ~5 GHz during the rising phase and back to ~30-100 GHz during the decay. This is the first time that these frequency shifts have been observed in a transient system containing a neutron star. A similar behaviour has been revealed in several black hole transients (even if for these sources a correlation of the radio peak with the X-ray spectrum holds, which is not detected in Aql X-1). This should indicate that physical processes at play for the jet formation depend only mildly on black hole mass or spin, or presence of a hard surface or the neutron star magnetic field, and should rely instead on the accretion disc and its corona properties.