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

TL;DR

This study tracks the jet spectrum changes in the neutron star X-ray binary Aql X-1 during its 2016 outburst, revealing a shifting spectral break that correlates with accretion states and supports similar jet physics in neutron stars and black holes.

Contribution

First measurement of the jet break frequency variation in a neutron star X-ray binary during an outburst, linking jet properties to accretion states and disc conditions.

Findings

Jet spectral break shifts from ~100 GHz to <5.5 GHz during state transition.

Highest radio flux density recorded for Aql X-1 at 0.82 mJy in the soft state.

Jet break reappears at 30-100 GHz during outburst decay.

Abstract

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 ~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$0.03 mJy, is the highest recorded to date for this source. During the decay of the outburst, the jet spectral break is detected again at a frequency of ~30-100 GHz. The flux density is 0.75$\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.