Quasi-simultaneous radio and X-ray observations of Aql X-1 probing low luminosities

Aql X-1 is one of the best-studied neutron star low-mass X-ray binaries. It was previously targeted using quasi-simultaneous radio and X-ray observations during at least seven different accretion outbursts. Such observations allow us to probe the interplay between accretion inflow (X-ray) and jet outflow (radio). Thus far, these combined observations have only covered one order of magnitude in radio and X-ray luminosity range; this means that any potential radio–X-ray luminosity correlation, L_R ∝ L_X^β, is not well constrained (β ≈ 0.4–0.9, based on various studies) or understood. Here we present quasi-simultaneous Very Large Array and Swift-XRT observations of Aql X-1’s 2016 outburst, with which we probe one order of magnitude fainter in radio and X-ray luminosity compared to previous studies (6 × 10³⁴ erg s⁻¹ < L_X <3 × 10³⁵ erg s⁻¹, i.e. the intermediate to low-luminosity regime between outburst peak and quiescence). The resulting radio non-detections indicate that Aql X-1’s radio emission decays more rapidly at low X-ray luminosities than previously assumed – at least during the 2016 outburst. Assuming similar behaviour between outbursts, and combining all available data in the hard X-ray state, this can be modelled as a steep β =1.17^+0.30_−0.21 power-law index or as a sharp radio cut-off at L_X ≲ 5 × 10³⁵ erg s⁻¹ (given our deep radio upper limits at X-ray luminosities below this value). We discuss these results in the context of other similar studies.