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 7 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 _ { \mathrm { R } } \propto L _ { \mathrm { X } } ^ { \beta } , is not well constrained ( \beta \approx 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 \times 10 ^ { 34 } < L _ { \mathrm { X } } < 3 \times 10 ^ { 35 } 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 \beta = 1.17 ^ { +0.30 } _ { -0.21 } power-law index or as a sharp radio cut-off at L _ { \mathrm { X } } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 5 % \times 10 ^ { 35 } 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 .