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

Abstract

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, $L_R\propto L_X^{\beta }$, 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 ($6\times 10^{34}<L_X<3\times 10^{35}$ 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, this can be modelled as a steep $\beta =1.17_{-0.21}^{+0.30}$ power-law index or as a sharp radio cut-off at $L_X\lesssim 5\times 10^{35}$ 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.

15 pages, 11 figures, 3 tables, accepted for publication in MNRAS