Evolution of the radio - X-ray coupling throughout an entire outburst of Aquila X-1

TL;DR

This study provides a comprehensive analysis of the radio and X-ray coupling during an entire outburst of Aquila X-1, revealing state-dependent jet activity, radio quenching at high luminosities, and steady, diffuse jets consistent with internal shock models.

Contribution

First detailed radio coverage of an entire outburst of Aquila X-1 with simultaneous X-ray data, confirming state-dependent jet behavior and steady diffuse jets.

Findings

Radio emission triggered at state transitions.

Radio quenching above 10% Eddington luminosity.

Detection of steady, diffuse jets with VLBI.

Abstract

The 2009 November outburst of the neutron star X-ray binary Aquila X-1 was observed with unprecedented radio coverage and simultaneous pointed X-ray observations, tracing the radio emission around the full X-ray hysteresis loop of the outburst for the first time. We use these data to discuss the disc-jet coupling, finding the radio emission to be consistent with being triggered at state transitions, both from the hard to the soft spectral state and vice versa. Our data appear to confirm previous suggestions of radio quenching in the soft state above a threshold X-ray luminosity of about 10% of the Eddington luminosity. We also present the first detections of Aql X-1 with Very Long Baseline Interferometry (VLBI), showing that any extended emission is relatively diffuse, and consistent with steady jets rather than arising from discrete, compact knots. In all cases where multi-frequency data were available, the source radio spectrum is consistent with being flat or slightly inverted, suggesting that the internal shock mechanism that is believed to produce optically thin transient radio ejecta in black hole X-ray binaries is not active in Aql X-1.