EVOLUTION OF THE RADIO-X-RAY COUPLING THROUGHOUT AN ENTIRE OUTBURST OF AQUILA X-1

Abstract

The 2009 November outburst of the neutron star X-ray binary Aquila X-1 was\nobserved with unprecedented radio coverage and simultaneous pointed X-ray\nobservations, tracing the radio emission around the full X-ray hysteresis loop\nof the outburst for the first time. We use these data to discuss the disc-jet\ncoupling, finding the radio emission to be consistent with being triggered at\nstate transitions, both from the hard to the soft spectral state and vice\nversa. Our data appear to confirm previous suggestions of radio quenching in\nthe soft state above a threshold X-ray luminosity of about 10% of the Eddington\nluminosity. We also present the first detections of Aql X-1 with Very Long\nBaseline Interferometry (VLBI), showing that any extended emission is\nrelatively diffuse, and consistent with steady jets rather than arising from\ndiscrete, compact knots. In all cases where multi-frequency data were\navailable, the source radio spectrum is consistent with being flat or slightly\ninverted, suggesting that the internal shock mechanism that is believed to\nproduce optically thin transient radio ejecta in black hole X-ray binaries is\nnot active in Aql X-1.\n