| Radio emission from sub-Eddington black hole X-ray binaries is usually associated with synchrotron emission from
steady collimated jets. This association relies on VLBA observations of two very bright sources, GRS 1915+105 and
Cyg X-1, emitting X-rays at or above a few per cent of the Eddington limit. It is unclear how the jet properties,
notably its size and collimation degree, might change at lower luminosities, in the so called `quiescent' regime. We
propose to observe two representative systems with the High Sensitivity Array as part of a wider program aiming to
constrain the nature and geometry of the radio emission from sub-Eddington X-ray binaries; A0620--00, the dimmest
and closest of all, and V404 Cyg, with the largest orbital period among the quiescent systems. Together with Cyg
X-1 and GRS 1915+105, high spatial resolution observations of A0620--00 and V404 Cyg would span more than 9
orders of magnitude in luminosity, allowing us to test theoretical predictions on how the jet angular size may scale
with the system parameters. |
