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Apep is the brightest and most luminous non-thermal colliding-wind binary byover an order of magnitude. It has been suggested from infrared observationsthat one of the Wolf-Rayet stars in Apep is launching an anisotropic wind. Herewe present radio observations of Apep from 0.2 to 20 GHz taken over 33 years.The spectrum reveals an extremely steep turnover in the flux density at lowfrequencies, where the flux density decreases by two orders of magnitude overonly 325 MHz of bandwidth. This exponential decline is best described byfree-free absorption, with a turnover frequency at 0.54 $\pm$ 0.01 GHz. Abovethe turnover, the spectrum is well described by a power-law and ahigh-frequency cut-off likely caused by inverse-Compton cooling. The lightcurveof Apep shows significant variation over the observing period, with Apepbrightening by over 50 mJy in a span of 25 years at 1.4 GHz. Models that assumespherical winds do not replicate all of the structure evident in the radiolightcurve. We derived a model that allows one of the winds in the system to beanisotropic. This anisotropic model recovers most of the structure of thelightcurve and is a significantly better statistical fit to the data than thespherical wind model. We suggest such a result is independent support that oneof the Wolf-Rayet stars in Apep is launching an anisotropic wind. If theanisotropic wind model is correct, we predict a ~25% decrease of the 1.4 GHzflux density of Apep over the next five years.

Radio modelling of the brightest and most luminous non-thermal colliding-wind binary Apep