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

TL;DR

This study presents 33 years of radio observations of Apep, revealing an exponential flux decline due to free-free absorption and supporting the presence of an anisotropic wind from one Wolf-Rayet star, improving modeling accuracy.

Contribution

It introduces an anisotropic wind model for Apep that better explains radio lightcurve variations than previous spherical wind models.

Findings

Radio spectrum shows steep turnover at 0.54 GHz due to free-free absorption.

Apep's radio flux increased by over 50 mJy at 1.4 GHz over 25 years.

An anisotropic wind model fits the data significantly better than spherical models.

Abstract

Apep is the brightest and most luminous non-thermal colliding-wind binary by over an order of magnitude. It has been suggested from infrared observations that one of the Wolf-Rayet stars in Apep is launching an anisotropic wind. Here we 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 low frequencies, where the flux density decreases by two orders of magnitude over only 325 MHz of bandwidth. This exponential decline is best described by free-free absorption, with a turnover frequency at 0.54 $\pm$ 0.01 GHz. Above the turnover, the spectrum is well described by a power-law and a high-frequency cut-off likely caused by inverse-Compton cooling. The lightcurve of Apep shows significant variation over the observing period, with Apep brightening by over 50 mJy in a span of 25 years at 1.4 GHz. Models that assume spherical winds do not replicate all of the structure evident in the radio lightcurve. We derived a model that allows one of the winds in the system to be anisotropic. This anisotropic model recovers most of the structure of the lightcurve and is a significantly better statistical fit to the data than the spherical wind model. We suggest such a result is independent support that one of the Wolf-Rayet stars in Apep is launching an anisotropic wind. If the anisotropic wind model is correct, we predict a ~25% decrease of the 1.4 GHz flux density of Apep over the next five years.