Radio modelling of the brightest and most luminous non-thermal colliding-wind binary Apep. (arXiv:2110.06154v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bloot_S/0/1/0/all/0/1">S. Bloot</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Callingham_J/0/1/0/all/0/1">J. R. Callingham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marcote_B/0/1/0/all/0/1">B. Marcote</a>
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.
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.
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