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23rd of June 2020
The variable radio counterpart of Swift J1858.6-0814
by van den Eijnden et al.
Swift J1858.6-0814 is a transient neutron star X-ray binary that was discovered in October 2018. Multiwavelength follow-up observations across the electromagnetic spectrum revealed many interesting properties, such as erratic flaring on minute timescales and evidence for wind outflows at both X-ray and optical wavelengths. Van den Eijnden report on a detailed radio observing campaign of the source using the Australia Telescope Compact Array and the Karl G. Jansky Very Large Array. A radio counterpart with a flat to inverted radio spectrum is detected in all observations, consistent with a compact jet being launched from the system.

The figure above shows the light curves of the ATCA 5.5 (red squares) and 9.0 (blue circles) GHz flux density of Swift J1858.6-0814, and the radio spectral index alpha ay a five-minutes resolution. The grey band shows the 9-GHz light curve of the nearby flat-spectrum background source. Near the start of the observation, a flare is observed, rising and decaying on a time scale of tens of minutes. During the rise and decay of this flare, the spectral index gradually decays from inverted (alpha > 1) to flat (alpha ~ 0). The flux densities across the observation span a dynamical range of more than four, varying between less than 200 µJy to more than 800 µJy on time scale of tens of minutes. They conclude that radio variability is consistent with being driven by variations in the accretion flow propagating down the compact jet. The paper has been accepted for publication in Monthly Notices of the Royal Astronomical Society.

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