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17th of September 2021
A highly-variable, highly polarised mystery
by Wang et al.
Wang et al. have reported the discovery with ASKAP of a highly-polarized, highly-variable, steep-spectrum radio source. The source, ASKAP J173608.2−321635, was detected six times in 2020 as part of the ASKAP Variables and Slow Transients (VAST) survey. It exhibited an unusually high degree (∼ 25%) of circular polarization when it was visible. Follow-up observations with the MeerKAT telescope in South Africa revealed strong time-variability, being undetectable at first, then brightening, before fading rapidly with a timescale of one day. The rotation measure of the source (which depends on the intervening plasma density and magnetic field) varied significantly over three days, which is also rare. No X-ray counterpart was found in follow-up X-ray observations, or in new or archival near-infrared observations. Various origins were considered, including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center Radio Transient. But none of these fully explains the observations, which suggests that this source may represent part of a new class of objects being discovered through radio imaging surveys.

The figure above shows the full radio lightcurve for ASKAP J173608.2−321635, including non-detections (times of X-ray observations are also indicated). The circular polarization fraction (Stokes V/Stokes I) is shown in the bottom panel for the detections. In the upper right panel, the detections with ASKAP from 2020 January are shown, and in the lower right panel, the observations close to the MeerKAT detections from 2021 February are shown. An exponential decay is fitted for the four 1.3 GHz detections (blue dashed line), yielding a decay timescale of ∼26 hours. The 800 MHz detection is scaled to 1.3 GHz assuming a spectral index of −2.7 and shown as the purple diamond.

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