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16th 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, is located toward the centre of our Galaxy and 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. 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 upper panels above show ASKAP images of ASKAP J173608.2−321635 (centered at 888 MHz). Each image is 10 arcminutes on a side. The “off” image observed on 2019 April 28 is shown in in panel (a), the “on” image observed on 2020 January 11 is shown in panel (b) and the circular polarisation (Stokes V) image from 2020 January 11 is shown in panel (c). images. The lower panels show MeerKAT L-band images of ASKAP J173608.2−321635. Each image is 10 arcminute on a side. The “off” image observed on 2021 January 19 is shown in panel (d), the “on” image observed on 2021 February 07 is shown in panel (e) and Stokes V image from 2021 February 07 in panel (f). The color scales are the same for all of these images.

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