Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder

We report the discovery of a highly-polarized, highly-variable, steep-spectrum radio source, ASKAP J173608.2-321635, located $\sim$4 deg from the Galactic center in the Galactic plane. The source was detected six times between 2020 January and 2020 September as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey at 888 MHz. It exhibited a high degree ($\sim 25$%) of circular polarization when it was visible. We monitored the source with the MeerKAT telescope from 2020 November to 2021 February on a 2--4 week cadence. The source was not detected with MeerKAT before 2021 February 07 when it appeared and reached a peak flux density of 5.6 mJy. The source was still highly circularly polarized, but also showed up to 80% linear polarization, and then faded rapidly with a timescale of one day. The rotation measure of the source varied significantly, from $-11.8\pm0.8$ rad m$^{-2}$ to $-64.0\pm1.5$ rad m$^{-2}$, over three days. No X-ray counterpart was found in follow-up \textit{Swift} or \textit{Chandra} observations about a week after the first MeerKAT detection, with upper limits of $\sim 5.0\times10^{31}$ erg s$^{-1}$ (0.3--8 keV, assuming a distance $\sim10$ kpc). No counterpart is seen in new or archival near-infrared observations down to $J=20.8$ mag. We discuss possible identifications for ASKAP J173608.2-321635 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: none of these fully explains the observations, which suggests that ASKAP J173608.2-321635 may represent part of a new class of objects being discovered through radio imaging surveys.