Multi-epoch variability of AT 2000ch

Archival radio continuum observations with the Very Large Array were reduced using standard calibration and imaging methods and is complemented by pre-reduced Continuum Halos in Nearby Galaxies - an Expanded Very Large Array Survey observations as well as the Westerbork Synthesis Radio Telescope and Low Frequency Array observations. For the analysis of AT 2000ch we consider the optical lightcurve and color evolution, its radio continuum brightness at different frequencies and times, and the corresponding spectral indices.

Mass loss rates and optical depths are estimated based on radio continuum brightnesses and H{alpha} fluxes. We report two newly detected outbursts of AT 2000ch similar to those found in the 2000s and 13 re-brightening events of which at least four are not conclusively detected because of insufficient sampling of the lightcurve. The dates of all outbursts and significant, well-sampled re-brightening events are consistent with a period of ~201+/-12 days over a total timespan of two decades.

Such a behavior has never been found for any supernova imposter, especially not for candidate SN 2009ip-analogues before. During 2010 to 2012 and 2014 to 2018 we only have a few detections insufficient to make conclusive remarks about a possible less eruptive phase of the transient. We find steady dimming after the most recent re-brightening events and hint at porosity in the circumstellar envelope suggesting AT 2000ch may currently be in transition to a state of relative calm.

We identified a second, unrelated source at a projected distance of ~23pc (~0.5") that has contaminated the optical measurements of AT 2000ch on its minimum luminosity over the last two decades probably on a 5-10% level but don't affect the overall findings and is negligible during re-brightening. We are able to restrict the mass loss rate of AT 2000ch to range between several 10^-6^M_{sun}_/yr to several 10^-5^M_{sun}_/yr.

The fresh ejecta appear to be optically thick to radio continuum emission at least within the first ~25 days after significant re-brightening. We suggest that other supernova imposter and probably also candidate SN 2009ip-analogues at comparable distances emit radio continuum fluxes on the order of a few to several tens of {mu}Jy at 6GHz. Deep and frequent continuum surveys in the radio range are needed to study these kind of objects in a broader context.