The Atacama Cosmology Telescope: Detection of Millimeter-wave Transient Sources

We report on the serendipitous discovery of three transient mm-wave sources using data from Atacama Cosmology Telescope. The first, detected at RA = 273.8138, dec -49.4628 ${\sim}50\sigma$ total, brightened less than 5 mJy to least 1100 150 GHz with an unknown rise time shorter thirteen days, during which increase 250 took only 8 minutes. Maximum flux was observed 2019-11-8. source's spectral index in between 90 and positive, $\alpha 1.5\pm0.2$. second, 105.1584, -11.2434 ${\sim}20\sigma$ 20 300 eight days. 2019-12-15. Its also 1.8\pm0.2$. third, 301.9952, 16.1652 ${\sim}40\sigma$ over a day or but decayed few 2018-9-11. spectrum approximately flat, -0.2\pm0.1$. None were polarized limits these measurements. two rising-spectrum are coincident position M K stars, while third is G star.