Probing the Transient Far-IR Sky with PRIMA

Abstract: The time variable far-IR/mm sky is largely unexplored. However, when PRIMA launches, next generation ground-based CMB experiments, including Simons Observatory and CMB-S4, will be operating. These will survey large areas of the sky for transient mm sources as a byproduct of their observations, producing regular mm-transient alerts. The results from current experiments show they can detect a wide variety of mm transients ranging from Galactic stars to extragalactic sources associated with AGNs and other energetic phenomena, and moving Solar System objects such as asteroids. These results, and theoretical predictions, indicate that future mm/submm facilities will detect many kinds of transient, including flaring stars, protostars, GRBs, TDEs, neutron star mergers, FBOTs, and SNe. New classes of mm-variable may be uncovered by CMB experiments, and transient searches at other wavelengths, such as the optical LSST survey, will produce additional targets to followup with PRIMA. Predicted rates for extragalactic mm transients to be detected by CMB experiments range from 10s to 1000s of events over the lifetime of these projects. CMB-S4 is most relevant for PRIMA, producing $\sim$100 extragalactic transients per year. Galactic transients and variable sources will also be detected, but the most common Galactic transients, flaring stars, operate on such short timescales that direct follow-up with PRIMA will not be feasible. Variable accretion rates in forming protostars, conversely, produce long term brightness variations that will be ideal monitoring targets. The addition of mid- and far-IR data points for all these sources can determine much about their radiation mechanisms and underlying physics. PRIMA followup of representative examples of various mm-transient and variable sources will thus have a powerful impact on our understanding of a wide range of astrophysical phenomena.