CMB-S4 will provide a unique platform to conduct a wide-field time-domain survey in the millimeter wavelength band, covering over half of the sky every day. In this waveband, the time-variable sky is largely unexplored, with only shallow surveys or those limited in area or scope. This is largely the result of limited observing time and fields of view for mm-band instruments which tend to focus on high-resolution observations of known objects. Despite this, a wide variety of sources are either known or believed to have particularly interesting time-variability in bands observed by CMB-S4. Expected sources include tidal disruption events, nearby supernovae, X-ray binaries, and classical novae. Particularly good candidates are gamma-ray bursts and active galaxies, such as the time-variable blazar that was identified as a possible source of high energy neutrinos. The combination of high sensitivity and wide area for CMB-S4 will open a new window for time domain astronomy and multi-messenger astrophysics.
CMB-S4 will be an excellent complement to other transient surveys, filling a gap between radio and optical searches. Gamma-ray burst afterglows can be detected within a few hours of the burst in many cases, and there is a possibility of capturing mm-wave afterglows that have no corresponding gamma-ray trigger either from the geometry of relativistic beaming and/or from sources being at very high redshift.
CMB-S4 could also find a hypothetical ninth planet in the outer reaches of the solar system, if one is out there. Thermal emission from known planets, dwarf planets, and asteroids have been measured at these wavelengths, and since their orbits give them fast proper motions, they should be easily differentiated from the extrasolar sky.
CMB-S4 will play an active role in multi-messenger astronomy. In the centers of galaxies, black holes that accrete gas are highly variable. CMB-S4 will provide long-duration and frequent measurements in intensity and linear polarization. This will create a mm-wave archive for multi-messenger astronomy, in particular for future sources that are discovered to be sources of high-energy neutrinos (such as the “blazar” galaxy associated with a detection by the IceCube neutrino experiment at the South Pole).
Additionally, the natural wide-area nature of the survey will make it straightforward to search for electromagnetic counterparts of gravitational wave events. Although the first binary neutron star merger discovered by gravitational waves was not detected at millimeter wavelengths, it was in a low density environment. Some mergers can occur in denser environments, which will make them fainter in visible light, but brighter for CMB-S4.