Multi-messenger astrophysics with continuous gravitational waves in the XG era

Continuous gravitational waves from spinning neutron stars are on the new frontiers of gravitational wave astrophysics and have strong connections to electromagnetic astronomy and nuclear astrophysics. I discuss prospects for detection of various populations, especially aided by electromagnetic observations, before and during the era of next generation (XG) detectors. Then I survey what can be learned from detections. Since continuous gravitational wave emission mechanisms depend on aspects of neutron star physics such as crustal stresses, which are not well constrained by current observations and nuclear experiments, gravitational waves may tell us a great deal that is new. Even more can be learned by combining gravitational wave observations with data from the Next Generation Very Large Array and other electromagnetic detectors operating in the XG era.