Intensity interferometry

Last, but not least, is the possibility of operating the CTAO telescope arrays in a different observing mode, that of optical intensity interferometry. This technique effectively turns the telescope array or any subset of it into a giant optical telescope, with albeit low signal-to-noise, but angular resolution far beyond any current facility. The pioneering work on using the same array as both an atmospheric Cherenkov telescope and an intensity interferometer goes back to Hanbury Brown and others in the 1960s, but was not competitive with other techniques till recently. Since 2020, VERITAS, MAGIC, and H.E.S.S. have all successfully implemented intensity interferometry to measure stellar diameters. With its much larger collecting area and km-scale distribution, CTAO would be ideal for indirect imaging at optical wavelengths of stars and stellar outflows. Especially exciting are processes that also produce gamma-rays, such as colliding winds in massive stellar binaries. The groups at UZH and UniGe are contributing to the stellar intensity interferometry science case being discussed for inclusion in CTAO.