Stellar Intensity Interferometry observations at the Cherenkov Telescope Array Observatory with the Medium-Sized Telescopes

The current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs: HESS, VERITAS and MAGIC) has led to a renaissance in the use of stellar intensity interferometry for sub-milliarcsecond optical astronomy. This technique, used over distances of O(100 m) between telescopes, enabled the measurement of stellar radii on the order of a few hundred micro-arcseconds with a ~10% resolution (assuming disk-like emission) for about 30 stars and binary systems. The new generation of IACTs, the Cherenkov Telescope Array Observatory (CTAO), is starting to follow this path. In particular, an SII observing system has been implemented in the camera of the prototype large-sized telescope at the CTAO North site, in conjunction with the MAGIC telescopes at La Palma (Spain). The larger the number and extent of the baselines, the better the angular resolution of SII measurements for smaller and/or fainter stellar objects. Thus, the addition of the medium-sized telescopes (MSTs) to SII observations at the CTAO sites will improve angular resolution to a few percent for the stars already measured. In addition, a relative precision comparable to that of previous generation IACTs will be achieved for a few hundred stars. SII observations with CTAO will also allow limb and gravity darkening, stellar ellipticity, etc., to be measured with a precision unattainable with previous generation IACTs. But the larger the extent of the baselines, the more difficult it is to combine the signals from telescopes up to a kilometer apart. Previous SII projects with IACTs and hardware from radio interferometry experiments provide the necessary technology to equip the NectarCAM camera of the MSTs with an SII observing system. Two technological solutions are investigated. One is a digital acquisition system integrating an improved WhiteRabbit node (IDROGEN), and the other is an analog acquisition system based on a vertical cavity surface emitting laser employed in the current-generation gamma-ray observatories. The first tests performed do not prefer one solution from the other.