Sub 35-ps Single-Photon Stellar Intensity Interferometry of Vega

Stellar intensity interferometry (SII) enables measurements of stellar photospheres by correlating intensity fluctuations, offering a robust alternative to amplitude-based techniques. With modern single-photon counting detectors and their ultra-high timing resolution, the method has re-emerged as a powerful tool in stellar interferometry. This work determines the stellar diameter of Vega at 405 nm using spatial intensity correlations measured at the C2PU facility, part of the Observatoire de la Côte d'Azur.

The setup was identical to Leopold et al. (2025). Two campaigns in April and August 2024 used hybrid photodetectors (HPDs) with 22.7 ps resolution and new PhotonPix detectors with 34.8 ps resolution. A Photonscore TDC integrated photon arrival times and IRIG-B timestamps for synchronization. The analysis calculated auto- and cross-correlation histograms from the recorded data, which were then normalized and corrected for optical path differences.

The temporal auto-correlations yielded a consistent coherence time of 0.21 ps. This confirms the effectiveness of the PhotonPix detectors despite a slight degradation in timing resolution relative to the HPDs. Combining data from the two measurement campaigns gives a uniform disk diameter of Vega of 2.97 +/- 0.23 mas, in agreement with the 3.047 mas diameter predicted by the stellar atmosphere model at the observation wavelength. This validates the technique and the new PhotonPix detectors for precision SII at sub-35 ps resolution.

Integrating advanced single-photon detectors like PhotonPix into existing IACT SII instruments promises substantial enhancements in signal-to-noise ratio and observational capabilities, particularly with CTA LSTs. Challenges remain, including night sky background saturation and precise positioning and guiding, but SII observations at apparent magnitudes up to 7.8 are within reach.