An intensity interferometry measurement of gamma Orionis (Bellatrix) with the VERITAS telescope array

In this talk I will describe the measurement of the B2V star gam Ori at visible wavelengths (417nm) using the VERITAS Stellar Intensity Interferometer. During the bright-moon periods Cherenkov observations at the VERITAS array are not feasible and intensity interferometry measurements take place as each of the four telescopes is instrumented with a single photomultiplier at the focal plane, digitized to 8-bits, and read out continuously at 250 MHZ. Offline, the recorded waveforms are correlated, revealing a small (~part per million) enhancement in the product of the currents for small relative time; this is the so-called Hanbury Brown-Twiss (HBT) effect. The size of the star may be extracted from the dependence of the correlation on telescope separation. In 2020-21, Bellatrix was observed for several hours, and its angular radius extracted with this technique. I will discuss this preliminary radius measurement, along with the convolution technique and the removal of a sizable noise component, which are used to obtain this measurement.