An Angular Diameter Measurement of β UMa with Stellar Intensity Interferometry at VERITAS

In this talk, I will present a measurement of the angular diameter of β UMa, the first scientific stellar intensity interferometry result from VERITAS, including results of an upcoming paper and details of observations and the analysis. VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array of four IACTs (Imaging Air Cherenkov Telescopes) used primarily for ultra high energy gamma ray astronomy. VERITAS stellar intensity interferometry exploits the bright period around the full moon to measure stars at a visual wavelength of 416 nm, with six pairs of telescopes providing a range of baselines approximately 35-170 m. SII is based on the Hanbury-Brown Twiss (HBT) effect, which is a part per million increase in probability in the product of the intensities of correlated light at the same relative time. The relationship between time correlations of the intensities of light between pairs of telescopes and their projected separation are used to produce a wavelength-dependent measurement of the angular diameter of a star. From nearly 40 hours of observations, we have measured the uniform disk angular diameter of β UMa to be 1.01 ± 0.03 mas. We also calculate the limb-darkened angular diameter of the star to be 1.06 ± 0.04 mas, to compare to previous measurements from other observatories at different wavelengths. I will discuss steps of the data analysis, including removal of contaminating frequencies, time-shifting of the correlation function to account for the change in optical path delay as the star travels across the sky, and time averaging to produce a correlation function, as well as the scientific importance of this measurement.