Nonequilibrium Hanbury-Brown-Twiss experiment: Theory and application to binary stars

In this work we consider the Hanbury-Brown and Twiss experiment for a configuration of two extended objects at different temperature. We show that intensity interference for these scenarios appears as an alternative method for measuring the features of the binary system. Unlike the case of equilibrium scenarios or the two-photon states, both temperatures and radii of each object take a role on the photon correlations. In addition to the interference oscillations on the second-order coherence, a long-baseline asymptotic value shows to depend on the observation frequency, temperatures and radii of both objects. We discuss the advantages of measuring each magnitude and also its combination according to the experimental possibilities. By finally including some aspects of the orbital motion to our approach, we apply it to the case of binary stars (in particular, Luhman 16 and Spica α Vir systems), showing that the method might be suitable for estimating not only the radii but the temperatures of the constituents. For the case of the Luhman 16 we show that measuring both magnitudes should be possible in the visible range with baselines of no more than hundreds of meters, while for the case of the Spica the same is possible in the ultraviolet regime but with baselines of tenths of meters. We believe that our work contributes to improve both fundamental and practical aspects of intensity interferometry as a tool for characterizing binary systems.