Stellar intensity interferometry over kilometer baselines: laboratory simulation of observations with the Cherenkov Telescope Array

Abstract

A long-held astronomical vision is to realize diffraction-limited optical aperture synthesis over kilometer baselines. This will enable imaging of stellar surfaces and their environments, show their evolution over time, and reveal interactions of stellar winds and gas flows in binary star systems. An opportunity is now opening up with the large telescope arrays primarily erected for measuring Cherenkov light in air induced by gamma rays. With suitable software, such telescopes could be electronically connected and used also for intensity interferometry. With no optical connection between the telescopes, the error budget is set by the electronic time resolution of a few nanoseconds. Corresponding light-travel distances are on the order of one meter, making the method practically insensitive to atmospheric turbulence or optical imperfections, permitting both very long baselines and observing at short optical wavelengths. Theoretical modeling has shown how stellar surface images can be retrieved from such observations and here we report on experimental simulations. In an optical laboratory, artificial stars (single and double, round and elliptic) are observed by an array of telescopes. Using high-speed photon-counting solid-state detectors and real-time electronics, intensity fluctuations are cross correlated between up to a hundred baselines between pairs of telescopes, producing maps of the second-order spatial coherence across the interferometric Fourier-transform plane. These experiments serve to verify the concepts and to optimize the instrumentation and observing procedures for future observations with (in particular) CTA, the Cherenkov Telescope Array, aiming at order-of-magnitude improvements of the angular resolution in optical astronomy.

Optical aperture synthesis with electronically connected telescopes

“…To cover also oblique and vertical baselines in the two-dimensional Fourier-transform plane, the position angle of the artificial 'star' was successively rotated relative to the plane of the telescopes. Various details of the overall laboratory arrangements and of measuring techniques are described elsewhere 14 . With N telescopes, N (N -1) / 2 different baselines can be constructed.…”

“…Such resolutions have hitherto been reached only in the radio, and it is awkward to predict what features on, or around stellar surfaces will appear in the optical 6 . However, to appreciate the meaning of such resolutions, we note that a hypothetical Jupiter-size exoplanet in transit across some nearby bright star such as Sirius would subtend an easily resolvable angle of some 350 µas 14 . While spatially resolving the disk of an exoplanet in its reflected light may remain unrealistic for the time being, the imaging of its dark silhouette on a stellar diskwhile certainly very challenging 19 could perhaps be not quite impossible.…”

Entanglement Enabled Intensity Interferometry

“…Another application is to the imaging of astrophysical sources. Since E 2 I 2 methodology could augment HBT, and it piggybacks naturally on HBT telescopes, it should be considered in conjunction with new proposed HBT experiments [26,27,28].…”