Astronomical Intensity Interferometry
Ribak, E.N.; Ribak, E.N.
Israel Institute of Technology

Intensity interferometry (II) was used for a while until thirty years ago to resolve 32 blue stars1. It had limited sensitivity and resolution, and when it exhausted its observation list it ceased operations. Some of these observations have not been surpassed since, because modern interfer-ometers operate at lower energies. Employing Galileo accurate timing and positioning and modern electronics and optics, it might be possible to rejuvenate II to fainter objects and obtain finer details than achievable before. Before embarking on technical details, it is essential to show that investment in II will yield significant scientific fruit, some of it unique to this special device. As has been mentioned above, II has access to shorter wave lengths and to higher resolution, something that is not achievable and not in planning in the near future. A multi-kilometer-size array will allow observations of fainter and finer details and measure their spectra2,3. Here are some interesting examples. Fast repeaters, such as pulsars, can be detected synchronously with their period. By slight de-lay the phase of the stellar signal can be measured. Cepheids could be better calibrated for their luminosity-period relation4. Close binaries will be characterized by astrometry, from interferome-try and imaging, and their radial velocity obtained from their spectra. Photometry, including tran-sits and lensing events, can easily be measured. Similar observables of course include also triples, bright compact objects and extra-solar planets. The detection of the latter will depend on the con-trast, or amount of light scattered in these short wave lengths from the central stars to overwhelm the planet signal. There are more interesting but less known targets, namely highly coherent objects. Masers are known to exist in galaxies, and probably visible lasers show up in more excited volumes5. These, and possible laser signals from extra-terrestrial civilizations can be found interferometrically, but the coherence of these objects (or the anti-bunching of their photons) can be ascertained using II6. Recent suggestions to rehabilitate II 2-4 are based on the advance of optics, electronics, and computers in the past thirty years. More and larger light collectors allow measurements down to limits barely accessible with amplitude interferometry. It is suggested here to use time signals from Galileo with position accuracy of AG to synchronise these collectors, raising the bandwidth to c/AG. Thus the locations of the collectors can be flexible, improving the spatial frequency cov-erage of the array and the final image quality. By using Galileo, the distance between light col-lecting dishes can be kilometers and tens of kilometers, getting an unprecedented resolution in the blue, at least an order of magnitude better than the original II experiment. Further enhancement can be achieved from higher order correlations between many collect-ing dishes, which drive the sensitivity even higher2,3. Novel optical and communications tech-nologies such as fibre optics and GHz transmitters allow improved transmission of the signals to a central correlation station. In summary, II is a low-risk, relatively low-price project, which banks on Galileo signals to achieve unprecedented results. It allows measurements essentially impossible today, on the ground or in space, of unique astrophysical targets. References. 1. R Hanbury Brown, The Intensity Interferometer – its application to astronomy. Taylor & Fran-cis (1974). 2. A Ofir and E N Ribak, Off-Line, Multi-Detector Intensity Interferometers I: Theory, Monthly Notices of the Royal Astronomical Society 368, 1646-51 (2006). 3. A Ofir and E N Ribak, Off-Line, Multi-Detector Intensity Interferometers II: Implications and Applications, Monthly Notices of the Royal Astronomical Society 386, 1651-6 (2006). 4. S Le Bohec and J Holder, Optical intensity interferometry with atmospheric cerenkov telescope arrays, The Astrophysical Journal 649, 399–405 (2006). 5. E N Ribak: Search for temporal coherence in the sky. SPIE 6268-160, Orlando (2006). 6. D Dravins "Quantum-optical signatures of stimulated emission", Astron. Soc. Pac. Conf. Proc. 242, 339. Ed T R Gull, S Johannson, and K Davidson (2001).