Letters of Intent received in 2022

LoI 2024-2159
Stellar Intensity Interferometry: Optical imaging toward micro-arcsecond resolution

Topics

1. Resolving persistent phenomena in stars: radii, shapes of rapid rotators, oscillations, orbits of non-interacting binaries

2. Imaging transient phenomena in stars: outflows, mass-transfer in binaries, novae

3. Current and future instruments and facilities: photo-detectors, correlators, current- and next-generation Cherenkov Telescopes.

4. Data, algorithms and software

Rationale

The basis of intensity interferometry, namely the Hanbury Brown and Twiss (HBT) effect, is the optical phenomenon of second-order coherence without classical first-order coherence. It is fundamental in quantum optics, and important in some other areas of physics as well. What is less known is that HBT originated in astronomy, and indeed Hanbury Brown was a past IAU President!

The lack of need for classical coherence means that intensity interferometry can operate without optical-quality mirrors, and in poor atmospheric seeing. The use of second-order coherence means that ultrafast (sub-ns) photon detection is desired while more photons are needed to get sufficient SNR. Time-domain measurements enable the important insensitivity to optical and atmospheric imperfections while reaching sufficient SNR has been a limitation. Thus the historical Narrabri Stellar Intensity Interferometer achieved sub-mas resolution in the 1960s, but the detector technology of the time limited it to about 30 bright stars.

Now, however, two developments are helping overcome the SNR challenge in intensity interferometry. One is vastly better photon detectors. The other is (surprisingly) the development of ground-based gamma-ray astronomy. Atmospheric Cherenkov telescopes measure the very short flashes of optical Cherenkov light generated in air and happen to have the same requirements of collecting many optical photons as fast as intensity interferometers. This makes it possible for Cherenkov telescopes to also operate as intensity interferometers.

The revival of intensity interferometry was initially led by a small number of advocates. The first scientific meeting on the topic since the 1970s was the "Workshop on Stellar Intensity Interferometry" in Salt Lake City in January 2009. Subsequent workshops have generated increasing interest. The next one is foreseen to be in Columbus, Ohio (April 2023).

Science results using intensity interferometry have been appearing since 2017. First from the twin 1m telescopes at Observatoire de la Côte d'Azur, Calern, and then from the Cherenkov telescopes of VERITAS in Arizona and MAGIC on La Palma. Observing campaigns at H.E.S.S. in Namibia have begun, with science results expected soon. With currently increasing activity, by mid-2024 one can confidently expect a body of results from facilities around the world, measuring stellar radii, resolving close binaries, perhaps spatially resolving stellar oscillation modes, at the sub-mas level.

In the late 2020s the Cherenkov Telescope Array, from its North and South locations in La Palma and Chile, will be a game changer. The baselines of 2 km or more would improve angular resolution by an order of magnitude. The dense coverage of the interferometric (u,v)-plane would be excellent for studying transient phenomena and two-dimensional image reconstruction for at least the brighter sources. The achievable resolution would reach about 30 micro-arcseconds, comparable to what recently was achieved by the Event Horizon Telescope in the short-wavelength radio.

Preparing for this astronomical window, where stellar outflows, mass transfer in binaries, perhaps the first hours of a nova, can be imaged, needs a community effort with diverse expertise, from MHD simulations of interacting binaries to novel detector technologies. A Focus Meeting at the IAU GA in 2024 would be a unique focus indeed for this emerging topic.