Stellar intensity interferometry: Prospects for sub-milliarcsecond optical imaging

Abstract

a b s t r a c tUsing kilometric arrays of air Cherenkov telescopes at short wavelengths, intensity interferometry may increase the spatial resolution achieved in optical astronomy by an order of magnitude, enabling images of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Intensity interferometry (once pioneered by Hanbury Brown and Twiss) connects telescopes only electronically, and is practically insensitive to atmospheric turbulence and optical imperfections, permitting observations over long baselines and through large airmasses, also at short optical wavelengths. The required large telescopes ($10 m) with very fast detectors ($ns) are becoming available as the arrays primarily erected to measure Cherenkov light emitted in air by particle cascades initiated by energetic gamma rays. Planned facilities (e.g., CTA, Cherenkov Telescope Array) envision many tens of telescopes distributed over a few square km. Digital signal handling enables very many baselines (from tens of meters to over a kilometer) to be simultaneously synthesized between many pairs of telescopes, while stars may be tracked across the sky with electronic time delays, in effect synthesizing an optical interferometer in software. Simulated observations indicate limiting magnitudes around m V = 8, reaching angular resolutions $30 larcsec in the violet. The signal-to-noise ratio favors high-temperature sources and emission-line structures, and is independent of the optical passband, be it a single spectral line or the broad spectral continuum. Intensity interferometry directly provides the modulus (but not phase) of any spatial frequency component of the source image; for this reason a full image reconstruction requires phase retrieval techniques. This is feasible if sufficient coverage of the interferometric (u, v)-plane is available, as was verified through numerical simulations. Laboratory and field experiments are in progress; test telescopes have been erected, intensity interferometry has been achieved in the laboratory, and first full-scale tests of connecting large Cherenkov telescopes have been carried out. This paper reviews this interferometric method in view of the new possibilities offered by arrays of air Cherenkov telescopes, and outlines observational programs that should become realistic already in the rather near future.

Long-baseline optical intensity interferometry

“…If, for example, a baseline of 2 km could be utilized with CTA at λ 350 nm, resolutions would approach 30 µas, which is an unparalleled spatial resolution in optical astronomy (Dravins et al, 2012(Dravins et al, , 2013.…”

“…Several studies have considered the observational challenges, such as the overall sensitivity and limiting stellar magnitudes, and those issues now appear to be consistently and generally understood (Dravins et al, 2012(Dravins et al, , 2013LeBohec & Holder, 2006;Trippe et al, 2014). For example, current types of electronics with current Cherenkov telescope designs should, during one night of observing in one single wavelength band, permit measurements of hotter stars down to a visual magnitude of about m V =8, giving access to many thousands of sources.…”

“…Of course, this enables an even richer (u,v)-plane coverage, and indeed it is the principle of aperture synthesis using Earth rotation (Saha, 2011). However, to retain a high Fourier-plane resolution in real-time correlation, this requires an electronic unit that implements a variable time delay onto the stream of photon pulses, compensating for the relative timings of the wavefront at the different telescopes, as the source moves across the sky (Dravins et al, 2012). In case the correlation is computed off-line, following a recording of the signal during observation, the variable delay would instead have to be applied in software.…”

“…Other candidate targets include rapidly rotating stars, with oblate shapes deformed by rotation, circumstellar disks, winds from hot stars, blue supergiants and extreme objects, such as η Carinae, interacting binaries, the hotter parts of [super]nova explosions, pulsating Cepheids, or other hotter variables (Dravins et al, 2012(Dravins et al, , 2013Trippe et al, 2014).…”

“…Among its envisioned uses is also that of an intensity interferometer, correlating intensity fluctuations between numerous telescope pairs across different baselines while applying timeshifts in software to track sources across the sky (Dravins et al, 2012(Dravins et al, , 2013. Since the signals are copied electronically (with no optical connection between telescopes), there is no loss of signal when forming additional baselines between any pairs of the numerous telescopes.…”