CTA Consortium meeting – DESY, Berlin/Zeuthen; May 2010 HUNDRED TIMES SHARPER THAN HUBBLE ! Intensity Interferometry with Various CTA Configurations Dainis Dravins & Hannes Jensen Lund Observatory, Sweden www.astro.lu.se/~dainis

ANGULAR RESOLUTION IN ASTRONOMY 1 arcsec 100 mas 10 mas 1 mas 100 µas 10 µas

(and not only starlight) Observing stars… (and not only starlight)

ESO Paranal

Actual image of the Mira-type variable T Leporis from VLTI Image obtained by combining hundreds of interferometric measurements Central disc shows stellar surface, surrounded by a spherical shell of expelled molecular material Infrared wavelengths color-coded: Blue = 1.4 – 1.6 µm Green = 1.6 – 1.75 µm Red = 1.75 – 1.9 µm In the green channel, the molecular envelope is thinner The size of Earth’s orbit is marked. Resolution = 4 milli-arcseconds (ESO press release 0906, Feb. 2009)

resolved surface objects Many stars become resolved surface objects for baselines 100-1000 m

Intensity interferometry Pro: Time resolution of 1 ns implies 30 cm light travel time; no need for any more accurate optics nor atmosphere. Short wavelengths no problem; hot sources observable Con: Signal comes from two-photon correlations, increases as signal squared; requires large flux collectors

Narrabri intensity interferometer with its circular railway track R.Hanbury Brown: BOFFIN. A Personal Story of the Early Days of Radar, Radio Astronomy and Quantum Optics (1991)

Flux collectors at Narrabri R.Hanbury Brown: The Stellar Interferometer at Narrabri Observatory Sky and Telescope 28, No.2, 64, August 1964

Intensity interferometry

OBSERVATIONS IN INTENSITY INTERFEROMETRY Visibility (solid) and squared visibility (dashed) as function of baseline at  500 nm. Inner curves are for a stellar disk of diameter 2 mas; outer for 1 mas.

OBSERVATIONS IN INTENSITY INTERFEROMETRY Squared visibility (“diffraction pattern”), of a stellar disk of angular diameter 0.5 mas. Z = normalized second-order coherence

OBSERVATIONS IN INTENSITY INTERFEROMETRY Squared visibility (“diffraction pattern”) from a close binary star. Left: Pristine image; Right: Logarithm of magnitude of Fourier transform

Different array layouts

OBSERVATIONS IN INTENSITY INTERFEROMETRY Projected baselines change with Earth rotation VERITAS Fourier plane coverage during 8 hours, as a star moves through the zenith

Simulated measurements of a binary star with CTA-B telescope array OBSERVATIONS IN INTENSITY INTERFEROMETRY Simulated measurements of a binary star with CTA-B telescope array Left: Short integration time (noisy); Right: Longer integration time. Color scale shows normalized correlation.

CTA B CTA D CTA I Left: Telescopes for CTA configurations B, D, and I. Center column: (u,v)-plane coverage for a star in zenith. Right: (u,v)-plane coverage for a star moving from zenith through 20 degrees west.

CTA B Simulated observations of binary stars with different sizes. (mV = 3; Teff = 7000 K; T = 10 h; t = 1 ns;  = 500 nm;  = 1 nm; QE = 0.7, array = CTA B) Top: Reconstructed and pristine images; Bottom: Fourier magnitudes. Already changes in stellar radii by only a few micro-arcseconds are well resolved.

CTA B Simulated observations of binary stars with different separations. (mV = 3; Teff = 7000 K; T = 10 h; t = 1 ns;  = 500 nm;  = 1 nm; QE = 0.7, array = CTA B) Top: Reconstructed and pristine images; Bottom: Fourier magnitudes. Stellar diameters and binary separations are well resolved.

Subsets of CTA B Subsets of CTA D Subsets of CTA I Left to right: About one half, one quarter, one eight of the telescopes retained.

CTA B CTA D CTA I Simulated observations in the (u,v)-plane of close binary stars. Full CTA configurations B (top row), D (middle), and I (bottom). Stellar magnitudes mV=3 (left column), mV=5, and mV=7 (right).

Simulated observations in the (u,v)-plane of close binary stars. Subsets of CTA B Subsets of CTA D Subsets of CTA I Simulated observations in the (u,v)-plane of close binary stars. Full CTA configurations B (top row), D (middle), and I (bottom). Half of all telescopes (left column), one quarter, and one eight (right).

CTA candidate configurations examined Conf.B Conf.D Conf.I Number of telescopes 42 57 77 Unique baselines 253 487 1606 Shortest baseline 32 170 90 meters Longest baseline 759 2180 2200 meters Resolution range @ 500 nm 0.16-3.9 0.05-0.75 0.06-1.4 mas Resolution range denotes smallest and largest angular sizes that can be resolved with the array (= 1.22 D/)

Evaluation: All configurations B, D, I provide dense sampling of the (u,v)-plane due to the sheer number of telescopes. Different declinations of the source or the geographic orientation of the array have negligible effects due to the large number of telescopes. but… Arrays such as D are severely crippled by lack of short baselines, limiting the instrument to studying sources smaller than 0.5 mas. Many telescopes are required for good Fourier-plane sampling, and too few telescopes provide poor data. Best performance among those examined: Configuration I.

Digital intensity interferometry Very fast digital detectors, very fast digital signal handling, and the quantum-optical theory of optical coherence now enable very-long-baseline optical interferometry by combining distant Cherenkov telescopes in software

OBSERVATIONS IN INTENSITY INTERFEROMETRY Diameters of brighter stars that are observable with intensity interferometry.

OBSERVATIONS IN INTENSITY INTERFEROMETRY Stellar diameters for different temperatures and different apparent magnitudes. Dashed lines show the baselines at which different diameters are resolved.