STELLAR INTENSITY INTERFEROMETRY. Dainis Dravins Lund Observatory

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1 CTA Stockholm STELLAR INTENSITY INTERFEROMETRY Optical imaging with sub-milliarcsecond resolution Dainis Dravins Lund Observatory

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

3 42-meter European Extremely Large Telescope

4 Seeing 8m HST E-ELT + AO limited (diffraction limit a few milliarcsec in the near-ir)

5 ESO Paranal

6 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 = µm Green = µm Red = 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)

7 Interferometric images of the F-type giant Aurigae during its month-long eclipse by an opaque disk, occurring every 27 years Infrared images of the transiting disk in the ϵ Aurigae system Kloppenborg et al., Nature 464, 870 (8 April 2010)

8 SHAPE OF ACHERNAR Image of the rapidly rotating ( Vsin i 250 km/s ) star Achernar ( Eri, B3 Vpe), from VLTI VINCI observations. Axis ratio = 1.56, the most flattened star seen until then. Because of the projection effect this ratio is a minimal value; the star could be even flatter. Individual diameter measurements are shown by points with error bars. A.Domiciano de Souza, P.Kervella, S.Jankov, L.Abe, F.Vakili, E.di Folco, F.Paresce: Astron.Astrophys. 407, L47

9 H.Jensen, D.Dravins, S.LeBohec, P.D.Nuñez: Stellar intensity interferometry: Optimizing air Cherenkov telescope array layouts, Proc. SPIE, 7734, 77341T, 2010

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11 Luciola* Hypertelescope * genus of fireflies The Luciola flotilla of many small collector mirrors operates like one giant diluted mirror. Focal beam-combiners independently exploit the sky image formed at the focal surface. A.Labeyrie et al., Luciola hypertelescope space observatory, Exp.Astron. 23, 463 (2009) & ESA Cosmic Vision proposal

12 INTENSITY INTERFEROMETRY

13 Intensity interferometry Pro: Time resolution of 10 ns, say, implies 3 m light travel time; no need for 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

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

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

16 PARTICLE PHYSICS

17 VERITAS MAGIC H.E.S.S. CANGAROO III AIR CHERENKOV TELESCOPES

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19 Digital intensity interferometry Cherenkov telescopes: Large flux collectors Fast digital detectors & high-speed signal handling Combine optical telescopes in software Huge number of baselines, no loss of digital signal 65 CTA telescopes: N (N-1)/2 = 2080 baselines Filled (u,v)-plane enables sub-milliarcsecond imaging

20 S/N in intensity interferometry PROPORTIONAL TO: Telescope areas (geometric mean) Detector quantum efficiency Square root of integration time Square root of electronic bandwidth

21 S/N in intensity interferometry PROPORTIONAL TO: Telescope areas (geometric mean) Detector quantum efficiency Square root of integration time Square root of electronic bandwidth Photon flux per optical frequency bandwidth

22 S/N in intensity interferometry PROPORTIONAL TO: Telescope areas (geometric mean) Detector quantum efficiency Square root of integration time Square root of electronic bandwidth Photon flux per optical frequency bandwidth INDEPENDENT OF: Width of optical passband

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24 SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY Squared visibility from a close binary star. Left: Pristine image; Right: Logarithm of magnitude of Fourier transform H.Jensen, D.Dravins, S.LeBohec, P.D.Nuñez: Stellar intensity interferometry: Optimizing air Cherenkov telescope array layouts, Proc. SPIE, 7734, 77341T, 2010

25 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. (Hannes Jensen, Lund Observatory, 2010)

26 H.Jensen, D.Dravins, S.LeBohec, P.D.Nuñez: Stellar intensity interferometry: Optimizing air Cherenkov telescope array layouts, Proc. SPIE, 7734, 77341T, 2010

27 SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY Limiting magnitude m v = 3 m v = 5 m v = 7 Simulated observations of binary stars of visual magnitudes 3, 5, and 7. Total integration time: 20 hours; wavelength 500 nm, time resolution 1 ns, quantum efficiency = 70% Array: CTA D H.Jensen, D.Dravins, S.LeBohec, P.D.Nuñez: Stellar intensity interferometry: Optimizing air Cherenkov telescope array layouts, Proc. SPIE, 7734, 77341T, 2010

28 Simulated observations of binary stars with different sizes. (m V = 3; T eff = 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. (Hannes Jensen, Lund Observatory, 2010)

29 Assumed close binary star of visual magnitude 6, and its reconstructed image from simulated observations during 12 hours with a CTA-like array. P.D.Nuñez, S.LeBohec, D.Kieda, R.Holmes, H.Jensen, D.Dravins: "Stellar Intensity Interferometry: Imaging capabilities of air Cherenkov telescope arrays", Proc. SPIE, 7734, 77341C, 2010

30 SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY S/N independent of spectral passband SIMULATED OBSERVATIONS OF ROTATIONALLY FLATTENED STAR WITH EMISSION-LINE DISK Left: Pristine image, 0.4 mas across with 10 µas equatorial emission-line disk, 6 times continuum intensity Center: Observed magnitude of the Fourier transform in continuum light Right: Same for a narrow-bandpass filter at He I 587 nm emission Stellar magnitude: m v = 6, T eff = 7000 K; T = 50 h, QE=70%; Array = CTA I D.Dravins, H.Jensen, S.LeBohec, P.D.Nuñez: Stellar Intensity Interferometry: Astrophysical targets for sub-milliarcsecond imaging, In Optical and Infrared Interferometry II, SPIE 7734, 77340A (2010)

31 ASTROPHYSICAL TARGETS FOR KILOMETRIC-SCALE INTENSITY INTERFEROMETRY (Dravins et al., SPIE Proc. 7734, 2010)

32 NON-RADIAL PULSATIONS & VELOCITIES ACROSS STELLAR SURFACES Observations through very narrow bandpass filters, spanning one spectral line (might require ordinary telescopes rather than Cherenkov ones) Simulated observations of a Cepheid-like star undergoing non-radial pulsations m V = 3.4; T eff = 7000 K; Δt = 1 ns; = 500 nm; Array = CTA B Left: Pristine image; Right: Observed Fourier magnitude (Hannes Jensen, Lund Observatory, 2010)

33 Experimental work

34 VERITAS telescopes at Basecamp, Mt.Hopkins, Arizona TESTING CONCEPTS FOR DIGITAL INTENSITY INTERFEROMETRY

35 The four 12-meter telescopes of the VERITAS array in Arizona offer baselines between m S.LeBohec, M.Daniel, W.J.de Wit, J.A.Hinton, E.Jose, J.A.Holder, J.Smith, R.J.White Stellar Intensity Interferometry with Air Cherenkov Telescope Arrays in D.Phelan, O.Ryan & A.Shearer, eds., The Universe at sub-second timescales, AIP Conf.Proc. 984, 205 (2008)

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39 Intensity interferometry can be carried out in moonlight when Cherenkov observations are not feasible

40 The 3 m air Cherenkov telescopes at StarBase (west of Salt Lake City, Utah) are protected by buildings which can be rolled open for observation, with the control room between them. The picture at left right shows one telescope before its camera was mounted. S.LeBohec et al.: Stellar intensity interferometry: Experimental steps toward long-baseline observations, Proc. SPIE 7734, 77341D, 2010

41 StarBase: Optics of the intensity-interferometry camera is mounted just above the focal plane of the telescope (AutoCAD drawing at left; actual camera at right). The light is reflected by a 45 mirror to a collimating lens and an interference filter. A beamsplitter can divide the light to two independent channels. The electronics are next to the photo-detectors, on the back of the camera. The lower-right image is of the star Capella; the 1 cm grid corresponds to 0.19 in the focal plane. S.LeBohec et al.: Stellar intensity interferometry: Experimental steps toward long-baseline observations, Proc. SPIE 7734, 77341D, 2010

42 A group from the Workshop on Stellar Intensity StarBase Utah, Grantsville (2009)

43 Stellar Intensity Interferometry Laboratory Lund Observatory An artificial star is observed by a pair of movable telescopes. Detected photon streams are cross correlated in real time.

44 Stellar Intensity Interferometry Laboratory Lund Observatory An artificial star is observed by a pair of movable telescopes across a 20 m long optics lab.

45 Single-photon-counting avalanche photodiode detectors being Lund Observatory for digital intensity interferometry ( made by: ID Quantique; Micro Photon Devices; PerkinElmer; SensL ) Analyzing photon-counting detectors Afterpulsing, afterglow and other signatures could mimic intensity correlations

46 Real-time digital photon correlators Permit to verify various observational modes, both in the lab, and at telescopes

47 DIGITAL PHOTON Lund Observatory 700 MHz clock rate (1.4 ns time resolution) 200 MHz maximum photon count rates per channel (pulse-pair resolution 5 ns) 8 input channels for photon pulses at TTL voltages Custom-made by Correlator.com for applications in intensity interferometry

48 Our local Universe is teeming with stars, but despite 400 years of telescopic observations, astronomy is still basically incapable of observing stars as such! Although we can observe the light radiated by them, we do not (with few exceptions) have the capability to observe the stars themselves, i.e., resolving their disks or viewing structures across and outside their surfaces (except for the Sun, of course!). In 2009, we celebrated 400 years of telescopic astronomy One can just speculate what new worlds will be revealed once stars no longer will be seen as mere point sources but as extended and irregular objects with magnetic or thermal spots, flattened or distorted by rapid rotation, and with mass ejections monitored in different spectral features as they flow towards their binary companions. It is not long ago that the satellites of the outer planets passed from being mere point sources to a plethora of different worlds, and one might speculate what meager state extragalactic astronomy would be in, were galaxies observed as point sources only. (Dravins & LeBohec, SPIE Proc. 6986, 2008)

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