Journal articles: 'Very Large Telescope Interferometer'

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Schöller, M. "The Very Large Telescope Interferometer." EAS Publications Series 28 (2008): 1–8. http://dx.doi.org/10.1051/eas:0828001.

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Lévêque, Samuel, Bertrand Koehler, and Oskar von der Lühe. "Longitudinal dispersion compensation for the Very Large Telescope Interferometer." Astrophysics and Space Science 239, no. 2 (September 1996): 305–14. http://dx.doi.org/10.1007/bf00645784.

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Schöller, Markus. "The Very Large Telescope Interferometer: Current facility and prospects." New Astronomy Reviews 51, no. 8-9 (October 2007): 628–38. http://dx.doi.org/10.1016/j.newar.2007.06.008.

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Bedding, T. R., J. M. Beckers, M. Faucherre, N. Hubin, B. Koehler, O. von der Lühe, F. Merkle, and N. Zhu. "The VLT Interferometer." Symposium - International Astronomical Union 158 (1994): 143–50. http://dx.doi.org/10.1017/s0074180900107454.

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One of the observing modes available with the ESO Very Large Telescope will be coherent combination of the light received by up to four 8 m unit telescopes and several 1.8 m auxiliary telescopes. The location of the main telescopes is fixed, while auxiliary telescopes can be moved among some 30 observing stations. The locations of these stations were chosen to augment the (u, v) coverage of the unit telescopes as well as to function as an independent interferometric array.The 8 m telescopes will be equipped with adaptive optics to correct for seeing-induced wavefront aberrations. This wavefront correction will be complete at near-infrared wavelengths, giving the interferometer very high sensitivity in this spectral regime. This paper gives a brief description of the VLT Interferometer and an update on its status.

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Woillez, J., J. A. Abad, R. Abuter, E. Aller Carpentier, J. Alonso, L. Andolfato, P. Barriga, et al. "NAOMI: the adaptive optics system of the Auxiliary Telescopes of the VLTI." Astronomy & Astrophysics 629 (September 2019): A41. http://dx.doi.org/10.1051/0004-6361/201935890.

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Context. The tip-tilt stabilisation system of the 1.8 m Auxiliary Telescopes of the Very Large Telescope Interferometer was never dimensioned for robust fringe tracking, except when atmospheric seeing conditions are excellent. Aims. Increasing the level of wavefront correction at the telescopes is expected to improve the coupling into the single-mode fibres of the instruments, and enable robust fringe tracking even in degraded conditions. Methods. We deployed a new adaptive optics module for interferometry (NAOMI) on the Auxiliary Telescopes. Results. We present its design, performance, and effect on the observations that are carried out with the interferometric instruments.

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Richichi, A. "The VLT Interferometer." Proceedings of the International Astronomical Union 3, S248 (October 2007): 44–47. http://dx.doi.org/10.1017/s1743921308018607.

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AbstractThe ESO Very Large Telescope Interferometer (VLTI) is arguably the most powerful optical interferometric facility available at present. In addition to the wide choice of baselines and the light collecting power of its 8.2 m and 1.8 m telescopes, the VLTI also offers a smooth and user-friendly operation which makes interferometry accessible to any astronomer and covers a wide range of scientific applications. Behind the routine scientific operations, however, the VLTI is in constant evolution. I will present some of the technological and instrumental improvements which are planned for the near and mid-term future, and discuss their implications for astrometry in particular. Among them, the PRIMA facility and the proposed GRAVITY instrument are designed to reach the level of 10 microarcseconds in the near-infrared.

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Abuter, R., M. Accardo, A. Amorim, N. Anugu, G. Ávila, N. Azouaoui, M. Benisty, et al. "First light for GRAVITY: Phase referencing optical interferometry for the Very Large Telescope Interferometer." Astronomy & Astrophysics 602 (June 2017): A94. http://dx.doi.org/10.1051/0004-6361/201730838.

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GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as mK ≈ 10 mag, phase-referenced interferometry of objects fainter than mK ≈ 15 mag with a limiting magnitude of mK ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5°, corresponding to a differential astrometric precision of better than ten microarcseconds (μas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 μas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few μas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.

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Kol'tso, N. E., S. A. Grenkov, and L. V. Fedotov. "Comparison of Radio Interferometers with Analog and Digital Extraction of Recorded Signal." Journal of the Russian Universities. Radioelectronics 23, no. 2 (April 28, 2020): 6–18. http://dx.doi.org/10.32603/1993-8985-2020-23-2-6-18.

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Introduction. Radio telescopes of Very Long Baseline Interferometry (VLBI) networks usually record several signals with relatively narrow (up to 32 MHz) bands, which are extracted by means of base band converters (BBC) from an analog noise signal of an intermediate frequency (IF) with bands up to 1 GHz. When processing data, frequency band synthesis is used. At new small radio telescopes (for example, RT-13), directly wideband IF signals are digitized. An ability to connect the RT-13 radio telescope to the “Quasar” VLBI complex and to international VLBI networks provides by a digital narrow-band signal extraction module developed in 2019.Aim. Determining the measuring accuracy of an interferometric group delay of a signal by a radio interferometer with a digital narrow-band signal extraction module and comparing the sensitivity of interferometers with analog and digital signal extraction systems.Materials and methods. Sensitivity losses of interferometers with different systems for detecting recorded signals were calculated. The accuracy of a multi-channel interferometer with the synthesis of a frequency band and of an interferometer with recording of digital broadband IF signals without band synthesis was compared. The results were confirmed by VLBI observations in the observatories of the “Quasar” complex.Results. When replacing the analog system of signal extraction with digital system the sensitivity losses of the interferometer were slightly reduced. The measurement accuracy of the interferometric group delay had not changed. Accuracy increased when digitally recording broadband IF signals and when synthesizing a frequency band significantly larger than the IF bandwidth. Conditions and minimum synthesized bands were determined under which the accuracy of the interferometer with the registration of narrowband signals can be higher than the accuracy of the interferometer with the registration of wideband IF signals.Conclusion. The problem of combining RT-13 radio telescopes with VLBI networks with recording of video frequency signals was solved. The efficiency of the installation of digital signal conversion systems at radio telescopes was shown.

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Merkle, Fritz. "Interferometry with the European Very Large Telescope." Optics News 14, no. 3 (March 1, 1988): 15. http://dx.doi.org/10.1364/on.14.3.000015.

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Stee, Ph, A. Meilland, and O. L. Creevey. "Interferometry of massive stars: the next step." Proceedings of the International Astronomical Union 9, S307 (June 2014): 480–89. http://dx.doi.org/10.1017/s174392131400742x.

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AbstractWe present some new and interesting results on the complementarity between asteroseismology and interferometry, the detection of non-radial pulsations in massive stars and the possibility for evidencing differential rotation on the surface of Bn stars. We also discuss the curretn interferometric facilities, namely the Very Large Telescope Interferometer (VLTI)/AMBER, VLTI/MIDI, VLTI/PIONIER within the European Southern Observatory (ESO) context and the Center for High Angular Resolution Astronomy (CHARA) array with their current limitations. The forthcoming second-generation VLTI instruments GRAVITY and MATISSE are presented as well as the FRIEND prototype in the visible spectral domain and an update of the Navy Precision Optical Interferometer (NPOI). A conclusion is presented with a special emphasis on the foreseen difficulties for a third generation of interferometric instruments within the (budget limited) Extremely Large Telescope framework and the need for strong science cases to push a future visible beam combiner.

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Glindemann, A., F. Delplancke, P. Kervella, F. Paresce, A. Richichi, and M. Schöller. "Ground interferometric searches." Symposium - International Astronomical Union 202 (2004): 417–24. http://dx.doi.org/10.1017/s007418090021841x.

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In this article, we discuss the instrumental requirements for planet searches from the ground. We review several methods and their fundamental limitations comparing the direct measurement of the visibility function (both modulus and phase) to astrometry and nulling interferometry. In particular, we will take the Very Large Telescope Interferometer (VLTI) as an example. The VLTI is an excellent facility for planet searches from the ground since it has both large apertures and long baselines, providing high SNR and milli-arcsecond resolution.

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Scott, Paul F. "The Very Small Array." Symposium - International Astronomical Union 201 (2005): 17–22. http://dx.doi.org/10.1017/s0074180900216057.

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The VSA is a 14-element aperture-synthesis telescope for mapping structure in the Cosmic Microwave Background which is now being commissioned at the Teide Observatory, Tenerife. It operates at frequencies between 26 and 36 GHz, with a bandwidth of 1.5GHz. Its elements track the sky, providing fringe-rate discrimination of any instrumental effects such as antenna cross-talk or ground radiation. The instrument incorporates a single-baseline interferometer comprising two large (3.7m) dishes, which is used to provide concurrent (and same frequency) pointed flux measurements of point sources in the VSA fields, the positions of these source having been obtained previously from survey observations made with the Cambridge Ryle Telescope at 15 GHz. The VSA is now completing its commissioning programme and it will start routine observations in September 2000.

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Hamaus, N., T. Paumard, T. Müller, S. Gillessen, F. Eisenhauer, S. Trippe, and R. Genzel. "PROSPECTS FOR TESTING THE NATURE OF Sgr A*'s NEAR-INFRARED FLARES ON THE BASIS OF CURRENT VERY LARGE TELESCOPE—AND FUTURE VERY LARGE TELESCOPE INTERFEROMETER—OBSERVATIONS." Astrophysical Journal 692, no. 1 (February 10, 2009): 902–16. http://dx.doi.org/10.1088/0004-637x/692/1/902.

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Li, Shuang-Liang. "A nearby luminous AGN sample optically selected from Hubble Space Telescope." Monthly Notices of the Royal Astronomical Society 490, no. 3 (October 12, 2019): 3793–98. http://dx.doi.org/10.1093/mnras/stz2864.

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ABSTRACT In this work, a nearby luminous active galactic nucleus (AGN) sample is selected from Hubble Space Telescope(HST), where only sources with both X-ray emission observed by Chandra/XMM–Newton and radio flux detected by Very Large Array (VLA)/Very Long Baseline Array (VLBA)/very long baseline interferometry (VLBI)/Multi-Element Radio Linked Interferometer Network (MERLIN) are adopted to keep high precision. Finally, we get a sample of 30 luminous AGNs, which consists of 11 radio-loud AGNs (RLAGN) and 19 radio-quiet AGNs (RQAGN). It is found that the relationship between RUV and αox, which was first reported by Li & Xie in low-luminosity AGNs (LLAGN), and other relationships are all absent in RLAGN, probably due to the complex physical process therein. Our results indicate that the X-ray emission from jet should play an important role in RLAGN and further support the transition of accretion mode between LLAGN and RLAGN. On the other hand, the traditional relationships in RQAGN, such as αox and λ, Γ and λ, are found to be well consistent with previous works.

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Baumgartner, Sandra, Mauro Bernardini, José R. Canivete Cuissa, Hugues de Laroussilhe, Alison M. W. Mitchell, Benno A. Neuenschwander, Prasenjit Saha, Timothée Schaeffer, Deniz Soyuer, and Lorenz Zwick. "Towards a polarization prediction for LISA via intensity interferometry." Monthly Notices of the Royal Astronomical Society 498, no. 3 (September 2, 2020): 4577–89. http://dx.doi.org/10.1093/mnras/staa2638.

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ABSTRACT Compact Galactic Binary Systems with orbital periods of a few hours are expected to be detected in gravitational waves (GW) by Laser Interferometer Space Antenna (LISA) or a similar mission. At present, these so-called verification binaries provide predictions for GW frequency and amplitude. A full polarization prediction would provide a new method to calibrate LISA and other GW observatories, but requires resolving the orientation of the binary on the sky, which is not currently possible. We suggest a method to determine the elusive binary orientation and hence predict the GW polarization, using km-scale optical intensity interferometry. The most promising candidate is CD–30° 11223, consisting of a hot helium subdwarf with mB = 12 and a much fainter white dwarf companion, in a nearly edge-on orbit with period 70.5 min. We estimate that the brighter star is tidally stretched by 6 per cent. Resolving the tidal stretching would provide the binary orientation. The resolution needed is far beyond any current instrument, but not beyond current technology. We consider scenarios where an array of telescopes with km-scale baselines and/or the Very Large Telescope (VLT) and Extremely Large Telescope (ELT) are equipped with recently developed kilopixel sub-ns single-photon counters and used for intensity interferometry. We estimate that a team-up of the VLT and ELT could measure the orientation to ±1° at 2σ confidence in 24 h of observation.

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Bright, S. N., O. Chesneau, G. C. Clayton, O. De Marco, I. C. Leão, J. Nordhaus, and J. S. Gallagher. "Very Large Telescope Interferometer observations of the dust geometry around R Coronae Borealis stars★." Monthly Notices of the Royal Astronomical Society 414, no. 2 (May 3, 2011): 1195–206. http://dx.doi.org/10.1111/j.1365-2966.2011.18449.x.

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Kervella, P., M. Schöller, R. van Boekel, E. di Folco, S. Correia, A. Glindemann, F. Paresce, A. Richichi, M. Tarenghi, and M. Wittkowski. "Interferometric Observations of η Carinae - the VLTI Takes Its First Glimpse at the Central Source." International Astronomical Union Colloquium 187 (2002): 99–105. http://dx.doi.org/10.1017/s0252921100001287.

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AbstractWe describe the first long-baseline interferometric measurement ever made of the luminous blue variable η Carinae. η Car was observed with the Very Large Telescope Interferometer (VLTI) with the 8.2 m Unit Telescopes as well as with the 40 cm test siderostats on three different baselines. We will detail the measurements and data analysis and discuss briefly the implications of the derived visibility on the morphology of the central source. Furthermore we will discuss the future possibilities of interferometric observations and will show that η Car is one of the prime candidates for imaging with the VLTI.

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Davis, John. "Commission 9: Instruments and Techniques (Instruments et Techniques)." Transactions of the International Astronomical Union 21, no. 1 (1991): 41–52. http://dx.doi.org/10.1017/s0251107x00009780.

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The period covered by this report has seen significant progress in the development of the new generation of telescopes with apertures in the 8 m plus range. The period has encompassed the major construction phase of the 10 m Keck Telescope, witnessed the commissioning of the European Southern Observatory’s (ESO) New Technology Telescope and the approval of funding for the ESO Very Large Telescope (VLT). Significant progress has been achieved in developing the necessary technology for manufacturing and figuring large mirrors. There have been major expansions of activity in the areas of active control of telescope optics and adaptive optics, and in high angular resolution interferometry with several new groups entering both fields. The use of optical fibers, particularly in the area of multiple-object spectroscopy, has continued to grow. Several telescopes can now be operated remotely and the control systems of new telescopes are being designed to facilitate remote operation.

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Von Der Lühe, O., S. Solanki, and Th Reinheimer. "Observing stellar surface structure with the ESO-VLT interferometer." Symposium - International Astronomical Union 176 (1996): 147–64. http://dx.doi.org/10.1017/s0074180900083182.

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The ESO Very Large Telescope (VLT) has the capability to coherently combine the light from four 8m telescopes and from smaller Auxiliary telescopes with baselines 4m and 200m (VLT Interferometer or VLTI). The resulting resolution approaches 0.5 milli-arcsec at visible wavelengths and 1 milli-arcsec in the near infra-red. We estimate that about 2000 nearby bright stars can be resolved with these baselines. The surface of about 400 stars, mainly K and M giants, can be mapped with a resolution of a dozen pixels or better across the stellar surface. This resolution permits detailed studies of the structure of stellar atmospheres, their hydrodynamics and magneto-hydrodynamics. VLTI can easily resolve starspots and very likely also convection cells on an active K-type giant at 10pc distance. A marginal detection of the largest starspots on a very active solar-type star at the same distance may be possible, but it appears very unlikely that the surface structure on an inactive solar-type star can be imaged.We present computer simulations of stellar surfaces to study in detail the response of the interferometer to an extended, complex source. First results which simulate the end-to-end process in a simplified manner indicate that useful maps can be produced provided that a sufficient number of baselines are combined in Earth-rotation synthesis mode. We also give arguments that high spectral resolution will prove essential to constrain reconstructed maps.

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Richichi, A., I. Percheron, and J. Davis. "A list of bright interferometric calibrators measured at the European Southern Observatory Very Large Telescope Interferometer." Monthly Notices of the Royal Astronomical Society 399, no. 1 (October 11, 2009): 399–409. http://dx.doi.org/10.1111/j.1365-2966.2009.15289.x.

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Kardashev, N. S., and V. I. Slysh. "The RADIOASTRON Project." Symposium - International Astronomical Union 129 (1988): 433–40. http://dx.doi.org/10.1017/s0074180900135211.

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The RADIOASTRON mission (Andreyanov et al. 1986) is designed to achieve angular resolutions as fine as 6 microarcsec and will be used to study radio sources with very high brightness temperature. It will form an orbiting radio interferometer between a satellite radio telescope 10 m in diameter and several large ground-based radio telescopes. The orbit of the satellite makes it possible to have baselines from several thousands to 80,000 km. The satellite will be equipped with four dual polarization receivers at 327, 1665, 4830, and 22235 MHz, with local oscillators phase-locked to a ground-based hydrogen maser frequency standard via an S-band microwave link. The IF signals from the receivers will be transmitted to the ground by an X-band high data rate link and recorded on magnetic tape in the VLBA format. Participation of large radio telescopes from many countries is envisaged and encouraged.

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Przygodda, F. "Computer simulations of interferometric imaging with the Very Large Telescope Interferometer and its Astronomical Multibeam Recombiner instrument." Optical Engineering 40, no. 5 (May 1, 2001): 753. http://dx.doi.org/10.1117/1.1361106.

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Ohnaka, K. "Spatially resolving the atmospheric dynamics over the surface of red supergiants with the Very Large Telescope Interferometer." EAS Publications Series 60 (2013): 121–29. http://dx.doi.org/10.1051/eas/1360013.

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Tubbs, R., N. M. Elias, R. Launhardt, S. Reffert, F. Delplancke, A. Quirrenbach, T. Henning, and D. Queloz. "ESPRI data-reduction strategy and error budget for PRIMA." Proceedings of the International Astronomical Union 3, S248 (October 2007): 132–33. http://dx.doi.org/10.1017/s1743921308018887.

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AbstractThe Exoplanet Search with PRIma (ESPRI) will use the PRIMA dual-feed astrometric capability on the Very Large Telescope Interferometer (VLTI) to perform astrometric detections of extra-solar planets. We present an overview of our data-reduction strategy for achieving 10-μarcsecond accuracy narrow-angle astrometry using the PRIMA instrument. We discuss the error budget for astrometric measurements, and those aspects of our strategy which are designed to minimise the astrometric measurement errors.

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Paladini, Claudia, Fabien Baron, A. Jorissen, J. B. Le Bouquin, B. Freytag, S. Van Eck, M. Wittkowski, et al. "Constraining convection across the AGB with high-angular-resolution observations." Proceedings of the International Astronomical Union 14, S343 (August 2018): 27–30. http://dx.doi.org/10.1017/s1743921318007378.

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AbstractWe present very detailed images of the photosphere of an AGB star obtained with the PIONIER instrument, installed at the Very Large Telescope Interferometer (VLTI). The images show a well defined stellar disc populated by a few convective patterns. Thanks to the high precision of the observations we are able to derive the contrast and granulation horizontal scale of the convective pattern for the first time in a direct way. Such quantities are then compared with scaling relations between granule size, effective temperature, and surface gravity that are predicted by simulations of stellar surface convection.

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Guinan, Edward F., Scott Engle, and Edward J. Devinney. "Advances in Telescope and Detector Technologies – Impacts on the Study and Understanding of Binary Star and Exoplanet Systems." Proceedings of the International Astronomical Union 7, S282 (July 2011): 11–20. http://dx.doi.org/10.1017/s1743921311026792.

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AbstractCurrent and planned telescope systems (both on the ground and in space) as well as new technologies will be discussed with emphasis on their impact on the studies of binary star and exoplanet systems. Although no telescopes or space missions are primarily designed to study binary stars (what a pity!), several are available (or will be shortly) to study exoplanet systems. Nonetheless those telescopes and instruments can also be powerful tools for studying binary and variable stars. For example, early microlensing missions (mid-1990s) such as EROS, MACHO and OGLE were initially designed for probing dark matter in the halos of galaxies but, serendipitously, these programs turned out to be a bonanza for the studies of eclipsing binaries and variable stars in the Magellanic Clouds and in the Galactic Bulge. A more recent example of this kind of serendipity is the Kepler Mission. Although Kepler was designed to discover exoplanet transits (and so far has been very successful, returning many planetary candidates), Kepler is turning out to be a “stealth” stellar astrophysics mission returning fundamentally important and new information on eclipsing binaries, variable stars and, in particular, providing a treasure trove of data of all types of pulsating stars suitable for detailed Asteroseismology studies. With this in mind, current and planned telescopes and networks, new instruments and techniques (including interferometers) are discussed that can play important roles in our understanding of both binary star and exoplanet systems. Recent advances in detectors (e.g. laser frequency comb spectrographs), telescope networks (both small and large – e.g. Super-WASP, HAT-net, RoboNet, Las Combres Observatory Global Telescope (LCOGT) Network), wide field (panoramic) telescope systems (e.g. Large Synoptic Survey Telescope (LSST) and Pan-Starrs), huge telescopes (e.g. the Thirty Meter Telescope (TMT), the Overwhelming Large Telescope (OWL) and the Extremely Large Telescope (ELT)), and space missions, such as the James Webb Space Telescope (JWST), the possible NASA Explorer Transiting Exoplanet Survey Satellite (TESS – recently approved for further study) and Gaia (due for launch during 2013) will all be discussed. Also highlighted are advances in interferometers (both on the ground and from space) and imaging now possible at sub-millimeter wavelengths from the Extremely Long Array (ELVA) and Atacama Large Millimeter Array (ALMA). High precision Doppler spectroscopy, for example with HARPS, HIRES and more recently the Carnegie Planet Finder Spectrograph, are currently returning RVs typically better than ~2-m/s for some brighter exoplanet systems. But soon it should be possible to measure Doppler shifts as small as ~10-cm/s – sufficiently sensitive for detecting Earth-size planets. Also briefly discussed is the impact these instruments will have on the study of eclipsing binaries, along with future possibilities of utilizing methods from the emerging field of Astroinformatics, including: the Virtual Observatory (VO) and the possibilities of analyzing these huge datasets using Neural Network (NN) and Artificial Intelligence (AI) technologies.

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Zhuravlev, Vladimir I., Yu I. Yermolaev, and A. S. Andrianov. "Probing the ionosphere by the pulsar B0950+08 with help of RadioAstron ground-space baselines." Monthly Notices of the Royal Astronomical Society 491, no. 4 (December 4, 2019): 5843–51. http://dx.doi.org/10.1093/mnras/stz3370.

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ABSTRACT The ionospheric scattering of pulses emitted by PSR B0950+08 is measured using the 10-mRadioAstron Space Radio Telescope, the 300-m Arecibo Radio Telescope, and the 14 x 25-m Westerbork Synthesis Radio Telescope (WSRT) at a frequency band between 316 and 332 MHz. We analyse this phenomenon based on a simulated model of the phase difference obtained between antennas that are widely separated by nearly 25 Earth diameters. We present a technique for processing and analysing the ionospheric total electron content (TEC) at the ground stations of the ground-space interferometer. This technique allows us to derive almost synchronous half-hour structures of the TEC in the ionosphere at an intercontinental distance between the Arecibo and WSRT stations. We find that the amplitude values of the detected structures are approximately twice as large as the values for the TEC derived in the international reference ionosphere (IRI) project. Furthermore, the values of the TEC outside these structures are almost the same as the corresponding values found by the IRI. According to a preliminary analysis, the detected structures were observed during a geomagnetic storm with a minimum Dst index of ∼75 nT generated by interplanetary disturbances, and may be due to the influence of interplanetary and magnetospheric phenomena on ionospheric disturbances. We show that the Space Very Long Baseline Interferometry provides us with new opportunities to study the TEC, and we demonstrate the capabilities of this instrument to research the ionosphere.

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Osmanov, Z. "Are the Dyson rings around pulsars detectable?" International Journal of Astrobiology 17, no. 2 (June 13, 2017): 112–15. http://dx.doi.org/10.1017/s1473550417000155.

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AbstractIn the previous paper ring (Osmanov 2016) (henceforth Paper-I) we have extended the idea of Freeman Dyson and have shown that a supercivilization has to use ring-like megastructures around pulsars instead of a spherical shell. In this work we reexamine the same problem in the observational context and we show that facilities of modern infrared (IR) telescopes (Very Large Telescope Interferometer and Wide-field Infrared Survey Explorer (WISE)) might efficiently monitor the nearby zone of the solar system and search for the IR Dyson-rings up to distances of the order of 0.2 kpc, corresponding to the current highest achievable angular resolution, 0.001 mas. In this case the total number of pulsars in the observationally reachable area is about 64 ± 21. We show that pulsars from the distance of the order of ~ 1 kpc are still visible for WISE as point-like sources but in order to confirm that the object is the neutron star, one has to use the ultraviolet telescopes, which at this moment cannot provide enough sensitivity.

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Cruzalèbes, P., R. G. Petrov, S. Robbe-Dubois, J. Varga, L. Burtscher, F. Allouche, P. Berio, et al. "A catalogue of stellar diameters and fluxes for mid-infrared interferometry★." Monthly Notices of the Royal Astronomical Society 490, no. 3 (October 7, 2019): 3158–76. http://dx.doi.org/10.1093/mnras/stz2803.

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Abstract We present the Mid-infrared stellar Diameters and Fluxes compilation Catalogue (MDFC) dedicated to long-baseline interferometry at mid-infrared wavelengths (3–13 $\mu$m). It gathers data for half a million stars, i.e. nearly all the stars of the Hipparcos-Tycho catalogue whose spectral type is reported in the SIMBAD data base. We cross-match 26 data bases to provide basic information, binarity elements, angular diameter, magnitude and flux in the near and mid-infrared, as well as flags that allow us to identify the potential calibrators. The catalogue covers the entire sky with 465 857 stars, mainly dwarfs and giants from B to M spectral types closer than 18 kpc. The smallest reported values reach 0.16 $\mu$Jy in L and 0.1 $\mu$Jy in N for the flux, and 2 microarcsec for the angular diameter. We build four lists of calibrator candidates for the L and Nbands suitable with the Very Large Telescope Interferometer (VLTI) sub- and main arrays using the MATISSE instrument. We identify 1621 candidates for L and 44 candidates for N with the Auxiliary Telescopes (ATs), 375 candidates for both bands with the ATs, and 259 candidates for both bands with the Unit Telescopes (UTs). Predominantly cool giants, these sources are small and bright enough to belong to the primary lists of calibrator candidates. In the near future, we plan to measure their angular diameter with 1 per cent accuracy.

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Groh, Jose H. "Zooming into Eta Carinae with interferometry." Proceedings of the International Astronomical Union 9, S307 (June 2014): 267–72. http://dx.doi.org/10.1017/s1743921314006917.

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AbstractShaped by strong mass loss, rapid rotation, and/or the presence of a close companion, the circumstellar environment around the most massive stars is complex and anything but spherical. Here we provide a brief overview of the high spatial resolution observations of Eta Carinae performed with the Very Large Telescope Interferometer (VLTI). Special emphasis is given to discuss VLTI/AMBER and VLTI/VINCI observations, which directly resolve spatial scales comparable to those where mass loss originates. Studying scales as small as a few milli-arcseconds allows us to investigate kinematical effects of rotation and binarity in more detail than ever before.

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Ohnaka, Keiichi, Thomas Driebe, Karl-Heinz Hofmann, Gerd Weigelt, and Markus Wittkowski. "Resolving the dusty torus and the mystery surrounding LMC red supergiant WOH G64." Proceedings of the International Astronomical Union 4, S256 (July 2008): 454–58. http://dx.doi.org/10.1017/s1743921308028858.

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AbstractWe present mid-IR long-baseline interferometric observations of the red supergiant WOH G64 in the Large Magellanic Cloud with MIDI at the ESO's Very Large Telescope Interferometer (VLTI). Our MIDI observations of WOH G64 are the first VLTI observations to spatially resolve an individual stellar source in an extragalactic system. Our 2-D radiative transfer modeling reveals the presence of a geometrically and optically thick torus seen nearly pole-on. This model brings WOH G64 in much better agreement with the current evolutionary tracks for a 25 M⊙ star — about a half of the previous estimate of 40 M⊙ — and solves the serious discrepancy between theory and observation which existed for this object.

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Gurvits, L. I. "VLBI from the Moon." Highlights of Astronomy 11, no. 2 (1998): 985–87. http://dx.doi.org/10.1017/s1539299600019377.

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Very Long Baseline Interferometry (VLBI) technique occupies a special place among tools for studying the Universe due to its record high angular resolution. The latter is in the inverse proportion to the length of interferometer baseline at any given wavelength. Until recently, the available angular resolution in radio domain of about 1 milliarcsecond at centimeter wavelengths was limited by the diameter of the Earth. However, many astrophysical problems require a higher angular resolution. The only way to achieve this at a given wavelength is to create an interferometer with the baseline larger than the Earth’s diameter by placing at least one telescope in space. In February 1997, the first dedicated Space VLBI mission, VLBI Space Observatory Program (VSOP), led by the Institute of Space and Astronautical Sciences (Japan) has been launched (Hirabayashi 1997). The VSOP mission opens a new dimension in the development of radio astronomy of extremely high angular resolution and will be followed by other Space VLBI missions. A review of scientific drives and technological challenges of the next generation Space VLBI mission have been discussed, for example, by Gurvits et al. (1996) and Ulvestad et al. (1997).

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Davis, John. "Overview of Multiple–Aperture Interferometry Binary Star Results from the Southern Hemisphere." Proceedings of the International Astronomical Union 2, S240 (August 2006): 45–53. http://dx.doi.org/10.1017/s174392130700378x.

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AbstractThe first multiple-aperture interferometric study of a binary system, in which the power of combining interferometric and spectroscopic data was demonstrated, was made from the Southern Hemisphere. The observations of α Vir with the Narrabri Stellar Intensity Interferometer (NSII) were combined with spectroscopic and photometric data to yield the mass, radius and luminosity of the primary as well as an accurate distance to the system. The NSII also revealed a number of stars, previously thought to be single, to be binary systems. Several of these systems have subsequently been shown to be spectroscopic binaries.The Sydney University Stellar Interferometer (SUSI) and the European Southern Observatory's Very Large Telescope Interferometer (VLTI) are the two current Southern Hemisphere multiple aperture interferometers. SUSI is being used to determine interferometric orbits for some of the binary systems discovered with the NSII including β Cen and λ Sco and, in combination with spectroscopy, to determine accurate masses for early-type stars and accurate dynamical parallaxes for the systems.The VLTI has operated with three beam-combining instruments, namely VINCI, MIDI and AMBER. The few observations of binary systems that have been made so far are summarised and, while in general they are of a preliminary nature, they demonstrate the potential of the VLTI for binary star studies.One double-lined spectroscopic binary that has been observed with all three Southern Hemisphere instruments is γ2 Vel, which has the brightest Wolf-Rayet star in the sky as its secondary. The observations and preliminary results for the masses of the O-type primary and WC8 secondary and for the distance to the system are summarised.

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Gallenne, A., G. Pietrzyński, D. Graczyk, N. Nardetto, A. Mérand, P. Kervella, W. Gieren, S. Villanova, R. E. Mennickent, and B. Pilecki. "Fundamental properties of red-clump stars from long-baseline H-band interferometry." Astronomy & Astrophysics 616 (August 2018): A68. http://dx.doi.org/10.1051/0004-6361/201833341.

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Observations of 48 red-clump stars were obtained in the H band with the PIONIER instrument installed at the Very Large Telescope Interferometer. Limb-darkened angular diameters were measured by fitting radial intensity profile I(r) to square visibility measurements. Half the angular diameters determined have formal errors better than 1.2%, while the overall accuracy is better than 2.7%. Average stellar atmospheric parameters (effective temperatures, metallicities and surface gravities) were determined from new spectroscopic observations and literature data and combined with precise Gaia parallaxes to derive a set of fundamental stellar properties. These intrinsic parameters were then fitted to existing isochrone models to infer masses and ages of the stars. The added value from interferometry imposes a better and independent constraint on the R −Teff plane. Our derived values are consistent with previous works, although there is a strong scatter in age between various models. This shows that atmospheric parameters, mainly metallicities and surface gravities, still suffer from a non-accurate determination, limiting constraints on input physics and parameters of stellar evolution models.

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Perraut, K., L. Jocou, J. P. Berger, A. Chabli, V. Cardin, G. Chamiot-Maitral, A. Delboulbé, et al. "Single-mode waveguides for GRAVITY." Astronomy & Astrophysics 614 (June 2018): A70. http://dx.doi.org/10.1051/0004-6361/201732544.

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Context. Within the framework of the second-generation instrumentation of the Very Large Telescope Interferometer of the European Southern Observatory we have developed the four-telescope beam combiner in integrated optics. Aims. We optimized the performance of such beam combiners, for the first time in the near-infrared K band, for the GRAVITY instrument dedicated to the study of the close environment of the galactic centre black hole by precision narrow-angle astrometry and interferometric imaging. Methods. We optimized the design of the integrated optics chip and the manufacturing technology as well, to fulfil the very demanding throughput specification. We also designed an integrated optics assembly able to operate at 200 K in the GRAVITY cryostat to reduce thermal emission. Results. We manufactured about 50 beam combiners by silica-on-silicon etching technology. We glued the best combiners to single-mode fluoride fibre arrays that inject the VLTI light into the integrated optics beam combiners. The final integrated optics assemblies have been fully characterized in the laboratory and through on-site calibrations: their global throughput over the K band is higher than 55% and the instrumental contrast reaches more than 95% in polarized light, which is well within the GRAVITY specifications. Conclusions. While integrated optics technology is known to be mature enough to provide efficient and reliable beam combiners for astronomical interferometry in the H band, we managed to successfully extend it to the longest wavelengths of the K band and to manufacture the most complex integrated optics beam combiner in this specific spectral band.

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Hippler, Stefan, Wolfgang Brandner, Silvia Scheithauer, Martin Kulas, Johana Panduro, Peter Bizenberger, Henry Bonnet, et al. "Infrared Wavefront Sensing for Adaptive Optics Assisted Galactic Center Observations with the VLT Interferometer and GRAVITY: Operation and Results." Instruments 4, no. 3 (July 15, 2020): 20. http://dx.doi.org/10.3390/instruments4030020.

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This article describes the operation of the near-infrared wavefront sensing based Adaptive Optics (AO) system CIAO. The Coudé Infrared Adaptive Optics (CIAO) system is a central auxiliary component of the Very Large Telescope (VLT) interferometer (VLTI). It enables in particular the observations of the Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly specialized beam combiner, a device that coherently combines the light of the four 8-m telescopes and finally records interferometric measurements in the K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT) experience during observation. Each of the four CIAO units generates an almost diffraction-limited image quality at its UT, which ensures that maximum flux of the observed stellar object enters the fibers of the GRAVITY beam combiner. We present CIAO performance data obtained in the first 3 years of operation as a function of weather conditions. We describe how CIAO is configured and used for observations with GRAVITY. In addition, we focus on the outstanding features of the near-infrared sensitive Saphira detector, which is used for the first time on Paranal, and show how it works as a wavefront sensor detector.

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Davis, J., M. J. Ireland, J. R. North, J. G. Robertson, W. J. Tango, and P. G. Tuthill. "The Angular Diameter and Fundamental Parameters of Sirius A." Publications of the Astronomical Society of Australia 28, no. 1 (2011): 58–65. http://dx.doi.org/10.1071/as10010.

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AbstractThe Sydney University Stellar Interferometer (SUSI) has been used to make a new determination of the angular diameter of Sirius A. The observations were made at an effective wavelength of 694.1 nm and the new value for the limb-darkened angular diameter is 6.048 ± 0.040 mas (± 0.66%). This new result is compared with previous measurements and is found to be in excellent agreement with a conventionally calibrated measurement made with the European Southern Observatory's Very Large Telescope Interferometer (VLTI) at 2.176 μm (but not with a second globally calibrated VLTI measurement). A weighted mean of the SUSI and first VLTI results gives the limb-darkened angular diameter of Sirius A as 6.041 ± 0.017 mas (± 0.28%). Combination with the Hipparcos parallax gives the radius equal to 1.713 ± 0.009 R⊙. The bolometric flux has been determined from published photometry and spectrophotometry and, combined with the angular diameter, yields the emergent flux at the stellar surface equal to (5.32 ± 0.14) × 108 W m−2 and the effective temperature equal to 9845 ± 64 K. The luminosity is 24.7 ± 0.7 L⊙.

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Porro, Irene L., Thomas Berkefeld, and Christoph Leinert. "Simulation of the effects of atmospheric turbulence on mid-infrared visibility measurements with the mid-infrared interferometric instrument for the Very Large Telescope Interferometer." Applied Optics 39, no. 10 (April 1, 2000): 1643. http://dx.doi.org/10.1364/ao.39.001643.

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Kundu, Mukul R., and Jeongwoo Lee. "Sunspots at centimeter wavelengths." Proceedings of the International Astronomical Union 6, S273 (August 2010): 265–71. http://dx.doi.org/10.1017/s1743921311015353.

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AbstractThe early solar observations of Covington (1947) established a good relation between 10.7 cm solar flux and the presence of sunspots on solar disk. The first spatially resolved observation with a two-element interferometer at arc min resolution by Kundu (1959) found that the radio source at 3 cm has a core-halo structure; the core is highly polarized and corresponds to the umbra of a sunspot with magnetic fields of several hundred gauss, and the halo corresponds to the diffuse penumbra or plage region. The coronal temperature of the core was interpreted as due to gyroresonance opacity produced by acceleration of electrons gyrating in a magnetic field. Since the opacity is produced at resonant layers where the frequency matches harmonics of the gyrofrequency, the radio observation could be utilized to measure the coronal magnetic field. Since this simple interferometric observation, the next step for solar astronomers was to use arc second resolution offered by large arrays at cm wavelengths such as Westerbrock Synthesis Radio Telescope and the Very Large Array, which were primarily built for cosmic radio research. Currently, the Owens Valley Solar Array operating in the range 1-18 GHz and the Nobeyama Radio Heliograph at 17 and 34 GHz are the only solar dedicated radio telescopes. Using these telescopes at multiple wavelengths it is now possible to explore three dimensional structure of sunspot associated radio sources and therefore of coronal magnetic fields. We shall present these measurements at wavelengths ranging from 1.7 cm to 90 cm and associated theoretical developments.

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Asmus, D. "New evidence for the ubiquity of prominent polar dust emission in AGN on tens of parsec scales." Monthly Notices of the Royal Astronomical Society 489, no. 2 (August 19, 2019): 2177–88. http://dx.doi.org/10.1093/mnras/stz2289.

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ABSTRACT The key ingredient of active galactic nuclei (AGN) unification, the dusty obscuring torus was so far held responsible for the observed mid-infrared (MIR) emission of AGN. However, the best studied objects with Very Large Telescope Interferometer (VLTI)/MID-infrared Interferometric instrument (MIDI) show that instead a polar dusty wind is dominating these wavelengths, leaving little room for a torus contribution. But is this wind a ubiquitous part of the AGN? To test this, we conducted a straightforward detection experiment, using the upgraded Very Large Telescope (VLT)/VLT Imager and Spectrometer for mid-InfraRed (VISIR) for deep subarcsecond resolution MIR imaging of a sample of nine [O iv]-bright, obscured AGN, all of which were predicted to have detectable polar emission. Indeed, the new data reveal such emission in all objects but one. We further estimate lower limits on the extent of the polar dust and show that the polar dust emission is dominating the total MIR emission of the AGN. These findings support the scenario that polar dust is not only ubiquitous in AGN but also an integral part of its structure, processing a significant part of the primary radiation. The polar dust has to be optically thin on average, which explains e.g. the small dispersion in the observed MIR–X-ray luminosity correlation. At the same time, it has to be taken into account when deriving covering factors of obscuring material from MIR to bolometric luminosity ratios. Finally, we find a new tentative trend of increasing MIR emission size with increasing Eddington ratio.

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Kovačević, Andjelka B., Yu-Yang Songsheng, Jian-Min Wang, and Luka Č. Popović. "Probing the elliptical orbital configuration of the close binary of supermassive black holes with differential interferometry." Astronomy & Astrophysics 644 (December 2020): A88. http://dx.doi.org/10.1051/0004-6361/202038733.

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Context. Obtaining detections of electromagnetic signatures from the close binaries of supermassive black holes (CB-SMBH) is still a great observational challenge. The Very Large Telescope Interferometer (VLTI) and the Extremely Large Telescope (ELT) will serve as a robust astrophysics suite offering the opportunity to probe the structure and dynamics of CB-SMBH at a high spectral and angular resolution. Aims. Here, we explore and illustrate the application of differential interferometry on unresolved CB-SMBH systems in elliptical orbital configurations. We also investigate certain peculiarities of interferometry signals for a single SMBH with clouds in elliptical orbital motion. Methods. Photocentre displacements between each SMBH and the regions in their disc-like broad line regions (BLR) appear as small interferometric differential phase variability. To investigate the application of interferometric phases for the detection of CB-SMBH systems, we simulated a series of differential interferometry signatures, based on our model comprising ensembles of clouds surrounding each supermassive black hole in a CB-SMBH. By setting the model to the parameters of a single SMBH with elliptical cloud motion, we also calculated a series of differential interferometry observables for this case. Results. We found various deviations from the canonical S-shape of the CB-SMBH phase profile for elliptically configured CB-SMBH systems. The amplitude and specific shape of the interferometry observables depend on the orbital configurations of the CB-SMBH system. We get distinctive results when considering anti-aligned angular momenta of cloud orbits with regard to the total CB-SMBH angular momentum. We also show that their velocity distributions differ from the aligned cloud orbital motion. Some simulated spectral lines from our model closely resemble observations from the Paα line obtained from near-infrared AGN surveys. We found differences between the “zoo” of differential phases of single SMBH and CB-SMBH systems. The “zoo” of differential phases for a single SMBH take a deformed S shape. We also show how their differential phase shape, amplitude, and slope evolve with various sets of cloud orbital parameters and the observer’s position. Conclusions. We calculate an extensive atlas of the interferometric observables, revealing distinctive signatures for the elliptical configuration CB-SMBH. We also provide an interferometry atlas for the case of a single SMBH with clouds with an elliptical motion, which differs from those of a CB-SMBH. These maps can be useful for extracting exceptional features of the BLR structure from future high-resolution observations of CB-SMBH systems, but also of a single SMBH with clouds in an elliptical orbital setup.

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Goddi, C., H. Falcke, M. Kramer, L. Rezzolla, C. Brinkerink, T. Bronzwaer, J. R. J. Davelaar, et al. "BlackHoleCam: Fundamental physics of the galactic center." International Journal of Modern Physics D 26, no. 02 (February 2017): 1730001. http://dx.doi.org/10.1142/s0218271817300014.

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Einstein’s General theory of relativity (GR) successfully describes gravity. Although GR has been accurately tested in weak gravitational fields, it remains largely untested in the general strong field cases. One of the most fundamental predictions of GR is the existence of black holes (BHs). After the recent direct detection of gravitational waves by LIGO, there is now near conclusive evidence for the existence of stellar-mass BHs. In spite of this exciting discovery, there is not yet direct evidence of the existence of BHs using astronomical observations in the electromagnetic spectrum. Are BHs observable astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Milky Way, which hosts the closest and best-constrained supermassive BH candidate in the universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with new-generation instruments. The first experiment consists of making a standard astronomical image of the synchrotron emission from the relativistic plasma accreting onto Sgr A*. This emission forms a “shadow” around the event horizon cast against the background, whose predicted size ([Formula: see text]as) can now be resolved by upcoming very long baseline radio interferometry experiments at mm-waves such as the event horizon telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the next-generation near-infrared (NIR) interferometer GRAVITY at the very large telescope (VLT). The third experiment aims to detect and study a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama large millimeter array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and contributes directly to them at different levels. These efforts will eventually enable us to measure fundamental BH parameters (mass, spin, and quadrupole moment) with sufficiently high precision to provide fundamental tests of GR (e.g. testing the no-hair theorem) and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*. We conclude that the Galactic center provides a unique fundamental-physics laboratory for experimental tests of BH accretion and theories of gravity in their most extreme limits.

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Perley, R. A. "Observations of Galactic and Extragalactic Jets." International Astronomical Union Colloquium 89 (1986): 403–24. http://dx.doi.org/10.1017/s025292110008619x.

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Since nearly all discrete radio sources of astronomical interest are of insufficient angular extent for their detailed structural properties to be accessible to single-dish radio telescopes, radio interferometry must be employed to gain information on the morphologies of these objects. Recently constructed imaging interferometer arrays which employ the technique of Fourier synthesis, particularly MERLIN and the VLA (Very Large Array), and the more recent VLBI arrays, have given unprecedented imaging capabilities, with the result that our knowledge, and hence perceptions, of discrete radio sources have vastly changed over the last few years. An equally important parallel development has been image processing algorithms. These have vastly improved the quality of information produced by these arrays, so that an instrument such as the VLA can now produce images with speed and quality exceeding original design specifications by factors of 100 to 1000.

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Markoff, Sera, David M. Russell, Jason Dexter, Oliver Pfuhl, Frank Eisenhauer, Roberto Abuter, James C. A. Miller-Jones, and Thomas D. Russell. "Infrared interferometry to spatially and spectrally resolve jets in X-ray binaries." Monthly Notices of the Royal Astronomical Society 495, no. 1 (April 30, 2020): 525–35. http://dx.doi.org/10.1093/mnras/staa1193.

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ABSTRACT Infrared interferometry is a new frontier for precision ground-based observing, with new instrumentation achieving milliarcsecond (mas) spatial resolutions for faint sources, along with astrometry on the order of 10 microarcseconds (μas). This technique has already led to breakthroughs in the observations of the supermassive black hole at the Galactic centre and its orbiting stars, active galactic nucleus, and exo-planets, and can be employed for studying X-ray binaries (XRBs), microquasars in particular. Beyond constraining the orbital parameters of the system using the centroid wobble and spatially resolving jet discrete ejections on mas scales, we also propose a novel method to discern between the various components contributing to the infrared bands: accretion disc, jets, and companion star. We demonstrate that the GRAVITY instrument on the Very Large Telescope Interferometer should be able to detect a centroid shift in a number of sources, opening a new avenue of exploration for the myriad of transients expected to be discovered in the coming decade of radio all-sky surveys. We also present the first proof-of-concept GRAVITY observation of a low-mass XRB transient, MAXI J1820+070, to search for extended jets on mas scales. We place the tightest constraints yet via direct imaging on the size of the infrared emitting region of the compact jet in a hard state XRB.

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Pfuhl, O., R. Davies, J. Dexter, H. Netzer, S. Hönig, D. Lutz, M. Schartmann, et al. "An image of the dust sublimation region in the nucleus of NGC 1068." Astronomy & Astrophysics 634 (January 28, 2020): A1. http://dx.doi.org/10.1051/0004-6361/201936255.

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We present near-infrared interferometric data on the Seyfert 2 galaxy NGC 1068, obtained with the GRAVITY instrument on the European Southern Observatory Very Large Telescope Interferometer. The extensive baseline coverage from 5 to 60 Mλ allowed us to reconstruct a continuum image of the nucleus with an unrivaled 0.2 pc resolution in the K-band. We find a thin ring-like structure of emission with a radius r = 0.24 ± 0.03 pc, inclination i = 70 ± 5°, position angle PA = −50 ± 4°, and h/r < 0.14, which we associate with the dust sublimation region. The observed morphology is inconsistent with the expected signatures of a geometrically and optically thick torus. Instead, the infrared emission shows a striking resemblance to the 22 GHz maser disc, which suggests they share a common region of origin. The near-infrared spectral energy distribution indicates a bolometric luminosity of (0.4–4.7) × 1045 erg s−1, behind a large AK ≈ 5.5 (AV ≈ 90) screen of extinction that also appears to contribute significantly to obscuring the broad line region.

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Sahlmann, J., R. Abuter, S. Ménardi, and G. Vasisht. "Astrometry with the VLTI: calibration of the Fringe Sensor Unit for the PRIMA astrometric camera." Proceedings of the International Astronomical Union 3, S248 (October 2007): 124–25. http://dx.doi.org/10.1017/s174392130801884x.

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AbstractThe future PRIMA facility at the Very Large Telescope Interferometer (VLTI) in astrometric mode offers the possibility to perform relative narrow-angle astrometry with 10 micro-arcsecond accuracy. This is achieved with a dual-beam interferometer concept, where a reference star and the scientific target, confined in a 60 arcsecond field, are observed simultaneously. The angular separation of the two stellar objects gives rise to an optical delay in the interferometer, which is measured by the Fringe Sensor Unit (FSU) and an internal laser metrology. PRIMA is using two FSU fringe detectors, each observing the interference of stellar beams coming from one of the two objects and measuring the corresponding phase and group delay. The astrometric observable, yielding the angular separation, is deduced from the group delay difference observed between the two objects. In addition, the FSU phase delay estimate is used as error signal for the fringe stabilisation loop of the VLTI. Both functions of the FSU require high precision fringe phase measurements with a goal of 1 nm rms (corresponding to λ/2000). These can only be achieved by applying a calibration procedure prior to the observing run. We discuss the FSU measurement principle and the applied algorithms. The calibration strategy and the methods used to derive the calibration parameters are presented. Special attention is given to the achieved measurement linearity and repeatability. The quality of the FSU calibration is crucial in order to achieve the ultimate accuracy and to fulfill the primary objective of PRIMA astrometry: the detection and characterisation of extrasolar planetary systems.

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Inoue, Makoto. "Sub-mm VLBI from the Arctic—Imaging Black Holes." Proceedings of the International Astronomical Union 8, S288 (August 2012): 200–203. http://dx.doi.org/10.1017/s1743921312016870.

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AbstractWe are deploying a new station for sub-millimeter Very Long Baseline Interferometry (VLBI) to obtain shadow images of Supermassive Black Hole (SMBH). Sub-mm VLBI is thought to be the only way so far to get the direct image of SMBH by its shadow, thanks to the superb angular resolution and high transparency against dense plasma around SMBH. At the Summit Station on Greenland, we have started monitoring the opacity at sub-mm region. The Summit Station subtends long baselines with the Atacama Large Milimeter/submillimeter Array (ALMA) in Chile and Submillimeter Array (SMA) in Hawaii. In parallel, we started retrofitting the ALMA North America prototype telescope (renamed as Greenland Telescope: GLT) for the cold environment.

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Linfield, R. P., G. S. Levy, J. S. Ulvestad, C. D. Edwards, J. F. Jordan, S. J. DiNardo, C. S. Christensen, et al. "Very Long Baseline Interferometry Observations Using the Tracking and Data Relay Satellite as an Orbiting Radio Telescope." Symposium - International Astronomical Union 129 (1988): 457–58. http://dx.doi.org/10.1017/s0074180900135259.

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An antenna in geostationary orbit was used for VLBI observations at 2.3 GHz, in combination with ground antennas in Australia and Japan. 23 of the 25 observed sources were detected on orbiter-ground baselines, with baseline lengths as large as 2.15 earth diameters. Brightness temperatures between 1012 K and 4 × 1012 K were measured for 10 sources.

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Kammerer, Jens, and Sascha P. Quanz. "Simulating the exoplanet yield of a space-based mid-infrared interferometer based on Kepler statistics." Astronomy & Astrophysics 609 (December 22, 2017): A4. http://dx.doi.org/10.1051/0004-6361/201731254.

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Aims. We predict the exoplanet yield of a space-based mid-infrared nulling interferometer using Monte Carlo simulations. We quantify the number and properties of detectable exoplanets and identify those target stars that have the highest or most complete detection rate. We investigate how changes in the underlying technical assumptions and uncertainties in the underlying planet population impact the scientific return. Methods. We simulated 2000 exoplanetary systems, based on planet occurrence statistics from Kepler with randomly orientated orbits and uniformly distributed albedos around each of 326 nearby (d< 20 pc) stars. Assuming thermal equilibrium and blackbody emission, together with the limiting spatial resolution and sensitivity of our simulated instrument in the three specific bands 5.6, 10.0, and 15.0 μm, we quantified the number of detectable exoplanets as a function of their radii and equilibrium temperatures. Results. Approximately [see formula in PDF] exoplanets, with radii 0.5 REarth ≤ Rp ≤ 6 REarth, were detected in at least one band and half were detected in all three bands during ~0.52 years of mission time assuming throughputs 3.5 times worse than those for the James Webb Space Telescope and ~40% overheads. Accounting for stellar leakage and (unknown) exozodiacal light, the discovery phase of the mission very likely requires 2−3 years in total. The uncertainties in planet yield are dominated by uncertainties in the underlying planet population, but the distribution of the Bond albedos also has a significant impact. Roughly 50% of the detected planets orbit M stars, which also have the highest planet yield per star; the other 50% orbit FGK stars, which show a higher completeness in the detectability. Roughly 85 planets could be habitable (0.5 REarth ≤ Rp ≤ 1.75 REarth and 200 K ≤ Teq ≤ 450 K) and are prime targets for spectroscopic observations in a second mission phase. Comparing these results to those of a large optical/near-infrared telescope, we find that a mid-infrared interferometer would detect more planets and the number of planets depends less strongly on the wavelength. Conclusions. An optimized space-based nulling interferometer operating in the mid-infrared would deliver an unprecedented dataset for the characterization of (small) nearby exoplanets including dozens of potentially habitable worlds.

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Burtscher, Leonard, Klaus Meisenheimer, Walter Jaffe, Konrad R. W. Tristram, and Huub J. A. Rottgering. "Resolving the Nucleus of Centaurus A at Mid-Infrared Wavelengths." Publications of the Astronomical Society of Australia 27, no. 4 (2010): 490–95. http://dx.doi.org/10.1071/as09068.

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AbstractWe have observed Centaurus A with the Mid-Infrared Interferometric Instrument (MIDI) at the Very Large Telescope Interferometer at resolutions of 7–15 mas (at 12.5 μm) and filled gaps in the (u, v) coverage in comparison to earlier measurements. We are now able to describe the nuclear emission in terms of geometric components and derive their parameters by fitting models to the interferometric data. With simple geometrical models, the best fit is achieved for an elongated disk with flat intensity profile with diameter 76 ± 9 × 35 ± 2 mas (1.41 ± 0.17 × 0.65 ± 0.03 pc) whose major axis is oriented at a position angle (PA) of 10.1 ± 2.2° east of north. A point source contributes 47 ± 11% of the nuclear emission at 12.5 μm. There is also evidence that neither such a uniform nor a Gaussian disk are good fits to the data. This indicates that we are resolving more complicated small-scale structure in active galactic nuclei with MIDI, as has been seen in Seyfert galaxies previously observed with MIDI. The PA and inferred inclination i = of the dust emission are compared with observations of gas and dust at larger scales.

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Journal articles on the topic 'Very Large Telescope Interferometer'

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