Stellar Intensity Interferometer Research Articles

Performance and first measurements of the MAGIC stellar intensity interferometer

ABSTRACT In recent years, a new generation of optical intensity interferometers has emerged, leveraging the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC telescopes host the MAGIC-SII system (Stellar Intensity Interferometer), implemented to investigate the feasibility and potential of this technique on IACTs. After the first successful measurements in 2019, the system was upgraded and now features a real-time, dead-time-free, 4-channel, GPU-based correlator. These hardware modifications allow seamless transitions between MAGIC’s standard very-high-energy gamma-ray observations and optical interferometry measurements within seconds. We establish the feasibility and potential of employing IACTs as competitive optical Intensity Interferometers with minimal hardware adjustments. The measurement of a total of 22 stellar diameters are reported, 9 corresponding to reference stars with previous comparable measurements, and 13 with no prior measurements. A prospective implementation involving telescopes from the forthcoming Cherenkov Telescope Array Observatory’s Northern hemisphere array, such as the first prototype of its Large-Sized Telescopes, LST-1, is technically viable. This integration would significantly enhance the sensitivity of the current system and broaden the UV-plane coverage. This advancement would enable the system to achieve competitive sensitivity with the current generation of long-baseline optical interferometers over blue wavelengths.

Wide-field intensity fluctuation imaging.

The temporal intensity fluctuations contain important information about the light source and light-medium interaction and are typically characterized by the intensity autocorrelation function, g2(τ). The measurement of g2(τ) is a central topic in many optical sensing applications, ranging from stellar intensity interferometer in astrophysics, to fluorescence correlation spectroscopy in biomedical sciences and blood flow measurement with dynamic light scattering. Currently, g2(τ) at a single point is readily accessible through high-frequency sampling of the intensity signal. However, two-dimensional wide-field imaging of g2(τ) is still limited by the cameras' frame rate. We propose and demonstrate a 2-pulse within-exposure modulation approach to break through the camera frame rate limit and obtain the quasi g2(τ) map in wide field with cameras of only ordinary frame rates.

Intensity interferometry and the second-order correlation function $g^{(2)}$ in astrophysics

Most observational techniques in astronomy can be understood as exploiting the various forms of the first-order correlation function g^(1). As however demonstrated by the Narrabri Stellar Intensity Interferometer back in the 1960's by Hanbury Brown & Twiss, and which is the first experiment to measure the second-order correlation function g^(2), light can carry more information than simply its intensity, spectrum and polarization. Since this experiment, theoretical and laboratory studies of non-classical properties of light have become a very active field of research, namely quantum optics. Despite the variety of results in this field, astrophysics remained focused essentially on first-order coherence. In this paper, we study the possibility that quantum properties of light could be observed in cosmic sources. We provide the basic mathematical ingredients about the first and the second order correlation functions, applied to the modern context of astronomical observations. The exploitation of g^(2) is certainly richer than what a modern intensity interferometer could bring and is particularly interesting for sources of non-thermal light. We conclude by briefly presenting why microquasars in our galaxy and their extragalactic parents can represent an excellent first target in the optical/near-infrared where to observe non-thermal light, and test the use of g^(2) in astrophysical sources.

Interferometric Constraints on Gravity Darkening with Application to the Modeling of Spica A & B

AbstractIn 2005 we obtained very precise interferometric measurements of the pole-on rapid rotator Vega (A0 V) with the longest baselines of the Center for High Angular Angular Resolution (CHARA) Array and the Fiber Linked Unit for Optical Recombination (FLUOR). For the analysis of these data, we developed a code for mapping sophisticated PHOENIX model atmospheres on to the surface of rotationally distorted stars described by a Roche-von Zeipel formalism. Given a set of input parameters for a star or binary pair, this code predicts the interferometric visibility, spectral energy distribution and high-resolution line spectrum expected for the system. For the gravity-darkened Vega, our model provides a very good match to the K-band interferometric data, a good match to the spectral energy distribution – except below 160 nm – and a rather poor match to weak lines in the high dispersion spectrum where the model appears overly gravity darkened. In 2006, we used the CHARA Array and FLUOR to obtain high precision measurements of the massive, non-eclipsing, double-line spectroscopic binary Spica, a 4-day period system where both components are gravity darkened rapid rotators. These data supplement recent data obtained with the Sydney University Stellar Interferometer (SUSI). Our study follows the classic 1971 study by Herbison-Evans et al. who resolved Spica as a binary with the Narrabri Stellar Intensity Interferometer (NSII). We will report on our progress modeling the new interferometric and archival spectroscopic data, with the goal towards better constraining the apsidal constant.

Overview of Multiple–Aperture Interferometry Binary Star Results from the Southern Hemisphere

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.

Forty Years of Progress in Long-Baseline Optical Interferometry: 2005 Robert Ellery Lecture

AbstractThe development of long-baseline optical interferometry in Australia from the Narrabri Stellar Intensity Interferometer (NSII) to the Sydney University Stellar Interferometer (SUSI) and the resulting technical and scientific achievements are described. Three examples of results from the SUSI programme, for a single star, a double-lined spectroscopic binary, and a Cepheid variable, are presented to illustrate the advances made in the past four decades. The leading role that Australia has played in the development of the field worldwide is discussed from a personal viewpoint. Long-baseline optical interferometry has promised much, has been slow to deliver, and has been restricted to black-belt interferometrists, but it has now matured to the point where it is becoming an observational technique for astronomers in general.

ζ Orionis A is a Double Star

A close, 4th magnitude companion to ζ Orionis A has been resolved with the Navy Prototype Optical Interferometer at Lowell Observatory. This confirms an indication of multiplicity from observations with the stellar intensity interferometer at Narrabri 26 years ago. The new component in the multiple system ADS 4263, ζ Orionis Ab, is 2 mag fainter than Aa, which is a supergiant of type O9.5. During 1998 February and March, the pair had a mean separation of 42 mas. Orbital motion was subsequently detected, but the corresponding arc allows only a preliminary ephemeris.

Bose statistics and the stars

A brief account is given of the early development of a new sources. Following the chance discovery that it was unaffected by scintillation it was proposed to apply the same principle to measuring visible stars. This proposal met with vigorous opposition from physicists when it was realised that it implied that the time of arrival of photons in two mutually coherent beams of light must be correlated. Two laboratory experiments were done to demonstrate that this correlation does in fact take place. Then, after a pilot model had measured the angular size of Sirius, a full scale stellar intensity interferometer was built and installed at Narrabri in Australia. In a programme lasting 12 years it measured the angular diameters of 32 single stars in the spectral range O to F and established the first wholly empirical temperature scale for stars in that range. For the last 10 years the work has been continued by the construction of the larger and more sensitive Sydney University Stellar Interferometer called SUSI.

Measuring Stars with High Angular Resolution: Results from Narrabri Observatory

The stellar intensity interferometer at Narrabri Observatory was used to measure the angular diameters of 32 stars in the spectral range 05f to F8 and brighter than about V = +2.6. The results were combined with measurements of absolute flux and, in a few cases parallax, to find the emergent flux, effective temperature, radius and luminosity of the stars. The methods used to calibrate these measurements and their uncertainties are discussed. It is shown that in measuring emergent flux and effective temperature the major uncertainties were most frequently in the measurements of angular size.

Measuring the sizes of stars

The Chatterton Astronomy Department aims to apply interferometers with very high resolving power to optical astronomy. The programme of the stellar intensity interferometer at Narrabri Observatory was completed in 1972 and since then the work has been directed towards building a more sensitive instrument with higher resolving power. As a first step a much larger intensity interferometer was designed but was not built because it was large, expensive and not as sensitive as desired. Efforts are now being made to design a more sensitive and cheaper instrument. A version of Michelson’s stellar interferometer is being built using modern techniques. It is hoped that it will reach stars of magnitude +8 and will work reliably in the presence of atmospheric scintillation. It is expected to cost considerably less than an intensity interferometer of comparable performance. The pilot model of this new instrument is almost complete and should be ready for test in 1984.

Long Baseline Interferometry and Binary Stars

AbstractThe observations of α Vir with the Narrabri Stellar Intensity Interferometer demonstrated the potential of long baseline interferometry for the determination of fundamental properties of double-lined spectroscopic binary systems. Since the completion of the programme with the Narrabri instrument the Chatterton Astronomy Department has been conducting a study aimed at developing a stellar interferometer with limiting magnitude V ≳ +8 and maximum baseline ≳ 1 km (resolution at 500 nm ≲ 7 × 10−5 seconds of arc). The way in which a long baseline interferometer may be used in the study of binary stars is outlined, the requirements for this work are discussed, and the current status and future plans of the Chatterton Astronomy Department’s programme to develop a new long baseline interferometer are summarised.

A Review of the Achievements and Potential of Intensity Interferometry

AbstractThe stellar intensity interferometer at Narrabri Observatory was operated for ten years. Measurements were made of 32 single stars of spectral type 0 to F and were used to establish the first completely empirical temperature scale for hot stars. The application of a high-resolution interferometer to stellar astronomy was explored: observations were made of binary, rotating and emission-line stars, limb-darkening, polarized light and of the effects of atmospheric scintillation and of Cerenkov radiation from the night sky. This programme was completed in 1973. An analysis has been made of the astronomical programme and value to stellar astronomy of a much larger and more sensitive instrument. An intensity interferometer capable of reaching stars of magnitude +7.3 with a baseline of 2 km has been designed and its cost has been estimated.

Measuring the angular diameters of stars

Abstract A difficult problem in observational optical astronomy is to measure the extremely small angles subtended at the Earth by the discs of stars. There are three techniques; Michelson's stellar interferometer, the stellar intensity interferometer at Narrabri, and lunar occultations. Michelson's interferometer has been used to measure a few cool giants and supergiants but, for technical reasons, attempts to develop it further have not, so far, proved successful. The stellar intensity interferometer is capable of very high resolving power and is unaffected by atmospheric seeing. It has been used to make all the existing measurements on hot stars. The present instrument at Narrabri in Australia is limited to stars brighter than magnitude + 2.5 and hotter than 6500°K (spectral type K5). A larger instrument could be built which would open up many new and interesting possibilities. The method of lunar occultations is comparatively simple and inexpensive. The accuracy of the results is still uncertain becau...

A Study of 2 Velorum with a Stellar Intensity Interferometer

The stellar intensity interferometer at Narrabri Observatory has been used, for the first time, to make observations of a multiple star ( γ Vel). Measurements have been made in the continuum at λ4430 and in the C III–IV emission feature at λ4650. The observations at λ4430 give the angular size of the semi-major axis of the binary system γ2 Vel and the angular diameter of the Wolf–Rayet component, while the observations at λ4650 give the angular size of the extended region responsible for the emission feature. The results have been combined with spectroscopic and photometric data to find the distance of γ Vel, the radius, absolute magnitude and emergent flux (λ5460) of the Wolf–Rayet star, and the radius and emergent flux (λ4650) of the emission region. The surface gravity and effective temperature of the Wolf–Rayet star have been estimated and the size of the emission region has been found to correspond to the critical Roche equipotential lobe around the Wolf–Rayet star. It is concluded that a stellar intensity interferometer can be applied profitably to the study of close binaries and emission-line stars.

Wideband multiplier correlator

Apparatus to measure the correlation between two wideband (10–100MHz) noise signals has been developed for the stellar intensity interferometer at the Narrabri observatory. This instrument is used to measure the angular diameters of stars in the range 0.01–0.0001″ of arc, with an accuracy within about 5%. To achieve this precision with the low signal/noise ratio of an intensity interferometer, it is necessary to integrate and average the data for periods up to 100h, which may involve observations spread over several weeks. The paper describes the novel techniques which have been used to obtain the necessary efficiency and stability in the electronic correlator.

Experimental tests of the theory of an amplitude-limited correlation detector

A sensitive correlation detector has been constructed in which the input signal and noise voltages are amplitude-limited. The correlator is described and experimental results are given in support of the theory. A method of analysing the data from the correlator to obtain the maximum output signal/noise ratio is discussed for the case where the signal and noise voltages depend on slowly varying functions of time. Particular reference is made to the analysis of data from observations with the stellar intensity interferometer for which the correlator was specifically designed. It is concluded that the dual-channel amplitude-limited correlator is stable and that it can be used to obtain accurate, meaningful and linear results despite the non-linear operations involved.

Stellar Intensity Interferometer I Signal-to-Noise Ratio for High-Intensity Radiation*

The signal-to-noise ratio (S/N) of the Brown–Twiss stellar intensity interferometer, in which the resolving time (τr) of the photodetectors is much longer than the coherence time of incident radiation (τc), is derived, especially for incident radiation with a very high effective temperature. The statistical properties of the wave amplitude of the incident radiation is assumed to be described by a gaussian distribution function. In order to evaluate the mean square error in the intensity-correlation measurement, the eighth-order moment of complex wave amplitudes is obtained by using the moment theorem for a gaussian random process. The mean square error of the high-intensity correlation experiment during a finite observation time (τ0) is determined chiefly by the random correlation of noise currents due to intensity fluctuations of the incident radiation. The S/N in the case of extremely high-intensity radiation, thus, is given by (τ0/τc)12∣γ12∣2, where γ12 is the complex degree of coherence between two observation points independent of the intensity of incident radiation. In the case of low-intensity radiation the mean square error of an intensity-correlation measurement is determined by the random correlation of shot noise in photoelectric currents. The S/N for low-intensity radiation, as originally discussed by Hanbury Brown and Twiss, is given by (τ0/τr)12∣γ12∣2ηnτc where n is the number of incident photons per second and η is the quantum efficiency of a photodetector. The S/N for the intermediate case is also discussed.