Intensity Interferometry Research Articles

An Angular Diameter Measurement of β UMa via Stellar Intensity Interferometry with the VERITAS Observatory

We use the Very Energetic Radiation Imaging telescope Array System (VERITAS) imaging air Cherenkov telescope array to obtain the first measured angular diameter of β UMa at visual wavelengths using stellar intensity interferometry (SII) and independently constrain the limb-darkened angular diameter. The age of the Ursa Major moving group has been assessed from the ages of its members, including nuclear member Merak (β UMa), an A1-type subgiant, by comparing effective temperature and luminosity constraints to model stellar evolution tracks. Previous interferometric limb-darkened angular-diameter measurements of β UMa in the near-infrared (Center for High Angular Resolution Astronomy (CHARA) Array, 1.149 ± 0.014 mas) and mid-infrared (Keck Nuller, 1.08 ± 0.07 mas), together with the measured parallax and bolometric flux, have constrained the effective temperature. This paper presents current VERITAS-SII observation and analysis procedures to derive squared visibilities from correlation functions. We fit the resulting squared visibilities to find a limb-darkened angular diameter of 1.07 ± 0.04 (stat) ± 0.05 (sys) mas, using synthetic visibilities from a stellar atmosphere model that provides a good match to the spectrum of β UMa in the optical wave band. The VERITAS-SII limb-darkened angular diameter yields an effective temperature of 9700 ± 200 ± 200 K, consistent with ultraviolet spectrophotometry, and an age of 390 ± 29 ± 32 Myr, using MESA Isochrones and Stellar Tracks. This age is consistent with 408 ± 6 Myr from the CHARA Array angular diameter.

Advancement of Analytical Model for Hydrophobic Rectangular Pillared Array on Al-Surface and Its Experimental Validation

AbstractOver the past few decades, self-cleaning surfaces have been significantly investigated due to their commercial applications in various fields. However, the researchers are still lagging in developing better mathematical models and fabricating hydrophobic surfaces for direct espousal in industry. In this study, a force-balanced system-based mathematical model is modified for a rectangular pillared array-based micro-structure and MATLAB simulations were used to validate it theoretically. The same pattern was developed on Al-surface using a single-point diamond turning (SPDT) machine experimentally. The experimental results were validated using coherence correlation interferometry (CCI), optical microscopy, drop shape analyser (DSA), and field emission scanning electron microscopy (FESEM). The experimentally estimated and theoretically predicted contact angles of the rectangular pillared array are found in close agreement. Further, the advancement in mathematical models and models-based surface manufacturing strategies can boost the research in this domain to develop robust self-cleaning hydrophobic surfaces.

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.

Multi-antenna probing of absorbing regions inside and outside Cassiopeia A

Context. Cassiopeia A occupies an important place among supernova remnants (SNRs) in low-frequency radio astronomy. Located in our Galaxy, this powerful radio source emits synchrotron radiation that propagates through the SNR environment and the ionized interstellar medium. The analysis of its continuum spectrum from low frequency observations reveals the evolution of the SNR absorption properties over time and suggests a method for probing unshocked ejecta and the SNR interaction with the circumstellar medium (CSM). Aims. In this paper, we present low-frequency measurements of the integrated spectrum of Cassiopeia A to find the typical values of free-free absorption parameters towards this SNR in the middle of 2023. We also add new results to track its slowly evolving and decreasing integrated flux density. Methods. We used the New Extension in Nançay Upgrading LOFAR (NenuFAR) and the Ukrainian Radio Interferometer of NASU (URAN–2, Poltava) for measuring the continuum spectrum of Cassiopeia A within the frequency range of 8–66 MHz. The radio flux density of Cassiopeia A relative to the calibration source, the radio galaxy Cygnus A, has been obtained on June–July, 2023 with two sub-arrays for each radio telescope, used as a two-element correlation interferometer. Results. We measured the values of parameters such as the emission measure, electron temperature and average number of ion charges, for both the internal and external absorbing ionized gas towards Cassiopeia A from its integrated spectrum. Generally, their values are comparable to those presented in the literature, but their slight differences demonstrate the evolution of free-free absorption parameters in this SNR. Based on high accuracy of the measurements, we have detected the SNR–CSM interaction. This led to the fact that the maximum of this continuous spectrum, decreasing in intensity, changed in frequency from past higher values to lower ones over time. Probably, such changes occur periodically in which the spectral peak can move in the direction of both higher frequencies and lower ones. This effect is mainly caused by the evolution of emission measure outside Cassiopeia A. Conclusions. The integrated flux-density spectrum of Cassiopeia A obtained with the NenuFAR and URAN–2 interferometric observations opens up new possibilities for continuous monitoring the ionized gas properties in and around Cassiopeia A to observe the evolution of unshocked ejecta and the SNR-CSM interaction in future studies.

Science with the ASTRI Mini-Array: From Experiment to Open Observatory

Although celestial sources emitting in the few tens of GeV up to a few TeV are being investigated by imaging atmospheric Čerenkov telescope arrays such as H.E.S.S., MAGIC, and VERITAS, at higher energies, up to PeV, more suitable instrumentation is required to detect ultra-high-energy photons, such as extensive air shower arrays, as HAWC, LHAASO, Tibet AS-γ. The Italian National Institute for Astrophysics has recently become the leader of an international project, the ASTRI Mini-Array, with the aim of installing and operating an array of nine dual-mirror Čerenkov telescopes at the Observatorio del Teide in Spain starting in 2025. The ASTRI Mini-Array is expected to span a wide range of energies (1–200 TeV), with a large field of view (about 10 degrees) and an angular and energy resolution of ∼3 arcmin and ∼10 %, respectively. The first four years of operations will be dedicated to the exploitation of Core Science, with a small and selected number of pointings with the goal of addressing some of the fundamental questions on the origin of cosmic rays, cosmology, and fundamental physics, the time-domain astrophysics and non γ-ray studies (e.g., stellar intensity interferometry and direct measurements of cosmic rays). Subsequently, four more years will be dedicated to Observatory Science, open to the scientific community through the submission of observational proposals selected on a competitive basis. In this paper, I will review the Core Science topics and provide examples of possible Observatory Science cases, taking into account the synergies with current and upcoming observational facilities.

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.

Photon counting intensity interferometry in the blue at a 0.5 m telescope

Abstract Intensity interferometry is a re-emerging interferometry tool that alleviates some of the challenges of amplitude interferometry at the cost of reduced sensitivity. We demonstrate the feasibility of intensity interferometry with fast single-photon counting detectors at small telescopes by utilizing a telescope of diameter of merely 0.5 m. The entire measurement set-up, including collimation, optical filtering, and two single-photon detectors, is attached directly to the telescope without the use of optical fibres, facilitated by the large area of our single-photon detectors. For digitization and timing, we utilize a time-to-amplitude-converter. Observing α Lyrae (Vega) for a total exposure time of 32.4 h over the course of six nights, an auto-correlation signal with a contrast of (9.5 ± 2.7) × 10−3 and a coherence time of (0.34 ± 0.12) ps at a signal-to-noise ratio of 2.8 is measured. The result fits well to preceding laboratory tests as well as expectations calculated from the optical and electronic characteristics of our measurement set-up. This measurement, to our knowledge, constitutes the first time that a bunching signal with starlight was measured in the B band with single-photon counting detectors. Simultaneously, this is to date the stellar intensity interferometry measurement utilizing the smallest telescope. Our successful measurement shows that intensity interferometry can be adopted not only at large-scale facilities, but also at readily available and inexpensive smaller telescopes.

Реалізація алгоритму пошуку кореляційного інтерферометричного пеленгатора засобами програмної платформи GNU радіо

In the realm of modern radio equipment development, the integration of Software Defined Radio (SDR) systems, encompassing both software and hardware components, has become widespread. SDR systems, particularly, find applications in the creation of direction finders for radio signal sources. The effectiveness of direction finding and the identification of radio radiation sources largely hinge on the technical specifications of the equipment. For contemporary systems aimed at detecting the location of radio radiation sources, direction finders based on a correlation interferometer prove to be highly suitable. These direction finders excel in performing direction finding for a wide array of radio signals, capable of recognizing broadband modulated signals. They can simultaneously process and identify several signals within a single frequency channel, originating either from a singular source of radio signals (coherent) or from multiple sources (incoherent). SDR technology enables programmable reconfiguration of a radio receiver through software. Such receivers boast a broadband radio frequency component with a significant dynamic range, a high-speed Analog-to-Digital Converter (ADC) path, a robust signal processor, and a specialized digital filtering path. SDR receivers are adaptable to signals of various standards and frequencies, depending on the specific tasks at hand. The GNU RADIO software platform stands out as the most flexible open-source software platform for SDR systems. This article delves into the utilization of the GNU RADIO software platform to implement the search algorithm for the direction finder of the correlation interferometer. The Ettus USRP N210 SDR platform was selected for implementing the direction finder using the correlation interferometer algorithm. This study introduces a two-channel software-controlled model of the direction finder, operating according to the correlation interferometer algorithm. The measurement results for the placement angle of radio signal sources yielded an individual absolute error ranging from 2 to 4 degrees, with an average absolute error of about 3 degrees. These results are quite commendable for such mobile two-channel systems. While systems with four or more channels boast higher accuracy, they tend to have lower economic feasibility.

First intensity interferometry measurements with the H.E.S.S. telescopes

ABSTRACT Intensity interferometry for astrophysical observations has gained increasing interest in the last decade. The method of correlating photon fluxes at different telescopes for high resolution astronomy without access to the phase of the incoming light is insensitive to atmospheric turbulence and does not require high-precision optical path control. The necessary large collection areas can be provided by Imaging Atmospheric Cherenkov Telescopes. Implementation of intensity interferometers to existing telescope systems such as VERITAS and MAGIC has proven to be successful for high-resolution imaging of stars. In 2022 April we equipped two telescopes of the H.E.S.S. array in Namibia with an intensity interferometry setup to measure southern sky stars and star systems during the bright moon period. We mounted an external optical system to the lid of the telescope cameras, which splits the incoming light and feeds it into two photomultipliers in order to measure the zero-baseline correlation within one telescope in addition to the cross-correlation between the telescopes. The optical elements are motorized, which enables live correction of tracking inaccuracies of the telescopes. During the campaign we measured the spatial correlation curves and thereby the angular diameters of λ Sco (Shaula) and σ Sgr (Nunki), while we also performed systematic studies of our interferometer using the multiple star system of α Cru (Acrux).

Electronics and Detectors for the Stellar Intensity Interferometer of the ASTRI Mini-Array Telescopes.

The ASTRI Mini-Array is an international collaboration led by the Italian National Institute for Astrophysics (INAF) that will operate nine telescopes to perform Cherenkov and optical stellar intensity interferometry (SII) observations. At the focal plane of these telescopes, we are planning to install a stellar intensity interferometry instrument. Here we present the selected design, based on Silicon Photomultiplier (SiPM) detectors matching the telescope point spread function together with dedicated front-end electronics.

Aperture-synthesis-based ptychography intensity interferometry imaging

To address the problem of instability in phase retrieval, ptychography technology was introduced to intensity interferometry (II), called ptychography II imaging (PIII). The resolution of PIII system is limited by the size of detection aperture. However, large-aperture detector means high cost. In this paper, the concept of aperture synthesis is brought to PIII. We theoretically derive the periodicity of speckle pattern produced by PIII, which can help guide the system design. Our findings also suggested that discontinuous speckle patterns can still be used to reconstruct the image of object. Additionally, the aperture-synthesis-based PIII system was found to have stronger tolerance to intensity noise and could reconstruct images at low signal-to-noise ratios. Finally, by conducting simulations and experiments, we demonstrated the effectiveness of the aperture-synthesis-based PIII system. These results indicate that aperture synthesis is a valuable approach for enhancing the performance of PIII system.

Intensity correlation holography for remote phase sensing and 3D imaging.

Holography is an established technique for measuring the wavefront of optical signals through interferometric combination with a reference wave. Conventionally the integration time of a hologram is limited by the interferometer coherence time, thus making it challenging to prepare holograms of remote objects, especially using weak illumination. Here, we circumvent this limitation by using intensity correlation interferometry. Although the exposure time of individual holograms must be shorter than the interferometer coherence time, we show that any number of randomly phase-shifted holograms can be combined into a single intensity-correlation hologram. In a proof-of-principle experiment, we use this technique to perform phase imaging and 3D reconstruction of an object at a ∼3 m distance using weak illumination and without active phase stabilization.

Harnessing data augmentation to quantify uncertainty in the early estimation of single-photon source quality

Novel methods for rapidly estimating single-photon source (SPS) quality have been promoted in recent literature to address the expensive and time-consuming nature of experimental validation via intensity interferometry. However, the frequent lack of uncertainty discussions and reproducible details raises concerns about their reliability. This study investigates the use of data augmentation, a machine learning technique, to supplement experimental data with bootstrapped samples and quantify the uncertainty of such estimates. Eight datasets obtained from measurements involving a single InGaAs/GaAs epitaxial quantum dot serve as a proof-of-principle example. Analysis of one of the SPS quality metrics derived from efficient histogram fitting of the synthetic samples, i.e. the probability of multi-photon emission events, reveals significant uncertainty contributed by stochastic variability in the Poisson processes that describe detection rates. Ignoring this source of error risks severe overconfidence in both early quality estimates and claims for state-of-the-art SPS devices. Additionally, this study finds that standard least-squares fitting is comparable to using a Poisson likelihood, and expanding averages show some promise for early estimation. Also, reducing background counts improves fitting accuracy but does not address the Poisson-process variability. Ultimately, data augmentation demonstrates its value in supplementing physical experiments; its benefit here is to emphasise the need for a cautious assessment of SPS quality.

Simultaneous Correlative Interferometer Technique for Direction Finding of Signal Sources.

In this paper, we propose a novel simultaneous Correlative Interferometer (CI) technique that elaborately estimates the Direction of Arrival (DOA) of multiple source signals incident on an antenna array. The basic idea of the proposed technique is that the antenna-array-based receiver compares the phase of the received signal with one of the candidates at each time sample and jointly exploits these multiple time samples to estimate the DOAs of multiple signal sources. The proposed simultaneous CI-based DOA estimation technique collectively utilizes multiple time-domain samples and can be regarded as a generalized version of the conventional CI algorithm for the case of multiple time-domain samples. We first thoroughly review the conventional CI algorithm to comprehensively explain the procedure of the direction-finding algorithm that adopts the phase information of received signals. We also discuss several technical issues of conventional CI-based DOA estimation techniques that are originally proposed for the case of a single time-domain sample. Then, we propose a simultaneous CI-based DOA estimation technique with multi-sample diversity as a novel solution for the case of multiple time-domain samples. We clearly compare the proposed simultaneous CI technique with the conventional CI technique and we compare the existing Multiple Signal Classification (MUSIC)-based DOA estimation technique with the conventional CI-based technique by using the DOA spectrum as well. To the best of our knowledge, the simultaneous CI-based DOA estimation technique that effectively utilizes the characteristics of multiple signal sources over multiple time-domain samples has not been reported in the literature. Through extensive computer simulations, we show that the proposed simultaneous CI technique significantly outperforms both the conventional CI technique in terms of DOA estimation even in harsh environments and with various antenna array structures. It is worth noting that the proposed simultaneous CI technique results in much better performance than the classical MUSIC algorithm, which is one of the most representative subspace-based DOA estimation techniques.

Detection of laser beams based on intensity interferometry

We describe an approach to detecting off-axis radiation of laser beams propagating in scattering media, especially in the atmosphere, in the presence of background (solar) radiation. The method relies on a generalization of the conventional intensity interferometry (II) theory to scenarios involving coexisting sources of relatively long (laser radiation) and much shorter (background) coherence times. In such circumstances, the high coherence of the laser light allows its discrimination against even much stronger, but low-coherence, background. We propose a simple detection system consisting of a small array of photodetectors (e.g. photodiodes) and estimate the ratio of the background-to-laser irradiances at which the laser radiation is expected to be detectable.

A Pipeline for Modeling Rapidly Rotating Stars

Recent stellar intensity interferometry observations of stars at 416 nm complement Michelson interferometry observations in the near-infrared where stellar surface intensity gradients (e.g., limb darkening) are weaker. Intensity gradients due to gravity darkening in rapidly rotating stars are also expected to show higher contrast at 416 nm relative to the near-infrared. We have created a software pipeline to model stellar photospheres of rapidly rotating stars to compare predictions at these wavelength regions. The pipeline can produce model images, visibility predictions, and synthetic spectra. Here we show examples for the Be star β CMi, which has not been interferometrically imaged. We see higher contrast from gravity darkening at 416 nm in model images for rapid rotators, but only for those stars that are not viewed equator-on.

Quantum Astronomy at the University and INAF Astronomical Observatory of Padova, Italy

Twenty years ago, we started to apply quantum optics to the astronomical research carried out inside the Department of Physics and Astronomy and the INAF Astronomical Observatory in Padova, Italy. The initial activities were stimulated by the project of the European Southern Observatory (ESO) to build a 100 m diameter telescope, the Overwhelmingly Large (OWL) telescope. The enormous photon flux expected from such an aperture suggested that quantum optics concepts be utilized in order to obtain novel astrophysical results. Following initial successful attempts to utilize the orbital angular momentum of the light beam to enhance the visibility of faint companions to bright stars, the Padova team concentrated its efforts on very high time resolution, in order to measure and store the arrival time of celestial photons to better than one nanosecond. To obtain observational results, we built two photon counting photometers (AquEye and IquEye) to be used with our telescopes of the Asiago Observatory and with 4 m class telescopes such as the ESO New Technology Telescope (NTT) in Chile. This paper firstly describes these two instruments and then expounds the results obtained on pulsar light curves, lunar occultations and the first photon counting intensity interferometry measurements of the bright star Vega. Indeed, the correlation of photon arrival times on two or more apertures can lead to extremely high angular resolutions, as shown around 1970 by Hanbury Brown and Twiss. Prospects for quantum intensity interferometry with arrays of Cherenkov light telescopes will also be described.

Migration of Mechanical Perturbations Estimated by Seismic Coda Wave Interferometry During the 2018 Pre‐Eruptive Period at Kīlauea Volcano, Hawaii

AbstractWe use seismic ambient noise correlation and coda wave interferometry to estimate velocity variations at high temporal resolution, during the pre‐eruptive period and the onset of the 2018 eruption of Kīlauea volcano. A progressive velocity increase is observed from March to the end of April. It is followed by rapid decrease starting a few days before the onset of the East Rift Zone (ERZ) eruption and then by sharp velocity drop when the eruption started. The change of trend from velocity increase to decrease is progressively delayed by a few days from the summit caldera toward the ERZ. The location of the velocity perturbations shows a migration of the sources of velocity changes from the summit caldera toward the ERZ before the eruption. Using a model of pressure source, we show that the simultaneous caldera inflation and velocity increase probably result from an anisotropic distribution of fault and crack orientations. The velocity decrease could be due to damaging processes above the shallow reservoir and to plastic deformations around the caldera. We introduce a forward model of rock damage associated with the volcano‐tectonic seismicity to calculate the velocity decrease. The good agreement between the calculated and the observed velocity variations shows that a large part of the velocity decrease results from damage of the medium. The delayed onsets of velocity decrease and the source migration of velocity perturbations are probably related to progressive fault openings in the Southern and Eastern parts of the caldera and to magma transfer toward the ERZ.

Intensity interferometry for holography with quantum and classical light.

As first demonstrated by Hanbury Brown and Twiss, it is possible to observe interference between independent light sources by measuring correlations in their intensities rather than their amplitudes. In this work, we apply this concept of intensity interferometry to holography. We combine a signal beam with a reference and measure their intensity cross-correlations using a time-tagging single-photon camera. These correlations reveal an interference pattern from which we reconstruct the signal wavefront in both intensity and phase. We demonstrate the principle with classical and quantum light, including a single photon. Since the signal and reference do not need to be phase-stable nor from the same light source, this technique can be used to generate holograms of self-luminous or remote objects using a local reference, thus opening the door to new holography applications.

Imaging via Correlation of X-Ray Fluorescence Photons.

We demonstrate that x-ray fluorescence emission, which cannot maintain a stationary interference pattern, can be used to obtain images of structures by recording photon-photon correlations in the manner of the stellar intensity interferometry of Hanbury Brown and Twiss. This is achieved utilizing femtosecond-duration pulses of a hard x-ray free-electron laser to generate the emission in exposures comparable to the coherence time of the fluorescence. Iterative phasing of the photon correlation map generated a model-free real-space image of the structure of the emitters. Since fluorescence can dominate coherent scattering, this may enable imaging uncrystallised macromolecules.