Deconvolution techniques for dynamical interferometric observations

La técnica de Interferometría de Muy Larga Baseline, o VLBI por sus siglas en inglés, está experimentando actualmente una notable era. Esta poderosa técnica consiste en conectar telescopios ubicados en distintas partes del mundo para crear un "super-telescopio" virtual. Al igual que los antiguos espartanos con su formación de falange, cada telescopio se convierte en parte de un sistema más grande, más sólido y más capaz. Gracias a VLBI, la colaboración internacional conocida como el Telescopio del Horizonte de Sucesos (EHT, por sus siglas en inglés), conformada por más de 300 astrónomos y 80 instituciones, ha mostrado al mundo lo invisbile: dos agujeros negros ubicados en los centros de la galaxia M87 y de nuestra propia Vía Láctea. Estos logros destacados han impulsado el futuro de VLBI hacia una trayectoria prometedora. Se están construyendo nuevos telescopios para expandir y mejorar la red global de observatorios, lo que permitirá no solo la observación de estos gigantes cósmicos, sino también una comprensión más profunda de su entorno circundante. Sin embargo, la conversión de los datos brutos recopilados por los telescopios en imágenes significativas requiere el desarrollo de modelos matemáticos complejos. De hecho, la llegada de telescopios de próxima generación plantea desafíos tecnológicos que exigen una reevaluación de las técnicas convencionales. Esta tesis presenta tres resultados significativos dirigidos específicamente a abordar las demandas únicas de esta nueva generación de telescopios. Entre los algoritmos desarrollados en este trabajo se encuentra un algoritmo de calibración que garantiza un tratamiento óptimo de los datos, así como dos algoritmos avanzados para la reconstrucción de imágenes y “películas” de objetos astronómicos, con un enfoque particular en los agujeros negros.Very Long Baseline Interferometry, or VLBI, is currently experiencing a remarkable era. This powerful technique involves connecting telescopes positioned across the globe to create a virtual “super-telescope”. Just like the ancient Spartans with their phalanx formation, each telescope becomes part of a larger, stronger, and more capable system. Thanks to VLBI, the international collaboration known as the Event Horizon Telescope (EHT) has achieved groundbreaking accomplishments. Comprising over 300 astronomers and 80 institutions, the EHT collaboration has unveiledthe unseenable: Two supermassive black holes located at the centers of the M87 galaxy and our own Milky Way. These remarkable achievements have propelled the future of VLBI into a promising trajectory. New telescopes are being constructed to expand and enhance the global network of observatories, allowing not only the observation of these cosmic giants but also a deeper understanding of their surrounding environments. However, the conversion of the raw data collected by telescopes into images necessitates the development of complex mathematical models. In fact, the advent of next-generation telescopes introduces technological challenges that demand a reevaluation of conventional techniques. This thesis presents three significant results specifically aimed at addressing the unique demands of this new generation of telescopes. Among the algorithms developed in this work is a calibration algorithm that ensures optimal data treatment, as well as two advanced algorithms for image and “movie” reconstruction of astronomical objects, with a particular focus on black holes

Using multiobjective optimization to reconstruct interferometric data (II): polarimetry and time dynamics

In Very Long Baseline Interferometry (VLBI), signals from multiple antennas combine to create a sparsely sampled virtual aperture, its effective diameter determined by the largest antenna separation. The inherent sparsity makes VLBI imaging an ill-posed inverse problem, prompting the use of algorithms like the Multiobjective Evolutionary Algorithm by Decomposition (MOEA/D), as proposed in the first paper of this series. This study focuses on extending MOEA/D to polarimetric and time dynamic reconstructions, particularly relevant for the VLBI community and the Event Horizon Telescope Collaboration (EHTC). MOEA/D's success in providing a unique, fast, and largely unsupervised representation of image structure serves as the basis for exploring these extensions. The extension involves incorporating penalty terms specific to total intensity imaging, time-variable, and polarimetric variants within MOEA/D's multiobjective, evolutionary framework. The Pareto front, representing non-dominated solutions, is computed, revealing clusters of proximities. Testing MOEA/D with synthetic datasets representative of EHTC's main targets demonstrates successful recovery of polarimetric and time-dynamic signatures despite sparsity and realistic data corruptions. MOEA/D's extension proves effective in the anticipated EHTC setting, offering an alternative and independent claim to existing methods. It not only explores the problem globally but also eliminates the need for parameter surveys, distinguishing it from Regularized Maximum Likelihood (RML) methods. MOEA/D emerges as a novel and useful tool for robustly characterizing polarimetric and dynamic signatures in VLBI datasets with minimal user-based choices. Future work aims to address the last remaining limitation of MOEA/D, specifically regarding the number of pixels and numerical performance, to establish it within the VLBI data reduction pipeline.Comment: Both first authors have contributed equally to this work. To appear in A&

A first search of transients in the Galactic Center from 230 GHz ALMA observations

The Galactic Center (GC) presents one of the highest stellar densities in our Galaxy, making its surroundings an environment potentially rich in radio transients, such as pulsars and different kinds of flaring activity. In this paper, we present the first study of transient activity in the region of the GC based on Atacama Large Millimeter/submillimeter (mm/submm) Array (ALMA) continuum observations at 230 GHz. This search is based on a new self-calibration algorithm, especially designed for variability detection in the GC field. Using this method, we have performed a search of radio transients in the effective field of view of~$\sim 30\,$arcseconds of the GC central supermassive black hole Sagittarius A* (SgrA*) using ALMA 230 GHz observations taken during the 2017 Event Horizon Telescope (EHT) campaign, which span several observing hours (5-10) on 2017 April 6, 7, and 11. This calibration method allows one to disentangle the variability of unresolved SgrA* from any potential transient emission in the wider field of view and residual effects of the imperfect data calibration. Hence, a robust statistical criterion to identify real transients can be established: the event should survive at least three times the correlation time and it must have a peak excursion of at least seven times the instantaneous root-mean-square between consecutive images. Our algorithms are successfully tested against realistic synthetic simulations of transient sources in the GC field. Having checked the validity of the statistical criterion, we provide upper limits for transient activity in the effective field of view of the GC at 230 GHz.Comment: Accepted for publication in Astronomy and Astrophysic

Identifying synergies between VLBI and STIX imaging

Reconstructing an image from sparsely sampled Fourier data is an ill-posed inverse problem that occurs in a variety of subjects within science, including the data analysis for Very Long Baseline Interferometry (VLBI) and the Spectrometer/Telescope for Imaging X-rays (STIX) for solar observations. Despite ongoing parallel developments of novel imaging algorithms, synergies remain unexplored. We study the synergies between the data analysis for the STIX instrument and VLBI, compare the methodologies and evaluate their potential. In this way, we identify key trends in the performance of several algorithmic ideas and draw recommendations for the future. To this end, we organized a semi-blind imaging challenge with data sets and source structures that are typical for sparse VLBI, specifically in the context of the Event Horizon Telescope (EHT), and for STIX observations. 17 different algorithms from both communities, from 6 different imaging frameworks, participated in the challenge, marking this work the largest scale code comparisons for STIX and VLBI to date. Strong synergies between the two communities have been identified, as can be proven by the success of the imaging methods proposed for STIX in imaging VLBI data sets and vice versa. Novel imaging methods outperform the standard CLEAN algorithm significantly in every test-case. Improvements over the performance of CLEAN make deeper updates to the inverse modeling pipeline necessary, or consequently replacing inverse modeling with forward modeling. Entropy-based and Bayesian methods perform best on STIX data. The more complex imaging algorithms utilizing multiple regularization terms (recently proposed for VLBI) add little to no additional improvements for STIX, but outperform the other methods on EHT data. This work demonstrates the great synergy between the STIX and VLBI imaging efforts and the great potential for common developments.Comment: accepted for publication in A&

Event Horizon Telescope observations of the jet launching and collimation in Centaurus A

Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimetre wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to 10-100 gravitational radii (rg = GM/c2) scales in nearby sources. Centaurus A is the closest radio-loud source to Earth. It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our Galactic Centre. A large southern declination of −43deg has, however, prevented VLBI imaging of Centaurus A below a wavelength of 1 cm thus far. Here we show the millimetre VLBI image of the source, which we obtained with the Event Horizon Telescope at 228 GHz. Compared with previous observations, we image the jet of Centaurus A at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that the source structure of Centaurus A resembles the jet in Messier 87 on ~500 rg scales remarkably well. Furthermore, we identify the location of Centaurus A's SMBH with respect to its resolved jet core at a wavelength of 1.3 mm and conclude that the source's event horizon shadow4 should be visible at terahertz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses

The Polarized Image of a Synchrotron-emitting Ring of Gas Orbiting a Black Hole

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov and conservation of the Walker-Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images

Constraints on black-hole charges with the 2017 EHT observations of M87*

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes

First Very Long Baseline Interferometry Detections at 870 μm

The first very long baseline interferometry (VLBI) detections at 870 μm wavelength (345 GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on intercontinental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in 2018 October. The longest-baseline detections approach 11 Gλ, corresponding to an angular resolution, or fringe spacing, of 19 μas. The Allan deviation of the visibility phase at 870 μm is comparable to that at 1.3 mm on the relevant integration timescales between 2 and 100 s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870 μm. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time