Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales

The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.Comment: 11 pages, accepted to Ap

VLBI Digital-Backend Intercomparison Test Report

Issues related to digital-backend (DBE) systems can be difficult to evaluate in either local tests or actual VLBI experiments. The 2nd DBE intercomparison workshop at Haystack Observatory on 25-26 October 2012 provided a forum to explicitly address validation and interoperability issues among independent global developers of DBE equipment. This special report discusses the workshop. It identifies DBE systems that were tested at the workshop, describes the test objectives and procedures, and reports and discusses the results of the testing

First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization

Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image (〈∣v∣〉 < 3.7%), consistent with the low image-integrated circular polarization fraction measured by the Atacama Large Millimeter/submillimeter Array (∣v int∣ < 1%). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent on the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*

VGOS: The Becoming of an Operational System

American Geophysical Union Annual Meeting (AGU2024), What's Next for Science, 9-13 December 2024, Washington, D.C.The S/X VLBI network has been the production system of the IVS since the Service's inception in 1999. In 2020, after a visionary journey from designing to prototyping to system rollout, the next-generation, broadband VLBI system called VGOS (VLBI Global Observing System) was declared operational and a (geographically) limited VGOS network of some eight (northern-hemisphere) stations started to contribute operationally to IVS products. That VGOS network has increased to currently 14 more globally-distributed stations, and it continues to grow with further seven stations expected to join between years 2024 and 2025. The current operational VGOS observing program includes two 24-hour session series (known as VGOS-OPS and VGOS-RD) for EOP determination and a 1-hour (Intensive), weekdaily series (VGOS-INT-A) for dUT1 determination. Further VGOS Intensive series are being validated. Beyond the network buildout activities, also other infrastructure components of the VLBI processing chain have been developed further (e.g., VGOS correlation and post-processing, VGOS analysis). In this presentation, we provide a status overview of the infrastructure realization of VGOS, outline current limitations and risks, and give an outlook as to the future prospects of VGOSPeer reviewe

Automatic Gravity Wave Detection on the Ross Ice Shelf Using Supervised Panoptic Spectrogram Segmentation

American Geophysical Union Fall Meeting, 12-16 December 2022, ChicagoIce shelves play a pivotal role in controlling the evolution of Antarctic glaciology by restraining, buttressing, and modulating the flow of grounded ice into the Southern Ocean. The stability of the Antarctic Ice Sheet thus depends critically on the stability of the ice shelves that fringe the continent. It is therefore important to understand how these shelves respond to environmental stresses, especially those as common as gravity wave forcings. This study focuses on applying machine learning to automatically detect, classify, and catalog low-frequency (0-70 mHz) gravity wave events impacting the Ross Ice Shelf (RIS) by panoptically segmenting seismic spectrograms. The data used to supervise training was collected by a broadband seismic array deployed on the RIS from November 2014 to November 2016 and was used to generate spectrograms of up to 70 mHz that were examined for infragravity waves and swell events. Our modified U-Net architecture achieved a Dice similarity coefficient (DSC) of over 0.73 during event detection, and its corresponding post processing pipeline recorded an accuracy of 94.4% during classification, outperforming alternative rule based techniques. This work serves as a proof-of-concept for using deep-learning algorithms to detect and catalog gravity wave events, a development that would allow for an improved understanding of the long-term stability of Antarctic ice shelvesPeer reviewe

Mixed-mode VLBI Observations: Implications for Terrestrial Reference Frame Stability

American Geophysical Union Annual Meeting (AGU2024), What's Next for Science, 9-13 December 2024, Washington, D.C.Very long baseline interferometry (VLBI) geodesy currently consists of two observing networks that run rather independently, in practice making them two unrelated techniques. The legacy VLBI network (aka S/X network) observes in two narrow frequency bands (S and X), and has been in operations since the 1980s. The new and expanding broadband VLBI network (aka VGOS network) observes in four frequency bands spanning 2-14 GHz and started operations in 2019. Regular observations from each network typically involve about a dozen stations, depending on the session. There is a real need to seamlessly combine the two networks to best exploit the time and frequency virtues that each network provides. A customary approach to network combination is the use of local ties at geodetic sites where antennas from both types of networks exist. These local ties can be measured using standard geodetic surveys and/or VLBI local-tie sessions, all of which are high-precision. An additional, less explored approach to network combination is the use of global ties. These can be obtained by conducting mixed-mode observations whereby the S/X and VGOS networks are run simultaneously. Although less precise than local ties, a significant benefit of the mixed-mode approach is that numerous globally-distributed baselines between S/X and VGOS stations can be formed thus also tying the two networks. This study explores the balance between both approaches focusing on the impact of the latter on the stability of a VLBI-only terrestrial reference framePeer reviewe

GNSS Reflectometry on Arctic Sea Ice using a small-scale network of GNSS sensors

American Geophysical Union Fall Meeting, 12-16 December 2022, ChicagoGNSS-Interferometric Reflectometry (GNSS-IR) utilizes the pattern in GNSS signal-to-noise ratio (SNR) observables resulting from interference between direct and reflected GNSS signals to probe the GNSS antenna’s surrounding physical environment. The technique is used as a sensor for sea level variations, soil moisture changes, snow depth accumulation, in addition to other geophysical applications. Traditionally, GNSS-IR has been applied with static GNSS stations. Here, we use it on a sea-ice floe in the Arctic Ocean drifting up to 25 km daily. A small-scale (~5 km) ice-strain network is formed with the 12 identical GNSS instruments deployed on the floe. This study describes the technique and this network, while also exploring how the redundancy afforded by the spatially-dense network can best be used to determine the estimation precision of snow accumulation changesPeer reviewe