Variable structures in M87* from space, time and frequency resolved interferometry

The immediate vicinity of an active supermassive black hole—with its event horizon, photon ring, accretion disk and relativistic jets—is an appropriate place to study physics under extreme conditions, particularly general relativity and magnetohydrodynamics. Observing the dynamics of such compact astrophysical objects provides insights into their inner workings, and the recent observations of M87* by the Event Horizon Telescope1–6 using very-long-baseline interferometry techniques allows us to investigate the dynamical processes of M87* on timescales of days. Compared with most radio interferometers, very-long-baseline interferometry networks typically have fewer antennas and low signal-to-noise ratios. Furthermore, the source is variable, prohibiting integration over time to improve signal-to-noise ratio. Here, we present an imaging algorithm7,8 that copes with the data scarcity and temporal evolution, while providing an uncertainty quantification. Our algorithm views the imaging task as a Bayesian inference problem of a time-varying brightness, exploits the correlation structure in time and reconstructs (2 + 1 + 1)-dimensional time-variable and spectrally resolved images. We apply this method to the Event Horizon Telescope observations of M87*9 and validate our approach on synthetic data. The time- and frequency-resolved reconstruction of M87* confirms variable structures on the emission ring and indicates extended and time-variable emission structures outside the ring itself.

The Celestial Frame and the Weighting of the Celestial Pole Offsets in the Computation of VLBI-Based Corrections for the Main Lunisolar Nutation Terms

Very long baseline interferometry (VLBI) is the only technique in space geodesy that can determine directly the celestial pole offsets (CPO). In this paper, we make use of the CPO derived from global VLBI solutions to estimate empirical corrections to the main lunisolar nutation terms included in the IAU 2006/2000A precession–nutation model. In particular, we pay attention to two factors that affect the estimation of such corrections: the celestial reference frame used in the production of the global VLBI solutions and the stochastic model employed in the least-squares adjustment of the corrections. In both cases, we have found that the choice of these aspects has an effect of a few μas in the estimated corrections.

Including precision clocks into space-based net as gravitational antennas

Here we propose to use a precision clock in a space-based ultra-precise clock network to register sources of low-frequency gravitational waves of cosmic origin in the range of 10-3 ÷ 0.1 Hz. We also show that the method of comparing clocks at inland and intercontinental distances (very long baseline interferometry), originally developed for radio astronomy and geodesy, can be used as a prototype method for recording gravitational waves. Estimates of the measurement accuracy are given. An analyse of precise clocks possibilities for experimental estimates for rotation parameter of Gödel universe and GW recordings is offered, which in particular opens up the prospect of registering circularly polarized gravitational waves. Some new problems of small time intervals registration from general relativity, thermodynamics and quantum mechanics points of view are discussed.

Water vapour radiometry in geodetic very long baseline interferometry telescopes: assessed through simulations

The accuracy of geodetic Very Long Baseline Interferometry (VLBI) is affected by water vapour in the atmosphere in terms of variations in the signal propagation delay at the different stations. This “wet” delay may be estimated directly from the VLBI data, as well as from independent instruments, such as collocated microwave radiometers. Rather than having stand-alone microwave radiometers we have, through simulations, evaluated the possibility to use radiometric data from the VLBI receiver in the VGOS telescopes at the Onsala Space Observatory. The advantage is that the emission from water vapour, as sensed by the radiometer, originates from the same atmospheric volume that delays the VLBI signal from the extra-galactic object. We use simulations of the sky brightness temperature and the wet delay together with an assumption of a root-mean-square (rms) noise of the receiver of 1 K, and observations evenly spread between elevation angles of 10$$^\circ $$ ∘ –90$$^\circ $$ ∘ . This results in an rms error of the estimated equivalent zenith wet delay of the order of 3 mm for a one frequency algorithm, used under cloud free conditions, and 4 mm for a two frequency algorithm, used during conditions with liquid water clouds. The results exclude rainy conditions when the method does not work. These errors are reduced by a factor of 3 if the receiver error is 0.1 K meaning that the receivers’ measurements of the sky brightness temperature is the main error source. We study the impact of ground-noise pickup by using a model of an existing wideband feed. Taking the algorithm uncertainty and the ground noise pickup into account we conclude that the method presented will be useful as an independent estimate of the wet delay to assess the quality of the wet delays and linear horizontal gradients estimated from the VLBI data themselves.

GNSS-based water vapor estimation and validation during the MOSAiC expedition

Within the transpolar drifting expedition MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate), the Global Navigation Satellite System (GNSS) was used among other techniques to monitor variations in atmospheric water vapor. Based on 15 months of continuously tracked GNSS data including GPS, GLONASS and Galileo, epoch-wise coordinates and hourly zenith total delays (ZTDs) were determined using a kinematic precise point positioning (PPP) approach. The derived ZTD values agree to 1.1 ± 0.2 mm (root mean square (rms) of the differences 10.2 mm) with the numerical weather data of ECMWF's latest reanalysis, ERA5, computed for the derived ship's locations. This level of agreement is also confirmed by comparing the on-board estimates with ZTDs derived for terrestrial GNSS stations in Bremerhaven and Ny-Ålesund and for the radio telescopes observing very long baseline interferometry in Ny-Ålesund. Preliminary estimates of integrated water vapor derived from frequently launched radiosondes are used to assess the GNSS-derived integrated water vapor estimates. The overall difference of 0.08 ± 0.04 kg m−2 (rms of the differences 1.47 kg m−2) demonstrates a good agreement between GNSS and radiosonde data. Finally, the water vapor variations associated with two warm-air intrusion events in April 2020 are assessed.

Symbiosis of radio-emitting winds and a jet in a super-Eddington active galactic nucleus

Super-critical accretion is the most powerful episode in nursing the black hole growth and works in several types of objects. Given that the inverse correlation between radio loudness and Eddington ratio, the super-Eddington active galactic nuclei (AGNs) hold the extremely radio-quiet end of AGNs. Regarding the existence of jet in super-Eddington or radio-quiet AGNs, it’s still unclear. Years of studies indicate nearly all types of super-Eddington accreting systems can launch a jet with one exception: no clear evidence to show jet in super-Eddington AGNs. Observations and theoretical works suggest that super-Eddington accretion can drive high-speed wind-like outflows, therefore produce radio emission through synchrotron (shocked wind) and bremsstrahlung mechanisms. However, such a radio-emitting wind has not been observed in super-Eddington systems except for the Galactic micro-quasar SS 433. In principle, high resolution very long baseline interferometry (VLBI) observation can directly map the inner structure of super-Eddington AGNs. Here, we report the discovery of the coupling of jet and radio-emitting winds in a nearby super-Eddington AGN, I Zw1. Its parsec-scale jet exhibits a wiggling, we interpret this as a jet precession. All the features make IZw1 act as a scaled-up version of SS 433. The observations favour that jet can be launched in extremely radio-quiet AGNs and ubiquitous in super-Eddington accreting systems. The jet wiggling or precession can produce a large aperture-angle shock, which emphasises the jet’s contribution to gas feedback. As the jet precession was also discovered in other super-Eddington systems such as SS 433 and V404 Cygni, it is possible that there is a correlation with each other.

Automated VLBI scheduling using AI-based parameter optimization

Within this work, a new geodetic very long baseline interferometry (VLBI) scheduling approach inspired by evolutionary processes based on selection, crossover and mutation is presented. It mimics the biological concept “surviving of the fittest” to iteratively explore the scheduling parameter space looking for the best solution. Besides providing high-quality results, one main benefit of the proposed approach is that it enables the generation of fully automated and individually optimized schedules. Moreover, it generates schedules based on transparent rules, well-defined scientific goals and by making decisions based on Monte Carlo simulations. The improvements in terms of precision of geodetic parameters are discussed for various observing programs organized by the International VLBI Service for Geodesy and Astrometry (IVS), such as the OHG, R1, and T2 programs. In the case of schedules with a difficult telescope network, an improvement in the precision of the geodetic parameters up to 15% could be identified, as well as an increase in the number of observations of up to 10% compared to classical scheduling approaches. Due to the high quality of the produced schedules and the reduced workload for the schedulers, various IVS observing programs are already making use of the evolutionary parameter selection, such as the AUA, INT2, INT3, INT9, OHG, T2 and VGOS-B program.

Short-baseline interferometry local-tie experiments at the Onsala Space Observatory

We present results from observation, correlation and analysis of interferometric measurements between the three geodetic very long baseline interferometry (VLBI) stations at the Onsala Space Observatory. In total, 25 sessions were observed in 2019 and 2020, most of them 24 h long, all using X band only. These involved the legacy VLBI station ONSALA60 and the Onsala twin telescopes, ONSA13NE and ONSA13SW, two broadband stations for the next-generation geodetic VLBI global observing system (VGOS). We used two analysis packages: $$\nu $$ ν Solve to pre-process the data and solve ambiguities, and ASCOT to solve for station positions, including modelling gravitational deformation of the radio telescopes and other significant effects. We obtained weighted root mean square post-fit residuals for each session on the order of 10–15 ps using group-delays and 2–5 ps using phase-delays. The best performance was achieved on the (rather short) baseline between the VGOS stations. As the main result of this work, we determined the coordinates of the Onsala twin telescopes in VTRF2020b with sub-millimetre precision. This new set of coordinates should be used from now on for scheduling, correlation, as a priori for data analyses, and for comparison with classical local-tie techniques. Finally, we find that positions estimated from phase-delays are offset $$\sim +3$$ ∼ + 3 mm in the up-component with respect to group-delays. Additional modelling of (elevation dependent) effects may contribute to the future understanding of this offset.

Observable quality assessment of broadband very long baseline interferometry system

The next-generation, broadband geodetic very long baseline interferometry system, named VGOS, is developing its global network, and VGOS networks with a small size of 3–7 stations have already made broadband observations from 2017 to 2019. We made quality assessments for two kinds of observables in the 21 VGOS sessions currently available: group delay and differential total electron content ($$\delta $$ δ TEC). Our study reveals that the random measurement noise of VGOS group delays is at the level of less than 2 ps ($$1\,\hbox {ps}\,=\,10^{-12}$$ 1 ps = 10 - 12 s), while the contributions from systematic error sources, mainly source structure related, are at the level of 20 ps. Due to the significant improvement in measurement noise, source structure effects with relatively small magnitudes that are not overwhelming in the S/X VLBI system, for instance 10 ps, are clearly visible in VGOS observations. Another critical error source in VGOS observations is discrete delay jumps, for instance, a systematic offset of about 310 ps or integer multiples of that. The predominant causative factor is found to be related to source structure. The measurement noise level of $$\delta $$ δ TEC observables is about 0.07 TECU, but the systematic effects are five times larger than that. A strong correlation between group delay and $$\delta $$ δ TEC observables is discovered with a trend of 40 ps/TECU for observations with large structure effects; there is a second trend in the range 60–70 ps/TECU when the measurement noise is dominant.

A Tri-Band Cooled Receiver for Geodetic VLBI

This paper shows a simultaneous tri-band (S: 2.2–2.7 GHz, X: 7.5–9 GHz and Ka: 28–33 GHz) low-noise cryogenic receiver for geodetic Very Long Baseline Interferometry (geo-VLBI) which has been developed at Yebes Observatory laboratories in Spain. A special feature is that the whole receiver front-end is fully coolable down to cryogenic temperatures to minimize receiver noise. It was installed in the first radio telescope of the Red Atlántica de Estaciones Geodinámicas y Espaciales (RAEGE) project, which is located in Yebes Observatory, in the frame of the VLBI Global Observing System (VGOS). After this, the receiver was borrowed by the Norwegian Mapping Autorithy (NMA) for the commissioning of two VGOS radiotelescopes in Svalbard (Norway). A second identical receiver was built for the Ishioka VGOS station of the Geospatial Information Authority (GSI) of Japan, and a third one for the second RAEGE VGOS station, located in Santa María (Açores Archipelago, Portugal). The average receiver noise temperatures are 21, 23, and 25 Kelvin and the measured antenna efficiencies are 70%, 75%, and 60% in S-band, X-band, and Ka-band, respectively.