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We present results on simultaneous observations of Class I methanol masers at 25, 36, and 44 GHz towards 22 Galactic targets carried out with the Effelsberg 100-m telescope. The study investigates relations between the hyperfine (HF) structure of the torsion–rotation transitions in CH |$_3$| OH and maser activity. By analysing the radial velocity shifts between different maser lines together with the patterns of the HF structure based on laboratory measurements and quantum-chemical calculations, we find that in any source only one specific HF transition forms the maser emission and that this transition changes from source to source. The physical conditions leading to this selective behaviour are still unclear. Using accurate laboratory rest frequencies for the 25 GHz transitions, we have refined the centre frequencies for the HF multiplets at 36, 44, and 95 GHz: |$f_{\scriptscriptstyle 36} = (36169.2488\pm 0.0002_{\scriptscriptstyle \rm stat} \pm 0.0004_{\scriptscriptstyle \rm sys})$|  MHz. |$f_{\scriptscriptstyle 44} = (44069.4176\pm 0.0002_{\scriptscriptstyle \rm stat} \pm 0.0004_{\scriptscriptstyle \rm sys})$|  MHz, and |$f_{\scriptscriptstyle 95} = (95169.4414\pm 0.0003_{\scriptscriptstyle \rm stat} \pm 0.0004_{\scriptscriptstyle \rm sys})$|  MHz. Comparison with previous observations of 44 GHz masers performed 6–10 yr ago with a Korean 21-m Korean Very Long Baseline Interferometry Network telescope towards the same targets confirms the kinematic stability of Class I maser line profiles during this time interval and reveals a systematic radial velocity shift of |$0.013\pm 0.005$|  km s |$^{-1}$| between the two telescopes. [ABSTRACT FROM AUTHOR]

The Event Horizon Telescope (EHT) is a ground-based array of Very Long Baseline Interferometry (VLBI) telescopes designed to image the event horizon of black holes. To overcome its limitations, this study explores a mission concept involving a two-satellite constellation of VLBI telescopes deployed in Medium Earth Orbit (MEO). Achieving high-resolution black hole images requires precise baseline determination at the millimetre level. To address this challenge, each satellite in the constellation is equipped with two Global Navigation Satellite System (GNSS) receivers and an optical Intersatellite Link (ISL) to enhance orbit determination. The results highlight the importance of integer ambiguity resolution and reveal that the ISL primarily improves baseline estimation along the link direction, with minimal impact along the black hole direction. Large intersatellite distances lead to sub-optimal relative orbit accuracy, challenging the attainment of the 3.5 mm relative position accuracy goal along the black hole direction. [ABSTRACT FROM AUTHOR]

The ability to distribute entanglement between quantum nodes may unlock new capabilities in the future that include teleporting information across multinode networks, higher resolution detection via entangled sensor arrays, and measurements beyond the quantum limit enabled by networked atomic clocks. These new quantum networks also hold promise for the Aerospace community in areas such as deep space exploration, improved satellite communication, and synchronizing drone swarms. Although exciting, these applications are a long way off from providing a "real-world" benefit, as they have only been theoretically explored or demonstrated in small-scale experiments. An outstanding challenge is to identify near-term use cases for quantum networks; this may be an intriguing new area of interest for the aerospace community, as the quantum networking field would benefit from more multidisciplinary collaborations. This paper introduces quantum networking, discusses the difficulties in distributing entanglement within these networks, highlights recent progress toward this endeavor, and features two current case studies on mobile quantum nodes and an entangled clock network, both of which are relevant to the aerospace community. [ABSTRACT FROM AUTHOR]

The upcoming European Space Agency (ESA) satellite mission GENESIS is an Earth-orbiting satellite carrying instruments of all four space geodetic techniques. The onboard transmitter for Very Long Baseline Interferometry (VLBI) will allow the observation of the satellite with VLBI radio telescopes. The objective of this study is to investigate the integration of VLBI observations of GENESIS into the operations of the VLBI Global Observing System (VGOS). Based on both current and foreseeable modern VGOS antenna networks, we consider the realistic observability of both geodetic radio sources and GENESIS. We conduct a comprehensive scheduling and perform extensive simulations of the VLBI observations. We assume that observations of GENESIS are scheduled within regular, geodetic experiments. The integration of GENESIS as an additional source in the scheduling results in a minimal degradation in the geodetic parameter estimation of station positions and dUT1 of less than 0.09 mm and 0.06 μs, respectively. The results suggest to schedule scans of GENESIS at intervals of about 5 min to limit the decrease in the number of observations of geodetic sources to less than 5% with respect to schedules containing only geodetic radio sources. The schedules for 24 h experiments comprise about 150 to 200 scans and 1000 to 5000 observations of GENESIS, depending on the size of the utilized network. The frame tie accuracy between the VLBI and GENESIS frames is assessed in the form of station positions, which are solely estimated from observations of GENESIS. Multiple 24 h experiments are simulated over 52 weeks with assumed session cadences of two to three experiments per week. By stacking the normal equations from three months of experiments, we obtain station position estimates with a precision of less than 10 mm. After 12 months, the repeatabilites are reduced to less than 5 mm. [ABSTRACT FROM AUTHOR]

The nearby ultraluminous infrared galaxy (ULIRG) IRAS F01004−2237 exhibits 100 kpc scale continuum emission at radio wavelengths. The absence of extended X-ray emission in IRAS F01004−2237 has suggested an active galactic nucleus (AGN) origin for the extended radio emission, whose properties and role in merging systems still need to be better understood. We present the results of multifrequency observations of IRAS F01004−2237 conducted by the Very Long Baseline Array at 2.3 and 8.4 GHz. Compact 8.4 GHz continuum emission was detected on a 1 pc scale in the nuclear region with an intrinsic brightness temperature of 108.1 K suggesting that the radio source originated from an AGN, potentially driving the extended emission. In contrast, no significant emission was observed at 2.3 GHz, indicating the presence of low-frequency absorption. This absorption cannot be attributed solely to synchrotron self-absorption; alternatively, free–free absorption due to thermal plasma is mainly at work in the spectrum. From combined perspectives, including mid-infrared and X-ray data, the AGN is obscured in a dense environment. The kinetic power of the nonthermal jet, as inferred from the extended emission, can play a more important role in dispersing the surrounding medium than the thermal outflow in IRAS F01004−2237. These findings hint that jet activities in ULIRGs may contribute to AGN feedback during galaxy evolution induced by merger events. [ABSTRACT FROM AUTHOR]

Radio galaxies harbouring jetted active galactic nuclei are a frequent target of very long baseline interferometry (VLBI) because they play an essential role in our exploration of how jets form and propagate. Hence, only a few have not yet been detected with VLBI; Fornax A was one of the most famous examples. Here we present the first detection of the compact core region of Fornax A with VLBI. At 8.4 GHz the faint core is consistent with an unresolved point source. We constrained its flux density to be S0 = 47.5 − 62.3 mJy and its diameter to be D0min ≤ 70 μas. The high values of the measured brightness temperature (TB ≳ 1011 K) imply that the observed radiation is of non-thermal origin, likely associated with the synchrotron emission from the active galactic nucleus. We also investigated the possibility of a second radio source being present within the field of view. Adding a second Gaussian component to the geometrical model fit does not significantly improve the quality of the fit, and we therefore, conclude that our detection corresponds to the compact core of Fornax A. Analysis of the non-trivial closure phases provides evidence for the detection of a more extended flux density, on the angular scale of ∼4000 μas. Finally, the fractional circular polarisation of the core is consistent with zero, with a conservative upper limit being mcirc ≤ 4%. [ABSTRACT FROM AUTHOR]

The Event Horizon Telescope (EHT) is an array of radio telescopes around the world that uses the technique of Very Long Baseline Interferometry (VLBI) to create a virtual Earth‐sized telescope. Using this technique, the EHT released the first image of a black hole on April 10, 2019, accelerating black hole science and creating enormous public impact. Now, a team of scientists and engineers is looking ahead to the next horizon: movies of black holes. In this paper, we give an overview of how the next‐generation EHT (ngEHT) Project has adopted a systems engineering approach to the design of future improvements to the EHT array, developing clear traceability from community‐defined key science goals. Scheduling, operations, site selection, observing bandwidth, data transport, and data management are some of the many factors that require careful consideration and balance and therefore benefit from intentional systems engineering methods underpinned by a "single source of truth" system model. [ABSTRACT FROM AUTHOR]

Very long baseline interferometry (VLBI) provides the highest-resolution images in astronomy. The sharpest resolution is nominally achieved at the highest frequencies, but as the observing frequency increases, so too does the atmospheric contribution to the system noise, degrading the sensitivity of the array and hampering detection. In this paper, we explore the limits of high-frequency VLBI observations using ngehtsim, a new tool for generating realistic synthetic data. ngehtsim uses detailed historical atmospheric models to simulate observing conditions, and it employs heuristic visibility detection criteria that emulate single- and multifrequency VLBI calibration strategies. We demonstrate the fidelity of ngehtsim's predictions using a comparison with existing 230 GHz data taken by the Event Horizon Telescope (EHT), and we simulate the expected performance of EHT observations at 345 GHz. Though the EHT achieves a nearly 100% detection rate at 230 GHz, our simulations indicate that it should expect substantially poorer performance at 345 GHz; in particular, observations of M87* at 345 GHz are predicted to achieve detection rates of ≲20% that may preclude imaging. Increasing the array sensitivity through wider bandwidths and/or longer integration times—as enabled through, e.g., the simultaneous multifrequency upgrades envisioned for the next-generation EHT—can improve the 345 GHz prospects and yield detection levels that are comparable to those at 230 GHz. M87* and Sgr A* observations carried out in the atmospheric window around 460 GHz could expect to regularly achieve multiple detections on long baselines, but analogous observations at 690 and 875 GHz consistently obtain almost no detections at all. [ABSTRACT FROM AUTHOR]

Objectives: The accuracy of relative track between orbiter and ascender can ensure success and safety during the meeting and docking process of Lunar. Methods: Based on dual-target joint measurement of Chang’E-5, we established the same beam interferomery(SBI) measurement and inter-satellite measure⁃ ment model, and designed a statistical orbit determination algorithm combined with orbiter and ascender. Results: We used the reference orbit to analyze delay and delay rate of SBI, and the range and range accuracy of microwave radar/laser radar. The accuracy of SBI is equivalent to that of very long baseline interferometry, and the range accuracy of microwave radar/laser radar is at meter level. Conclusions: By establishing the relative motion relationship between the two targets, the relative orbit accuracy can be sig⁃ nificantly improved in orbit determination. [ABSTRACT FROM AUTHOR]

This article is an interview between Sara Seager and Ignas Snellen, an astronomer. Snellen discusses his background and love for astronomy, as well as his experiences with telescopes and observing celestial objects. He shares his journey to becoming an astronomer, including his struggles with grades in university and his transition from studying radio galaxies to exoplanet research. Snellen explains his work in Very Long Baseline Interferometry (VLBI) and his transition to studying exoplanets. He also discusses his interest in young radio sources and his struggle to find a job in his niche field. Snellen mentions his previous publications and obtaining telescope time for his research. The interview concludes with Snellen reflecting on the imposter syndrome and the importance of building confidence in one's abilities. [Extracted from the article]

There is still a limited number of high-redshift (z > 3) active galactic nuclei (AGNs) whose jet kinematics have been studied with very long baseline interferometry (VLBI). Without a dedicated proper motion survey, regularly conducted astrometric VLBI observations of bright radio-emitting AGN with sensitive arrays can be utilized to follow changes in the jets, by means of high-resolution imaging and brightness distribution modelling. Here, we present a first-time VLBI jet kinematic study of NVSS J080518 + 614423 (z = 3.033) and NVSS J165844 − 073918 (z = 3.742), two flat-spectrum radio quasars that display milliarcsecond-scale jet morphology. Archival astrometric observations carried out mainly with the Very Long Baseline Array, supplemented by recent data taken with the European VLBI Network, allowed us to monitor changes in their radio structure in the 7.6−8.6 GHz frequency band, covering almost two decades. By identifying individual jet components at each epoch, we were able to determine the apparent proper motion for multiple features in both sources. Apparent superluminal motions range |$(1-14)\, c$|⁠ , and are found to be consistent with studies of other high-redshift AGN targets. Using the physical parameters derived from the brightness distribution modelling, we estimate the Doppler-boosting factors (δ ≈ 11.2 and δ ≈ 2.7), the Lorentz factors (Γ ≈ 7.4 and Γ ≈ 36.6), and the jet viewing angles (θ ≈ 4.4° and θ ≈ 8.0°), for NVSS J080518 + 614423 and NVSS J165844 − 073918, respectively. The data revealed a stationary jet component with negligible apparent proper motion in NVSS J165844 − 073918. [ABSTRACT FROM AUTHOR]

We report sub-parsec-scale observations of the 321 GHz H2O emission line in the radio galaxy NGC 1052. The H2O line emitter size is constrained in <0.6 mas distributed on the continuum core component. The brightness temperature exceeding 106 K and the intensity variation indicate certain evidence for maser emission. The maser spectrum consists of redshifted and blueshifted velocity components spanning ∼400 km s−1, separated by a local minimum around the systemic velocity of the galaxy. The spatial distribution of maser components shows a velocity gradient along the jet direction, implying that the population-inverted gas is driven by the jets interacting with the molecular torus. We identified a significant change of the maser spectra between two sessions separated by 14 days. The maser profile showed a radial velocity drift of 127 ± 13 km s−1 yr−1 implying inward gravitational acceleration at 5000 Schwarzschild radii. The results demonstrate the feasibility of future very long baseline interferometry observations to resolve the jet–torus interacting region. [ABSTRACT FROM AUTHOR]

The paper examines the possibilities of using multi-frequency synthesis methods for very long baseline (VLBI) space radio interferometers to improve the coverage and the quality of the resulting synthesized images. To evaluate the contribution of multi-frequency synthesis methods, simulations of VLBI observations were performed using the example of the space VLBI concept that is based on a combination of circular near-Earth orbits. [ABSTRACT FROM AUTHOR]

Earth-based very long baseline interferometry (VLBI) has made rapid advances in imaging black holes. However, due to the limitations imposed on terrestrial VLBI by the Earth's finite size and turbulent atmosphere, it is imperative to have a space-based component in future VLBI missions. This paper investigates the effect of the Earth's oblateness, also known as the J 2 effect, on orbiters in Earth–space and space–space VLBI. The paper provides an extensive discussion on how the J 2 effect can directly impact orbit selection for black hole observations and how, through informed choices of orbital parameters, the effect can be used to a mission's advantage, a fact that has not been addressed in previous space VLBI investigations. We provide a comprehensive study of how the orbital parameters of several current space VLBI proposals will vary specifically due to the J 2 effect. For black hole accretion flow targets of interest, we demonstrate how the J 2 effect leads to a modest increase in shorter-baseline coverage, filling gaps in the (u, v) plane. Subsequently, we construct a simple analytical formalism that allows isolation of the impact of the J 2 effect on the (u, v) plane without requiring computationally intensive orbit propagation simulations. By directly constructing (u, v) coverage using J 2-affected and J 2-invariant equations of motion, we obtain distinct coverage patterns for M87* and Sgr A* that show extremely dense coverage on short baselines as well as long-term orbital stability on longer baselines. [ABSTRACT FROM AUTHOR]

The article discusses the passing of Professor Peter Fulde, a renowned physicist and science administrator. Born in Germany in 1936, Fulde's family had to flee during World War II. He studied physics in East Berlin before escaping to West Germany. Fulde made significant contributions to the field of physics and held various positions at research institutes in Germany and Korea. The article also highlights the Universiti Malaya Center for Astronomy and Astrophysics Research (CAAR) in Malaysia, which focuses on radio astronomy, optical astronomy, and theoretical astronomy. CAAR has made significant contributions to the field and aims to inspire future astronomers. [Extracted from the article]

Accretion of black holes at near-Eddington or super-Eddington rates represents the most powerful episode driving black hole growth, potentially occurring across various types of objects. However, the physics governing accretion and jet–disk coupling in such states remains unclear, primarily due to the difficulty in detecting associated jets, which may emit extremely weakly or exhibit episodic behavior. Only a few near/super-Eddington systems have demonstrated radio activity, and it remains uncertain whether jets exist and what their properties are in super-Eddington active galactic nuclei (AGNs) and ultraluminous X-ray sources. This uncertainty stems mainly from the complex radio emission mix, which includes contributions from jets, star formation activity, photoionized gas, accretion disk wind, and coronal activity. In this work, we conducted high-resolution, very long baseline interferometry observations to investigate jets in the highly accreting narrow-line Seyfert I system I Zw 1. Our observations successfully revealed small-scale jets (with a linear size of ∼45 pc) at both 1.5 and 5 GHz, based on the high radio brightness temperature, radio morphology, and spectral index distribution. Additionally, the parsec-scale jet observed in I Zw 1 displays a knotted morphology reminiscent of other sources accreting at similar rates. In summary, the high accretion rates and jet properties observed in the AGN I Zw 1 may support the AGN/X-ray binary analogy in this extreme state. [ABSTRACT FROM AUTHOR]

Context. Understanding the mechanisms that launch and shape powerful relativistic jets from supermassive black holes (SMBHs) in high-redshift active galactic nuclei (AGNs) is crucial for probing the co-evolution of SMBHs and galaxies over cosmic time. Aims. We focus on the high-redshift (z = 3.396) blazar OH 471 to explore the jet launching mechanism in the early Universe. Methods. Using multi-frequency radio monitoring observations and high-resolution Very Long Baseline Interferometry (VLBI) imaging over three decades, we studied the milliarcsecond structure and long-term variability of OH 471. Results. Our spectral modeling of the radio flux densities revealed a synchrotron self-absorbed spectrum, indicating strong magnetic fields within the compact core. By applying the flux freezing approximation, we estimated the magnetic flux carried by the jet. We found that it reaches or exceeds theoretical predictions for jets powered by black hole spin energy via the Blandford-Znajek mechanism. This implies that OH 471 is in a magnetically arrested disk (MAD) state, where the magnetic flux accumulated near the horizon regulates the accretion flow, allowing for an efficient extraction of black hole rotational energy. Conclusions. Our study demonstrates the dominance of MAD accretion in powering the prominent radio flares and relativistic jets observed in the radio-loud AGN named OH 471. Statistical studies of larger samples of high-redshift AGNs will shed light on the role of MAD accretion in launching and accelerating the earliest relativistic jets. [ABSTRACT FROM AUTHOR]

Aims. The X-ray luminous and radio-loud active galactic nucleus (AGN) SRGE J170245.3+130104 discovered at z ∼ 5.5 provides unique chances to probe the growth and evolution of a supermassice black hole (SMBH) with powerful jets in the early Universe. Methods. We present 1.35 − 5.1 GHz Very Long Baseline Array (VLBA) results regarding the radio continuum emission and a spectral analysis of this quasar in a low-flux-density state. Results. This source is unresolved at three frequencies, with total flux densities of 8.35 ± 0.09 Jy beam−1, 7.47 ± 0.08 Jy beam−1, and 6.57 ± 0.02 Jy beam−1 at 1.73 GHz, 2.26 GHz, and 4.87 GHz, respectively. Meanwhile, the brightness temperature is higher than 109 K. Conclusions. Compared with previous radio observations with arcsecond-scale resolution, nearly all the radio emission from this source is concentrated in the very central milliarcsecond (mas) scale area. We confirm this source is a bright blazar at z > 5. This young AGN provides a great opportunity to understand the mechanisms behind the generation of strong jets in the early Universe. [ABSTRACT FROM AUTHOR]

The past decade has seen significant advances in wide-field cm-wave very long baseline interferometry (VLBI), which is timely given the wide-area, synoptic survey-driven strategy of major facilities across the electromagnetic spectrum. While wide-field VLBI poses significant post-processing challenges that can severely curtail its potential scientific yield, many developments in the km-scale connected-element interferometer sphere are directly applicable to addressing these. Here we present the design, processing, data products, and source counts from a deep (11 μ Jy beam−1), quasi-uniform sensitivity, contiguous wide-field (160 arcmin2) 1.6 GHz VLBI survey of the CANDELS GOODS-North field. This is one of the best-studied extragalactic fields at milli-arcsecond resolution and, therefore, is well-suited as a comparative study for our Tera-pixel VLBI image. The derived VLBI source counts show consistency with those measured in the COSMOS field, which broadly traces the AGN population detected in arcsecond-scale radio surveys. However, there is a distinctive flattening in the S 1.4GHz ∼100–500  μ Jy flux density range, which suggests a transition in the population of compact faint radio sources, qualitatively consistent with the excess source counts at 15 GHz that is argued to be an unmodelled population of radio cores. This survey approach will assist in deriving robust VLBI source counts and broadening the discovery space for future wide-field VLBI surveys, including VLBI with the Square Kilometre Array, which will include new large field-of-view antennas on the African continent at ≳1000 km baselines. In addition, it may be useful in the design of both monitoring and/or rapidly triggered VLBI transient programmes. [ABSTRACT FROM AUTHOR]

We present multiband follow-up observations of the supermassive binary black hole (BBH) candidate and misaligned double–double radio galaxy, J1328+2752. To investigate its parsec-scale structure, we have carried out observations with the Very Long Baseline Array (VLBA) and the European Very Long Baseline Interferometry (VLBI) Network. Additionally, we have obtained optical spectroscopic observations with the 3.6 m Devasthal Optical Telescope. Within 3.5 yr of our previous VLBI observations, the central parsec-scale radio structure of J1328+2752 has changed from a double component to a single central component and its flux density has increased by a factor of 3 in the 5 GHz VLBA observations. The new radio component is largely unresolved at 3, 5, and 8 GHz. The multifrequency radio data shows a convex-shaped spectrum for this unresolved component. The turnover frequency is at ∼3 GHz. While the total intensity image does not reveal any details, a super-resolved VLBA 5 and 8 GHz spectral index image does indicate the presence of two components at a separation of ∼4.2 pc with spectral indices −0.5 ± 0.3 and −0.9 ± 0.3. We have not observed a simultaneous change in the optical line profiles or intensities over the past few years. The radio structure, the variation of the flux density, and spectral shape can potentially be consistent both with the signature of a young radio source or a BBH at the center of J1328+2752. [ABSTRACT FROM AUTHOR]

International terrestrial reference frame (ITRF) input data, generated by Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radiopositioning integrated by satellite (DORIS) combination centers (CCs), are considered to be relatively high-quality and accurate solutions. Every few years, these input data are submitted to the three ITRS combination centers, namely Institut Géographique National (IGN), Deutsches Geodätisches Forschungsinstitut at the Technische Universität München (DGFI-TUM), and Jet Propulsion Laboratory (JPL), to establish a multi-technique combined terrestrial reference frame (TRF). Generally, these solutions have undergone three rounds of outlier removal: the first at the technique analysis centers during solution generations and the second during the technique-specific combination by the CCs; ITRS CCs then perform a third round of outlier removal and preprocessing during the multi-technique combination of TRFs. However, since the primary objective of CCs is to release the final TRF product, they do not emphasize the publication of analytical preprocessing results, such as the outlier rejection rate. In this paper, our specific focus is on assessing the precision improvement of ITRF input data from 2014 to 2020, which includes evaluating the accuracy of coordinates, the datum accuracy, and the precision of the polar motions, for all four techniques. To achieve the above-mentioned objectives, we independently propose a TRF stacking approach to establish single technical reference frameworks, using software developed by us that is different from the ITRF generation. As a result, roughly 0.5% or less of the SLR observations are identified as outliers, while the ratio of DORIS, GNSS, and VLBI observations are below 1%, around 2%, and ranging from 1% to 1.2%, respectively. It is shown that the consistency between the SLR scale and ITRF has improved, increasing from around −5 mm in ITRF2014 datasets to approximately −1 mm in ITRF2020 datasets. The scale velocity derived from fitting the VLBI scale parameter series with all epochs in ITRF2020 datasets differs by approximately 0.21 mm/year from the velocity obtained by fitting the data up to 2013.75 because of the scale drift of VLBI around 2013. The decreasing standard deviations of the polar motion parameter (XPO, YPO) offsets between Stacking TRFs and 14C04 (20C04) indicate an improvement in the precision of polar motion observations for all four techniques. From the perspective of the weighted root mean square (WRMS) in station coordinates, since the inception of the technique, the station coordinate WRMS of DORIS decreased from 30 mm to 5 mm for X and Y components, and 25 mm to 5 mm for the Z component; SLR WRMS decreased from 20 mm to better than 10 mm (X, Y and Z); GNSS WRMS decreased from 4 mm to 1.5 mm (X and Y) and 5 mm to 2 mm (Z); while VLBI showed no significant change. [ABSTRACT FROM AUTHOR]

Radio telescopes with dual linearly polarized feeds regularly participate in Very Long Baseline Interferometry. One example is the VLBI Global Observing System (VGOS), which is employed for high‐precision geodesy and astrometry. In order to achieve the maximum signal‐to‐noise ratio, the visibilities of all four polarization products are combined to Stokes I before fringe‐fitting. Our aim is to improve cross‐polarization bandpass calibration, which is an essential processing step in this context. Here we investigate the shapes of these station‐specific quantities as a function of frequency and time. We observed the extra‐galactic source 4C 39.25 for 6 hours with a VGOS network. We correlated the data with the DiFX software and analyzed the visibilities with PolConvert to determine the complex cross‐bandpasses with high accuracy. Their frequency‐dependent shape is to first order characterized by a group delay between the two orthogonal polarizations, in the order of several hundred picoseconds. We find that this group delay shows systematic variability in the range of a few picoseconds, but can remain stable within this range for several years, as evident from earlier sessions. On top of the linear phase‐frequency relationship there are systematic deviations of several tens of degrees, which in addition are subject to smooth temporal evolution. The antenna cross‐bandpasses are variable on time scales of ∼1 hr, which defines the frequency of necessary calibrator scans. The source 4C 39.25 is confirmed as an excellent cross‐bandpass calibrator. Dedicated surveys are highly encouraged to search for more calibrators of similar quality. Key Points: The new‐generation geodetic radio telescopes observe two orthogonal linear polarization directionsCalibration of the gain differences between the two polarizers is necessary to maximize the signal‐to‐noise ratio of observationsWe investigate these cross‐polarization gain differences and their temporal evolution for selected antennas [ABSTRACT FROM AUTHOR]

The Earth's Free Core Nutation (FCN) causes Earth tides and forced nutation with frequencies close to the FCN that exhibit resonance effects. High-precision superconducting gravimeter (SG) and very long baseline interferometry (VLBI) provide good observation techniques for detecting the FCN parameters. However, some choices in data processing and solution procedures increase the uncertainty of the FCN parameters. In this study, we analyzed the differences and the effectiveness of weight function and ocean tide corrections in the FCN parameter detection using synthetic data, SG data from thirty-one stations, and the 10 celestial pole offset (CPO) series. The results show that significant discrepancies are caused by different computing options for a single SG station. The stacking method, which results in a variation of 0.24–5 sidereal days (SDs) in the FCN period (T) and 103-104 in the quality factor (Q) due to the selection of the weighting function and the ocean tide model (OTM), can effectively suppress this influence. The statistical analysis results of synthetic data shows that although different weight choices, while adjusting the proportion of diurnal tidal waves involved, do not significantly improve the accuracy of fitted FCN parameters from gravity observations. The study evaluated a series of OTMs using the loading correction efficiency. The fitting of FCN parameters can be improved by selecting the mean of appropriate OTMs based on the evaluation results. Through the estimation of the FCN parameters based on the forced nutation, it was found that the weight function P1 is more suitable than others, and different CPO series (after 2009) resulted in a difference of 0.4 SDs in the T and of 103 in the Q. We estimated the FCN parameters for SG (T = 430.4 ± 1.5 SDs and Q = 1.52 × 104 ± 2.5 × 103) and for VLBI (T = 429.8 ± 0.7 SDs, Q = 1.88 × 104 ± 2.1 × 103).

Imaging interferometric data in radio astronomy requires the use of non-linear algorithms that rely on different assumptions on the source structure and may produce non-unique results. This is especially true for very long baseline interferometry (VLBI) observations, where the sampling of Fourier space is very sparse. A basic tenet in standard VLBI imaging techniques is to assume that the observed source structure does not evolve during the observation. However, the recent VLBI results of the supermassive black hole at our Galactic Centre (Sagittarius A*), recently reported by the Event Horizon Telescope Collaboration, require the development of dynamic imaging algorithms, since it exhibits variability at minute time-scales. In this paper, we introduce a new non-convex optimization problem that extends the standard maximum entropy method (MEM), for reconstructing intra-observation dynamical images from interferometric data that evolve in every integration time. We present a rigorous mathematical formalism to solve the problem via the primal–dual approach. We build a Newton strategy and we give its numerical complexity. We also give a strategy to iteratively improve the obtained solution and, finally, we define a novel figure of merit to evaluate the quality of the recovered solution. Then, we test the algorithm, called the new-generation MEM (ngMEM), in different synthetic data sets, with increasing difficulty. Finally, we compare it with another well-established dynamical imaging method. Within this comparison, we have identified a significant improvement of the ngMEM reconstructions. Moreover, the evaluation of the integration time evolution scheme and the time contribution showed that this method can play a crucial role in obtaining good dynamic reconstructions. [ABSTRACT FROM AUTHOR]

• White noise domains in VLBI position time series. • White and flicker noise amplitudes show a pattern that depends on latitude. • Colored noise significantly impacts the estimation of velocity and its uncertainty. Identification of the optimal noise model for space geodetic time series is crucial for various studies in geodynamics and geodesy. However, the position time series of Very Long Baseline Interferometry (VLBI) are typically irregularly sampled. There is no clear evidence from previous studies to support that the Maximum Likelihood Estimate (MLE) method can allow for stable and reliable noise estimates for irregularly sampled data. To address this issue, we first conducted simulation studies. Results indicate that when the number of data points in the VLBI position time series reaches 300, reliable noise estimations result can be obtained using the MLE. We also found that the higher proportion of white noise a station has, the less influences the data gaps would be on noise estimations. Second, we apply the MLE to investigate the noise properties in the 32 VLBI position time series. About 12 (37.5%) stations exhibit pure white noise, and 15 (46.9%) stations display a combination of white noise and flicker noise. For these stations, the median amplitudes of white noise in the East, North, and Up components are 3.9, 4.6, and 11.9 mm, respectively. Moreover, white and flicker noise amplitudes show a latitude-dependent pattern, with stations located in the southern hemisphere and low latitudes (0-30°N) in the northern hemisphere exhibiting higher noise amplitudes. The remaining approximately 5 stations (15.6%) have the optimal noise model of power law noise with the mean spectral index of −0.32, −0.36, and −0.53 for the East, North, and Up components, respectively. Simply assuming pure white noise can significantly affect velocity uncertainty estimates when compared to using a noise model that includes colored noise. The velocity uncertainty will be underestimated by a factor of 2.9 on average. The effects of a small proportion of colored noise in VLBI data on velocity uncertainty estimation can be ignored only when the time series length exceeds 32.6 years. Additionally, the choice of different noise model can lead to a maximum difference of 0.84 mm for the annual signal of the Up component. [ABSTRACT FROM AUTHOR]

GW170817 is a binary neutron star merger that exhibited a gravitational wave (GW) and a gamma-ray burst, followed by an afterglow. In this work, we estimate the Hubble constant (H 0) using broad-band afterglow emission and relativistic jet motion from the Very Long Baseline Interferometry and HST images of GW170817. Compared to previous attempts, we combine these messengers with GW in a simultaneous Bayesian fit. We probe the H 0 measurement robustness depending on the data set used, the assumed jet model, the possible presence of a late time flux excess. Using the sole GW leads to a 20 per cent error (⁠|$77^{+21}_{-10}$|   |$\rm km\, s^{-1}\, Mpc^{-1}$|⁠ , medians, 16th–84th percentiles), because of the degeneracy between viewing angle (θv) and luminosity distance (d L). The latter is reduced by the inclusion in the fit of the afterglow light curve, leading to |$H_0=96^{+13}_{-10}$|   |$\rm km\, s^{-1}\, Mpc^{-1}$|⁠ , a large value, caused by the fit preference for high viewing angles due to the possible presence of a late-time excess in the afterglow flux. Accounting for the latter by including a constant flux component at late times brings |$H_0=78.5^{+7.9}_{-6.4}$| |$\rm km\, s^{-1}\, Mpc^{-1}$|⁠. Adding the centroid motion in the analysis efficiently breaks, the d L − θv degeneracy and overcome the late-time deviations, giving |$H_0 = 69.0^{+4.4}_{-4.3}$|   |$\rm km\, s^{-1}\, Mpc^{-1}$| (in agreement with Planck and SH0ES measurements) and |$\theta _{\rm v} = 18.2^{+1.2}_{-1.5}$| °. This is valid regardless of the jet structure assumption. Our simulations show that for next GW runs radio observations are expected to provide at most few other similar events. [ABSTRACT FROM AUTHOR]

Jets from active galactic nuclei are thought to play a role in the evolution of their host and local environments, but a detailed prescription is limited by the understanding of the jets themselves. Proper motion studies of compact bright components in radio jets can be used to produce model-independent constraints on their Lorentz factor, necessary to understand the quantity of energy deposited in the intergalactic medium. We present our initial work on the jet of radio–galaxy 3C 78, as part of Catalogue of proper motions in active galactic nuclei using Very Large Array Studies (CAgNVAS), with a goal of constraining nature of jet plasma on larger (>100 parsec) scales. In 3C 78, we find three prominent knots (A, B, and C), where knot B undergoes sub-luminal longitudinal motion (∼0.6 c at ∼ 200 pc), while knot C undergoes extreme (apparent) backward motion and eventual forward motion (∼−2.6 c , 0.5 c , at ∼ 300 pc). Assuming knots are shocks, we infer the bulk speeds from the pattern motion of Knots B and C. We model the spectral energy distribution of the large-scale jet and observe that a physically motivated two-zone model can explain most of the observed emission. We also find that the jet profile remains approximately conical from parsec to kiloparsec scales. Using the parsec-scale speed from very long baseline interferometry studies (∼0.1 c) and the derived bulk speeds, we find that the jet undergoes bulk acceleration between the parsec and the kiloparsec scales providing the first direct evidence of jet acceleration in a conical and matter-dominated jet. [ABSTRACT FROM AUTHOR]

Multi-epoch very long baseline interferometry (VLBI) observations measure three-dimensional water maser motions in protostellar outflows, enabling analysis of inclination and velocity. However, these analyses assume that water masers and shock surfaces within outflows are co-propagating. We compare VLBI data on maser-traced bow shocks in the high-mass protostar AFGL 5142-MM1, from seven epochs of archival data from the VLBI Exploration of Radio Astrometry (VERA), obtained from 2014 April to 2015 May, and our newly conducted data from the KVN and VERA Array (KaVA), obtained in 2016 March. We find an inconsistency between the expected displacement of the bow shocks and the motions of individual masers. The separation between two opposing bow shocks in AFGL 5142-MM1 was determined to be 337.17 ± 0.07 mas in the KaVA data, which is less than an expected value of 342.1 ± 0.7 mas based on extrapolation of the proper motions of individual maser features measured by VERA. Our measurements imply that the bow shock propagates at a velocity of 24 ± 3 km s−1, while the individual masing gas clumps move at an average velocity of 55 ± 5 km s−1; that is ,the water masers are moving in the outflow direction at double the speed at which the bow shocks are propagating. Our results emphasize that investigations of individual maser features are best approached using short-term high-cadence VLBI monitoring, while long-term monitoring on timescales comparable to the lifetimes of maser features is better suited to tracing the overall evolution of shock surfaces. Observers should be aware that masers and shock surfaces can move relative to each other, and that this can affect the interpretation of protostellar outflows. [ABSTRACT FROM AUTHOR]

Recent studies suggest that high-energy neutrinos can be produced in the jets of blazars, radio-loud active galactic nuclei (AGN) with jets pointing close to the line of sight. Due to the relatively poor angular resolution of current neutrino detectors, several sources can be regarded as the possible counterpart of a given neutrino event. Therefore, follow-up observations of counterpart candidates in the electromagnetic regime are essential. Since the Very Long Baseline Interferometry (VLBI) technique provides the highest angular resolution to study the radio jets of blazars, a growing number of investigations are being conducted to connect individual blazars to given high-energy neutrino events. We analyzed more than 20 years of available archival VLBI data of the blazar CTD 74, which has been listed as a possible counterpart of a neutrino event. Using cm-wavelength data, we investigated the jet structure, determined the apparent speed of jet components, and the core flux density before and after the neutrino event. Our results indicate stationary jet features and a significant brightening of the core after the neutrino event. [ABSTRACT FROM AUTHOR]

LAMOST J024048.51+195226.9 (J0240+1952) was recently identified as the second AE Aquarii (AE Aqr)-type cataclysmic variable, possessing the fastest known rotating white dwarf. We performed a Very Long Baseline Interferometry (VLBI) observation of J0240+1952 utilizing the European VLBI Network at 1.7 GHz, to obtain the first view of the radio morphology on mas scale. Our high-resolution VLBI image clearly shows that the radio emission is compact on mas scale (≲2 AU), with no evidence for a radio jet or extended emission. The compact radio source has an average flux density of ∼0.37 mJy, and its brightness temperature is given at ≳2.3 × 107 K, confirming a non-thermal origin. The emission exhibits irregular variations on a time-scale of tens of minutes, similar to the radio flares seen in AE Aqr. The measured VLBI position of J0240+1952 is consistent with that derived from Gaia. Our results favour the model in which the radio emission is attributed to a superposition of synchrotron radiation from expanding magnetized blobs of this system. [ABSTRACT FROM AUTHOR]

Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active galactic nuclei (AGN). Among them, TXS 1206+549 belongs to a small group of radio-loud and γ -ray-emitting NLS1 galaxies. We focus on the radio properties of this galaxy by analysing archival, high-resolution, very long baseline interferometry (VLBI) imaging observations taken at 8 GHz frequency in six epochs between 1994 and 2018. Using the milliarcsecond-scale radio structure, we can resolve a core and a jet component whose angular separation increases by (0.055 ± 0.006) mas yr−1. This corresponds to an apparent superluminal jet component motion of (3.5 ± 0.4) c . From the core brightness temperature and the jet component proper motion, we determine the characteristic Doppler-boosting factor, the bulk Lorentz factor, and the jet viewing angle. We find no compelling evidence for a very closely aligned blazar-type jet. The parameters for TXS 1206+549 resemble those of radio-loud quasar jets with a moderate Lorentz factor ( Γ ≈ 4 ) and ϑ ≈ 24 ∘ inclination to the line of sight. [ABSTRACT FROM AUTHOR]

The difference between observed and modelled precession/nutation reveals unmodelled signals commonly referred to as Celestial Pole Offsets (CPO), denoted by dX and dY. CPO are currently observed only by Very Long Baseline Interferometry (VLBI), but there is nearly 4 weeks of latency by which the data centers provide the most accurate, final CPO series. This latency problem necessitates predicting CPO for high-accuracy, real-time applications that require information regarding Earth rotation, such as spacecraft navigation. Even though the International Earth Rotation and Reference Systems Service (IERS) provides so-called rapid CPO, they are usually less accurate and therefore, may not satisfy the requirements of the mentioned applications. To enhance the quality of CPO predictions, we present a new methodology based on Neural Additive Models (NAMs), a class of interpretable machine learning algorithms. We formulate the problem based on long short-term memory neural networks and derive simple analytical relations for the quantification of prediction uncertainty and feature importance, thereby enhancing the intelligibility of predictions made by machine learning. We then focus on the short-term prediction of CPO with a forecasting horizon of 30 days. We develop an operational framework that consistently provides CPO predictions. Using the CPO series of Jet Propulsion Laboratory as the input to the algorithm, we show that NAMs predictions improve the IERS rapid products on average by 57% for dX and 25% for dY under fully operational conditions. Our predictions are both accurate and overcome the latency issue of final CPO series and thus, can be used in real-time applications. [ABSTRACT FROM AUTHOR]

Context. The colliding-wind region in binary systems made up of massive stars allows us to investigate various aspects of shock physics, including particle acceleration. Particle accelerators of this kind are mainly identified thanks to their synchrotron radio emission and dubbed particle-accelerating colliding-wind binaries. Aims. Our objective is first to validate the notion that obtaining snapshot high-resolution radio images of massive binaries constitutes a relevant approach to unambiguously identifying particle accelerators. Second, we intend to exploit these images to characterise the synchrotron emission of two specific targets, HD 167971 and HD 168112, which are known particle accelerators. Methods. We traced the radio emission from the two targets at 1.6 GHz with the European Very Long Baseline Interferometry (VLBI) Network, with an angular resolution of a few milli-arcseconds. Results. Our measurements allowed us to obtain images for both targets. For HD 167971, our observation occurs close to apastron, at an orbital phase where the synchrotron emission is at minimum. For HD 168112, we resolved for the very first time the synchrotron emission region. The emission region appears slightly elongated, in agreement with the expectations for a colliding-wind region. In both cases, the measured emission is significantly stronger than the expected thermal emission from the stellar winds, lending strong support to their non-thermal nature. Conclusions. Our study offers a significant contribution to the still poorly addressed question of high angular resolution radio imaging of colliding-wind binaries. We show that snapshot VLBI measurements constitute an efficient approach to investigate these objects, with promising results in terms of the identification of additional particle accelerators, coupled with their applicability in revealing long-period binaries. [ABSTRACT FROM AUTHOR]

Earth rotation parameters (ERPs) are fundamental to geodetic and astronomical studies. With its high measurement accuracy and stability, the Very Long Baseline Interferometry (VLBI) plays an irreplaceable role in estimating the ERPs and maintaining the earth reference frame. However, the imperfect global station distribution, observation discontinuity, and vast cost of the VLBI make the GNSS a more attractive technique. In 2020, the third generation of the BeiDou Navigation System (BDS), namely BDS-3, was constructed completely. In this study, we conducted a series of experiments to estimate Earth's rotation parameters based on the continuous BDS-3 observation data, the discontinuous VLBI observation data, and the combined BDS-3 and discontinuous VLBI observation data. We used two methods, namely the weighted averaging method and the normal equation combination method, to obtain ERP combination solutions. The results are compared with the International Earth Rotation and Reference Systems Service (IERS) EOP 20C04 at 00:00:00 UTC. Final results show that (a) the estimation accuracy becomes stable when the number of BDS-3 tracking stations is more than 40. At the same time, both the number of stations and the volume of polyhedrons formed by the observing stations affect the accuracy of the ERPs estimated by the BDS-3 or VLBI. (b) Results have also shown that the inclusion of the BDS-3 IGSO and GEO satellites contributes little to the ERP estimation. (c) For the BDS-3-only MEO satellites solution, the root mean square (RMS) was 113.2 µas, 102.8 µas, and 13.1 µs/day for X-pole coordinate, Y-pole coordinate, and length of day (LOD), respectively. For the VLBI solution, the RMSs of the X-pole, Y-pole, and LOD were 100.4 µas for the X-pole, 94.2 µas for the Y-pole, and 14.1 µs/day. The RMS was 82.6 µas, 70.3 µas, and 10.5 µs/day for the combined X-pole, Y-pole, and LOD using the weighted averaging method. It was 78.2 µas, 62.6 µas, and 8.6 µs/day when the normal equation combination method was applied. This demonstrates that by taking advantage of the BDS-3 and VLBI technique combinations, accuracy in estimating the ERPs can be improved over that using either of them, in addition to enhanced stability and reliability. [ABSTRACT FROM AUTHOR]

We present here uGMRT band 4 (∼650 MHz) polarization images of 8 BL Lac objects belonging to the Palomar-Green (PG) 'blazar' sample. A large fraction of the sources (⁠|$\sim 63~{{\ \rm per\ cent}}$|⁠) reveal core-halo radio structures with most of the polarization detected in the inner core-jet regions. PG1101 + 385 and PG2254 + 075 exhibit a 'spine-sheath structure' in polarization. The core-halo and 'spine-sheath' structures are consistent with the Unified Scheme suggestion that BL Lacs are the pole-on beamed counterparts of Fanaroff–Riley (FR) type I radio galaxies. PG1418 + 546 and PG0851 + 203 (OJ287) show the presence of terminal hotspots similar to FR type II radio galaxies. They were also found to be low-spectrally peaked BL Lacs, supportive of the 'blazar envelope' scenario for BL Lacs and quasars. Fractional polarization ranges from 1 to 13 per cent in the cores and 2 to 26 per cent in the inner jets/lobes of the sample BL Lacs. Compared to the varied radio morphology of quasars from the PG 'blazar' sample, the BL Lacs appear to be less diverse. A comparison of the inferred core magnetic (B -) field structures on arcsec- (kpc-) scales w.r.t. the Very Long Baseline Interferometry jet direction does not reveal any preferred orientation, suggesting that if large-scale ordered B -fields exist, they do so on scales smaller than probed by the current observations. However, the presence of polarized emission on arcsec-scales suggests that any mixing of thermal plasma with the synchrotron emitting plasma is insufficient to fully depolarize the emission via the internal depolarization process. [ABSTRACT FROM AUTHOR]

Celestial Pole Offsets (CPO), denoted by dX and dY, describe the differences in the observed position of the pole in the celestial frame with respect to a certain precession-nutation model. Precession and nutation components are part of the transformation matrix between terrestrial and celestial systems. Therefore, various applications in geodetic science such as high-precision spacecraft navigation require information regrading precession and nutation. For this purpose, CPO can be added to the precession-nutation model to precisely describe the motion of the celestial pole. However, as Very Long Baseline Interferometry (VLBI)—currently the only technique providing CPO—requires long data processing times resulting in several weeks of latency, predictions of CPO become necessary. Here we present a new methodology named Deep Ensemble Geophysics-Informed Neural Networks (DEGINNs) to provide accurate CPO predictions. The methodology has three main elements: (1) deep ensemble learning to provide the prediction uncertainty; (2) broad-band Liouville equation as a geophysical constraint connecting the rotational dynamics of CPO to the atmospheric and oceanic Effective Angular Momentum (EAM) functions and (3) coupled oscillatory recurrent neural networks to model the sequential characteristics of CPO time-series, also capable of handling irregularly sampled time-series. To test the methodology, we use the newest version of the final CPO time-series of International Earth Rotation and Reference Systems Service (IERS), namely IERS 20 C04. We focus on a forecasting horizon of 90 days, the practical forecasting horizon needed in space-geodetic applications. Furthermore, for validation purposes we generate an independent global VLBI solution for CPO since 1984 up to the end of 2022 and analyse the series. We draw the following conclusions. First, the prediction performance of DEGINNs demonstrates up to 25 and 33 percent improvement, respectively, for dX and dY, with respect to the rapid data provided by IERS. Secondly, predictions made with the help of EAM are more accurate compared to those without EAM, thus providing a clue to the role of atmosphere and ocean on the excitation of CPO. Finally, free core nutation period shows temporal variations with a dominant periodicity of around one year, partially excited by EAM. [ABSTRACT FROM AUTHOR]

M60, an elliptical galaxy located 16.5 Mpc away, has an active nucleus with a very low luminosity and an extremely low accretion rate. Its central supermassive black hole (SMBH) has a mass of M BH ∼ 4.5 × 109 M ⊙ and a Schwarzschild radius corresponding to R S ∼ 5.4 μ as. To investigate the nature of its innermost radio nucleus, data from the Very Long Baseline Array (VLBA) at 4.4 and 7.6 GHz were reduced. The VLBA images reveal a compact component with total flux densities of ∼20 mJy at both frequencies, a size of ≤0.27 mas (99.7% confidence level), about 0.022 pc (50 R S) at 7.6 GHz, and a brightness temperature of ≥6 × 109 K. This suggests that the observed centiparsec-scale compact core could be attributed to a nonthermal jet base or an advection-dominated accretion flow (ADAF) with nonthermal electrons. The extremely compact structure also supports the presence of an SMBH in the center. Our results indicate that M60 is a promising target for broadband very long baseline interferometry observations at millimeter wavelengths to probe ADAF scenarios and tightly constrain the potential photon ring (about 28 μ as) around its SMBH. [ABSTRACT FROM AUTHOR]

In this paper we significantly improve estimates of distance to the X-ray binary systems Cyg X-3 and GRS 1915+105. We report a highly accurate trigonometric parallax measurement for Cyg X-3 using the Very Long Baseline Array at 43 GHz, placing the source at a distance of 9.67 − 0.48 + 0.53 kpc. We also use Galactic proper motions and line-of-sight radial velocity measurements to determine three-dimensional (3D) kinematic distances to both systems, under the assumption that they have low peculiar velocities. This yields distances of 8.95 ± 0.96 kpc for Cyg X-3 and 9.4 ± 0.6 (statistical) ± 0.8 (systematic) for GRS 1915+105. The good agreement between parallax and 3D kinematic distances validates the assumption of low peculiar velocities, and hence small natal kicks, for both of the systems. For a source with a low peculiar velocity, given its parallax distance, Cyg X-3 should have a V LSR near −64 ± 5 km s−1. Our measurements imply a slightly higher inclination angle, and hence lower black hole mass, for GRS 1915+105 than found from previous work by Reid et al. and strengthen arguments from X-ray polarization that Cyg X-3 would be an ultraluminous X-ray source if viewed face-on. [ABSTRACT FROM AUTHOR]

Studying a centi-parsec supermassive black hole binary (SMBHB) would allow us to explore a new parameter space in active galactic nuclei, and these objects are also potential sources of gravitational waves. We report evidence that an SMBHB with an orbital period of ∼30 yr may be resident in the nearby galactic nucleus M81. This orbital period and the known mass of M81 imply an orbital separation of ∼0.02 pc. The jet emanating from the primary black hole showed a short period of jet wobbling at ∼16.7 yr, superposing a long-term precession at a timescale of several hundred years. Periodic radio and X-ray outbursts were also found two times per orbital period, which could be explained by a double-peaked mass accretion rate variation per binary orbit. If confirmed, M81 would be one of the closest SMBHB candidates, providing a rare opportunity to study the final parsec problem. [ABSTRACT FROM AUTHOR]

Planetary Radio Interferometry and Doppler Experiment (PRIDE) is a multi-purpose experimental technique aimed at enhancing the science return of planetary missions. The technique exploits the science payload and spacecraft service systems without requiring a dedicated onboard instrumentation or imposing on the existing instrumentation any special for PRIDE requirements. PRIDE is based on the near-field phase-referencing Very Long Baseline Interferometry (VLBI) and evaluation of the Doppler shift of the radio signal transmitted by spacecraft by observing it with multiple Earth-based radio telescopes. The methodology of PRIDE has been developed initially at the Joint Institute for VLBI ERIC (JIVE) for tracking the ESA's Huygens Probe during its descent in the atmosphere of Titan in 2005. From that point on, the technique has been demonstrated for various planetary and other space science missions. The estimates of lateral position of the target spacecraft are done using the phase-referencing VLBI technique. Together with radial Doppler estimates, these observables can be used for a variety of applications, including improving the knowledge of the spacecraft state vector. The PRIDE measurements can be applied to a broad scope of research fields including studies of atmospheres through the use of radio occultations, the improvement of planetary and satellite ephemerides, as well as gravity field parameters and other geodetic properties of interest, and estimations of interplanetary plasma properties. This paper presents the implementation of PRIDE as a component of the ESA's Jupiter Icy Moons Explorer (JUICE) mission. [ABSTRACT FROM AUTHOR]

The latest results of the most detailed analysis of multi-epoch polarization-sensitive observations of active galactic nuclei (AGN) jets at parsecs scales by very long baseline interferometry (VLBI) reveal several characteristic patterns of linear polarization distribution and its variability [1, 2]. Some of the observed profiles can be reproduced by a simple model of a jet threaded by a helical magnetic field. However, none of the models presented to date can explain the observed polarization profiles with an increase in its degree towards the edges of the jet, and accompanied by a "fountain" type electrical vector pattern and its high temporal variability in the center. Based on simulations of the VLBI observations of relativistic jets, we show here that the observed transverse linear polarization profiles can be naturally produced in a model of jets precessing on ten-years scales. In this scenario, due to the finite resolution of VLBI arrays, the initially strong polarization along the jet axis is blurred due to the superposition of regions whose polarization angle changes significantly when projected onto the sky plane. In our simulations, we qualitatively reproduce the distribution of the electric vector and its variability, though the polarization distribution images are characterized by a bright spine due to weak suppression of polarized emission, which is poorly consistent with observations of quasars. More effective depolarization can be obtained in models with the suppressed emission of the jet spine. [ABSTRACT FROM AUTHOR]

With the imaging and characterization of the horizon-scale images of M87* and Sgr A* by the Event Horizon Telescope (EHT), it has become possible to resolve the near-horizon region of astrophysical black holes. As a result, there has been considerable interest in the implications of the measurement of the shadow size, i.e., the asymptotic photon ring. We explore the general implications of such a measurement, identifying what is and, more importantly, is not constrained by such measurements, with applications to EHT and future instruments. We consider a general spherically symmetric metric, which effectively applies for a polar observer (appropriate for M87*) in the slow rotation limit. We propose a nonperturbative, nonparametric spacetime-domain characterization of shadow size and related measurements that makes explicit the nature and power (or lack thereof) of shadow-size-based constraints, and facilitates comparisons among observations and targets. [ABSTRACT FROM AUTHOR]

Class II CH3OH masers are used as a convenient tracer of disc-like structures in high-mass star formation. However, more than half of them show a complex distribution in Very Long Baseline Interferometry (VLBI) maps. The origin of such a complex distribution is still unknown. We conducted VLBI monitoring observations to unveil the origin of a complex class II CH3OH maser in the high-mass star-forming region G59.783+0.065. We observed the CH3OH maser at 6.7 GHz and the H2O maser at 22 GHz to probe detailed circumstellar kinematics and structures by the Japanese VLBI network and the VLBI Exploration of Radio Astrometry. We found similar bipolar distributions in both masers, specifically two clusters located 2000 au apart along the east–west direction. We detected a linear distribution of CH3OH masers in the western cluster. A position–velocity diagram shows that the western CH3OH masers trace a rotating disc-wind or infalling component inside an edge-on disc-like structure. In contrast to the simple bipolar expanding motions of the H2O masers, the CH3OH masers exhibited complex motions despite their spatial coincidence. Some of the eastern CH3OH masers showed bipolar expansions similar to the H2O masers, while others displayed random or even inward motions. Such complex kinematics and their close association with the H2O maser could occur at the boundary between outflow and inflow. We suggest that the complex distribution of class II CH3OH masers, like G59.783+0.065 arises from several distinct circumstellar structures that simultaneously achieve maser excitation. [ABSTRACT FROM AUTHOR]

Equipping Galileo satellites with a VLBI transmitter (VT) will allow to observe satellites next to quasars with Very Long Baseline Interferometry (VLBI) radio telescopes. This concept will facilitate the direct estimation of the satellite orbits in the celestial reference frame. Moreover, these observations along with usual Galileo observations can be used to transfer the space tie between the VT and the antenna on the Galileo satellite to the Earth surface realizing the frame tie at the geodetic site with VLBI radio telescope and Galileo antenna. In this study, we assess the accuracy of that frame tie by simulating the estimation of station coordinates from VLBI observations to Galileo satellites next to quasars. We find that at least two or three satellites need to be equipped with a VT with the best results if all satellites with a VT are placed in the same plane. Concerning the ratio between satellite and quasar observations within a schedule, the results suggest that the optimal ratio is around 30% to 40% satellite observations out of the total number of observations in order to have enough observations for the estimation of the station coordinates but still enough quasar observations to ensure a sufficient sky-coverage for the estimation of troposphere parameters. The best scenario with two satellites yields repeatabilities for the east and north components between 7.5 and 10 mm, and for the up component between 9.5 and 12 mm. In case there is a third satellite with a VLBI transmitter in the same plane, the repeatabilities are reduced by up to 2 mm for the horizontal components and up to 3 to 4 mm for the up component. Rotating the schedules over the constellation repeat cycle of Galileo of 10 days reveals that there are differences between the individual days, but there are no days with a significantly worse precision of the estimated station coordinates. [ABSTRACT FROM AUTHOR]

This study uses multifrequency very long baseline interferometry (VLBI) to study the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loud quasars (RLQs). The diverse morphologies, radio spectra, and brightness temperatures observed in the VLBI images of these RQQs, together with the variability in their GHz spectra and VLBI flux densities, shed light on the origins of their nuclear radio emission. The total radio emission of RQQs appears to originate from non-thermal synchrotron radiation due to a combination of active galactic nuclei and star formation activities. However, our data suggest that the VLBI-detected radio emission from these RQQs is primarily associated with compact jets or corona, with extended emissions such as star formation and large-scale jets being resolved by the high resolution of the VLBI images. Wind emission models are not in complete agreement the VLBI observations. Unlike RLQs, where the parsec-scale radio emission is dominated by a relativistically boosted core, the radio cores of RQQs are either not dominant or are mixed with significant jet emission. RQQs with compact cores or core-jet structures typically have more pronounced variability, with flat or inverted spectra, whereas jet-dominated RQQs have steep spectra and unremarkable variability. Future high-resolution observations of more RQQs could help to determine the fraction of different emission sources and their associated physical mechanisms. [ABSTRACT FROM AUTHOR]

Studying the long-term radio variability (time-scales of months to years) of blazars enables us to gain a better understanding of the physical structure of these objects on subparsec scales, and the physics of supermassive black holes. In this study, we focus on the radio variability of 1157 blazars observed at 15 GHz through the Owens Valley Radio Observatory Blazar Monitoring Program. We investigate the dependence of the variability amplitudes and time-scales, characterized based on model fitting to the structure functions, on the milliarcsecond core sizes measured by Very Long Baseline Interferometry. We find that the most compact sources at milliarcsecond scales exhibit larger variability amplitudes and shorter variability time-scales than more extended sources. Additionally, for sources with measured redshifts and Doppler boosting factors, the correlation between linear core sizes against variability amplitudes and intrinsic time-scales is also significant. The observed relationship between variability time-scales and core sizes is expected, based on light travel-time arguments. This variability versus core size relation extends beyond the core sizes measured at 15 GHz; we see significant correlation between the 15 GHz variability amplitudes (as well as time-scales) and core sizes measured at other frequencies, which can be attributed to a frequency–source size relationship arising from the intrinsic jet structure. At low frequencies of 1 GHz where the core sizes are dominated by interstellar scattering, we find that the variability amplitudes have significant correlation with the 1 GHz intrinsic core angular sizes, once the scatter broadening effects are deconvoluted from the intrinsic core sizes. [ABSTRACT FROM AUTHOR]

This paper examines options for orbit configurations for a space interferometer. In contrast to previously presented concepts for space very long baseline interferometry, we propose a combination of regular and retrograde near-Earth circular orbits in order to achieve a faster filling of (u , v) coverage. With the rapid relative motion of the telescopes, it will be possible to quickly obtain high quality images of supermassive black holes. As a result of such an approach, it will be possible for the first time to conduct high quality studies of the supermassive black hole close surroundings in dynamics. • A new configuration for the space interferometer is proposed. • This configuration uses near-Earth regular and retrograde orbits. • Calculated orbits are stable more than 10 years. • The supermassive black hole can be imaged within 20 h. • Synthetic simulations show observations of black holes are possible in dynamics. [ABSTRACT FROM AUTHOR]