Space VLBI Research Articles

Unveiling the bent-jet structure and polarization of OJ 287 at 1.7 GHz with space VLBI (Corrigendum)

Unveiling the bent-jet structure and polarization of OJ 287 at 1.7 GHz with space VLBI (Corrigendum)

Imaging the event horizon of M87* from space on different timescales

Imaging the event horizon of M87* from space on different timescales

Effects of Earth’s Oblateness on Black Hole Imaging through Earth–Space and Space–Space Very Long Baseline Interferometry

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.

Unveiling the bent-jet structure and polarization of OJ 287 at 1.7 GHz with space VLBI

We present total intensity and linear polarization images of OJ 287 at 1.68 GHz, obtained through space-based very long baseline interferometry (VLBI) observations with RadioAstron on April 16, 2016. The observations were conducted using a ground array consisting of the Very Long Baseline Array (VLBA) and the European VLBI Network (EVN). Ground-space fringes were detected with a maximum projected baseline length of ∼5.6 Earth’s diameter, resulting in an angular resolution of ∼530 μas. With this unprecedented resolution at such a low frequency, the progressively bending jet structure of OJ 287 has been resolved up to ∼10 parsec of the projected distance from the radio core. In comparison with close-in-time VLBI observations at 15, 43, 86 GHz from MOJAVE and VLBA-BU-BLAZAR monitoring projects, we obtain the spectral index map showing the opaque core and optically thin jet components. The optically thick core has a brightness temperature of ∼1013 K, and is further resolved into two sub-components at higher frequencies labeled C1 and C2. These sub-components exhibit a transition from optically thick to thin, with a synchrotron self-absorption (SSA) turnover frequency estimated to be ∼33 and ∼11.5 GHz, and a turnover flux density ∼4 and ∼0.7 Jy, respectively. Assuming a Doppler boosting factor of 10, the SSA values provide the estimate of the magnetic field strengths from SSA of ∼3.4 G for C1 and ∼1.0 G for C2. The magnetic field strengths assuming equipartition arguments are also estimated as ∼2.6 G and ∼1.6 G, respectively. The integrated degree of linear polarization is found to be approximately ∼2.5%, with the electric vector position angle being well aligned with the local jet direction at the core region. This alignment suggests a predominant toroidal magnetic field, which is in agreement with the jet formation model that requires a helical magnetic field anchored to either the black hole ergosphere or the accretion disk. Further downstream, the jet seems to be predominantly threaded by a poloidal magnetic field.

Gravitational redshift test of EEP with RadioAstron from near Earth to the distance of the Moon

The Einstein Equivalence Principle (EEP) is a cornerstone of general relativity and predicts the existence of gravitational redshift. We report on new results of measuring this shift with RadioAstron (RA), a space very long baseline interferometry (VLBI) spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353 000 km. The spacecraft and ground tracking stations at Pushchino, Russia, and Green Bank, USA, were each equipped with a hydrogen maser frequency standard allowing a possible violation of the predicted gravitational redshift, in the form of a violation parameter ɛ, to be measured. By alternating between RA’s frequency referencing modes during dedicated sessions between 2015 and 2017, the recorded downlink frequencies can essentially be corrected for the non-relativistic Doppler shift. We report on an analysis using the Doppler-tracking frequency measurements made during these sessions and find . We also discuss prospects for measuring ɛ with a significantly smaller uncertainty using instead the time-domain recordings of the spacecraft signals and envision how 10−7 might be possible for a future space VLBI mission.

Design and Analysis of LPDA Fed Parabolic Reflector Antenna

In this research, a parabolic reflector antenna fed by a log-periodic dipole array (LPDA) was designed for the broadband requirement in the space very long baseline interferometry (SVLBI) project. Geometric parameters for LPDA and reflector were calculated and selected based on the major performance indicators, then the simulation and results were obtained in the software ANSYS Electronics 2021. The realized antenna gain exceeded the engineering requirement of 15 dB, and related issues were also studied and analyzed.

OmniUV: A Multipurpose Simulation Toolkit for VLBI Observation

We present OmniUV, a multipurpose simulation toolkit for space and ground VLBI observations. It supports various kinds of VLBI stations, including Earth (ground) fixed, Earth orbit, Lunar fixed, Lunar orbit, Moon–Earth, and Earth–Sun Lagrange 1 and 2 points, etc. The main functionalities of this toolkit are: (1) trajectory calculation; (2) baseline uv calculation, by taking the availability of each station into account; (3) visibility simulation for the given uv distribution, source structure, and system noise; (4) image and beam reconstruction. Two scenarios, namely the space VLBI network and the wide-field array, are presented as the application of the toolkit at completely different scales. OmniUV is the acronym of “Omnipotent UV.” We hope it could work as a general framework, in which various kinds of stations could be easily incorporated and the functionalities could be further extended. The toolkit has been made publicly available.

The science case and challenges of space-borne sub-millimeter interferometry

Ultra-high angular resolution in astronomy has always been an important vehicle for making fundamental discoveries. Recent results in direct imaging of the vicinity of the supermassive black hole in the nucleus of the radio galaxy M87 by the millimeter VLBI system Event Horizon Telescope and various pioneering results of the Space VLBI mission RadioAstron provided new momentum in high angular resolution astrophysics. In both mentioned cases, the angular resolution reached the values of about 10–20 microarcseconds (0.05–0.1 nanoradian). Further developments towards at least an order of magnitude “sharper” values, at the level of 1 microarcsecond are dictated by the needs of advanced astrophysical studies. The paper emphasis that these higher values can only be achieved by placing millimeter and submillimeter wavelength interferometric systems in space. A concept of such the system, called Terahertz Exploration and Zooming-in for Astrophysics, has been proposed in the framework of the ESA Call for White Papers for the Voyage 2050 long term plan in 2019. In the current paper we present new science objectives for such the concept based on recent results in studies of active galactic nuclei and supermassive black holes. We also discuss several approaches for addressing technological challenges of creating a millimeter/sub-millimeter wavelength interferometric system in space. In particular, we consider a novel configuration of a space-borne millimeter/sub-millimeter antenna which might resolve several bottlenecks in creating large precise mechanical structures. The paper also presents an overview of prospective space-qualified technologies of low-noise analogue front-end instrumentation for millimeter/sub-millimeter telescopes. Data handling and processing instrumentation is another key technological component of a sub-millimeter Space VLBI system. Requirements and possible implementation options for this instrumentation are described as an extrapolation of the current state-of-the-art Earth-based VLBI data transport and processing instrumentation. The paper also briefly discusses approaches to the interferometric baseline state vector determination and synchronisation and heterodyning system. The technology-oriented sections of the paper do not aim at presenting a complete set of technological solutions for sub-millimeter (terahertz) space-borne interferometers. Rather, in combination with the original ESA Voyage 2050 White Paper, it sharpens the case for the next generation microarcsecond-level imaging instruments and provides starting points for further in-depth technology trade-off studies.

Effects of Surface Errors of Antennas on Detection Performance of Space VLBI

This work investigates the effects of the surface errors of the satellite antenna on the detection performance of the space VLBI system. First, the relationship between the surface errors and the antenna gain loss is analyzed, and then the influence of the gain loss on the detection performance of the VLBI system is analyzed. Both the uniform and nonuniform distributions of errors are studied, and the second-order Taylor expansion is performed on the errors to simplify the calculation. When the errors distribute nonuniformly, the solver SCIP is adopted to solve the corresponding distribution, which leads to the maximum gain loss of the antenna. Taking the VLBI system with two base stations as the object, and each station’s radio telescope is a hoop truss deployable antenna with 30 m aperture, the effects of antenna gain loss on the detection capability of the radio telescope and the delay error of the VLBI system are studied. The study of extreme working conditions will have a higher guiding significance for the overall link analysis of practical projects.

Laboratory Demonstration of the Local Oscillator Concept for the Event Horizon Imager

Black hole imaging challenges the third-generation space VLBI, the Very Long Baseline Interferometry, to operate on a 500[Formula: see text]GHz band. The coherent integration time needed here is 450[Formula: see text]s though the available space oscillators cannot offer more than 10[Formula: see text]s. Self-calibration methods might solve this issue in an interferometer formed by three antenna/satellite systems, but the need for the third satellite increases the mission costs. A frequency transfer is of special interest to alleviate both performance and cost issues. A concept of two-way optical frequency transfer is examined to investigate its suitability to enable space-to-space interferometry, in particular, to image the “shadows” of black holes from space. The concept, promising on paper, has been demonstrated by tests. The laboratory test set-up is presented and the verification of the temporal stability using standard analysis tool as TimePod has been passed. The resulting Allan Deviation is dominated by the 1/[Formula: see text] phase noise trend since the frequency transfer timescale of interest is shorter than 0.2[Formula: see text]s. This trend continues into longer integration times, as proven by the longest tests spanning over a few hours. The Allan Deviation between derived 103.2[Formula: see text]GHz oscillators is [Formula: see text]/[Formula: see text] within 10[Formula: see text][Formula: see text][Formula: see text]s that degrades twice towards the longest delay of 0.2[Formula: see text]s. The worst case satisfies the requirement with a margin of 11 times. The obtained coherence in the range of 0.997[Formula: see text]0.9998 is beneficial for space VLBI at 557[Formula: see text]GHz. The result is of special interest to future science missions for black hole imaging from space.

Black hole parameter estimation with synthetic very long baseline interferometry data from the ground and from space

Context.The Event Horizon Telescope (EHT) has imaged the shadow of the supermassive black hole in M 87. A library of general relativistic magnetohydrodynamics (GMRHD) models was fit to the observational data, providing constraints on black hole parameters.Aims.We investigate how much better future experiments can realistically constrain these parameters and test theories of gravity.Methods.We generated realistic synthetic 230 GHz data from representative input models taken from a GRMHD image library for M 87, using the 2017, 2021, and an expanded EHT array. The synthetic data were run through an automated data reduction pipeline used by the EHT. Additionally, we simulated observations at 230, 557, and 690 GHz with the Event Horizon Imager (EHI) Space VLBI concept. Using one of the EHT parameter estimation pipelines, we fit the GRMHD library images to the synthetic data and investigated how the black hole parameter estimations are affected by different arrays and repeated observations.Results.Repeated observations play an important role in constraining black hole and accretion parameters as the varying source structure is averaged out. A modest expansion of the EHT already leads to stronger parameter constraints in our simulations. High-frequency observations from space with the EHI rule out all but ∼15% of the GRMHD models in our library, strongly constraining the magnetic flux and black hole spin. The 1σconstraints on the black hole mass improve by a factor of five with repeated high-frequency space array observations as compared to observations with the current ground array. If the black hole spin, magnetization, and electron temperature distribution can be independently constrained, the shadow size for a given black hole mass can be tested to ∼0.5% with the EHI space array, which allows tests of deviations from general relativity. With such a measurement, high-precision tests of the Kerr metric become within reach from observations of the Galactic Center black hole Sagittarius A*.

THEZA: TeraHertz Exploration and Zooming-in for Astrophysics

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above sim 300 GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline Interferometry) and mm- and sub-mm astrophysical studies with “single dish” instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) – the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum.

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron

Context. Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated. Aims. We study the innermost jet morphology and magnetic field strength in the AGN 3C 345 with an unprecedented resolution using images obtained within the framework of the key science programme on AGN polarisation of the Space VLBI mission RadioAstron. Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016 March 30 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode. Results. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300 μas angular resolution, corresponding to a projected linear scale of about 2 pc or a few thousand gravitational radii. We identify the synchrotron self-absorbed core at the jet base and find the brightest feature in the jet 1.5 mas downstream of the core. Several polarised components appear in the Space VLBI images that cannot be seen from ground array-only images. Except for the core, the electric vector position angles follow the local jet direction, suggesting a magnetic field perpendicular to the jet. This indicates the presence of plane perpendicular shocks in these regions. Additionally, we infer a minimum brightness temperature at the largest (u, v)-distances of 1.1 × 1012 K in the source frame, which is above the inverse Compton limit and an order of magnitude larger than the equipartition value. This indicates locally efficient injection or re-acceleration of particles in the jet to counter the inverse Compton cooling or the geometry of the jet creates significant changes in the Doppler factor, which has to be > 11 to explain the high brightness temperatures.

The optimization of satellite orbit for Space-VLBI observation

By sending one or more telescopes into space, Space-VLBI (SVLBI) is able to achieve even higher angular resolution and is therefore the trend of the VLBI technique. For the SVLBI program, the design of satellite orbits plays an important role for the success of planned observation. In this paper, we present our orbit optimization scheme, so as to facilitate the design of satellite orbits for SVLBI observation. To achieve that, we characterize the uv coverage with a measure index and minimize it by finding out the corresponding orbit configuration. In this way, the design of satellite orbit is converted to an optimization problem. We can prove that, with an appropriate global minimization method, the best orbit configuration can be found within the reasonable time. Besides that, we demonstrate that this scheme can be used for the scheduling of SVLBI observations.

Compact Bright Radio-loud AGNs. III. A Large VLBA Survey at 43 GHz

Abstract We present the results from the 43 GHz Very Long Baseline Array (VLBA) observations of 124 compact radio-loud active galactic nuclei (AGNs) that were conducted between 2014 November and 2016 May. The typical dimensions of the restoring beam in each image are about 0.5 mas × 0.2 mas. The highest resolution of 0.2 mas corresponds to a physical size of 0.02 pc for the lowest redshift source in the sample. The 43 GHz very long baseline interferometry (VLBI) images of 97 AGNs are presented for the first time. We study the source compactness on milliarcsecond and submilliarcsecond scales, and suggest that 95 sources in our sample are suitable for future space VLBI observations. By analyzing our data supplemented with other VLBA AGN surveys from the literature, we find that the core brightness temperature increases with increasing frequency below a break frequency ∼7 GHz, and decreases between ∼7 and 240 GHz but increases again above 240 GHz in the rest frame of the sources. This indicates that the synchrotron opacity changes from optically thick to thin. We also find a strong statistical correlation between radio and γ-ray flux densities. Our correlation is tighter than those in the literature derived from lower-frequency VLBI data, suggesting that the γ-ray emission is produced more cospatially with the 43 GHz VLBA core emission. This correlation can also be extrapolated to the unbeamed AGN population, implying that a universal γ-ray production mechanism might be at work for all types of AGNs.

Probing the Innermost Regions of AGN Jets and Their Magnetic Fields with RadioAstron. III. Blazar S5 0716+71 at Microarcsecond Resolution

Abstract We present RadioAstron Space VLBI imaging observations of the BL Lac object S5 0716+71 made on 2015 January 3–4 at a frequency of 22 GHz (wavelength λ = 1.3 cm). The observations were made in the framework of the AGN Polarization Key Science Program. The source was detected on projected space–ground baselines up to 70,833 km (5.6 Earth diameters) for both parallel-hand and cross-hand interferometric visibilities. We have used these detections to obtain a full-polarimetric image of the blazar at an unprecedented angular resolution of 24 μas, the highest for this source to date. This enabled us to estimate the size of the radio core to be <12 × 5 μas and to reveal a complex structure and a significant curvature of the blazar jet in the inner 100 μas, which is an indication that the jet viewing angle lies inside the opening angle of the jet conical outflow. Fairly highly (15%) linearly polarized emission is detected in a jet region 19 μas in size, located 58 μas downstream from the core. The highest brightness temperature in the source frame is estimated to be >2.2 × 1013 K for the blazar core. This implies that the inverse-Compton limit must be violated in the rest frame of the source, even for the largest Doppler factor δ ∼ 25 reported for 0716+714.

Substructure of Visibility Functions from Scattered Radio Emission of Pulsars through Space VLBI

Abstract We report on the substructure of visibility functions in the delay domain of PSRs B0329+54, B0823+26, B0834+06, B1933+16, and B0833–45 (Vela) observed with Earth–Earth and RadioAstron space–Earth two-element interferometers at frequencies of 324 and 1668 MHz. All visibility functions display unresolved spikes distributed over a range of delays. They are due to band-limited scintillation noise and related to the scattering time. The envelopes for each but the Vela pulsar are well fit by a single Lorentzian, which we interpret as being indicative of isotropic scattering on the plane of the sky due to a thin scattering screen between the pulsar and us. In contrast, the envelope for the Vela pulsar needs to be mostly fit by at least two Lorentzians, a narrow and a broad one at the same zero delay. We interpret this characteristic as indicative of anisotropic scattering due to a more complex structure of scattering screens in the supernova remnant. The possibility of describing the delay visibility functions by Lorentzians is likely a general property of pulsars and offers a new way of describing the scattering parameters of the intervening interstellar medium. Furthermore, for all of our pulsars, the unresolved spikes in visibility functions of similar projected baselines were well correlated, indicating that the telescopes are located in the same diffraction spot. The correlation vanished for visibilities from largely different baselines, when some radio telescopes are not in the same spot.

The space VLBI mission RadioAstron: AGN results

AbstractThe RadioAstron Space VLBI mission utilized the 10-m radio telescope on board the dedicated Spektr-R spacecraft to observe cosmic radio sources with an unprecedented angular resolution at centimeter wave lengths in total and polarized light. The longest baseline of the space-ground interferometer is about 350000 km. It operated in 2011–2019 together with 58 largest ground radio telescopes. Resolution as high as 10 microarcsec has been achieved. An overview of its AGN science results will be presented in the talk. It includes a probe of jet emission mechanism through brightness temperature measurements, reconstruction of magnetic field structure close to the jet origin using polarization data, jet formation and collimation study for well resolved nearby AGN, as well as observations and analysis of jet precession and plasma instabilities. We will also discuss a new scattering effect which was discovered by RadioAstron to affect high resolution radio measurements of AGN and SgrA*.

Detection statistics of the RadioAstron AGN survey

The largest Key Science Program of the RadioAstron space VLBI mission is a survey of active galactic nuclei (AGN). The main goal of the survey is to measure and study the brightness of AGN cores in order to better understand the physics of their emission while taking interstellar scattering into consideration. In this paper we present detection statistics for observations on ground-space baselines of a complete sample of radio-strong AGN at the wavelengths of 18, 6, and 1.3 cm. Two-thirds of them are indeed detected by RadioAstron and are found to contain extremely compact, tens to hundreds of μas structures within their cores.

Metrics and Motivations for Earth–Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope

Abstract Very-long-baseline interferometry (VLBI) at frequencies above 230 GHz with Earth-diameter baselines gives spatial resolution finer than the ∼50 μas “shadow” of the supermassive black hole at the Galactic Center, Sagittarius A* (Sgr A*). Imaging static and dynamical structure near the “shadow” provides a test of general relativity and may allow measurement of black hole parameters. However, traditional Earth-rotation synthesis is inapplicable for sources (such as Sgr A*) with intraday variability. Expansions of ground-based arrays to include space-VLBI stations may enable imaging capability on timescales comparable to the prograde innermost stable circular orbit (ISCO) of Sgr A*, which is predicted to be 4–30 minutes, depending on black hole spin. We examine the basic requirements for space VLBI, and we develop tools for simulating observations with orbiting stations. We also develop a metric to quantify the imaging capabilities of an array irrespective of detailed image morphology or reconstruction method. We validate this metric on example reconstructions of simulations of Sgr A* at 230 and 345 GHz, and use these results to motivate expanding the Event Horizon Telescope to include small dishes in Low Earth Orbit (LEO). We demonstrate that high-sensitivity sites such as the Atacama Large Millimeter/Submillimeter Array (ALMA) make it viable to add small orbiters to existing ground arrays, as space-ALMA baselines would have sensitivity comparable to ground-based non-ALMA baselines. We show that LEO-enhanced arrays sample half of the diffraction-limited Fourier plane of Sgr A* in less than 30 minutes, enabling reconstructions of near-horizon structure with a normalized root-mean-square error ≲0.3 on sub-ISCO timescales.