VLBI Array Research Articles

Thai National Radio Astronomy Observatory Project and a future South-East Asian VLBI Network

Abstract NARIT initiated a national flagship project in 2017 for development of radio astronomy and geodesy in Thailand. In this project, a 40-m Thai National Radio Telescope (TNRT) and a 13-m VLBI Global Observing System radio telescope are constructed in Chiang Mai. The 40-m TNRT is the largest telescope for radio astronomy in South-East Asia. Its flexible operation with a wide-frequency coverage 0.3-115 GHz will allow us to uniquely contribute to the time-domain astronomy as well as carry out unbiased surveys for a wide variety of research fields, which were published in a white paper. Within the framework of collaboration with VLBI arrays in the world, TNRT will drastically improve the imaging quality and performances based on its unique geographical location. Through commissioning of the L-band system, the 1st Call for Proposals of the 40-m TNRT in the L-band1 has been internationally announced on 10th October 2023, TST 10 am. Future vision for establishment of forthcoming regional VLBI networks based on TNRT is also introduced: Thai National VLBI Array and South-East Asian VLBI Network in collaboration with Indonesia, Malaysia, and Vietnam.

The cylindrical jet base of M 87 within 100 μas of the central engine

A recent article on high-resolution 86 GHz observations with the Global Millimeter VLBI Array, the phased Atacama Large Millimeter/submillimeter Array, and the Greenland Telescope describes the detection of a limb-brightened cylindrical jet, 25 μas < z < 100 μas, where z is the axial displacement from the supermassive black hole in the sky plane. It was shown to be much wider and much more collimated than 2D simulations of electromagnetic (Blandford-Znajek) jets from the event horizon predicted. This was an unanticipated discovery. The claimed detection of a jet connected to the accretion flow provides a direct observational constraint on the geometry and physics of the jet launching region for the first time in any black hole jetted system. This landmark detection warrants further analysis. This Letter focuses on the most rudimentary properties, the shape and size of the source of the detected jet emission, the determination of which is not trivial due to line-of-sight effects. Simple thick-walled cylindrical shell models for the source were analyzed to constrain the thickness of the jet wall. The analysis indicates a tubular jet source with a radius R ≈ 144 μas ≈ 38M and that the tubular jet walls have a width W ≈ 36 μas ≈ 9.5M, where M is the geometrized mass of the black hole (a volume comparable to that of the interior cavity). The observed cylindrical jet connects continuously to the highly limb-brightened jet (previously described as a thick-walled tubular jet) that extends to z > 0.65 mas, and the two are likely in fact the same outflow (i.e., from the same central engine).

The Past and Future of East Asia to Italy: Nearly Global VLBI

We present here the East Asia to Italy Nearly Global VLBI (EATING VLBI) project. How this project started and the evolution of the international collaboration between Korean, Japanese, and Italian researchers to study compact sources with VLBI observations is reported. Problems related to the synchronization of the very different arrays and technical details of the telescopes involved are presented and discussed. The relatively high observation frequency (22 and 43 GHz) and the long baselines between Italy and East Asia produced high-resolution images. We present example images to demonstrate the typical performance of the EATING VLBI array. The results attracted international researchers and the collaboration is growing, now including Chinese and Russian stations. New in progress projects are discussed and future possibilities with a larger number of telescopes and a better frequency coverage are briefly discussed herein.

Overview of the Observing System and Initial Scientific Accomplishments of the East Asian VLBI Network (EAVN)

The East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three correlators located in China, Japan, and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43 GHz with the longest baseline length of 5078 km, resulting in the highest angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct capabilities of EAVN is multi-frequency simultaneous data reception at nine telescopes, which enable us to employ the frequency phase transfer technique to obtain better sensitivity at higher observing frequencies. EAVN started its open-use program in the second half of 2018, providing a total observing time of more than 1100 h in a year. EAVN fills geographical gap in global VLBI array, resulting in enabling us to conduct contiguous high-resolution VLBI observations. EAVN has produced various scientific accomplishments especially in observations toward active galactic nuclei, evolved stars, and star-forming regions. These activities motivate us to initiate launch of the ’Global VLBI Alliance’ to provide an opportunity of VLBI observation with the longest baselines on the earth.

The 40-m Thai National Radio Telescope with its key sciences and a future South-East Asian VLBI Network

AbstractNational Astronomical Research Institute of Thailand (Public Organization) initiated a national flagship project in 2017 for development of radio astronomy and geodesy in Thailand. In this project, a 40-m Thai National Radio Telescope (TNRT) and a 13-m VLBI Global Observing System (VGOS) radio telescope as its co-location are constructed in Chiang Mai. The 40-m TNRT is the largest telescope for radio astronomy in South-East Asia. Its flexible operation with a wide-coverage of observable frequencies 0.3-115 GHz will allow us to uniquely contribute to the time-domain astronomy as well as carry out unbiased surveys for a wide variety of science research fields, which were published in a white paper. Within the framework of collaboration with VLBI arrays in the world, TNRT will drastically improve the imaging quality and performances based on its unique geographical location, for both radio astronomy and geodetic VLBI studies in South-East Asia for the first time. On-going commissioning of TNRT particularly in the L-band system (1.0–1.8 GHz) is introduced as well as vision for establishment of forthcoming regional VLBI networks based on TNRT: Thai National VLBI Array and South-East Asian VLBI Network in collaboration with Indonesia, Malaysia, and Vietnam.

Southern Hemisphere Maser Astrometry

AbstractMany astrophysical phenomena can only be studied in detail for objects in our galaxy, the Milly Way, but we know much more about the structure of thousands of nearby galaxies than we do about our own Galaxy. Accurate distance measurements in the Milky Way underpin our ability to understand a wide range of astrophysical phenomena and this requires observations from both the northern and southern hemisphere. Our ability to measure accurate parallaxes to southern masers has been hampered a range of factors, in particular the absence of a dedicated, homogeneous VLBI array in the south. We have recently made significant advances in astrometric calibration techniques which allow us to achieve trigonometric parallax accuracies of around 10 micro-arcseconds (μas) for 6.7 GHz methanol masers with a hetrogeneous array of 4 antennas. We outline the details of this new “multiview” technique and present the first trigonometric parallax measurements that utilise this approach.

Properties of pulsating OH/IR stars revealed from astrometric VLBI observation

AbstractWe aim to reveal properties of evolution stages in AGB phase; Mira, OH/IR stars, and non-variable OH/IR stars. We presented results of our VLBI observations of four stars; NSV17351, OH39.7+1.5, IRC–30363, and AW Tau. We used the VERA VLBI array to observe 22 GHz H2O masers. Parallaxes of the four sources were obtained to be 0.247±0.035 mas (4.05±0.59 kpc), 0.54±0.03 mas (1.85±0.10 kpc), 0.562±0.201 mas (1.78±0.73 kpc), and 0.449±0.032 mas (2.23±0.16 kpc). Determination of pulsation period of NSV17351 was done for the first time. We revealed the position and kinematics of NSV17351 in our Galaxy and found that NSV17351 is located in an interarm region. A new period-magnitude relationship was indicated in the infrared region. Various other properties based on the distance measurements are also discussed. We have to emphasize that the VLBI astrometry is effective and the only way for parallax measurements of dust obscured OH/IR stars.

Preliminary results on SiO maser emission from the AGB binary system: R Aqr.

AbstractBinary/multiple stellar systems are as abundant as single stars. They are very interesting cosmic laboratories as they are related to many astrophysical phenomena. In the case of AGB (and post-AGB) stars, binaries are the most likely explanation for the shaping of non spherical PNe, a long standing problem in stellar evolution. While many binaries are known in the PNe phase, there are few examples in the previous AGB phase. Hydrodynamical models of the binary interaction are available, but well known parameters for orbits are non-existent. We observed the AGB binary system R Aqr at 7 mm using the Global VLBI array in wide-band continuum and SiO masers. The strong SiO masers were used to self-calibrate the observations and pinpoint the location of the AGB component. We used the continuum to try to directly detect the WD companion or its close environments (non-thermal emission from accretion disks/jets). We present our preliminary results for R Aqr on the relative positions of the J=1−0 SiO masers (v=1) with respect to the white dwarf position. This opens a new way to determine orbits in binaries at the AGB phase and better study their role in late stellar evolution and PNe shaping.

Fine structure and refractive scattering of the H2O maser in star-forming region W49N

AbstractWe used the unprecedented resolution of ∼25 μas of the VLBI array formed with the RadioAstron satellite to study the structures of H2O maser spots in the star forming region W49N. We found that anisotropic diffractive scattering of the ISM dominates the images of the maser spot, but does not completely blur them. The refractive scattering floor is about 0.001 in visibility at a baseline of 8 Gλ.

Resolving the Inner Parsec of the Blazar J1924–2914 with the Event Horizon Telescope

The blazar J1924–2914 is a primary Event Horizon Telescope (EHT) calibrator for the Galactic center’s black hole Sagittarius A*. Here we present the first total and linearly polarized intensity images of this source obtained with the unprecedented 20 μas resolution of the EHT. J1924–2914 is a very compact flat-spectrum radio source with strong optical variability and polarization. In April 2017 the source was observed quasi-simultaneously with the EHT (April 5–11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline Array (April 28), giving a novel view of the source at four observing frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties from the subparsec to 100 pc scales. We combine the multifrequency images of J1924–2914 to study the source morphology. We find that the jet exhibits a characteristic bending, with a gradual clockwise rotation of the jet projected position angle of about 90° between 2.3 and 230 GHz. Linearly polarized intensity images of J1924–2914 with the extremely fine resolution of the EHT provide evidence for ordered toroidal magnetic fields in the blazar compact core.

A persistent double nuclear structure in 3C 84

ABSTRACT 3C 84 (NGC 1275) is the radio source at the centre of the Perseus cluster and exhibits a bright radio jet. We observed the source with the Global Millimeter VLBI Array (GMVA) between 2008 and 2015, with a typical angular resolution of ∼50 $\mu$as. The observations revealed a consistent double nuclear structure separated by ∼770 gravitational radii assuming a black hole mass of 3.2 × 108 M⊙. The region is likely too broad and bright to be the true jet base anchored in the accretion disc or black hole ergosphere. A cone and parabola were fit to the stacked (time averaged) image of the nuclear region. The data did not strongly prefer either fit, but combined with a jet/counter-jet ratio analysis, an upper limit on the viewing angle to the inner jet region of ≤35° was found. This provides evidence for a variation of the viewing angle along the jet (and therefore a bent jet) within ∼0.5 pc of the jet launching region. In the case of a conical jet, the apex is located ∼2400 gravitational radii upstream of the bright nuclear region and up to ∼600 gravitational radii upstream in the parabolic case. We found a possible correlation between the brightness temperature and relative position angle of the double nuclear components, which may indicate rotation within the jet.

An 86 GHz Search for Pulsars in the Galactic Center with the Atacama Large Millimeter / submillimeter Array

Abstract We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using time series of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745−2900 at millimeter wavelength for the first time, which turns out to be nearly 100% linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.

The milliarcsecond-scale radio structure of AB Doradus A

Context. The fast rotator, pre-main sequence star AB Dor A is a strong and persistent radio emitter. The extraordinary coronal flaring activity is thought to be the origin of compact radio emission and other associated phenomena, such as large slingshot prominences. Aims. We aim to investigate the radio emission mechanism and the milliarcsecond radio structure around AB Dor A. Methods. We performed phase-referenced VLBI observations at 22.3 GHz, 8.4 GHz, and 1.4 GHz over more than one decade using the Australian VLBI array. Results. Our 8.4 GHz images show a double core-halo morphology, similar at all epochs, with emission extending at heights between 5 and 18 stellar radii. Furthermore, the sequence of the 8.4 GHz maps shows a clear variation of the source structure within the observing time. However, images at 1.4 GHz and 22.3 GHz are compatible with a compact source. The phase-reference position at 8.4 GHz and 1.4 GHz are coincident with those expected from the well-known milliarcsecond-precise astrometry of this star, meanwhile the 22.3 GHz position is 4σ off the prediction in the north-west direction. The origin of this offset is still unclear. Conclusions. We have considered several models to explain the morphology and evolution of the inner radio structure detected in AB Dor A. These models include emission from the stellar polar caps, a flaring, magnetically-driven loop structure, and the presence of helmet streamers. We also investigated a possible close companion to AB Dor A. Our results confirm the extraordinary coronal magnetic activity of this star, capable of producing compact radio structures at very large heights that have so far only been seen in binary interacting systems.

Annual parallax measurement of the Mira variable star BX Camelopardalis with VERA

Abstract We report the results of astrometric VLBI observations toward the Mira variable star BX Cam using the VERA VLBI array. The observation was performed from 2012 February to 2014 November. The parallax obtained is 1.73 ± 0.03 mas corresponding to a distance of 0.58 ± 0.01 kpc. The parallax of this source was reported in Gaia DR2 as 4.13 ± 0.25 mas, and there is a 240% difference between these two measurements. Astrometric results from our VLBI observations show that we exactly traced the angular motions of the seven maser spots in BX Cam. We calculated the stellar luminosities using both parallaxes, and obtained luminosities of $L_{\ast }^{\mathrm{VERA}} = 4950\pm 170\, L_{\odot }$ and $L_{\ast }^{\mathrm{Gaia}} = 870\pm 110\, L_{\odot }$. These deduced luminosities also support the validity of the parallax that we determined with VERA. Evaluating the two parallaxes, we conclude that the parallax of 1.73 ± 0.03 mas from the VERA observations is correct for BX Cam. We obtained the systemic motion of BX Cam as (μαcos δsys, ${\mu }_{\delta }^{\mathrm{sys}}$) = (13.48 ± 0.14, −34.30 ± 0.18) mas yr−1. The total of 73 H2O maser spots detected from our VLBI observations shows a spatial distribution of 30 au × 80 au, with a strong elongation along the north–south direction. They show outflows with a three-dimensional velocity of 14.79 ± 1.40 km s−1. From a comparison between the time variations of the V-band magnitudes and the H2O masers, we found that the variation of the H2O masers is relevant to that seen in the V band even though the H2O masers do not recover their maximum flux in each cycle.

Long-term multi-frequency maser observations of the intermediate-mass young stellar object G107.298+5.639

Context. Periodic flares of maser emission are thought to be induced either by variations of the seed photon flux in young binary systems or the pump rate regulated by stellar and accretion luminosities. Aims. We seek to study the variability of four maser transitions of three different species in G107.298+5.639 to constrain the dominant mechanism of periodic flares. Methods. Light curves of the 6.7 GHz methanol and 22.2 GHz water vapour maser were obtained with the Torun 32 m radio telescope over 39 and 34 cycles, respectively. The target was also monitored at the 1.6 GHz hydroxyl transitions with the Nançay radio telescope over 13 cycles. All these maser lines were imaged using VLBI arrays. Results. The study confirms alternating flares of the methanol and water masers with a period of 34.4 d and reveals the synchronised behaviour of the methanol and hydroxyl masers in this source. The observed spatial distribution of the methanol maser cloudlets and the measured time delays of the flares of individual features imply a ring-like structure of radius 240 au and thickness 30 au. Internal proper motions indicate that the velocity of methanol cloudlets is dominated by a disc-wind component of about 5 km s−1. The methanol emission detected during only one VLBI observation is located in a region about 550 au from a central star, which also exhibits OH maser flares. The erratic appearance of methanol features can be related to a powering object of relatively low luminosity which, during some variability cycles, can excite molecules only in the nearest part of the disc. A careful analysis of the maser and infrared light curves reveal a strong correlation between the 6.7 GHz line and the infrared flux densities supporting a radiative pumping of the maser. Conclusions. The synchronised behaviour of the hydroxyl 1665/1667 MHz and 6.7 GHz methanol transitions indicates a common pumping mechanism for the periodic flares of G107.298+5.639.

Testing general relativity with the Event Horizon Telescope

The Event Horizon Telescope is a millimeter VLBI array that is taking the first horizon-scale pictures of the black hole in the center of the M87 galaxy and, in the near future, of the one in the center of the Milky Way. Measurements of the shape and size of the shadows cast by the black holes on the surrounding emission can test the cosmic censorship conjecture and the no-hair theorem and may find evidence for classical effects of the quantum structure of black holes. Observations of coherent structures in the accretion flows may lead to accurate measurements of the spins of the black holes and of other properties of their spacetimes. For Sgr A*, the black hole in the center of the Milky Way, measurements of the precession of stellar orbits and timing monitoring of orbiting pulsars offer complementary avenues to the gravitational tests with the Event Horizon Telescope.

Spatially resolved origin of millimeter-wave linear polarization in the nuclear region of 3C 84

We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultrahigh angular resolution of 50 μas (corresponding to ∼200Rs). We also add complementary multiwavelength data from the Very Long Baseline Array (VLBA; 15 and 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0 and 343.5 GHz). At 86 GHz, we measured a fractional linear polarization of ∼2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (∼0.3−0.7% and <0.1%, respectively). This suggests an increasing linear polarization degree toward shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ∼105–6 rad m2in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.

Inside view

Inside view

Imaging black holes: past, present and future

This paper briefly reviews past, current, and future efforts to image black holes. Black holes seem like mystical objects, but they are an integral part of current astrophysics and are at the center of attempts to unify quantum physics and general relativity. Yet, nobody has ever seen a black hole. What do they look like? Initially, this question seemed more of an academic nature. However, this has changed over the past two decades. Observations and theoretical considerations suggest that the supermassive black hole, Sgr A*, in the center of our Milky Way is surrounded by a compact, foggy emission region radiating at and above 230 GHz. It has been predicted that the event horizon of Sgr A* should cast its shadow onto that emission region, which could be detectable with a global VLBI array of radio telescopes. In contrast to earlier pictures of black holes, that dark feature is not supposed to be due to a hole in the accretion flow, but would represent a true negative image of the event horizon. Currently, the global Event Horizon Telescope consortium is attempting to make such an image. In the future those images could be improved by adding more telescopes to the array, in particular at high sites in Africa. Ultimately, a space array at THz frequencies, the Event Horizon Imager, could produce much more detailed images of black holes. In combination with numerical simulations and precise measurements of the orbits of stars – ideally also of pulsars – these images will allow us to study black holes with unprecedented precision.

Astrometric VLBI Observations of the Galactic LPVs, Miras, and OH/IR stars

AbstractStudies of Galactic LPVs based on astrometric VLBI are presented. We use a VLBI array, “VERA”, to measure parallaxes and calibrate the K-band period luminosity relation (PLR) of the Galactic Miras. Since the PLR offers a distance indicator, its calibration is crucial to reveal their spatial distribution. Parallaxes of dozens of LPVs are presented. For the longer period stars, the mass-loss is high and the stars are obscured and recognized as OH/IR stars. We estimated mid-infrared absolute magnitudes of dozens of OH/IR stars and found that they show a loose concentration around −14 mag at λ of 11.6 μm, indicating an existence of PLR for OH/IR stars. Astrometry of OH/IR stars will also help us to study non-steady spiral arms as proposed from the latest simulation study of the galactic dynamics. We will start astrometric VLBI observation of two OH/IR stars NSV25875 and OH127.8+0.0 at 43 GHz with VERA.