Very Long Baseline Interferometer Research Articles

Using the bootstrap to assess uncertainties of VLBI results – I. The method and image-based errors

Very Long Baseline Interferometric (VLBI) observations of quasar jets enable one to measure many theoretically expected effects. Estimating the significance of observational findings is complicated by the correlated noise in the image plane. A reliable and well justified approach to estimate the uncertainties of VLBI results is needed as well as significance testing criteria. We propose to use bootstrap for both tasks. Using simulations we find that bootstrap-based errors for the full intensity, rotation measure, and spectral index maps have coverage closer to the nominal values than conventionally obtained errors. The proposed method naturally takes into account heterogeneous interferometric arrays (such as Space VLBI) and can be easily extended to account for instrumental calibration factors.

A radio structure resolved at the deca-parsec scale in the radio-quiet quasar PDS 456 with an extremely powerful X-ray outflow

Active galactic nuclei (AGN) accreting at rates close to the Eddington limit can host radiatively driven mildly relativistic outflows. Some of these X-ray absorbing but powerful outflows may produce strong shocks resulting in a significant non-thermal emission. This outflow-driven radio emission may be detectable in the radio-quiet quasar PDS 456 since it has a bolometric luminosity reaching the Eddington limit and a relativistic wide-aperture X-ray outflow with a kinetic power high enough to quench the star formation in its host galaxy. To investigate this possibility, we performed very-long-baseline interferometric (VLBI) observations of the quasar with the European VLBI Network (EVN) at 5 GHz. The EVN image with the full resolution reveals two faint and diffuse radio components with a projected separation of about 20 pc and an average brightness temperature of around two million Kelvin. In relation to the optical sub-mas-accuracy position measured by the Gaia mission, the two components are very likely on opposite sides of an undetected radio core. The VLBI structure at the deca-pc scale can thus be either a young jet or a bidirectional radio-emitting outflow, launched in the vicinity of a strongly accreting central engine. Two diffuse components at the hecto-pc scale, likely the relic radio emission from the past AGN activity, are tentatively detected on each side in the low-resolution EVN image.

High cadence, linear, and circular polarization monitoring of OJ 287

Aims. We present a multifrequency, dense radio monitoring program of the blazar OJ 287 using the 100-m Effelsberg radio telescope. The program aims to test different binary supermassive black hole (SMBH) scenarios and studying the physical conditions in the central region of this bright blazar. Here, we analyze the evolution in total flux density, linear and circular polarization as a means to study the OJ 287 jet structure and its magnetic field geometry. Methods. We used a recently developed, high-precision data analysis methodology to recover all four Stokes parameters. We measured the total flux density of OJ 287 at nine bands from 2.64 GHz to 43 GHz, the linear polarization parameters at four bands between 2.64 GHz and 10.45 GHz, and the circular polarization at two bands, 4.85 GHz and 8.35 GHz. The mean cadence of our measurements is ten days. Results. Between December 2015 and January 2017 (MJD 57370–57785), OJ 287 showed flaring activity and complex linear and circular polarization behavior. The radio electric vector position angle (EVPA) showed a large clockwise (CW) rotation by ∼340° with a mean rate of −1.04°/day. Based on concurrent very long baseline interferometric (VLBI) polarization data at 15 GHz and 43 GHz, the rotation seems to originate within the jet core at 43 GHz (projected angular size ≤0.15 mas or 0.67 pc at the redshift of the source). Moreover, optical polarization data show a similar monotonic CW rotation with a rate of about −1.1°/day which is superposed with shorter and faster rotations that exhibit rates of about 7.8°/day, mainly in the CW sense. Conclusions. The flux density and polarization variability of the single dish, VLBI and optical data is consistent with a polarized emission component propagating on a helical trajectory within a bent jet. We constrained the helix arc length to 0.26 pc and radius to ≤0.04 pc as well as the jet bending arc length projected on the plane of the sky to ≤1.9–7.6 pc. A similar bending has been observed also in high angular resolution VLBI images of the OJ 287 jet at its innermost regions. The helical trajectory covers only a part of the jet width, possibly its spine. In addition, our results indicate the presence of a stable polarized emission component. Its EVPA (−10°) is oriented perpendicular to the large scale jet, suggesting dominance of the poloidal magnetic field component. Finally, the EVPA rotation begins simultaneously with an optical flare and hence the two might be physically connected. That optical flare has been suggested to be linked to the interaction of a secondary SMBH with the inner accretion disk or originating in the jet of the primary.

Detection of Intrinsic Source Structure at ∼3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*

Abstract We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional uv coverage in the N–S direction, and leads to a spatial resolution of ∼30 μas (∼3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of ∼4%–13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of ∼3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.

A Fast Radio Burst Search Method for VLBI Observation

Abstract We introduce the cross-spectrum-based fast radio burst (FRB) search method for Very Long Baseline Interferometer (VLBI) observation. This method optimizes the fringe fitting scheme in geodetic VLBI data post-processing, which fully utilizes the cross-spectrum fringe phase information and therefore maximizes the power of single-pulse signals. Working with cross-spectrum greatly reduces the effect of radio frequency interference compared with using auto-power spectrum. Single-pulse detection confidence increases by cross-identifying detections from multiple baselines. By combining the power of multiple baselines, we may improve the detection sensitivity. Our method is similar to that of coherent beam forming, but without the computational expense to form a great number of beams to cover the whole field of view of our telescopes. The data processing pipeline designed for this method is easy to implement and parallelize, which can be deployed in various kinds of VLBI observations. In particular, we point out that VGOS observations are very suitable for FRB search.

The 999thSwiftgamma-ray burst: Some like it thermal

We present a multiwavelength study of GRB 151027A. This is the 999th gamma-ray burst detected by the Swift satellite and it has a densely sampled emission in the X-ray and optical band and has been observed and detected in the radio up to 140 days after the prompt. The multiwavelength light curve from 500 s to 140 days can be modelled through a standard forward shock afterglow, but it requires an additional emission component to reproduce the early X-ray and optical emission. We present optical observations performed with the Telescopio Nazionale Galileo (TNG) and the Large Binocular Telescope (LBT) 19.6, 33.9, and 92.3 days after the trigger which show a bump with respect to a standard afterglow flux decay and are interpreted as possibly due to the underlying supernova and host galaxy (at a level of ~0.4 μJy in the optical R band, RAB ~ 25). Radio observations, performed with the Sardinia Radio Telescope (SRT) and Medicina in single-dish mode and with the European Very Long Baseline Interferometer (VLBI) Network and the Very Long Baseline Array (VLBA), between day 4 and 140 suggest that the burst exploded in an environment characterized by a density profile scaling with the distance from the source (wind profile). A remarkable feature of the prompt emission is the presence of a bright flare 100 s after the trigger, lasting ~70 s in the soft X-ray band, which was simultaneously detected from the optical band up to the MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, the broadband (0.3–1000 keV) time resolved spectral analysis of the flare reveals the coexistence of a non-thermal (power law) and thermal blackbody components. The blackbody component contributes up to 35% of the luminosity in the 0.3–1000 keV band. The γ-ray emission observed in Swift-BAT and Fermi-GBM anticipates and lasts less than the soft X-ray emission as observed by Swift-XRT, arguing against a Comptonization origin. The blackbody component could either be produced by an outflow becoming transparent or by the collision of a fast shell with a slow, heavy, and optically thick fireball ejected during the quiescent time interval between the initial and later flares of the burst.

Radio VLBI and the quantum interference paradox

We address here the question of interference of radio signals from astronomical sources like distant quasars, in a very long baseline interferometer (VLBI), where two (or more) distantly located radio telescopes (apertures), receive a simultaneous signal from the sky. In an equivalent optical two-slit experiment, it is generally argued that for the photons involved in the interference pattern on the screen, it is not possible, even in principle, to know which of the two slits a particular photon went through and that any procedure to ascertain this destroys the interference pattern. But in the case of the modern radio VLBI, it is a routine matter to record the phase and amplitude of the voltage outputs from the two radio antennas on a recording media separately and then do the correlation between the two recorded signals later in an off-line manner. Does this not violate the quantum interference principle? We provide a resolution of this problem here.

VLBI Observation of Pulsar B0329+54 with the CVN at S/X Bands

A VLBI (Very Long Baseline Interferometer) observation of pulsar B0329+54 was carried out with the CVN (Chinese VLBI Network) in February 2015. It is the first time for the CVN to observe the pulsar B0329+54 in the S/X dual frequency bands. The observation was performed in the phase referencing mode. The observational data, including 16 channels of 16MHz bandwidth, were collected by the CDAS (Chinese Data Acquirement System). The correlation was made by the DiFX (Distributed FX) correlator at Shanghai Astronomical Observatory. The pulsar binning mode of DiFX was used to increase the signal to noise ratio of pulsar fringes. We obtained the accurate position of Pulsar B0329+54 from its VLBI image. Our result is in good agreement with the previous VLBI results.

Testing General Relativity with Accretion-Flow Imaging of SgrA*

The Event Horizon Telescope is a global, very long baseline interferometer capable of probing potential deviations from the Kerr metric, which is believed to provide the unique description of astrophysical black holes. Here, we report an updated constraint on the quadrupolar deviation of Sagittarius A^{*} within the context of a radiatively inefficient accretion flow model in a quasi-Kerr background. We also simulate near-future constraints obtainable by the forthcoming eight-station array and show that in this model already a one-day observation can measure the spin magnitude to within 0.005, the inclination to within 0.09°, the position angle to within 0.04°, and the quadrupolar deviation to within 0.005 at 3σ confidence. Thus, we are entering an era of high-precision strong gravity measurements.

Time and Frequency Synchronization on the Virac Radio Telescope RT-32

Abstract One of the main research directions of Ventspils International Radio Astronomy Centre (VIRAC) is radio astronomy and astrophysics. The instrumental base for the centre comprised two fully steerable parabolic antennas, RT-16 and RT-32 (i.e. with the mirror diameter of 16 m and 32 m). After long reconstruction, radio telescope RT-32 is currently equipped with the receiving and data acquisition systems that allow observing in a wide frequency range from 327 MHz to 9 GHz. New Antenna Control Unit (ACU) allows stable, fast and precise pointing of antenna. Time and frequency distribution service provide 5, 10 and 100 MHz reference frequency, 1PPS signals and precise time stamps by NTP protocol and in the IRIG-B format by coaxial cable. For the radio astronomical observations, main requirement of spatially Very Long Base Line Interferometric (VLBI) observations for the observatory is precise synchronization of the received and sampled data and linking to the exact time stamps. During October 2015, radio telescope RT-32 performance was tested in several successful VLBI experiments. The obtained results confirm the efficiency of the chosen methods of synchronization and the ability to reproduce them on similar antennas.

MODELING SEVEN YEARS OF EVENT HORIZON TELESCOPE OBSERVATIONS WITH RADIATIVELY INEFFICIENT ACCRETION FLOW MODELS

ABSTRACT An initial three-station version of the Event Horizon Telescope, a millimeter-wavelength very-long baseline interferometer, has observed Sagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in the measurement of a variety of interferometric quantities. Of particular importance is that there is now a large set of closure phases measured over a number of independent observing epochs. We analyze these observations within the context of a realization of semi-analytic radiatively inefficient disk models, implicated by the low luminosity of Sgr A*. We find a broad consistency among the various observing epochs and between different interferometric data types, with the latter providing significant support for this class of model of Sgr A*. The new data significantly tighten existing constraints on the spin magnitude and its orientation within this model context, finding a spin magnitude of a = 0.10 − 0.10 − 0.10 + 0.30 + 0.56 , an inclination with respect to the line of sight of θ = 60 ° − 8 ° − 13 ° + 5 ° + 10 ° , and a position angle of ξ = 156 ° − 17 ° − 27 ° + 10 ° + 14 ° east of north. These are in good agreement with previous analyses. Notably, the previous 180° degeneracy in the position angle has now been conclusively broken by the inclusion of the closure-phase measurements. A reflection degeneracy in the inclination remains, permitting two localizations of the spin vector orientation, one of which is in agreement with the orbital angular momentum of the infrared gas cloud G2 and the clockwise disk of young stars. This may support a relationship between Sgr A*'s accretion flow and these larger-scale features.

Caravan-Submm, Black Hole Imager in the Andes

Imaging a black hole horizon as a shadow at the center of black hole accretion disk is another method to prove/check Einstein’s general relativity at strong gravitational fields. Such black hole imaging is expected to be achievable using a submillimeter wavelength VLBI (very long baseline interferometer) technique. Here, we introduce a Japanese black hole imaging project, Caravan-submm undertaken in the Andes.

PAGAN I: MULTI-FREQUENCY POLARIMETRY OF AGN JETS WITH KVN

Active Galactic Nuclei (AGN) with bright radio jets offer the opportunity to study the structure of and physical conditions in relativistic outflows. For such studies, multi-frequency polarimetric very long baseline interferometric (VLBI) observations are important as they directly probe particle densities, magnetic field geometries, and several other parameters. We present results from first-epoch data obtained by the Korean VLBI Network (KVN) within the frame of the Plasma Physics of Active Galactic Nuclei (PAGaN) project. We observed seven radio-bright nearby AGN at frequencies of 22, 43, 86, and 129 GHz in dual polarization mode. Our observations constrain apparent brightness temperatures of jet components and radio cores in our sample to $>10^{8.01}$ K and $>10^{9.86}$ K, respectively. Degrees of linear polarization $m_{L}$ are relatively low overall: less than 10%. This indicates suppression of polarization by strong turbulence in the jets. We found an exceptionally high degree of polarization in a jet component of BL Lac at 43 GHz, with $m_{L} \sim$ 40%. Assuming a transverse shock front propagating downstream along the jet, the shock front being almost parallel to the line of sight can explain the high degree of polarization.

THE VLBI MONITORING PROJECT FOR 6.7 GHz METHANOL MASERS USING THE JVN/EAVN

We have initiated a Very Long Baseline Interferometer (VLBI) monitoring project of 36 methanol maser sources at 6.7 GHz using the Japanese VLBI Network (JVN) and East-Asian VLBI Network (EAVN), starting in August 2010. The purpose of this project is to systematically reveal 3-dimensional (3-D) kinematics of rotating disks around forming high-mass protostars. As an initial result, we present proper motion detections for two methanol maser sources showing an elliptical spatial morphology, G 002.53+00.19 and G 006.79−00.25, which could be the best candidates associated with the disk. The detected proper motions indicate a simple rotation in G 002.53+00.19 and rotation with expansion in G 006.79−00.25, respectively, on the basis of disk model fits with rotating and expanding components. The expanding motions might be caused by the magnetic-centrifugal wind on the disk.

Modern observations of the effect of earthquakes on the Chandler wobble

Earthquakes have long been postulated as the source of excitation of the Chandler wobble (Mansinha and Smylie, 1967). More recently, the classical astronometric observations of the polar motion have been replaced by very long baseline interferometric (VLBI) observations with an improvement in accuracy by a factor of several thousand. We analyze the record of nearly 29 years of VLBI polar motion observations from the Goddard Space Flight Center.In addition to the Chandler wobble, the polar motion has annual components making the analysis more difficult. The present study extends the polar motion sequence in both directions by the maximum entropy method (MEM). This allows the annual components, both the prograde motion and a weaker retrograde motion, to be identified and removed, leaving a pure Chandler wobble and secular polar shift. In the absence of excitation, the free Chandler wobble is closely a prograde circular motion. Circular arcs are fitted to the pole path, free of the annual components, to determine breaks corresponding to sudden excitations. The event times of earthquakes of magnitude greater than or equal to 7.5 are shown on the plotted pole paths. Often, the effects on the pole path precede the earthquake by many days, confirming the establishment of the far-field displacements in advance of the earthquake. The precursory rise in P-wave attenuation before the 2004 Parkfield earthquake, as discovered by Chun et al. (2010), may indicate a similar effect from local deformations.

The KaVA and KVN pulsar project

Abstract We present our work towards using the Korean VLBI (Very Long Baseline Interferometer) Network (KVN) and VLBI Exploration of Radio Astronomy (VERA) arrays combined into the KVN and VERA Array (KaVA) for observations of radio pulsars at high frequencies (≃ 22 GHz). Pulsar astronomy is generally focused at frequencies approximately 0.3 to several GHz and pulsars are usually discovered and monitored with large, single-dish, radio telescopes. For most pulsars, reduced radio flux is expected at high frequencies due to their steep spectrum, but there are exceptions where high frequency observations can be useful. Moreover, some pulsars are observable at high frequencies only, such as those close to the Galactic Center. The discoveries of a radio-bright magnetar and a few dozen extended Chandra sources within 15′ of the Galactic Center provide strong motivations to make use of the KaVA frequency band to search for pulsars in this region. Here, we describe the science targets and report progress made from the KVN test observations for known pulsars. We then discuss why KaVA pulsar observations are compelling.

The Chinese space Millimeter-wavelength VLBI array—A step toward imaging the most compact astronomical objects

The Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS) is studying a space VLBI (Very Long Baseline Interferometer) program. The ultimate objective of the program is to image the immediate vicinity of the supermassive black holes (SMBHs) in the hearts of galaxies with a space-based VLBI array working at sub-millimeter wavelengths and to gain ultrahigh angular resolution. To achieve this ambitious goal, the mission plan is divided into three stages. The first phase of the program is called the Space Millimeter-wavelength VLBI Array (SMVA) consisting of two satellites, each carrying a 10-m diameter radio telescope into elliptical orbits with an apogee height of 60,000km and a perigee height of 1200km. The VLBI telescopes in space will work at three frequency bands, 43, 22 and 8GHz. The 43- and 22-GHz bands will be equipped with cryogenic receivers. The space telescopes, observing together with ground-based radio telescopes, enable the highest angular resolution of 20μarcsec at 43GHz. The SMVA is expected to conduct a broad range of high-resolution research, e.g. imaging the shadow of (dark region surrounding) the supermassive black hole in the heart of the galaxy M87 for the first time, studying the kinematics of water megamasers surrounding the SMBHs, and exploring the power source of active galactic nuclei. Pre-research funding has been granted by the CAS in October 2012, to support scientific and technical feasibility studies. These studies also include the manufacturing of a prototype of the deployable 10-m space-based telescope and a 22-GHz receiver. Here we report on the latest progress of the SMVA project.

SHORT-TERM RADIO VARIABILITY AND PARSEC-SCALE STRUCTURE IN A GAMMA-RAY NARROW-LINE SEYFERT 1 GALAXY 1H 0323+342

We made simultaneous single-dish and very long baseline interferometer (VLBI) observations of a narrow-line Seyfert 1 galaxy (NLS1) 1H 0323+342, showing gamma-ray activity revealed by Fermi/LAT observations. We found significant variation of the total flux density at 8 GHz on the time scale of one month by the single-dish monitoring. The total flux density varied by 5.5% in 32 days, which is comparable to the gamma-ray variability time scale, corresponding to the variability brightness temperature of $7.0 \times 10^{11}$ K. The source consists of central and southeastern components on the parsec (pc) scale. The flux of only the central component decreased in the same way as the total flux density, indicating that the short-term radio variability, and probably the gamma-ray emitting region, is associated with this component. From the VLBI observations we obtained the brightness temperatures of greater than $(5.2 \pm 0.3) \times 10^{10}$ K, and derived the equipartition Doppler factor of greater than 1.7, the variability Doppler factor of 2.2, and the 8 GHz radio power of $10^{24.6}$ W Hz$^{-1}$. Combining them we conclude that acceleration of radio jets and creation of high-energy particles are ongoing in the central engine, and that the apparent very radio-loud feature of the source is due to the Doppler-boosting effect, resulting in the intrinsic radio loudness to be an order of magnitude smaller than the observed values. We also conclude that the pc-scale jet represents recurrent activity from the the spectral fitting and the estimated kinematic age of pc- and kpc-scale extended components with different position angle.

Demonstration of a 16 Gbps Station-1 Broadband-RF VLBI System

ABSTRACTA relatively inexpensive 16 Gbps data-recording system based on commercial off-the-shelf technology and open-source software has recently been developed. Combining this recorder with the parallel development of broadband Very Long Baseline Interferometer (VLBI) instrumentation is enabling dramatically improved sensitivity for both astronomical and geodetic VLBI. In this article, we describe the VLBI system and the results of a demonstration experiment that illustrates a number of cutting-edge technologies that can be deployed in the near future to significantly enhance the power of the VLBI technique.

Pilot VLBI Survey of SiO v = 3 J = 1 → 0 Maser Emission around Evolved Stars

Abstract In this letter, we report on detections of SiO $ v$$ =$ 3 $ J$$ =$ 1$ \rightarrow$ 0 maser emission in very long-baseline interferometric (VLBI) observations towards 4 out of 12 long-period variable stars: WX Psc, R Leo, W Hya, and T Cep. The detections towards WX Psc and T Cep are new ones. We also present successful astrometric observations of SiO $ v$$ =$ 2 and $ v$$ =$ 3 $ J$$ =$ 1$ \rightarrow$ 0 maser emissions associated with two stars: WX Psc and W Hya and their position-reference continuum sources: J010746.0$ +$ 131205 and J135146.8$-$ 291218 with the VLBI Exploration of Radio Astrometry (VERA). The relative coordinates of the position-reference continuum source and SiO $ v$$ =$ 3 maser spots were measured with respect to those of an SiO $ v$$ =$ 2 maser spot adopted as a fringe-phase reference. Thus, the faint continuum sources were inversely phase-referenced to the bright maser sources. This implies possible registration of multiple SiO maser line maps onto a common coordinate system with 10 microarcsecond-level accuracy.