VLBI Data Research Articles

Jupiter spin-pole precession rate and moment of inertia from Juno radio-science observations

Through detailed and realistic numerical simulations, the present paper assesses the precision with which the Juno spacecraft can measure the normalized polar moment of inertia (MOI) of Jupiter. Based on Ka-band Earth-based Doppler data, created with realistic 10μm/s of white noise at 60s of integration, this analysis shows that the determination of the precession rate of Jupiter is by far more efficient than the Lense–Thirring effect previously proposed to determine the moment of inertia and therefore to constrain the internal structure of the giant planet with Juno.We show that the Juno mission will allow the estimation of the precession rate of Jupiter׳s pole with an accuracy better than 0.1%. We provide an equation relating the pole precession rate and the normalized polar moment of inertia of Jupiter. Accounting for the uncertainty in the parameters affecting precession, we show that the accuracy of the MOI inferred from the precession rate is also better than 0.1%, and at least 50 times better than inferred from the Lense–Thirring acceleration undergone by Juno. This accuracy of the MOI determination should provide tight constraints on the interior structure of Jupiter, especially the core size and mass, helping to distinguish among competing scenarios of formation and evolution of the giant planet.In addition, though the Juno mission operations are already defined, the exact duration of the tracking and its occurrence with respect to the spacecraft pericenter pass are not definitely scheduled. The simulations performed here quantify the impact of this aspect of the mission on the Juno sensitivity to (in particular) the spin-pole precession rate of Jupiter.Finally, additional simulations have been performed to test the usefulness of combining Doppler data with VLBI data, showing the latter measurements to be 104–105 times less sensitive than the former to our parameters of interest and therefore, obviously, totally needless.

Pulsar lensing geometry

Our analysis of archival VLBI data of PSR 0834+06 revealed that its scintillation properties can be precisely modelled using the inclined sheet model (Pen & Levin 2014), resulting in two distinct lens planes. These data strongly favour the grazing sheet model over turbulence as the primary source of pulsar scattering. This model can reproduce the parameters of the observed diffractive scintillation with an accuracy at the percent level. Comparison with new VLBI proper motion results in a direct measure of the ionized ISM screen transverse velocity. The results are consistent with ISM velocities local to the PSR 0834+06 sight-line (through the Galaxy). The simple 1D structure of the lenses opens up the possibility of using interstellar lenses as precision probes for pulsar lens mapping, precision transverse motions in the ISM, and new opportunities for removing scattering to improve pulsar timing. We describe the parameters and observables of this double screen system. While relative screen distances can in principle be accurately determined, a global conformal distance degeneracy exists that allows a rescaling of the absolute distance scale. For PSR B0834+06, we present VLBI astrometry results that provide (for the fist time) a direct measurement of the distance of the pulsar. For targets where independent distance measurements are not available, which are the cases for most of the recycled millisecond pulsars that are the targets of precision timing observations, the degeneracy presented in the lens modelling could be broken if the pulsar resides in a binary system.

Precise three-way Doppler of spacecraft based on VLBI data and its application

We have researched the software phase-locked loop (SPLL) technique to extract three way Doppler of spacecraft and the propagation medium error correction method. The method is used on Chang’E 3 X-band DOR main carrier observed by Chinese VLBI Network. The three way Doppler noise is 0.64 mm/s. Orbit determination experiment results of using three way Doppler and range data show that the corresponding Doppler and range rms are about 0.55 mm/s and 0.41 m, and there is ~17 m difference away from the reference precise ephemeris. The difference is within the inner precision of the precise ephemeris. The SPLL has also been used to research the interplanetary scintillation using European Space Agency’s spacecraft MEX. Our results are in good agreement with the results from JIVE with high and low solar elongation. The result also reflects the effect of solar wind plasma on Doppler from MEX. The SPLL based on VLBI is useful for the future deep space exploration projects and radio science.

The Study of Radio Flux Density Variations of the Quasar OJ 287 by the Wavelet and the Singular Spectrum Methods

AbstractFlux density variations of the extragalactic radio source OJ 287 are studied by applying the wavelet and the singular spectrum methods to the long-term monitoring data at 14.5, 8.0 and 4.8 GHz acquired at the University of Michigan Radio Astronomy Observatory during 40 years. This monitoring significantly supplements the episodic VLBI data. The wavelet analysis at all three frequencies revealed the presence of quasiperiods within the intervals 6.0–7.4 and 1.2–1.8 years. The singular spectrum analysis revealed the presence of quasiperiods within the intervals 6–10 and 1.6–4.0 years. For each quasiperiod the time interval of its existence was determined.

RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz

Polarization of radio emission in extragalactic jets at a sub-milliarcsecond angular resolution holds important clues for understanding the structure of the magnetic field in the inner regions of the jets and in close vicinity of the supermassive black holes in the centers of active galaxies. Space VLBI observations provide a unique tool for polarimetric imaging at a sub-milliarcsecond angular resolution and studying the properties of magnetic field in active galactic nuclei on scales of less than 10^4 gravitational radii. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471), made at a wavelength of 18 cm (frequency of 1.6 GHz) as part of the Early Science Programme (ESP) of the RadioAstron} mission, is used here to test the polarimetric performance of the orbiting Space Radio Telescope (SRT) employed by the mission, to establish a methodology for making full Stokes polarimetry with space VLBI at 1.6 GHz, and to study the polarized emission in the target object on sub-milliarcsecond scales. Polarization leakage of the SRT at 18 cm is found to be within 9 percents in amplitude, demonstrating the feasibility of high fidelity polarization imaging with RadioAstron at this wavelength. A polarimetric image of 0642+449 with a resolution of 0.8 mas (signifying an ~4 times improvement over ground VLBI observations at the same wavelength) is obtained. The image shows a compact core-jet structure with low (~2%) polarization and predominantly transverse magnetic field in the nuclear region. The VLBI data also uncover a complex structure of the nuclear region, with two prominent features possibly corresponding to the jet base and a strong recollimation shock. The maximum brightness temperature at the jet base can be as high as 4*10^13 K.

VLBI MEASUREMENT OF WEAK SOURCES WITH IMPROVED SENSITIVITY

Compared with traditional analog system, the Chinese VLBI Data Acquisition System (CDAS) is a digital one with better bandpass and wider bandwidth which allow weaker sources to be detected and measured by VLBI techniques. After optimizing and verifying the performance of CDAS in wide bandwidth observing mode, we performed an experiment by observing 85 weak sources along the ecliptic with Chinese VLBI stations located at Shanghai, Kunming and Urumqi. The capability of CDAS has been demonstrated for the detection of weak sources with improved sensitivity.

Statistical characteristics of atmospheric phase fluctuations observed by using same-beam VLBI

When the signals from spacecrafts and radio sources transmits through the earths atmosphere and ionosphere, their phase occurs fluctuation, which has great influence on the measurement precision of the ground observing system such as VLBI. Using same beam VLBI data of 1 year which were got from two satellites (Rstar and Vstar) in Japanese lunar exploration project SELENE, we studied the Allan standard deviation and the structure function of correlation phase fluctuations of Rstar and Vstar, and differential correlation phase fluctuation between Rstar and Vstar on six baselines. We also obtained Allan standard deviation of phase fluctuations of Rstar and differential phase fluctuation between Rstar and Vstar at each station based on the 6 baseline data. In addition, we obtained the relationship model between differential phase fluctuations of Rstar and Vstar and its separation angle at each station. In the range of time interval 8 τ τ 0, which obeys Kolmogorov disturbance theory. The relationship model between the root mean square of differential phase fluctuations of Rstar and Vstar and its separation angle at each station is φ =1.967 θ +0.738 deg/deg, and that on six baselines is φ =2.759 θ +1.069 deg/deg.

Atmospheric modeling for co-located VLBI antennas and twin telescopes

In the next generation VLBI network, the VLBI global observing system (VGOS), there will be several twin telescopes, i.e. stations equipped with a pair of VLBI telescopes with identical design. In this work we test the possibility of combining the tropospheric parameters of these two telescopes within the VLBI data analysis. This is done through simulations of a possible future VGOS network containing one twin telescope. We simulate the tropospheric delays with the help of a turbulence model, approximately taking into account the distance between the antennas. The results show that the combination of tropospheric delays can improve the station position repeatability by about 15 % as long as the distance is smaller than 1 km. The main improvement is in the repeatability of the baseline vector between the antennas. However, the results are strongly dependent on how the observations are scheduled for the twin telescope. The simulation results are confirmed by an analysis of the CONT14 campaign, where the tropospheric parameters of the two Hobart antennas are combined. Furthermore, we also discuss the study of combining other parameters for the twin telescope, i.e. the clocks and/or the station positions.

The intra-hour variable quasar J1819+3845: 13-year evolution, jet polarization structure, and interstellar scattering screen properties

We examine the long-term evolution of the intra-hour variable quasar, J1819+3845, whose variations have been attributed to interstellar scintillation by extremely local turbulent plasma, located only 1-3pc from Earth. The variations in this source ceased some time between June 2006 and February 2007. The evolution of the source spectrum and the long-term lightcurve, and the persistent compactness of the source VLBI structure indicates that the cessation of rapid variability was associated with the passage of the scattering material out of the line of sight to the quasar. We present an analysis of the linear polarization variations and their relation to total intensity variations. The proper motion of polarized features in the quasar jet is found to be subluminal. Systematic time delays between Stokes I, Q and U, in combination with the structure of the source obtained from 8.4GHz VLBI data, confirm the estimate of the screen distance: 1-2pc, making the screen one of the nearest objects to the Solar System. We determine the physical properties of this scattering material. The electron density in the scattering region is extremely high with respect to the warm ionized ISM, with an estimated density of $n_e \sim 97 \, l_0^{1/3} {\Delta L}_{100}^{-1/2}$cm$^{-3}$, where $l_0$ is the outer scale of the turbulence in AU and $\Delta L = 100 \Delta L_{100}$ AU is the depth of the scattering region. If this plasma is in pressure balance with the local magnetic field, one expects a ~2 rad/m^2 rotation measure change associated with the passage of this material past the quasar. We examine the rotation measures of sources and the diffuse polarized emission in the surrounding region. We place a limit of 10 rad/m^2 on the RM change. The variability of sources near J1819+3845 is used to deduce that the screen must therefore be either very small (~100 AU) or patchy.

FIRST-EPOCH VLBA IMAGING OF 20 NEW TeV BLAZARS

We present Very Long Baseline Array (VLBA) images of 20 TeV blazars (HBLs) not previously well-studied on the parsec scale. Observations were made between August and December 2013, at a frequency of 8.4 GHz. These observations represent the first epoch of a VLBA monitoring campaign on these blazars, and they significantly increase the fraction of TeV HBLs studied with high-resolution imaging. The peak VLBI flux densities of these sources range from about 10 to 100 mJy/beam, and parsec-scale jet structure is detected in all sources. About half of the VLBI cores are resolved, with brightness temperature upper limits of a few times 10^10 K, and we find that a brightness temperature of about 2x10^10 K is consistent with the VLBI data for all but one of the sources. Such brightness temperatures do not require any relativistic beaming to reduce the observed value below commonly invoked intrinsic limits; however, the lack of detection of counter-jets does place a modest limit on the bulk Lorentz factor of greater than about 2. These data are thus consistent with a picture where weak-jet sources like the TeV HBLs develop significant velocity structures on parsec-scales. We also extend consideration to the full sample of TeV HBLs by combining the new VLBI data with VLBI and gamma-ray data from the literature. By comparing measured VLBI and TeV fluxes to samples with intrinsically uncorrelated luminosities generated by Monte Carlo simulations, we find a marginally significant correlation between the VLBI and TeV fluxes for the full TeV HBL sample.

Fourier plane modeling of the jet in the galaxy M81

AbstractThe nearby spiral galaxy M81 has a low-luminosity Active Galactic Nucleus in its center with a core and a one-sided curved jet, dubbed M81*, that is barely resolved with VLBI. To derive basic parameters such as the length of the jet, its orientation and curvature, the usual method of model-fitting with point sources and elliptical Gaussians may not always be the most appropriate one. We are developing Fourier-plane models for such sources, in particular an asymmetric triangle model to fit the extensive set of VLBI data of M81* in the u-v plane. This method may have an advantage over conventional ones in extracting information close to the resolution limit to provide us with a more comprehensive picture of the structure and evolution of the jet. We report on preliminary results.

May PKS 1155+251 be the habitat of a binary black hole?

AbstractClose binary black holes (BBH) are important not only in astrophysics but they would be the strongest gravitational wave sources in the universe. Galaxy-galaxy merging systems are mostly found in optical and X-ray images. In radio, however, the VLBI can resolve the close binary system at pc scale, if their nuclei are radio loud. Recently we analyzed the archive VLBI data of PKS 1155+251, it shows twin core-jets like VLBI structure. In this poster, we present preliminary result from analyzing of the archive data. Further investigations with high frequency VLBI observations are required to confirm if it is a true BBH system.

On the accuracy of the theory of precession and nutation

Corrections to the IAU 2000/2006 parameters of the theory of precession and nutation are calculated using five different series—two individual series and three combined series that have been used in the literature for this purpose. A comparison of the sets of corrections obtained using the different datasets indicates significance systematic differences between them, which often substantially exceed the corresponding random errors. At the same time, existing studies have usually used data obtained from one or two series chosen by the authors without special justification. When refining the theory of precession and nutation, it is necessary to consider and compare various available series of VLBI data if one wishes to reduce the systematic errors in an improved model.

The unusual multiwavelength properties of the gamma-ray source PMN J1603−4904

We investigate the nature and classification of PMNJ1603-4904, a bright radio source close to the Galactic plane, which is associated with one of the brightest hard-spectrum gamma-ray sources detected by Fermi/LAT. It has previously been classified as a low-peaked BL Lac object based on its broadband emission and the absence of optical emission lines. Optical measurements, however, suffer strongly from extinction and the absence of pronounced short-time gamma-ray variability over years of monitoring is unusual for a blazar. We are combining new and archival multiwavelength data in order to reconsider the classification and nature of this unusual gamma-ray source. For the first time, we study the radio morphology at 8.4GHz and 22.3GHz, and its spectral properties on milliarcsecond (mas) scales, based on VLBI observations from the TANAMI program. We combine the resulting images with multiwavelength data in the radio, IR, optical/UV, X-ray, and gamma-ray regimes. PMNJ1603-4904 shows a symmetric brightness distribution at 8.4GHz on mas-scales, with the brightest, and most compact component in the center of the emission region. The morphology is reminiscent of a Compact Symmetric Object (CSO). Such objects have been predicted to produce gamma-ray emission but have not been detected as a class by Fermi/LAT so far. Sparse (u, v)-coverage at 22.3GHz prevents an unambiguous modeling of the source morphology. IR measurements reveal an excess in the spectral energy distribution (SED), which can be modeled with a blackbody with a temperature of about 1600K, and which is usually not present in blazar SEDs. The VLBI data and the shape of the SED challenge the current blazar classification. PMNJ1603-4904 seems to be either a highly peculiar BL Lac object or a misaligned jet source. In the latter case, the intriguing VLBI structure opens room for a possible classification as a gamma-ray bright CSO.

A revised rotation curve of the Milky Way with maser astrometry

We reconstruct the rotation curve of the Milky Way using the new trigonometric parallax and proper motion data for masers in 43 high-mass star-forming regions obtained by VLBI, as well as the existing data from the literature, based on a new set of galactic constants (R0, Θ0) = (8.4 kpc, 254 km s−1) measured by Reid et al. The revised rotation curve of the Milky Way is almost flat or slightly rising in the region from about 15 to 6 kpc. The rotation velocities within 5 kpc of the Galactic center, as determined by VLBI, differ from those obtained by measurement of the HI- and CO-line tangent velocities. We fitted the revised rotation curve arising from three mass components: the bulge, disk and dark matter halo. The total mass of the Milky Way is found to be 2.3 × 1011 M⊙ (20 kpc). This is about 10% larger than that from Sofue et al, and is comparable with the mass of M31, 3.4 × 1011 M⊙ (35 kpc), given by Carignan et al. The limited accurate observational data, especially the VLBI data, do not permit a fully satisfactory fit to the rotation curve. The extensive parallax and proper motion data that will be produced by the Bar and Spiral Structure Legacy Survey project in the next few years should lead to considerable progress in understanding the rotation curve and dark matter halo of the Milky Way.

The proper motion of PSR J0205+6449 in 3C 58

We report on sensitive phase-referenced and gated 1.4-GHz VLBI radio observations of the pulsar PSR J0205+6449 in the young pulsar-wind nebula 3C 58, made in 2007 and 2010. We employed a novel technique where the ~105-m Green Bank telescope is used simultaneously to obtain single-dish data used to determine the pulsar's period as well as to obtain the VLBI data, allowing the VLBI correlation to be gated synchronously with the pulse to increase the signal-to-noise. The high timing noise of this young pulsar precludes the determination of the proper motion from the pulsar timing. We derive the position of the pulsar accurate at the milliarcsecond level, which is consistent with a re-determined position from the Chandra X-ray observations. We reject the original tentative optical identification of the pulsar by Shearer and Neustroev (2008), but rather identify a different optical counterpart on their images, with R-band magnitude ~24. We also determine an accurate proper motion for PSR J0205+6449 of (2.3 +- 0.3) mas/yr, corresponding to a projected velocity of only (35 +- 6) km/s for a distance of 3.2 kpc, at p.a. -38 deg. This projected velocity is quite low compared to the velocity dispersion of known pulsars of ~200 km/s. Our measured proper motion does not suggest any particular kinematic age for the pulsar.

Jet dynamics and stability

The dynamics and stability of extragalactic jets may be strongly influenced by small (and probable) differences in pressure between the jet and the ambient and within the jet itself. The former give rise to expansion and recollimation of the jet. This occurs in the form of conical shocks, or Mach disks, if the pressure difference is large enough. Pressure asymmetries within the jet may trigger the development of helical patterns via coupling to kink current-driven instability, or to helical Kelvin-Helmholtz instability, depending on the physical conditions in the jet. I summarize here the evidence collected during the last years on the presence of recollimation shocks and waves in jets. In the jet of CTA 102 evidence has been found for (traveling)shock-(standing)shock interaction in the core-region (0.1 mas from the core), using information from the light-curve of the source combined with VLBI data. The conclusions derived have been confirmed by numerical simulations combined with emission calculations that have allowed to study the spectral evolution of the perturbed jet. Helical structures can also be identified in radio-jets. The ridge-line of emission of the jet of S5~0836+710 has been identified as a physical structure corresponding to a wave developing in the jet flow. I review here the evidence that has allowed to reach this conclusion, along with an associated caveat. Current data do not allow to distinguish between magnetic or hydrodynamical instabilities. I finally discuss the importance of these linear and non-linear waves for jet evolution.

Constraints on Blazar Jet Conditions During Gamma-Ray Flaring from Radiative Transfer Modeling

As part of a program to investigate jet flow conditions during GeV gamma-ray flares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band total flux density and linear polarization monitoring observations to constrain radiative transfer models incorporating propagating shocks orientated at an arbitrary angle to the flow direction. We describe the characteristics of the model, illustrate how the data are used to constrain the models, and present results for three program sources with diverse characteristics: PKS 0420-01, OJ 287, and 1156+295. The modeling of the observed spectral behavior yields information on the sense, strength and orientation of the shocks producing the radio-band flaring; on the energy distribution of the radiating particles; and on the observer's viewing angle with respect to the jet independent of VLBI data. We present evidence that, while a random component dominates the jet magnetic field, a distinguishing feature of those radio events with an associated gamma-ray flare is the presence of a weak but non-negligible ordered magnetic field component along the jet axis.

Precise positioning of the Chang’E-3 lunar lander using a kinematic statistical method

A kinematic statistical method is proposed to determine the position for Chang’E-3 (CE-3) lunar lander. This method uses both ranging and VLBI measurements to the lander for a continuous arc, combing with precise knowledge about the motion of the moon as provided by planetary ephemeris, to estimate the lander’s position on the lunar surface with high accuracy. Accuracy analyses are carried out with simulation data using the software developed at Shanghai Astronomical Observatory in this study to show that measurement errors will dominate the position accuracy. Application of lunar digital elevation model (DEM) as constraints in the lander positioning is also analyzed. Simulations show that combing range/doppler and VLBI data, single epoch positioning accuracy is at several hundred meters level, but with ten minutes data accumulation positioning accuracy is able to be achieved with several meters. Analysis also shows that the information given by DEM can provide constraints in positioning, when DEM data reduce a 3-dimensional positioning problem to 2-dimensional. Considering the Sinus Iridum, CE-3 lander’s planned landing area, has been observed with dedicated details during the CE-1 and CE-2 missions, and its regional DEM model accuracy may be higher than global models, which will certainly support CE-3’s lander positioning.

Celestial reference frames at multiple radio wavelengths

AbstractIn 1997 the IAU adopted the International Celestial Reference Frame (ICRF) built from S/X VLBI data. In response to IAU resolutions encouraging the extension of the ICRF to additional frequency bands, VLBI frames have been made at 24, 32, and 43 GHz. Meanwhile, the 8.4 GHz work has been greatly improved with the 2009 release of the ICRF-2. This paper discusses the motivations for extending the ICRF to these higher frequency radio bands. Results to date will be summarized including evidence that the high frequency frames are rapidly approaching the accuracy of the 8.4 GHz ICRF-2. We discuss current limiting errors and prospects for the future accuracy of radio reference frames. We note that comparison of multiple radio frames is characterizing the frequency dependent systematic noise floor from extended source morphology and core shift. Finally, given Gaia's potential for high accuracy optical astrometry, we have simulated the precision of a radio-optical frame tie to be ~10–15 μas (1-σ, per component).