Very Long Baseline Interferometry Astrometry Research Articles

Toward Microarcsecond Astrometry for the Innermost Wobbling Jet of the BL Lacertae Object OJ 287

The BL Lacertae object OJ 287 is a very unusual quasar producing a wobbling radio jet and some double-peaked optical outbursts with a possible period of about 12 yr for more than one century. This variability is widely explained by models of binary supermassive black holes (SMBHs) or precessing jets/disks from a single SMBH. To enable an independent and nearly bias-free investigation on these possible scenarios, we explored the feasibility of extremely high-precision differential astrometry on its innermost restless jet at millimeter wavelengths. Through revisiting some existing radio surveys and very long baseline interferometry (VLBI) data at frequencies from 1.4 to 15.4 GHz and performing new Very Long Baseline Array observations at 43.2 GHz, we find that the radio source J0854+1959, 7.′1 apart from OJ 287 and with no clearly seen optical and infrared counterparts, could provide a nearly ideal reference point to track the complicated jet activity of OJ 287. The source J0854+1959 has a stable GHz-peaked radio spectrum and shows a jet structure consisting of two discrete, milliarcsecond-scale-compact and steep-spectrum components and showing no proper motion over about 8 yr. The stable VLBI structure can be interpreted by an episodic, optically thin, and one-sided jet. With respect to its 4.1 mJy peak feature at 43.2 GHz, we have achieved an astrometric precision at the state-of-art level, about 10 μas. These results indicate that future VLBI astrometry on OJ 287 could allow us to accurately locate its jet apex and activity boundary, align its restless jet structure over decades without significant systematic bias, and probe various astrophysical scenarios.

VLBI astrometry on the white dwarf pulsar AR Scorpii

ABSTRACT AR Scorpii (AR Sco), the only-known radio-pulsing white dwarf binary, shows unusual pulsating emission at the radio, infrared, optical, and ultraviolet bands. To determine its astrometric parameters at the radio band independently, we conducted multi-epoch Very Long Baseline Interferometry (VLBI) phase-referencing observations with the European VLBI Network at 5 GHz and the Chinese VLBI Network plus the Warkworth 30-m telescope (New Zealand) at 8.6 GHz. By using the differential VLBI astrometry, we provide high-precision astrometric measurements on the parallax ($\pi =8.52_{-0.07}^{+0.04}$ mas) and proper motion ($\mu _{\alpha }=9.48_{-0.07}^{+0.04}$ mas yr−1, $\mu _{\delta }=-51.32_{-0.38}^{+0.22}$ mas yr−1). The new VLBI results agree with the optical Gaia astrometry. Our kinematic analysis reveals that the Galactic space velocities of AR Sco are quite consistent with that of both intermediate polars and polars. Combined with the previous tightest VLBI constraint on the size, our parallax distance suggests that the radio emission of AR Sco should be located within the light cylinder of its white dwarf.

East Asian VLBI Network astrometry toward the extreme outer Galaxy: Kinematic distance with the proper motion of G034.84−00.95

Abstract We aim to reveal the structure and kinematics of the Outer Scutum–Centaurus (OSC) arm located on the far side of the Milky Way through very long baseline interferometry (VLBI) astrometry using KaVA, which is composed of KVN (the Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We report the proper motion of a 22 GHz H2O maser source, which is associated with the star-forming region G034.84−00.95, to be (μαcos δ, μδ) = (−1.61 ± 0.18, −4.29 ± 0.16) mas yr−1 in equatorial coordinates (J2000.0). We estimate the 2D kinematic distance to the source to be 18.6 ± 1.0 kpc, which is derived from the variance weighted average of kinematic distances with local standard of rest velocity and the galactic-longitude component of the measured proper motion. Our result places the source in the OSC arm and implies that G034.84−00.95 is moving away from the Galactic plane with a vertical velocity of −38 ± 16 km s−1. Since the H i supershell GS033+06−49 is located at a kinematic distance roughly equal to that of G034.84−00.95, it is expected that gas circulation occurs between the outer Galactic disk around G034.84−00.95 with a Galactocentric distance of $12.8^{+1.0}_{-0.9}\:$kpc and the halo. We evaluate possible origins of the fast vertical motion of G034.84−00.95, which are (1) supernova explosions and (2) cloud collisions with the Galactic disk. However, neither of these possibilities are matched with the results of VLBI astrometry or the spatial distributions of H ii regions and H i gas.

Multiwavelength View of the Close-by GRB 190829A Sheds Light on Gamma-Ray Burst Physics

We monitored the position of the close-by (about 370 Mpc) gamma-ray burst GRB 190829A, which originated from a massive star collapse, through very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array, carrying out a total of nine observations between 9 and 117 days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of a few milliarcseconds. From a state-of-the art analysis of these data, we obtained valuable limits on the source size and expansion rate. The limits are in agreement with the size evolution entailed by a detailed modeling of the multiwavelength light curves with a forward-plus-reverse shock model, which agrees with the observations across almost 18 orders of magnitude in frequency (including the HESS data at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to the multiwavelength, high-cadence coverage of the afterglow, inherent degeneracies in the afterglow model are broken to a large extent, allowing us to capture some unique physical insights; we find a low prompt emission efficiency of ≲10−3, a low fraction of relativistic electrons in the forward shock downstream χ e < 13% (90% credible level), and a rapid decay of the magnetic field in the reverse shock downstream after the shock crossing. While our model assumes an on-axis jet, our VLBI astrometry is not sufficiently tight as to exclude any off-axis viewing angle, but we can exclude the line of sight to have been more than ∼2° away from the border of the gamma-ray-producing region based on compactness arguments.

New timing measurement results of 16 pulsars

Abstract A pulsar’s position, proper motion, and parallax are important parameters in timing equations. It is challenging to fit astrometric parameters accurately through pulsar timing, especially for pulsars that show irregular timing properties. With the fast development of related techniques, it is possible to measure astrometric parameters of more and more pulsars in a model-independent manner with very-long-baseline interferometry (VLBI). In this work we select 16 normal pulsars, whose parallax and proper motion have not been successfully fitted with timing observations or show obvious differences from the corresponding latest VLBI solutions, and do further studies on their timing properties. After updating the astrometric parameters in pulsar ephemerides with the latest VLBI measurements, we derive the latest rotation solutions of these pulsars with observation data in the S and C bands obtained from the Shanghai Tian Ma Radio Telescope. Compared with the spin frequency ν inferred from previous rotation solutions, the newly fitted ν show differences larger than 10−9 Hz for most pulsars. The contribution of the Shklovsky effect to the period derivative $\dot{P}$ can be properly removed by taking advantage of the accurate proper motion and distance of target pulsars measured by VLBI astrometry. This further leads to a precise estimate of the intrinsic characteristic age τc. The differences between the newly measured τc and corresponding previous results are as large as 2% for some pulsars. VLBI astrometric parameter solutions also lead to better measurements of timing irregularities. For PSR B0154+61, the glitch epoch (MJD 58279.5) measured with a previous ephemeris is about 13 d later than the result (MJD 58266.4) obtained with VLBI astrometric parameter solutions.

The Gaia reference frame for bright sources examined using VLBI observations of radio stars

Context. Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude, this is achieved through Gaia’s direct observations of quasars. At brighter magnitudes, it is difficult to validate the quality of the reference frame because comparison data are scarce. Aims. The aim of this paper is to examine the use of very long baseline interferometry (VLBI) observations of radio stars to determine the spin and orientation of the bright reference frame of current and future Gaia data releases. Methods. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results. The VLBI data for the best-fitting 26 sources indicate that the bright reference frame of Gaia DR2 rotates relative to the faint quasars at a rate of about 0.1 mas yr−1, which is significant at the 2σ level. This supports a similar conclusion based on a comparison with stellar positions in the HIPPARCOS frame. The accuracy is currently limited because only a few radio sources are included in the solution, by uncertainties in the Gaia DR2 proper motions, and by issues related to the astrophysical nature of the radio stars. Conclusions. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.

MultiView High Precision VLBI Astrometry at Low Frequencies

Abstract The arrival of the Square Kilometer Array (SKA) will revitalize all aspects of Very Long Baseline Interferometry (VLBI) astronomy at lower frequencies. In the last decade, there have been huge strides toward routinely achieving high precision VLBI astrometry at frequencies dominated by tropospheric contributions, most notably at 22 GHz, using advanced phase-referencing techniques. Nevertheless, to increase the capability for high precision astrometric measurements at low radio frequencies (&lt;8 GHz), an effective calibration strategy of the systematic ionospheric propagation effects that is widely applicable is required. Observations at low frequencies are dominated by distinct direction-dependent ionospheric propagation errors, which place a very tight limit on the angular separation of a suitable phase-referencing calibrator. The MultiView technique holds the key to compensating for atmospheric spatial-structure errors, by using observations of multiple calibrators and two-dimensional interpolation in the visibility domain. In this paper we present the first demonstration of the power of MultiView using three calibrators, several degrees from the target, along with a comparative study of the astrometric accuracy between MultiView and phase-referencing techniques. MultiView calibration provides an order of magnitude improvement in astrometry with respect to conventional phase referencing, achieving ∼100 μas astrometry errors in a single epoch of observations, effectively reaching the thermal noise limit. MultiView will achieve its full potential with the enhanced sensitivity and multibeam capabilities of SKA and the pathfinders, which will enable simultaneous observations of the target and calibrators. Our demonstration indicates that the 10 μas goal of astrometry at ∼1.6 GHz using VLBI with SKA is feasible using the MultiView technique.

What can VLBI astrometry tell us about AR Sco?

AbstractWe proposed to carry out high precision very long baseline interferometry (VLBI) observations of AR Sco, a pulsing white dwarf (WD) - M dwarf (MD) binary, to provide a direct distance measurement with the e-EVN (European VLBI Network) at 5 GHz. By the proposed parallax measurement on AR Sco, not only the precise distance will be determined, but also some physical parameters, such as the luminosity, the mass and the magnetic field will be significantly tightened accordingly, even the gravitational wave amplitude from this unique binary system can be tightly constrained. In addition, the EVN observations will allow us to answer that whether there is an extended emission structure associated with AR Sco, which will help us to explain the stable continuum radio emission observed during the pulse-off state.

RADIO ASTROMETRIC OBSERVATIONS AND THE GALACTIC CONSTANT AS THE BASIS OF A GALACTIC KINEMATICS STUDY

We made phase-referencing Very Long Baseline Interferometry (VLBI) observations of Galactic 22 GHz H2O maser sources with VLBI Exploration of Radio Astrometry (VERA). We measured the parallax dis-tances of G48.61+0.02, G48.99-0.30, G49.19-0.34, ON1, IRAS 20056+3350, IRAS 20143+3634, ON2N, and IRAS 20126+4104, which are located near the tangent point and the Solar circle. The angular ve-locity of the Galactic rotation at the LSR (i.e. the ratio of the Galactic constants) is derived using the measured parallax distances and proper motions of these sources. The derived value of Ω 0 = 28.8 ± 1.7 km s -1 kpc -1 is consistent with recent values obtained using VLBI astrometry but 10% larger than the International Astronomical Union (IAU) recommended value of 25.9 km s -1 kpc -1 = (220 km s -1 ) / (8.5 kpc).

Determining dynamical parameters of the Milky Way Galaxy based on high-accuracy radio astrometry

Abstract In this paper we evaluate how the dynamical structure of the Galaxy can be constrained by high-accuracy VLBI (Very Long Baseline Interferometry) astrometry such as VERA (VLBI Exploration of Radio Astrometry). We generate simulated samples of maser sources which follow the gas motion caused by a spiral or bar potential, with their distribution similar to those currently observed with VERA and VLBA (Very Long Baseline Array). We apply the Markov chain Monte Carlo analyses to the simulated sample sources to determine the dynamical parameter of the models. We show that one can successfully determine the initial model parameters if astrometric results are obtained for a few hundred sources with currently achieved astrometric accuracy. If astrometric data are available from 500 sources, the expected accuracy of R0 and Θ0 is ∼ 1% or higher, and parameters related to the spiral structure can be constrained by an error of 10% or with higher accuracy. We also show that the parameter determination accuracy is basically independent of the locations of resonances such as corotation and/or inner/outer Lindblad resonances. We also discuss the possibility of model selection based on the Bayesian information criterion (BIC), and demonstrate that BIC can be used to discriminate different dynamical models of the Galaxy.

VLBI observations of H2O maser annual parallax and proper motion in IRAS 20143+3634: Reflection on the Galactic constants

Abstract We report the results of very long baseline interferometry (VLBI) observations of H2O masers in the IRAS 20143+3634 star-forming region (SFR) using VERA (VLBI Exploration of Radio Astrometry). By tracking masers for a period of over two years we measured a trigonometric parallax of π = 0.367 ± 0.037 mas, corresponding to a source distance of $D = 2.72 ^{+0.31}_{-0.25}$ kpc and placing it in the Local spiral arm. Our trigonometric distance is just 60% of the previous estimation based on radial velocity, significantly impacting the astrophysics of the source. We measured proper motions of −2.99 ± 0.16 mas yr−1 and −4.37 ± 0.43 mas yr−1 in RA and Dec, respectively, which were used to estimate the peculiar motion of the source as (Us, Vs, Ws) = (−0.9 ± 2.9, −8.5 ± 1.6, +8.0 ± 4.3) km s−1 for R0 = 8 kpc and Θ0 = 221 km s−1, and (Us, Vs, Ws) = (−1.0 ± 2.9, −9.3 ± 1.5, +8.0 ± 4.3) km s−1 for R0 = 8.5 kpc and Θ0 = 235 km s−1. IRAS 20143+3634 was found to be located near the tangent point in the Cygnus direction. Using our observations we derived the angular velocity of Galactic rotation of the local standard of rest (LSR), Ω0 = 27.3 ± 1.6 km s−1 kpc−1, which is consistent with previous values derived using VLBI astrometry of SFRs at the tangent points and Solar circle. It is higher than the value recommended by the International Astronomical Union of Ω0 = 25.9 km s−1 kpc−1 which was calculated using the Galactocentric distance of the Sun and circular velocity of the LSR.

THE BENEFITS OF VLBI ASTROMETRY TO PULSAR TIMING ARRAY SEARCHES FOR GRAVITATIONAL RADIATION

Precision astrometry is an integral component of successful pulsar timing campaigns. Astrometric parameters are commonly derived by fitting them as parameters of a timing model to a series of pulse times of arrival (TOAs). TOAs measured to microsecond precision over several-year spans can yield position measurements with sub-milliarcsecond precision. However, timing-based astrometry can become biased if a pulsar displays any red spin noise, which can be compared to the red noise signal produced by the stochastic gravitational wave background. We investigate how noise of different spectral types is absorbed by timing models, leading to significant estimation errors in the astrometric parameters. We find that commonly used techniques for fitting timing models in the presence of red noise (Cholesky whitening) prevent the absorption of noise into the timing model remarkably well if the time baseline of observations exceeds several years, but are inadequate for dealing with shorter pulsar data sets. Independent of timing, pulsar-optimized very long baseline interferometry (VLBI) is capable of providing position estimates precise to the sub-milliarcsecond levels needed for high-precision timing. We compute a necessary transformation between the International Celestial Reference Frame and pulsar timing frames and quantitatively discuss how the transformation will improve in coming years. We find that incorporating VLBI astrometry into the timing models of pulsars for which only a couple of years of timing data exist will lead to more realistic assessments of red spin noise and could enhance the amplitude of gravitational wave signatures in post-fit timing residuals by factors of 20 or more.

VLBI ASTROMETRY OF PSR J2222-0137: A PULSAR DISTANCE MEASURED TO 0.4% ACCURACY

The binary pulsar J2222−0137 is an enigmatic system containing a partially recycled millisecond pulsar and a companion of unknown nature. While the low eccentricity of the system favors a white dwarf companion, an unusual double neutron star system is also a possibility, and optical observations will be able to distinguish between these possibilities. In order to allow the absolute luminosity (or upper limit) of the companion object to be properly calibrated, we undertook astrometric observations with the Very Long Baseline Array to constrain the system distance via a measurement of annual geometric parallax. With these observations, we measure the parallax of the PSR J2222–0137 system to be 3.742 mas, yielding a distance of 267.3 pc, and measure the transverse velocity to be 57.1 km s−1. Fixing these parameters in the pulsar timing model made it possible to obtain a measurement of Shapiro delay and hence the system inclination, which shows that the system is nearly edge-on (sin i = 0.9985 ± 0.0005). Furthermore, we were able to detect the orbital motion of PSR J2222–0137 in our very long baseline interferometry (VLBI) observations and measure the longitude of ascending node Ω. The VLBI astrometry yields the most accurate distance obtained for a radio pulsar to date, and is furthermore the most accurate parallax for any radio source obtained at "low" radio frequencies (below ∼5 GHz, where the ionosphere dominates the error budget). Using the astrometric results, we show that the companion to PSR J2222–0137 will be easily detectable in deep optical observations if it is a white dwarf. Finally, we discuss the implications of this measurement for future ultra-high-precision astrometry, in particular in support of pulsar timing arrays.

Very long baseline interferometry astrometry of PSR B1257+12, a pulsar with a planetary system

An astrometry project on PSR B1257+12, a millisecond pulsar with a planetary system, has been carried out with the Very Long Baseline Array and the European Very Long Baseline Interferometry (VLBI) Network at the central frequencies of 1465 and 1540 MHz, respectively. This project consists of nine epochs of phase-referenced observations spanning about two years. Model-independent estimates of the distance and velocity of this pulsar have been obtained using VLBI astrometry for the first time. The precise distance to PSR B1257+12 is 710(-38)(+43) pc. This is in agreement with the timing results, but is larger than the predictions from dispersion measurements using Galactic electron density distribution models. The measured proper-motion components of PSR B1257+12 in right ascension and declination are 46.44 +/- 0.08 and -84.87 +/- 0.32 mas yr(-1), respectively. The corresponding transverse velocity of PSR B1257+12 is similar to 326 km s(-1), making it the only millisecond pulsar known so far with a velocity clearly exceeding 300 km s(-1). Based on these results, the apparent X-ray efficiency of this pulsar is estimated to be in the same range as other millisecond pulsars, which is not as low as previously thought. The planets around PSR B1257+12 should be formed in a protoplanetary disc. We review possible evolutionary scenarios for the protoplanetary disc, and conclude that the protoplanetary disc of this pulsar has most probably been formed by the evaporation or disruption of its companion in a binary.

Radio astrometry with chromatic AGN core positions

Aims: The effect of frequency-dependent AGN core positions (``core-shifts'') on radio Very Long Baseline Interferometry (VLBI) global astrometry measurements is investigated. Methods: The basic equations relating to VLBI astrometry are reviewed, including the effects of source structure. A power-law representation of core-shifts, based on both observations and theoretical considerations of jet conditions, is incorporated. Results: It is shown that, in the presence of core-shifts, phase and group-delay astrometry measurements yield different positions. For a core displacement from the jet base parametrized by Delta x (lambda) = k lambda^beta group delays measure a ``reduced'' core-shift of (1-beta) Delta x (lambda). For the astrophysically-significant case of beta = 1, group delays measure no shift at all, giving the position of the jet base. At 8.4 GHz an estimated typical offset between phase and group-delay positions of ~170 uas is smaller than the current ~250 uas precision of group-delay positions of the sources used to define the ICRF; however, this effect must be taken into account for future measurements planned with improved accuracy when comparing with optical positions of AGN to be obtained with the GAIA mission.

Astrometry of H2O Masers in Nearby Star-Forming Regions with VERA II SVS 13 in NGC 1333

Abstract We report on the results of multiepoch very long baseline interferometry (VLBI) observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H$_{2}$O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We carried out phase-referencing VLBI astrometry, and measured the annual parallax of the maser features in SVS 13 of 4.25$\pm$0.32mas, corresponding to a distance of 235$\pm$18pc from the Sun. Our result is consistent with a photometric distance of 220 pc, previously reported. Even though the maser features were detectable only for 6 months, the present result provides the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H$_{2}$O masers with LSR (local standard of rest) velocities of 7–8kms$^{-1}$ are most likely associated with VLA4A, which is a radio counterpart of SVS 13. It is currently difficult to attribute the observed proper motions of the maser features to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

Improved VLBI astrometry of OH maser stars

Aims. Accurate distances to evolved stars with high mass loss rates are needed for studies of many of their fundamental properties. However, as these stars are heavily obscured and variable, optical and infrared astrometry is unable to provide enough accuracy. Methods. Astrometry using masers in the circumstellar envelopes can be used to overcome this problem. We have observed the OH masers of a number of Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long Baseline Array (VLBA). We have used the technique of phase referencing with in-beam calibrators to test the improvements this technique can provide to Very Long Baseline Interferometry (VLBI) OH maser astrometric observations. Results. We have significantly improved the parallax and proper motion measurements of the Mira variable stars U Her, S CrB and RR Aql. Conclusions. It is shown that both in-beam phase-referencing and a decrease in solar activity during the observations significantly improves the accuracy of the astrometric observations. The improved distances to S CrB ($418^{+21}_{-18}$ pc) and RR Aql ($633^{+214}_{-128}$ pc) are fully consistent with published P – L relations, but the distance to U Her ($266^{+32}_{-28}$ pc) is significantly smaller. We conclude that for sources that are bright and have a nearby in-beam calibrator, VLBI OH maser astrometry can be used to determine distances to OH masing stars of up to ~2 kpc.

Improving the Accuracy of Radio Astrometry

AbstractThe International Celestial Reference Frame (ICRF) is currently realized by the radio positions of compact extragalactic objects obtained by the technique of Very Long Baseline Interferometry (VLBI). The positions of the defining sources with the highest quality are estimated to be accurate at the 0.25 milliarcsecond (mas) level. The limiting accuracy is thought to be due mostly to tropospheric propagation effects and by the apparent motions of the sources due to variable intrinsic structure. Increasing the accuracy of VLBI astrometry beyond current levels will require an improved understanding of the variable emission structures associated with compact extragalactic sources as well as improvements in the modeling of the Earth’s troposphere.