Very Long Baseline Array Data Research Articles

The VLBA CANDELS GOODS-North Survey. II – Wide-field source catalogue comparison between the VLBA, EVN, e-MERLIN, and VLA

ABSTRACT Deep radio surveys of extragalactic legacy fields trace a large range of spatial and brightness temperature sensitivity scales, and therefore have differing biases to radio-emitting physical components within galaxies. This is particularly true of radio surveys performed at $\lesssim 1 \ \mathrm{arcsec}$ angular resolutions, and so robust comparisons are necessary to better understand the biases present in each survey. We present a multiresolution and multiwavelength analysis of the sources detected in a new Very Long Baseline Array (VLBA) survey of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey Great Observatories Origins Deep Survey-North field. For the 24 VLBA-selected sources described in Paper I, we augment the VLBA data with EVN data, and ∼0.1–1 arcsec angular resolution data provided by Very Large Array (VLA) and enhanced-Multi Element Remotely Linked Interferometry Network. This sample includes new active galactic nuclei (AGN) detected in this field, thanks to a new source extraction technique that adopts priors from ancillary multiwavelength data. The high brightness temperatures of these sources (TB ≳ 106 K) confirm AGN cores, that would often be missed or ambiguous in lower-resolution radio data of the same sources. Furthermore, only 15 sources are identified as ‘radiative’ AGN based on available X-ray and infrared constraints. By combining VLA and VLBA measurements, we find evidence that the majority of the extended radio emission is also AGN dominated, with only three sources with evidence for extended potentially star formation-dominated radio emission. We demonstrate the importance of wide-field multiresolution (arcsecond–milliarcsecond) coverage of the faint radio source population, for a complete picture of the multiscale processes within these galaxies.

Parsec-scale evolution of the gigahertz-peaked spectrum quasar PKS 0858 − 279

ABSTRACT We conducted multi-epoch, multifrequency parsec-scale studies on the gigahertz-peaked spectrum quasar PKS 0858 − 279 with the Very Long Baseline Array (VLBA). Our observations on 2005 November 26 elucidated a weak core, characterized by an inverted spectrum, and a distinctly bent jet that exhibited a notable bright feature in its Stokes I emission. Through comprehensive analysis of polarization and spectral data, we inferred the formation of a shock wave within this feature, stemming from interactions with a dense cloud in the ambient medium. In this paper, Very Long Baseline Interferometry-Gaia astrometry further reinforces the core identification. With a deep analysis of six additional VLBA epochs spanning from 2007 to 2018, we observed that while the quasar’s parsec-scale structure remained largely consistent, there were discernible flux density changes. These variations strongly imply the recurrent ejection of plasma into the jet. Complementing our VLBA data, RATAN-600 observations of the integrated spectra suggested an interaction between standing and travelling shock waves in 2005. Moreover, our multi-epoch polarization analysis revealed a drastic drop in rotation measure values from 6000 to 1000 rad m−2 within a single year, attributable to diminishing magnetic fields and particle density in an external cloud. This change is likely instigated by a shock in the cloud, triggered by the cloud’s interaction with the jet, subsequently prompting its expansion. Notably, we also observed a significant change in the magnetic field direction of the jet, from being perpendicular post its observed bend to being perpendicular prior to the bend – an alteration possibly induced by the dynamics of shock waves.

Calibrating VLBI Polarization Data Using GPCAL. II. Time-dependent Calibration

We present a new method of time-dependent instrumental polarization calibration for very long baseline interferometry (VLBI). This method has been implemented in the recently developed polarization calibration pipeline GPCAL. Instrumental polarization, also known as polarimetric leakage, is a direction-dependent effect, and it is not constant across the beam of a telescope. Antenna pointing model accuracy is usually dependent on time, resulting in off-axis polarimetric leakages that can vary with time. The method is designed to correct for the off-axis leakages with large amplitudes that can severely degrade linear polarization images. Using synthetic data generated based on real Very Long Baseline Array (VLBA) data observed at 43 GHz, we evaluate the performance of the method. It was able to reproduce the off-axis leakages assumed in the synthetic data, particularly those with large amplitudes. The method has been applied to two sets of real VLBA data, and the derived off-axis leakages show very similar trends over time for pairs of nearby sources. Furthermore, the amplitudes of the off-axis leakages are strongly correlated with the antenna gain correction factors. The results demonstrate that the method is capable of correcting for the off-axis leakages present in VLBI data. By calibrating time-dependent instrumental polarization, the rms noise levels of the updated linear polarization images have been significantly reduced. The method is expected to substantially enhance the quality of linear polarization images obtained from existing and future VLBI observations.

Calibrating VLBI Polarization Data Using GPCAL. I. Frequency-dependent Calibration

In this series of papers, we present new methods of frequency- and time-dependent instrumental polarization calibration for very long baseline interferometry (VLBI). In most existing calibration tools and pipelines, it has been assumed that instrumental polarization is constant over frequency within the instrument bandwidth and over time. The assumption is not always true and may prevent an accurate calibration, which can result in degradation of the quality of linear polarization images. In this paper, we present a method of frequency-dependent instrumental polarization calibration that is implemented in GPCAL, a recently developed polarization calibration pipeline. The method is tested using simulated data sets generated from real Very Long Baseline Array (VLBA) data. We present the results of applying the method to real VLBA data sets observed at 15 and 43 GHz. We were able to eliminate significant variability in cross-hand visibilities over frequency that is caused by frequency-dependent instrumental polarization. As a result of the calibration, linear polarization images were slightly to modestly improved as compared to those obtained without frequency-dependent instrumental polarization calibration. We discuss the reason for the minor impact of frequency-dependent instrumental polarization calibration on existing VLBA data sets and prospects for applying the method to future VLBI data sets, which are expected to provide very large bandwidths.

Multiple imaging of the quasar 2005 + 403 formed by anisotropic scattering

ABSTRACT We report on the low Galactic latitude (b = 4${_{.}^{\circ}}$3) quasar 2005 + 403, the second active galactic nuclei, in which we detected a rare phenomenon of multiple imaging induced by refractive-dominated scattering. The manifestation of this propagation effect is revealed at different frequencies (≲ 8 GHz) and epochs of Very Long Baseline Array (VLBA) observations. The pattern formed by anisotropic scattering is stretched out along the line of constant Galactic latitude with a local position angle, PA ≈ 40° showing 1–2 sub-images, often on either side of the core. Analysing the multifrequency VLBA data ranging from 1.4 to 43.2 GHz, we found that both the angular size of the apparent core component and the separation between the primary and secondary core images follow a wavelength squared dependence, providing convincing evidence for a plasma scattering origin for the multiple imaging. Based on the Owens Valley Radio Observatory long-term monitoring data at 15 GHz obtained for 2005 + 403, we identified the characteristic flux density excursions occurred in 2019 April and May and attributed to an extreme scattering event (ESE) associated with the passage of a plasma lens across the line of sight. Modelling the ESE, we determined that the angular size of the screen is 0.4 mas and it drifts with the proper motion of 4.4 mas yr−1. Assuming that the scattering screen is located in the highly turbulent Cygnus region, the transverse linear size and speed of the lens with respect to the observer are 0.7 au and 37 km s−1, respectively.

Cold gas in the heart of Perseus A

We present new Karl G. Jansky Very Large Array (JVLA) and Very Long Baseline Array (VLBA) observations that traced the H I in the central region of 3C 84 (Perseus A). This radio source is hosted by the bright cluster galaxy NGC 1275 in the centre of the iconic Perseus cluster. With the JVLA, we detected very broad (FWHM ∼500 km s−1) H I absorption at arcsecond resolution (∼300 pc) that is centred at the systemic velocity of NGC 1275 against the bright radio continuum, suggesting that the detected gas is very close to the supermassive black hole (SMBH). However, we did not detect any absorption in the higher-resolution VLBA data against the parsec-scale radio core and jet. Based on a comparison of the properties of the H I absorption with those of the molecular circum-nuclear disc (CND) known to be present in NGC 1275, we argue that the H I seen in absorption arises from H I in this fast-rotating CND, and that neutral atomic hydrogen is present as close as ∼20 pc from the SMBH. The radio continuum providing the background for absorption arises from non-thermal synchrotron emission from the star formation activity in the CND, whose presence has been reported by earlier VLBA studies. We did not detect any signature that the H I gas is affected by an interaction with the radio jet. Thus, at this stage of the evolution of the source, the impact of the radio jet on the gas in NGC 1275 mainly creates cavities on much larger galaxy scales. This prevents the circum-galactic gas from cooling, and it does not produce gas outflows. Overall, the properties of the CND in Perseus A present a very similar case to that of Mrk 231, suggesting that, unlike often assumed, H I absorption can arise against the radio emission from star formation in a CND and is perhaps common in radio AGN. With the JVLA, we serendipitously detected a new, faint absorbing system that is redshifted by ∼2660 km s−1, in addition to the already known high-velocity absorption system that is redshifted 2850 km s−1 with respect to NGC 1275. We identify this new system as gas that is stripped from a foreground galaxy falling into the Perseus cluster. This new absorption remains undetected with the VLBA.

Looking for Signatures of AGN Feedback in Radio-Quiet AGN

In this article, we discuss the state of “AGN feedback” in radio-quiet (RQ) AGN. This study involves heterogeneous samples of nearby Seyfert and LINER galaxies as well as quasi-stellar objects (QSOs) that have been observed at low radio frequencies (few ∼100 MHz) with the upgraded Giant Meterwave Radio Telescope (GMRT) and ∼GHz frequencies with the Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA). These multi-frequency, multi-resolution observations detect a range of arcsecond-scale radio spectral indices that are consistent with the presence of multiple contributors including starburst winds and AGN jets or winds; steep spectrum “relic” emission is observed as well. Polarization-sensitive data from the VLA and GMRT suggest that the radio outflows are stratified (e.g., in IIIZw2, Mrk231); distinct polarization signatures suggest that there could either be a “spine + sheath” structure in the radio outflow, or there could be a “jet + wind” structure. Similar nested biconical outflows can also explain the VLBA and SDSS emission-line data in the KISSR sample of double-peaked emission-line Seyfert and LINER galaxies. Furthermore, the modeling of the emission-lines with plasma modeling codes such as MAPPINGS indicates that parsec-scale jets and winds in these sources can disturb or move the narrow-line region (NLR) gas clouds via the “shock + precursor” mechanism. Apart from the presence of “relic” emission, several Seyfert and LINER galaxies show clear morphological signatures of episodic jet activity. In one such source, NGC2639, at least four distinct episodes of jets are observed, the largest one of which was only detectable at 735 MHz with the GMRT. Additionally, a ∼6 kpc hole in the CO molecular gas along with a dearth of young stars in the center of its host galaxy is observed. Multiple jet episodes on the 10–100 parsec scales and a ∼10 parsec hole in the molecular gas is also observed in the Seyfert galaxy NGC4051. This suggests a link between episodic jet activity in RQ AGN and “AGN feedback” influencing the evolution of their host galaxies. However, a similar simple relationship between radio outflows and molecular gas mass is not observed in the Palomar–Green (PG) QSO sample, indicating that “AGN feedback” is a complex phenomenon in RQ AGN. “AGN feedback” must occur through the local impact of recurring multi-component outflows in RQ AGN. However, global feedback signatures on their host galaxy properties are not always readily evident.

Deep Very Long Baseline Interferometry Observations Challenge Previous Evidence of a Binary Supermassive Black Hole Residing in Seyfert Galaxy NGC 7674

Previous Ku-band (15 GHz) imaging with data obtained from the Very Long Baseline Array (VLBA) had shown two compact, subparsec components at the location of a presumed kiloparsec-scale radio core in Seyfert galaxy NGC 7674. It was then presumed that these two unresolved and compact components were dual radio cores corresponding to two supermassive black holes (SMBHs) accreting surrounding gas and launching radio-bright relativistic jets. However, utilizing the original VLBA data set used to claim the detection of a binary SMBH, in addition to later multiepoch/multifrequency data sets obtained from both the VLBA and the European very long baseline interferometry (VLBI) network, we find no evidence to support the presence of a binary SMBH. We place stringent upper limits to the flux densities of any subparsec-scale radio cores that are at least an order of magnitude lower than the original VLBI radio-core detections, directly challenging the original binary SMBH detection claim. With this in mind, we discuss the possible reasons for the nondetection of any VLBI radio cores in our imaging, the possibility of a binary SMBH still residing in NGC 7674, and the prospect of future observations shedding further light on the true nature of this active galactic nucleus.

The Search for Binary Supermassive Black Holes among Quasars with Offset Broad Lines Using the Very Long Baseline Array

Abstract In several previous studies, quasars exhibiting broad emission lines with ≳1000 km s−1 velocity offsets with respect to the host galaxy rest frame have been discovered. One leading hypothesis for the origin of these velocity-offset broad lines is the dynamics of a binary supermassive black hole (SMBH). We present high-resolution radio imaging of 34 quasars showing these velocity-offset broad lines with the Very Long Baseline Array (VLBA), aiming to find evidence for the putative binary SMBHs (such as dual radio cores), and testing the competing physical models. We detect exactly half of the target sample from our VLBA imaging, after implementing a 5σ detection limit. While we do not resolve double radio sources in any of the targets, we obtain limits on the instantaneous projected separations of a radio-emitting binary for all of the detected sources under the assumption that a binary still exists within our VLBA angular resolution limits. We also assess the likelihood that a radio-emitting companion SMBH exists outside of our angular resolution limits, but its radio luminosity is too weak to produce a detectable signal in the VLBA data. Additionally, we compare the precise sky positions afforded by these data to optical positions from both the SDSS and Gaia DR2 source catalogs. We find projected radio/optical separations on the order of ∼10 pc for three quasars. Finally, we explore how future multi-wavelength campaigns with optical, radio, and X-ray observatories can help discriminate further between the competing physical models.

A ring accelerator? Unusual jet dynamics in the IceCube candidate PKS 1502+106

ABSTRACT On 2019/07/30.86853 ut, IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 per cent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure that develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C iv line emission (Sloan Digital Sky Survey) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the γ-ray emission, with radio lagging the γ-rays. Based on the γ-ray variability time-scale, we constrain the γ-ray emission zone to the BLR (30–200 rg) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for γ-ray production via external Compton scattering. The neutrino is most likely produced by proton–proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR.

The JPL Planetary and Lunar Ephemerides DE440 and DE441

Abstract The planetary and lunar ephemerides called DE440 and DE441 have been generated by fitting numerically integrated orbits to ground-based and space-based observations. Compared to the previous general-purpose ephemerides DE430, seven years of new data have been added to compute DE440 and DE441, with improved dynamical models and data calibration. The orbit of Jupiter has improved substantially by fitting to the Juno radio range and Very Long Baseline Array (VLBA) data of the Juno spacecraft. The orbit of Saturn has been improved by radio range and VLBA data of the Cassini spacecraft, with improved estimation of the spacecraft orbit. The orbit of Pluto has been improved from use of stellar occultation data reduced against the Gaia star catalog. The ephemerides DE440 and DE441 are fit to the same data set, but DE441 assumes no damping between the lunar liquid core and the solid mantle, which avoids a divergence when integrated backward in time. Therefore, DE441 is less accurate than DE440 for the current century, but covers a much longer duration of years −13,200 to +17,191, compared to DE440 covering years 1550–2650.

GPCAL: A Generalized Calibration Pipeline for Instrumental Polarization in VLBI Data

Abstract We present the Generalized Polarization CALibration pipeline (GPCAL), an automated pipeline for instrumental polarization calibration of very long baseline interferometry (VLBI) data. The pipeline is designed to achieve a high calibration accuracy by means of fitting the instrumental polarization model, including the second-order terms, to data from multiple calibrator sources simultaneously. It also allows for using more accurate linear polarization models of calibrators for D-term estimation compared to the conventional way that assumes similar linear polarization and total intensity structures. This assumption has been widely used in the existing packages for instrumental polarization calibration but could be a source of significant uncertainties when there is no suitable calibrator satisfying the assumption. We demonstrate the capabilities of GPCAL by using simulated data, archival Very Long Baseline Array (VLBA) data of many active galactic nucleus (AGN) jets at 15 and 43 GHz, and our Korean VLBI Network (KVN) observations of many AGN jets at 86, 95, 130, and 142 GHz. The pipeline could reproduce the complex linear polarization structures of several sources shown in the previous studies using the same VLBA data. In addition, GPCAL reveals a complex linear polarization structure in the flat-spectrum radio quasar 3C 273 from the KVN data at all four frequencies. These results demonstrate that GPCAL can achieve a high calibration accuracy for various VLBI arrays.

A VLBA Survey of Radio Stars in the Orion Nebula Cluster. II. Astrometry

Abstract From Very Long Baseline Array (VLBA) observations we previously identified a population of 123 young stellar systems with nonthermal radio emission toward the core of the Orion Nebula Cluster (ONC). We find optical sources in the Gaia DR2 catalog for 34 of them within 0.″2 of the radio positions. Most of the radio sources are likely companions of Gaia detections. However, there are 11 stars whose VLBA positions differ from Gaia by <1.6 mas, and some of these radio sources probably are the direct counterparts of the optical stars. We are able to obtain radio proper motions for 23 stars. Combining the stellar proper motions derived from the VLBA and Gaia DR2 data we find the global motion and velocity dispersion of the ONC to be (μ α ∗, μ δ ) = (1.20 ± 0.09, 0.18 ± 0.09) mas yr−1 and ( σ μ α ∗ , σ μ δ ) = (0.84 ± 0.09, 1.30 ± 0.09) mas yr−1. Finally, we looked for ordered motions by estimating the means of scalar and vectorial products, which results in v · r ˆ ¯ = − 0.61 ± 1.00 km s−1 and v × r ˆ ¯ = 0.57 ± 0.95 km s−1. These do not show indications that the young stellar cluster is in expansion, contraction, or rotation.

NGC 3894: a young radio galaxy seen by Fermi-LAT

Context. According to radiative models, radio galaxies may produce γ-ray emission from the first stages of their evolution. However, very few such galaxies have been detected by the Fermi Large Area Telescope (LAT) so far. Aims. NGC 3894 is a nearby (z = 0.0108) object that belongs to the class of compact symmetric objects (CSOs, i.e., the most compact and youngest radio galaxies), which is associated with a γ-ray counterpart in the Fourth Fermi-LAT source catalog. Here we present a study of the source in the γ-ray and radio bands aimed at investigating its high-energy emission and assess its young nature. Methods. We analyzed 10.8 years of Fermi-LAT data between 100 MeV and 300 GeV and determined the spectral and variability characteristics of the source. Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz over a period of 35years were used to study the radio morphology of NGC 3894 and its evolution. Results. NGC 3894 is detected in γ-rays with a significance >9σ over the full period, and no significant variability has been observed in the γ-ray flux on a yearly time-scale. The spectrum is modeled with a flat power law (Γ = 2.0 ± 0.1) and a flux on the order of 2.2 × 10−9 ph cm−2 s−1. For the first time, the VLBA data allow us to constrain with high precision the apparent velocity of the jet and counter-jet side to be βapp, NW = 0.132 ± 0.004 and βapp, SE = 0.065 ± 0.003, respectively. Conclusions. Fermi-LAT and VLBA results favor the youth scenario for the inner structure of this object, with an estimated dynamical age of 59 ± 5 years. The estimated range of viewing angle (10° < θ < 21°) does not exclude a possible jet-like origin of the γ-ray emission.

Very Long Baseline Array Astrometry of Cassini: The Final Epochs and an Improved Orbit of Saturn

Abstract We report results from multi-epoch radio astrometry of the Cassini spacecraft with the Very Long Baseline Array (VLBA). These observations are part of a program to determine a series of accurate positions for the Saturn system barycenter in the inertial International Celestial Reference Frame (ICRF) and to use these position measurements to improve our knowledge of Saturn’s orbit in the planetary ephemeris. Our VLBA observations cover the full duration of the orbital phase of the Cassini mission, from Saturn orbit insertion in 2004 to the end of mission in 2017. This period covers more than one-third of Saturn’s orbital period, allowing us to obtain good orbit constraints for Saturn, particularly on the inclination and ascending node longitude. During the early years of Cassini’s orbital mission our VLBA data dominated the determination of orbit orientation, while later in the mission range measurements become more significant. The orientation of Saturn’s orbit is now known to approximately 0.25 milli-arcseconds (1.25 nrad), an order of magnitude improvement since the start of Cassini observations. Continuing improvements in the ICRF position accuracy for our phase reference sources, and possible improvements in the final orbit solutions for Cassini, may lead to a still better Saturn orbit over the coming years.

Gaia-DR2 Confirms VLBA Parallaxes in Ophiuchus, Serpens, and Aquila

Abstract We present Gaia-DR2 astrometry of a sample of YSO candidates in Ophiuchus, Serpens Main, and Serpens South/W40 in the Aquila Rift, which had been mainly identified by their infrared excess with Spitzer. We compare the Gaia-DR2 parallaxes against published and new parallaxes obtained from our Very Long Baseline Array (VLBA) program Gould’s Belt Distances Survey. We obtain consistent results between Gaia and the VLBA for the mean parallaxes in each of the regions analyzed here. We see small offsets, when comparing mean values, of a few tens of microarcseconds in the parallaxes, which are either introduced by the Gaia zero-point error or due to a selection effect by Gaia toward the brightest, less obscured stars. Gaia-DR2 data alone conclusively places Serpens Main and Serpens South at the same distance, as we first inferred from VLBA data alone in a previous publication. Thus, Serpens Main, Serpens South, and W40 are all part of the same complex of molecular clouds, located at a mean distance of 436 ± 9 pc. In Ophiuchus, both Gaia and VLBA suggest a small parallax gradient across the cloud, and the distance changes from 144.2 ± 1.3 to 138.4 ± 2.6 pc when going from L1689 to L1688.

THE GOULD’S BELT DISTANCES SURVEY (GOBELINS). III. THE DISTANCE TO THE SERPENS/AQUILA MOLECULAR COMPLEX

We report on new distances and proper motions to seven stars across the Serpens/Aquila complex. The observations were obtained as part of the Gould's Belt Distances Survey (GOBELINS) project between September 2013 and April 2016 with the Very Long Baseline Array (VLBA). One of our targets is the proto-Herbig AeBe object EC 95, which is a binary system embedded in the Serpens Core. For this system, we combined the GOBELINS observations with previous VLBA data to cover a total period of ~8 years, and derive the orbital elements and an updated source distance. The individual distances to sources in the complex are fully consistent with each other, and the mean value corresponds to a distance of $436.0\pm9.2$~pc for the Serpens/W40 complex. Given this new evidence, we argue that Serpens Main, W40 and Serpens South are physically associated and form a single cloud structure.

HIGH-PRECISION RADIO AND INFRARED ASTROMETRY OF LSPM J1314+1320AB. I. PARALLAX, PROPER MOTIONS, AND LIMITS ON PLANETS

ABSTRACT We present multi-epoch astrometric radio observations with the Very Long Baseline Array (VLBA) of the young ultracool-dwarf binary LSPM J1314+1320AB. The radio emission comes from the secondary star. Combining the VLBA data with Keck near-infrared adaptive-optics observations of both components, a full astrometric fit of parallax (π abs = 57.975 ± 0.045 mas, corresponding to a distance of d = 17.249 ± 0.013 pc), proper motion (μ αcos δ = −247.99 ± 0.10 mas yr−1, μ δ = −183.58 ± 0.22 mas yr−1), and orbital motion is obtained. Despite the fact that the two components have nearly identical masses to within ±2%, the secondary’s radio emission exceeds that of the primary by a factor of ≳30, suggesting a difference in stellar rotation history, which could result in different magnetic field configurations. Alternatively, the emission could be anisotropic and beamed toward us for the secondary but not for the primary. Using only reflex motion, we exclude planets of mass 0.7–10 M jup with orbital periods of 600–10 days, respectively. Additionally, we use the full orbital solution of the binary to derive an upper limit for the semimajor axis of 0.23 au for stable planetary orbits within this system. These limits cover a parameter space that is inaccessible with, and complementary to, near-infrared radial velocity surveys of ultracool dwarfs. Our absolute astrometry will constitute an important test for the astrometric calibration of Gaia.

Possible Detection of Quasi-Periodic Oscillations from Sgr A* at 43 GHz

AbstractQuasi-periodic oscillations (QPOs) are believed to be indirect evidence for black holes. Several authors have reported detections of QPOs from Sgr A*, the nucleus of our Galaxy, in infrared and X-ray wavelength during flare-ups. Miyoshi et al. (2011) reported a tentative detection of QPOs in the 43 GHz light curve of Sgr A* obtained with the Very Long Baseline Array (VLBA). To confirm their detection, we reanalysed their VLBA data very conservatively. The 43 GHz flux was calculated for every 15 seconds by assuming a two-dimensional Gaussian-shape spatial structure. The Lomb-Scargle periodogram of the 43 GHz flux just after a millimeter wave flare of Sgr A*, shows three apparent peaks at 10.2, 14.6 and 32.1 min. Two of them are barely consistent with the previously reported QPOs. Using the resonant oscillation model, we estimated the spin parameter of the Sgr A* black hole to be 0.56 assuming the mass of 4.3 × 106M⊙.

Water Fountains in Pre-Planetary Nebulae: The Case of IRAS16342–3814

AbstractWe present a brief review of Very Long Baseline Array (VLBA) observations of water masers in the so-called water fountain pre-planetary nebulae, and report on new VLBA and Very Large Array (VLA) data for the water masers in the prototypical water fountain source IRAS16342–3814, taken approximately monthly in 2008–2009. A new and very strong water maser is found at an LSR velocity of −3 km s−1, which is offset from the central (likely stellar) velocity. The new VLBA observations still show a similar general structure as was observed in 2002, but there are details which are difficult to explain.