Very Long Baseline Array Data Research Articles

Transient relativistic ejections and stationary core in XTE J1752−223

ABSTRACT The Galactic X-ray transient XTE J1752−223 was shown to have properties of black hole binary candidates. As reported in our previous paper, we identified transient and decelerating ejecta in multi-epoch Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) and the NRAO Very Long Baseline Array (VLBA). Here we present new EVN and VLBA data in which a new transient ejection event and later a stationary component are identified. The latter is interpreted as a reappearance of the radio core/compact jet during the transition from soft to hard X-ray state. This component appears to be highly variable in brightness although effects of tropospheric instabilities might play a role too. We also re-analyse the earlier VLBI data and find that the transient ejecta closer to the core position has significantly higher proper motion, further strengthening the case for strongly decelerating ejecta on the scale of several hundred milliarcsecond, never observed in X-ray binaries before. Although the distance of the source is not well constrained, it is clear that these ejectas are at least mildly relativistic at the early stages. Moreover, we show the large scale environment of the transient from the Westerbork synthesis array data recorded in parallel during the EVN run.

Flares and proper motions of ground-state OH masers in W75N

The star-forming region W75N hosts bright OH masers that are observed to be variable. We present observations taken in 2008 of the ground-state OH maser transitions with the Very Long Baseline Array (VLBA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN) and with the Nancay Radio Telescope in 2011. Several of the masers in W75N were observed to be flaring, with the brightest 1720-MHz maser in excess of 400 Jy. The 1720-MHz masers appear to be associated with the continuum source VLA 1, unlike the bright flaring 1665- and 1667-MHz masers, which are associated with VLA 2. The 1720-MHz masers are located in an outflow traced by water masers and are indicative of very dense molecular material near the H II region. The magnetic field strengths are larger in the 1720-MHz maser region than in most regions hosting only main-line OH masers. The density falls off along the outflow, and the order of appearance of different transitions of OH masers is consistent with theoretical models. The 1665- and 1667-MHz VLBA data are compared against previous epochs over a time baseline of over 7 years. The median maser motion is 3.5 km/s, with a scatter that is comparable to thermal turbulence. The general pattern of maser proper motions observed in the 1665- and 1667-MHz transitions is consistent with previous observations.

V-FASTR: THE VLBA FAST RADIO TRANSIENTS EXPERIMENT

Recent discoveries of dispersed, non-periodic impulsive radio signals with single-dish radio telescopes have sparked significant interest in exploring the relatively uncharted space of fast transient radio signals. Here we describe V-FASTR, an experiment to perform a blind search for fast transient radio signals using the Very Long Baseline Array (VLBA). The experiment runs entirely in a commensal mode, alongside normal VLBA observations and operations. It is made possible by the features and flexibility of the DiFX software correlator that is used to process VLBA data. Using the VLBA for this type of experiment offers significant advantages over single-dish experiments, including a larger field of view, the ability to easily distinguish local radio-frequency interference from real signals and the possibility to localize detected events on the sky to milliarcsecond accuracy. We describe our software pipeline, which accepts short integration (~ms) spectrometer data from each antenna in real time during correlation and performs an incoherent dedispersion separately for each antenna, over a range of trial dispersion measures. The dedispersed data are processed by a sophisticated detector and candidate events are recorded. At the end of the correlation, small snippets of the raw data at the time of the events are stored for further analysis. We present the results of our event detection pipeline from some test observations of the pulsars B0329+54 and B0531+21 (the Crab pulsar).

VERY LONG BASELINE ARRAY ASTROMETRIC OBSERVATIONS OF THECASSINISPACECRAFT AT SATURN

The planetary ephemeris is an essential tool for interplanetary spacecraft navigation, studies of solar system dynamics (including, for example, barycenter corrections for pulsar timing ephemeredes), the prediction of occultations, and tests of general relativity. We are carrying out a series of astrometric VLBI observations of the Cassini spacecraft currently in orbit around Saturn, using the Very Long Baseline Array (VLBA). These observations provide positions for the center of mass of Saturn in the International Celestial Reference Frame (ICRF) with accuracies ~0.3 milli-arcsecond (1.5 nrad), or about 2 km at the average distance of Saturn. This paper reports results from eight observing epochs between 2006 October and 2009 April. These data are combined with two VLBA observations by other investigators in 2004 and a Cassini-based gravitational deflection measurement by Fomalont et al. in 2009 to constrain a new ephemeris (DE 422). The DE 422 post-fit residuals for Saturn with respect to the VLBA data are generally 0.2 mas, but additional observations are needed to improve the positions of all of our phase reference sources to this level. Over time we expect to be able to improve the accuracy of all three coordinates in the Saturn ephemeris (latitude, longitude, and range) by a factor of at least three. This will represent a significant improvement not just in the Saturn ephemeris but also in the link between the inner and outer solar system ephemeredes and in the link to the inertial ICRF.

The Parsec‐Scale Jets of the TeV Blazars H1426+428, 1ES 1959+650, and PKS 2155−304: 2001–2004

We present Very Long Baseline Array (VLBA) observations of the TeV blazars H1426+428, 1ES 1959+650, and PKS 2155–304 obtained during the years 2001-2004. We observed H1426+428 at four epochs at 8 GHz and found that its parsec-scale structure consisted of a ~17 mJy core and a single ~3 mJy jet component with an apparent speed of 2.09c ± 0.53c. The blazar 1ES 1959+650 was observed at three epochs at frequencies of 15 and 22 GHz. Spectral index information from these dual-frequency observations was used to definitively identify the core of the parsec-scale structure. PKS 2155–304 was observed at a single epoch at 15 GHz with dual-circular polarization, and we present the first VLBI polarimetry image of this source. For 1ES 1959+650 and PKS 2155–304, the current observations are combined with the VLBA observations from our earlier paper to yield improved apparent speed measurements for these sources with greatly reduced measurement errors. The new apparent speed measured for component C2 in 1ES 1959+650 is 0.00c ± 0.04c (stationary), and the new apparent speed measured for component C1 in PKS 2155–304 is 0.93c ± 0.31c. We combine the new apparent speed measurements from this paper with the apparent speeds measured in TeV blazar jets from our earlier papers to form a current set of apparent speed measurements in TeV high-frequency peaked BL Lac objects (HBLs). The mean peak apparent pattern speed in the jets of the TeV HBLs is about 1c. We conclude the paper with a detailed discussion of the interpretation of the collected VLBA data on TeV blazars in the context of current theoretical models for the parsec-scale structure of TeV blazar jets.

The dual-frequency calibration of ionosphere influence in VLBA data processing

Some results of processing data of Very Long Baseline Array (VLBA) experiments titled RDV14, RDV15, RDV16, and RDV17 are presented. Observations had been carried out on 15/04/1999, 10/05/1999, 21/06/1999, and 02/08/1999. The Flexible Image Transport System (FITS) files have been created with a VLBA correlator and then placed into archive. Later, these data have been kindly placed at my disposal by Dr. Leonid Petrov (Goddard Space Centre). The software project titled “Astro Space Locator” (ASL for Windows) has been used for post-correlation data processing.

Multifrequency VLBA monitoring of 3C 273 during the INTEGRAL Campaign in 2003

In this first of a series of papers describing polarimetric multifrequency Very Long Baseline Array (VLBA) monitoring of 3C 273 during a simultaneous campaign with the INTEGRAL gamma-ray satellite in 2003, we present 5 Stokes I images and source models at 7 mm. We show that a part of the inner jet (1-2 milliarcseconds from the core) is resolved in a direction transverse to the flow, and we analyse the kinematics of the jet within the first 10 mas. Based on the VLBA data and simultaneous single-dish flux density monitoring, we determine an accurate value for the Doppler factor of the parsec scale jet, and using this value with observed proper motions, we calculate the Lorentz factors and the viewing angles for the emission components in the jet. Our data indicates a significant velocity gradient across the jet with the components travelling near the southern edge being faster than the components with more northern path. We discuss our observations in the light of jet precession model and growing plasma instabilities.

The extremely asymmetric radio structure of the z = 3.1 radio galaxy B3 J2330 + 3927

Abstract We report on 1.7- and 5.0-GHz observations of the z= 3.087 radio galaxy B3 J2330 + 3927, using the Very Long Baseline Array (VLBA), and archival 1.4- and 8.4-GHz Very Large Array (VLA) data. Our VLBA data identify a compact, flat-spectrum (α5 GHz1.7 GHz=−0.2 ± 0.1; Sν∝να) radio component as the core. The VLA images show that the fraction of core emission is very large (fc≈ 0.5 at 8.4 GHz), and reveal a previously undetected, very faint counterjet, implying a radio lobe flux density ratio R≳ 11 and a radio lobe distance ratio Q≈ 1.9. Those values are much more common in quasars than in radio galaxies, but the optical/near-infrared spectra show a clear type II active galactic nucleus for B3 J2330 + 3927, confirming that it is indeed a radio galaxy. Unlike all other radio galaxies, the bright Lyα-emitting gas is located towards the furthest radio arm. We argue against environmental and relativistic beaming effects being the cause of the observed asymmetry, and suggest this source has intrinsically asymmetric radio jets. If this is the case, B3 J2330 + 3927 is the first example of such a source at high redshift, and seems to be difficult to reconcile with the unified model, which explains the differences between quasars and radio galaxies as being due to orientation effects.

Spatial and Temporal Variations in Small-Scale Galactic HiStructure toward 3C 138

Several recent studies of Galactic H I absorption toward background quasars and pulsars have provided evidence that there are opacity changes in the neutral Galactic interstellar medium on size scales as small as a few AU. The nature of these opacity variations has remained a matter of debate but could reflect a variety of physical processes, including changes in the H I spin temperature or gas density. We present three epochs of Very Long Baseline Array (VLBA) observations of Galactic H I absorption toward the quasar 3C 138 with resolutions of 20 mas (~10 AU). This analysis includes VLBA data from observations in 1999 and 2002 along with a reexamination of the 1995 VLBA data, reported by Faison and coworkers. Improved data reduction and imaging techniques have led to an order of magnitude improvement in sensitivity compared to previous work. With these new data we confirm the previously detected milliarcsecond-scale spatial variations in the H I opacity at the level of Δτmax = 0.50 ± 0.05. The typical size scale of the optical depth variations is ~50 mas or 25 AU. In addition, for the first time we see clear evidence for temporal variations in the H I opacity over the 7 yr time span of our three epochs of data. We also attempt to detect the magnetic field strength in the H I gas using the Zeeman effect. From this analysis we have been able to place a 3 σ upper limit on the magnetic field strength per pixel of ~45 μG. We have also been able to calculate for the first time the plane-of-sky covering fraction of the small-scale H I gas of ~10%. This small covering fraction suggests that the filling factor of such gas is quite low, in agreement with recent optical observations. We also find that the line widths of the milliarcsecond-scale H I features are comparable to those determined from previous single-dish measurements toward 3C 138, suggesting that the opacity variations cannot be due to changes in the H I spin temperature. From these results we favor a density enhancement interpretation for the small-scale H I structures, although these enhancements appear to be of short duration and are unlikely to be in equilibrium.

Structure of OJ 287 from Geodetic VLBA Data

We have used the Radio Reference Frame Image Database (RRFID) data of the US Naval Observatory (USNO) at 8 GHz to study the structure of OJ 287. The data comprise Very Long Baseline Array (VLBA) observations made during the epochs from 1994 July to 2002 January. The imaging of these data reveals that the position angle of the more collimated unresolved jet rotated clockwise ~30° in 8 yr. The best explanation for this result is given by a ballistic precessing model. The aperture angle of the precessing cone is 3°, the axis of the precessing cone forms an angle of 11° with the line of sight and has a projected position angle of -107° in the plane of sky. The outer structure (~1.5 mas) of the source is also consistent with a ballistic precessing jet if the velocity of the ballistic component decreases with distance.

The kinematics of water masers in the stellar molecular outflow source, IRAS 19134+2131

Using the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), we have observed water maser emission in the proto-planetary nebula candidate IRAS 19134+2131, in which the water maser spectrum has two groups of emission features separated in radial velocity by ~100 km s -1 . The blue-shifted and red-shifted clusters of maser features are clearly separated spatially by ~150 mas, indicative of a fast collimated flow. However, not all of the maser features are aligned along the axis of the flow, as is seen in the similar high-velocity water maser source, W43A. Comparing the VLA and VLBA maps of the water maser source, we find 4 maser features that were active for 2 years. Using only VLBA data, we identified proper motions for 8 maser features. The full 3D outflow velocity is estimated to be ~130 km s -1 , indicating that the dynamical age of the flow is only ~50 yr. On the basis of the relative positions with respect to the nearby extragalactic reference source, J1925+2106, we also obtain a secular motion of IRAS 19134+2131of $\mu_{l}=-4.6\pm0.7$ mas yr -1 along the Galactic plane toward the Galactic centre. This indicates a “far distance ( ≥ 16 kpc) for IRAS 19134+2131 if the Galactic rotation curve remains flat at 220 km s -1 .

R Aquarii: Evidence for Differential Rotation of the S[CLC]i[/CLC]O Maser Shell

We previously reported Very Large Array and Berkeley-Illinois-Maryland Association (BIMA) array observations that suggested rotation of the SiO maser shell surrounding the long-period variable (LPV) in the R Aquarii binary system. In the present Very Long Baseline Array (VLBA) work, we report high spatial and spectral resolution observations of the v = 1, J = 1-0, SiO maser line that confirm our previous result and further suggest that the LPV maser shell is undergoing differential rotation. The 8-34 yr range of rotational periods resulting from differential rotation of the maser shell contains the ~18 yr period reported previously. The velocity structure of the VLBA data suggests a rotation symmetry axis oriented at a position angle of ~150°. The differential rotation model can be envisioned as a series of nested thin spherical shells that have a common rotation axis; each thin shell is characterized by its radius, r, with the innermost shell rotating fastest and the outermost shell slowest, in accordance with an equatorial plane velocity law of the form v ∝ 1/2. We find that q ≈ 1.09 is necessary to approximate the VLBA data, suggesting that the differential rotation is approximately Keplerian.

Multiepoch Very Long Baseline Array Observations of EGRET‐detected Quasars and BL Lacertae Objects: Connection between Superluminal Ejections and Gamma‐Ray Flares in Blazars

We examine the coincidence of times of high γ-ray flux and ejections of superluminal components from the core in EGRET blazars based on a Very Long Baseline Array (VLBA) monitoring program at 22 and 43 GHz from 1993 November to 1997 July. In 23 cases of γ-ray flares for which sufficient VLBA data exist, 10 of the flares (in eight objects) fall within 1 σ uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730-130), two successive γ-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations that show that if the number of coincidences is ≥10, the radio and γ-ray events are associated with each other at greater than 99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the γ-ray flares suggests that the γ-ray events occur in the superluminal radio knots. This implies that the γ-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine.

The Structure of the Cold Neutral Interstellar Medium on 10–100 AU Scales

We have used the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) to image Galactic neutral hydrogen in absorption towards four compact extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA data by Faison et al (1998) have shown the existence of prominent structures in the direction of the extragalactic source 3C~138 with scale sizes of 10-20 AU with changes in HI optical depth in excess of 0.8 $\pm$ 0.1. In this paper we confirm the small scale \hi optical depth variations toward 3C~147 suggested earlier at a level up to 20 % $\pm$ 5% . The sources 3C~119, 2352+495 and 0831+557 show no significant change in \hi optical depth across the sources with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently investigated with the VLBA and VLA, only 3C~138 and 3C~147 show statistically significant variations in HI opacities. Deshpande (2000) have attempted to explain the observed small-scale structure as an extension of the observed power spectrum of structure on parsec size scales. The predictions of Deshpande (2000) are consistent with the VLBA HI data observed in the directions of a number of sources, including 3C~147, but are not consistent with our previous observations of the HI opacity structure toward 3C~138.