Very Long Baseline Array Observations Research Articles

Measurement of the Distance and Proper Motions of the H2O Masers in the Young Planetary Nebula K 3−35

Abstract In this paper we present the results of very long baseline interferometry (VLBI) observations carried out with the VLBI Exploration of Radio Astrometry (VERA) array and the Very Long Baseline Array (VLBA) toward H$_{2}$O masers in a young planetary nebula K 3$-$35. From the VERA observations we measured the annual parallax and proper motion of a bright water maser spot in K 3$-$35. The resulting distance is $D$$=$ 3.9$^{+0.7}_{\hskip1.11pt-0.5}$ kpc. This is the first time that the parallax of a planetary nebula is obtained by observations of its maser emission. On the other hand, the proper motion of K 3$-$35 as a whole was estimated to be $\mu_{\alpha}$$=$$-$3.34 $\pm$ 0.10 mas yr$^{-1}$, $\mu_{\delta}$$=$$-$5.93 $\pm$ 0.07 mas yr$^{-1}$. From these results we determined the position and velocity of K 3$-$35 in Galactic cylindrical coordinates: ($R,\theta,\textsl{z}$) $=$ (7.11$^{+0.08}_{\hskip1.11pt-0.06}$ kpc, 27$^{\circ}$$\pm$ 5$^{\circ}$, 140$^{+25}_{\hskip1.11pt-18}$ pc) and ($V_{R}, V_{\theta}, V_{{\sl z}}$) $=$ (33 $\pm$ 16, 233 $\pm$ 11, 11 $\pm$ 2) km s$^{-1}$. Additionally, from our VLBA observations we measured the relative proper motions among the water maser spots located in the central region of the nebula, which have been proposed to be tracing a toroidal structure. The distribution and relative proper motions of the masers, compared with previous reported observed epochs, suggest that such structure could be totally destroyed within a few years, due to the action of high-velocity winds and the expansion of the ionization front in the nebula.

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.

Wide-field VLBA observations of theChandradeep field South

Wide-field surveys are a commonly-used method for studying thousands of objects simultaneously, to investigate, e.g., the joint evolution of star-forming galaxies and active galactic nuclei. VLBI observations can yield valuable input to such studies because they are able to identify AGN. However, VLBI observations of large swaths of the sky are impractical using standard methods, because the fields of view of VLBI observations are of the order of 10" or less. We have embarked on a project to carry out Very Long Baseline Array (VLBA) observations of all 96 known radio sources in one of the best-studied areas in the sky, the Chandra Deep Field South (CDFS). The challenge was to develop methods which could significantly reduce the amount of observing (and post-processing) time. We have developed an extension to the DiFX software correlator which allows one to correlate hundreds of positions within the primary beams. This extension enabled us to target many sources, at full resolution and high sensitivity, using only a small amount of observing time. The combination of wide fields-of-view and high sensitivity across the field in this survey is unprecedented. We have observed with the VLBA a single pointing containing the Chandra Deep Field South, in which 96 radio sources were known from previous observations with the ATCA. From our input sample, 20 were detected with the VLBA. The majority of objects have flux densities in agreement with arcsec-scale observations, implying that their radio emission comes from very small regions. One VLBI-detected object had earlier been classified as a star-forming galaxy. Comparing the VLBI detections to sources found in sensitive, co-located X-ray observations we find that X-ray detections are not a good indicator for VLBI detections. Wide-field VLBI survey science is now coming of age.

Radio polarimetry of 3C 119, 3C 318, and 3C 343 at milliarcsecond resolution

<i>Aims. <i/>We report new Very Long Baseline Array (VLBA) polarimetric observations of the compact steep-spectrum (CSS) sources 3C 119, 3C 318, and 3C 343 at 5 and 8.4 GHz. <i>Methods. <i/>We analysed our VLBA observations and derived milliarcsecond-resolution images of the total intensity, polarisation, and rotation measure (<i>RM<i/>) distributions. <i>Results. <i/> The CSS source 3C 119, associated with a possible quasar, has source rest-frame <i>RM<i/> values up to ~10 200 rad m<sup>-2<sup/> in a region that coincides with a change in the direction of the inner jet. This component is located ~325 pc from the core, which is a variable source with a peaked radio spectrum. For 3C 318, which is associated with a galaxy, a rest-frame <i>RM<i/> of ~3030 rad m<sup>-2<sup/> was estimated for the brightest component contributing almost all of the polarised emission. Two more extended components were detected, that contain “wiggles” in the jet towards the southern side of the source. The CSS source 3C 343 contains two peaks of emission and a curved jet embedded in more diffuse emission. It exhibits complex field directions close to the emission peaks, which are indicative of rest-frame <i>RM<i/> values in excess of <i>≈<i/>6000 rad m<sup>-2<sup/>. The locations of the cores in 3C 318 and 3C 343 are unclear. <i>Conclusions. <i/>The available data about mas-scale rest-frame <i>RM<i/> estimates for CSS sources show that these have a wide range of values extending up to ~40 000 rad m<sup>-2<sup/> in the central region of OQ172, and may be located at projected distances from the core of up to ~1600 pc, as in 3C 43 where this feature has a rest-frame <i>RM<i/> of ~14 000 rad m<sup>-2<sup/>. The <i>RM<i/> estimates for the cores of core-dominated radio sources indicate that in addition to responding to an overall density gradient of the magneto-ionic medium, geometry, orientation, and modes of fuelling may also play a significant role. In addition to these effects, the high values of <i>RM<i/> in CSS sources are possibly caused by dense clouds of gas interacting with the radio jets. The observed distortions in the radio structures of many CSS sources are consistent with this interpretation.

Understanding BL Lacertae objects

Context. We present evidence that parsec-scale jets in BL Lac objects may be significantly distinct in kinematics from their counterparts in quasars. We argued this previously for the BL lac sources 1803+784 and 0716+714, report here a similar pattern for another well-known BL Lac object, PKS 0735+178, whose nuclear jet is found to exhibit kinematics atypical of quasars. Aims. A detailed study of the jet components' motion reveals that the standard AGN paradigm of apparent superluminal motion does not always describe the kinematics in BL Lac objects. We study 0735+178 here to augment and improve the understanding of the peculiar motions in the jets of BL Lac objects as a class. Methods. We analyzed 15 GHz VLBA (Very Long Baseline Array) observations (2cm/MOJAVE survey) performed at 23 epochs between 1995.27 and 2008.91. Results. We found a drastic structural mode change in the VLBI jet of 0735+178, between 2000.4 and 2001.8 when its twice sharply bent trajectory turned into a linear shape.We further found that this jet had undergone a similar transition sometime between December 1981 and June 1983. A mode change, occurring in the reverse direction (between mid-1992 and mid-1995) has already been reported in the literature. These structural mode changes are found to be reflected in changed kinematical behavior of the nuclear jet, manifested as an apparent superluminal motion and stationarity of the radio knots. In addition, we found the individual mode changes to correlate in time with the maxima in the optical light curve. The last two transitions occurred before a (modest) radio flare. The behavior of this pc-scale jet appears to favor a scenario involving non-ballistic motions of the radio knots, produced by the precession of a continuous jet within the ambient medium.

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.

Imaging the oxygen-rich disk toward the silicate carbon star EU Andromedae

Context. Silicate carbon stars are characterized by oxygen-rich circumstellar environments as revealed by prominent silicate emission, despite their carbon-rich photospheres. While the presence of a circumbinary disk or a disk around an unseen, low-luminosity companion has been suggested to explain the peculiar dust chemistry, the origin of silicate carbon stars is still a puzzle to date. Aims. We present multi-epoch, high-angular resolution observations of 22 GHz H2O masers toward the silicate carbon star EU And to probe the spatio-kinematic distribution of oxygen-rich material. Methods. EU And was observed at three epochs (maximum time interval of 14 months) with the Very Long Baseline Array (VLBA). Results. Our VLBA observations of the 22 GHz H2O masers have revealed that the maser spots are distributed along a straight line across ∼20 mas, with a slight hint of an S-shaped structure. The observed spectra show three prominent velocity components at VLSR= −42, −38, and −34 km s −1 , with the masers in SW redshifted and those in NE blueshifted. The maser spots located in the middle of the overall distribution correspond to the component at VLSR = −38 km s −1 , which approximately coincides with the systemic velocity. These observations can be interpreted as either an emerging helical jet or a disk viewed almost edge-on (a circumbinary or circum-companion disk). However, the outward motion measured in the VLBA images taken 14 months apart is much smaller than what is expected from the jet scenario. Furthermore, the mid-infrared spectrum obtained with the Spitzer Space Telescope indicates that the 10 µm silicate emission is optically thin and the silicate grains are of sub-micron size. This lends support to the presence of a circum-companion disk, because an optically thin circumbinary disk consisting of such small grains would be blown away by the intense radiation pressure of the primary (carbon-rich) star. If we assume Keplerian rotation for the circum-companion disk, the mass of the companion is estimated to be 0.5–0.8 M� . We also identify CO2 emission features at 13–16 µm in the Spitzer spectrum of EU And–the first unambiguous detection of CO2 in silicate carbon stars.

Multi-epoch VLBA observations of 3C 66A

We present the results of six-epoch Very Long Baseline Array (VLBA) observations of 3C 66A. The high-resolution Very Long Baseline Interferometer (VLBI) maps obtained at multi-frequency (2.3, 8.4, and 22.2 GHz) simultaneously enabled us to identify the brightest compact component with the core. We find that the spectrum of the core can be reasonably fitted by the synchrotron self-absorption model. Our VLBA maps show that the jet of 3C 66A has two bendings at about 1.2 and 4 mas from the core. We also give possible identifications of our jet components with the components in previous VLBA observations by analysing their proper motions. We find consistent differences of the position from the core in one component between different frequencies at six epochs.

VLBA Measurement of the Transverse Velocity of the Magnetar XTE J1810−197

We have obtained observations of the magnetar XTE J1810-197 with the Very Long Baseline Array (VLBA) at two epochs separated by 106 days, at wavelengths of 6 and 3.6 cm. Comparison of the positions yields a proper-motion value of 13.5 ± 1.0 mas yr-1 at an equatorial position angle of 209.4° ± 2.4° (east of north). This value is consistent with a lower significance proper-motion value derived from infrared observations of the source over the past 3 years, also reported here. Given its distance of 3.5 ± 0.5 kpc, the implied transverse velocity corrected to the local standard of rest is 212 ± 35 km s-1 (1 σ). The measured velocity is slightly below the average for normal young neutron stars, indicating that the mechanism(s) of magnetar birth need not lead to high neutron star velocities. We also use Australia Telescope Compact Array, Very Large Array, and these VLBA observations to set limits on any diffuse emission associated with the source on a variety of spatial scales, concluding that the radio emission from XTE J1810-197 is >96% pulsed.

A documentary of high-mass star formation: Probing the dynamical evolution of Orion Source I on 10–100 AU scales using SiO masers

AbstractA comprehensive picture of high-mass star formation has remained elusive, in part because examples of high-mass young stellar objects (YSOs) tend to be relatively distant, deeply embedded, and confused with other emission sources. These factors have impeded dynamical investigations within tens of AU of high-mass YSOs—scales that are critical for probing the interfaces where outflows from accretion disks are launched and collimated. Using observations of SiO masers obtained with the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), the KaLYPSO project is overcoming these limitations by mapping the structure and dynamical/temporal evolution of the material 10-1000 AU from the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data include ~40 epochs of VLBA observations over a several-year period, allowing us to track the proper motions of individual SiO maser spots and to monitor changes in the physical conditions of the emitting material with time. Ultimately these data will provide 3-D maps of the outflow structure over approximately 30% of the outflow crossing time. Here we summarize recent results from the KaLYPSO project, including evidence that high-mass star formation occurs via disk-mediated accretion.

Quasi-simultaneous five-frequency VLBA observations of PKS 0528+134

We present results of Very Long Baseline Array (VLBA) observations of PKS 0528+134 at five frequencies (2.3, 5.0, 8.4, 15.4, and 22.2 GHz). These quasi-simultaneous data enable us to study the spectral distribution of Very Long Baseline Interferometer (VLBI) components for the first time in this highly variable source, from which the central compact core is identified. Our observations indicate that there are two bendings for the jet motion at parsec scale. We provide an approximate spatial fit to the curved jet trajectory using the Steffen et al. (\cite{Steffen95}) helical model. We further investigate the proper motions of three jet components, which all show superluminal motion. At high frequencies (15.4 and 22.2 GHz) we detected a new component, which is estimated to be related to a radio burst peaking at about 2000.

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine-Scale Structure

We have examined the compact structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz. With projected baselines out to 440 Mλ, we are able to investigate source structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for short-wavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes. We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron–emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 1011 and 1013 K, but they extend up to 5 × 1013 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.

The low-power nucleus of PKS 1246-410 in the Centaurus cluster

We present Chandra, Very Large Array (VLA), and Very Long Baseline Array (VLBA) observations of the nucleus of NGC 4696, a giant elliptical in the Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS 1246-410 in the Centaurus cluster is a nearby example of a radio galaxy in a dense cluster environment. In analyzing the new X-ray data we have found a compact X-ray feature coincident with the optical and radio core. While nuclear emission from the X-ray source is expected, its luminosity is low, <10^{40} erg/s. We estimate the Bondi accretion radius to be 30 pc and the accretion rate to be 0.01 M_sun/yr which under the canonical radiative efficiency of 10% would overproduce by 3.5 orders of magnitude the radiative luminosity. Much of this energy can be directed into the kinetic energy of the jet, which over time inflates the observed cavities seen in the thermal gas. The VLBA observations reveal a weak nucleus and a broad, one-sided jet extending over 25 parsecs in position angle -150 degrees. This jet is deflected on the kpc-scale to a more east-west orientation (position angle of -80 degrees).

VSOP Observations of the Gamma-Ray Sources J1733−1304, J1625−2527, and J1015+4926

Abstract We present images from VSOP (VLBI Space Observatory Programme) and VLBA (Very Long Baseline Array) observations of the active galactic nuclei J1733$-$1304, J1625$-$2527, and J1015$+$4926. These three sources have been identified as possible counterparts to gamma-ray sources in the third EGRET catalog. In general, the counterparts to extragalactic EGRET sources have very compact structure, with brightness temperatures ($T_{\mathrm{b}}$) greater than $10^{11} \hbox{--} 10^{12} \,\mathrm{K}$. Evidence for compactness in gamma-ray emitting AGNs is also given by the variability seen from gamma-ray to radio frequencies. In this report, we compare the images of the three sources with previously published images and also examine jet motions and spectral indices. The measured $T_{\mathrm{b}}$ of J1733$-$1304 are an order of magnitude lower than previous $T_{\mathrm{b}}$ measurements, suggesting that the VSOP observations were made while the source was in a relatively quiescent state after the radio outburst of 1995–1997. The relatively low $T_{\mathrm{b}}$ of J1625$-$2527 is also consistent with the highly variable nature of this source. A comparison of the VSOP observation of J1015$+$4926 with a VLBA image made 2.2 yr earlier suggests the existence of stationary jet components in this source.

High‐Resolution Radio Observations of the Colliding‐Wind Binary WR 140

Milliarcsecond resolution Very Long Baseline Array (VLBA) observations of the archetype W-R+O star colliding-wind binary (CWB) system WR 140 are presented for 23 epochs between orbital phases 0.74 and 0.97. At 8.4 GHz, the emission in the wind-collision region (WCR) is clearly resolved as a bow-shaped arc that rotates as the orbit progresses. We interpret this rotation as due to the O star moving from southeast to approximately east of the W-R star, which leads to solutions for the orbital inclination of 122° ± 5°, longitude of the ascending node of 353° ± 3°, and an orbit semimajor axis of 9.0 ± 0.5 mas. The distance to WR 140 is determined to be 1.85 ± 0.16 kpc, which requires the O star to be a supergiant. The inclination implies that the mass of the W-R and O star is 20 ± 4 and 54 ± 10 M☉, respectively. We determine a wind momentum ratio of 0.22, with an expected half-opening angle for the WCR of 63°, consistent with 65° ± 10° derived from the VLBA observations. Total flux measurements from Very Large Array (VLA) observations show that the radio emission from WR 140 is very closely the same from one orbit to the next, pointing strongly toward emission, absorption, and cooling mechanism(s) that are controlled largely by the orbital motion. The synchrotron spectra evolve dramatically through the orbital phases observed, exhibiting both optically thin and optically thick emission. We discuss a number of absorption and cooling mechanisms that may determine the evolution of the synchrotron spectrum with orbital phase.

Multiepoch VLBA Observations of T Tauri South

In this Letter, we present a series of seven observations of the compact, nonthermal radio source associated with T Tauri South made with the Very Long Baseline Array (VLBA) over the course of 1 year. The emission is found to be composed of a compact structure most certainly originating from the magnetosphere of an underlying pre-main-sequence star and a low brightness extension that may result from reconnection flares at the star-disk interface. The accuracy of the absolute astrometry offered by the VLBA allows us to make very precise determinations of the trigonometric parallax and proper motion of T Tau South. The proper motion derived from our VLBA observations agrees with that measured with the Very Large Array over a similar period to better than 2 mas yr-1, and it is fully consistent with the infrared proper motion of T Tau Sb, the pre-main-sequence M star with which the radio source has traditionally been associated. The parallax, π = 7.07 ± 0.14 mas, corresponds to a distance of 141.5 pc.

VLBA Polarization Observations of Markarian 421 after a Gamma‐Ray High State

We present four high dynamic range, dual-circular polarization, Very Long Baseline Array (VLBA) observations at 22 GHz of Markarian 421, taken throughout the year following the source's unprecedented gamma-ray high state in early 2001. These four new VLBA observations are combined with data from our earlier 1999 paper and archival VLBA data-sets that have become available since 1999 to produce a combined 28 epoch VLBA data-set on Mrk 421 spanning the years 1994 to 2002. No new component associated with the 2001 flares was seen on the total intensity images, but the combined data-set allowed precise measurements of the apparent speeds of the existing components. The peak measured apparent speed was for component C5, which has an apparent speed of 0.1 +/- 0.02 c (H_0=71 km s^{-1} Mpc^{-1}, Omega_m=0.27, and Omega_Lambda=0.73). No counterjet is seen with a limit on the jet to counterjet brightness ratio J >~ 100. These observed VLBI properties of Markarian 421 are consistent with a jet with a bulk Lorentz factor of about 2 and an angle to the line-of-sight of about 1 degree, suggesting a jet that decelerates between the gamma-ray producing region and the parsec scale. The VLBI core and inner jet (component C7) have fractional polarizations of about 5%, and an electric vector position angle (EVPA) aligned with the jet axis. Component C5 (at 1.5 mas from the core) has a higher fractional polarization of about 15%, and an EVPA nearly orthogonal to the jet axis. Significant variability is detected in the EVPA of component C6, which at two of the four epochs shows an EVPA aligned with the jet axis, possibly a sign of propagating disturbances that are only visible on the polarization images. If these propagating disturbances are linked to the 2001 gamma-ray high state, then their inferred apparent speed is between 1 and 3 c.

Time‐sequenced Multi–Radio Frequency Observations of Cygnus X‐3 in Flare

Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over three decades in frequency, over a period of six days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to nu^{-0.6}, after which the spectral index remains at this latter terminal value; a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are approximately 8 milliarcseconds. The light-crossing time within these plasmons is of the same order as the time-scale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons which are initially optically thick, but which after expansion become optically thin, explains the key features of the spectral evolution, for example the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is roughly beta = 0.63 and the precession period is about 5 days, significantly lower than fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero-separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cygnus X-3.

Parsec-Scale Radio Structure of the Double Active Nucleus of NGC 6240

NGC 6240 is a luminous infrared galaxy that, based on recent MERLIN and Chandra observations, appears to host a pair of active galactic nuclei. We present new multiepoch and multifrequency radio observations of NGC 6240 obtained using the Very Long Baseline Array (VLBA). These observations resolve out all of the diffuse radio continuum emission related to starburst activity to reveal three compact sources, two associated with the infrared and X-ray nuclei: components N1, the northern nucleus, and S, the southern nucleus; and component S1, a weak point source lying northeast of component S. The radio continuum properties of N1 and S both resemble those of compact radio sources in Seyfert nuclei, including an inverted spectrum at low frequencies and high brightness temperatures. Component N1 further resolves into a 9 pc linear source, again resembling compact jets in some well-studied Seyfert nuclei. The inverted radio spectrum is most likely caused by free-free absorption through foreground plasma with emission measures on the order of 107 cm-6 pc, consistent with the X-ray spectroscopy, which implies the presence of a foreground, Compton-thick absorber. Synchrotron self-absorption may also contribute to the flat spectrum of component S. Component S1 better resembles a luminous radio supernova such as those detected in Arp 220, although we cannot specifically rule out the possibility that S1 may be ejected material from the southern active nucleus. We demonstrate that the light curve of S1 is at least consistent with current radio supernova (RSN) models, and given the high star formation rate in the nuclear region and the (possible) proclivity of infrared merger galaxies to produce luminous RSNe, it is not surprising that one such supernova is detected serendipitously in the VLBA observations.

Phase‐Referenced Very Long Baseline Array Observations of OH Masers at 4765 MHz

We report Very Long Baseline Array (VLBA) observations of maser emission from the rotationally excited 2 � 1=2; J ¼ 1=2 state of OH at 4765 MHz. We made phase-referenced observations of W3(OH) at both 4765 and 1720 MHz and found emission in three fields within a � 2000 AU diameter region and verified that in two of the three fields, 4765 and 1720 MHz emission arise from the same position to within � 4 mas (� 8 AU). We imaged DR 21EX without phase referencing and detected six � 5 AU diameter emission regions along an approximately north-south arc with linear extent � 500 AU. In addition, we carried out phase-referenced observations of 4765 MHz emission from K3-50. We searched for the 4765 MHz line in W49 (without phase referencing) and W75N (phase-referenced to the strongest 4765 MHz maser feature in DR 21EX); we were unable to detect these sources with the VLBA. For 2 1 years (including the dates of the VLBA observations), we carried out monitoring observations of 4765 MHz emission with the Very Lary Array. Constraints on models for maser emission at 1720 and 4765 MHz are derived from the observations. These observations are then briefly compared with existing models. Subject headings: masers — techniques: interferometric — radio lines: general