GHz VLBI Images Research Articles

VLBI FOR GRAVITY PROBE B . II. MONITORING OF THE STRUCTURE OF THE REFERENCE SOURCES 3C 454.3, B2250+194, AND B2252+172

We used 8.4 GHz VLBI images obtained at up to 35 epochs between 1997 and 2005 to examine the radio structures of the main reference source, 3C 454.3, and two secondary reference sources, B2250+194 and B2252+172, for the guide star for the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in 2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz images for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We identified at each epoch a component, C1, near the easternmost edge of the core region. Simulations of the core region showed that C1 is located, on average, 0.18 +- 0.06 mas west of the unresolved "core" identified in 43 GHz images. We also identified in 3C 454.3 at 8.4 GHz several additional components which moved away from C1 with proper motions ranging in magnitude between 0.9c and 5c. The detailed motions of the components exhibit two distinct bends in the jet axis located ~3 and ~5.5 mas west of C1. The spectra between 5.0 and 15.4 GHz for the "moving" components are steeper than that for C1. The 8.4 GHz images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal compact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 is inverted while that for B2252+172 is flat. Based on its position near the easternmost edge of the 8.4 GHz radio structure, close spatial association with the 43 GHz core, and relatively flat spectrum, we believe 3C 454.3 component C1 to be the best choice for the ultimate reference point for the GP-B guide star. The compact structures and inverted to flat spectra of B2250+194 and B2252+172 make these objects valuable secondary reference sources

The follow-up European VLBI Network observations of twelve GPS radio sources at 5 GHz

We defined a sub-sample of twelve GPS sources which have not been observed with the VLBI before, from the Parkes half-Jansky sample, and carried out VLBI observations at 1.6 GHz and 5 GHz with the European VLBI Network (EVN) in 2006 and 2008, respectively, to classify the source structure and to find compact symmetric objects (CSOs). Additionally, we carried out the 4.85 GHz flux density observations for these sources with the Urumqi 25-m telescope between the years 2007 and 2009 to study whether there is any variability in the total flux density of the GPS sources. The results of the 5 GHz VLBI observations and total flux densities of these sources are presented in this paper. From the VLBI morphologies, the spectral indices of components and the total flux variability of the twelve targets, we firmly classify three sources J0210+0419, J1135$-$0021, and J2058+0540 as CSOs, and classify J1057+0012, J1203+0414, and J1600$-$0037 as core-jet sources. The others J0323+0534, J0433$-$0229, J0913+1454, J1109+1043, and J1352+0232 are labelled CSO candidates, and J1352+1107 is a complex feature. Apart from core-jet sources, the total flux densities of the CSOs and candidates are quite stable at 5 GHz both during a long-term of $\sim$20 years relative to the PKS90 data and in a period between 2007 and 2009. The total flux densities are resolved-out by more than 20\% in the 5 GHz VLBI images for 6 sources, probably because of diffuse emission. In addition, we estimated the jet viewing angles $\Theta$ for the confirmed CSOs by using the double-lobe flux ratio of the sources, the result being indicative of relatively large $\Theta$ for the CSOs.

VLBI observations of nineteen GHz-peaked-spectrum radio sources at 1.6 GHz

. We present the results of VLBI observations of nineteen GHz-peaked-spectrum (GPS) radio sources at 1.6 GHz. Of them, 15 sources are selected from the Parkes half-jansky (PHJ) sample (Snellen et al. 2002, MNRAS, 337, 981), and 4 others are from our previous observation list. We aimed at imaging the structure of GPS sources, searching for compact symmetric objects (CSOs) and studying the absorption for the convex radio spectra of GPS sources. . We obtained total-intensity 1.6 GHz VLBI images of 17 sources for the first time. Of them, 80% show a mini-double-lobe radio structure, indicating that they are CSOs or candidates, and their host AGNs could be edge-on to us. This result suggests that there is a high incidence of mini double-lobe sources (or CSOs) in the PHJ sample. The sources J0323+0534, J1135-0021, J1352+0232, J2058+0540, J2123-0112, and J2325-0344 are classified as CSOs with measured redshift, showing double-lobe structure with sizes of <1 kpc. Three sources J1057+0012, J1600-0037, and J1753+2750 are considered as core-jet sources according to their morphologies and flux variability.

Ten Milliparsec-Scale Structure of the Nucleus Region in Centaurus A

Abstract We present the results of a VLBI Space Observatory Programme (VSOP) observation of the subparsec structure in Cen A at 4.9 GHz. The observation produced an image of the subparsec jet components with a resolution of three-times better than images from previous VLBI monitoring campaigns at 8.4 GHz, and twice better than the previous 22 GHz studies. Owing to its proximity, our Cen A space-VLBI image is one of the highest spatial-resolution images of an AGN ever made—0.01 pc per beam—comparable only to the recent 43 GHz VLBI images of M87. The elongated core region is resolved into several components of over 10 milliarcsec long (0.2 pc), including a compact component of brightness temperature $2.2 \times 10^{10} \,\mathrm{K}$. A counterjet was detected: if we assume jet-counterjet symmetry, a relatively slow jet speed, and a large viewing angle, as derived from previous observations, the image allows us to investigate the distribution of ionized gas around the core, which is opaque at this frequency due to free-free absorption. We also analyzed the jet geometry in terms of collimation. Assuming the strongest component to be the core, the jet opening angle at $\sim 5000 \,r_{\mathrm{S}}$ from the core is estimated to be $\sim 12^\circ$, with the collimation of the jet to $\sim 3^\circ$ continuing out to $\sim 20000 \,r_{\mathrm{S}}$. This result is consistent with previous studies of the jet in M87, which favor MHD disk outflow models. Future space VLBI observations at higher frequencies will probably be able to image the collimation region, within $1000 \, r_{\mathrm{S}}$ of the center of Cen A, together with the accretion disk itself.

Using VLBI to Probe the Orion KL Outflow on AU Scales

We present the first contemporaneous 43GHz and 86GHz VLBI images of the v=1 J=2-1 and J=1-0 SiO masers in the Orion-KL nebula. Both maser species exhibit the same general morphology of earlier J=1-0 maser images which appear to trace the edges of a bi-polar conical outflow. Surprisingly, the J=2-1 masers form further from the central protostar than the J=1-0 masers, a fact not readily explained by current SiO maser pumping models. The average magnitude of offsets between corresponding regions of the two masing transitions is approximately 14% of the total radial extent of the SiO maser emission. This offset indicates that each transition must trace different physical conditions.

Deep 8.4 GHz VLBI Images of Seven Faint Nuclei in Lobe‐dominated Quasars

We present deep 8.4 GHz VLBI images of the faint nuclei in seven lobe-dominated quasars (LDQs): 3C 175, 3C 204, 3C 205, 3C 215, 3C 351, 0821)447, and 0839)616 . These images, made from data obtained in 1988 and 1991 with high-sensitivity VLBI arrays, have a typical rms noise level of D0.03 mJy per beam and resolution of D0.5 mas. The parsec-scale jet structures are generally greater in extent and complexity than those seen on many previous, less sensitive maps of other LDQ nuclei; the jet in 3C 204 bends through at least 10i, and the jet in 3C 205 extends out to D5 mas. Observations of 3C 204, 3C 205, and 0839)616 at two epochs reveal clear —ux density variability in 3C 204 and 3C 205, but none of these sources shows any evidence for outward motion of jet components; their mean appar- ent transverse velocity is ((0.47 ^ 0.43)h~1c, from which we determine a 3 p upper limit on their mean expansion speed of D0.8 h~1c km s~1 Mpc~1, The innermost jet knot in 3C 204 (H 0 100 hq 0 0.5). underwent a marginal position angle swing away from the mean jet axis. We also present the —rst 8.4 GHz A-array VLA images for all the objects except 3C 351. All the sources except 0821)447 display one-sided VLBI and VLA jets, which lie on the same side of the VLBI core and are generally well aligned (to within D5i). In addition, we analyze the statistics of projected linear size, relative nuclear strength, and misalignment angle for the 3CR and Jodrell Bank LDQ samples. We conclude that our results are generally consistent with a fairly simple relativistic jet model and with the uni—cation of core- dominated quasars, lobe-dominated quasars, and possibly even powerful radio galaxies; there is a mar- ginal suggestion that the quasars in our samples have a restricted range of angles to the line of sight (e.g., 0i to D45i). Subject headings: galaxies: jetsgalaxies: structurequasars: generalradio continuum: galaxies ¨ techniques: interferometric

Geodetic VLBI Observations of EGRET Blazars

We present VLBI observations of the EGRET quasars 0202+149, CTA 26, and 1606+106, as well as additional analysis of VLBI observations of 1156+295 presented in Piner & Kingham (1997b). We have produced 8 and 2 GHz VLBI images at 11 epochs, 8 epochs, and 12 epochs, spanning the years 1989 to 1996, of 0202+149, CTA 26, and 1606+106 respectively. The VLBI data have been taken from the Washington VLBI correlator's geodetic database. We have measured the apparent velocities of the jet components and find that CTA 26 and 1606+106 are superluminal sources, with average apparent speeds of 8.9 and 2.9 h^{-1}c respectively (H_{0}=100h km s^{-1} Mpc^{-1}, q_{0}=0.5). The components in 0202+149 are stationary, and we identify this source as a compact F double. These sources all have apparently bent jets, and we detected non-radial motion of components in CTA 26 and 1156+295. We have not yet detected any components emerging subsequent to the gamma-ray flares in CTA 26, 1156+295, and 1606+106, and we derive lower limits on the ejection times of any such components. The misalignment angle distribution of the EGRET sources is compared to the distribution for blazars as a whole, and we find that EGRET sources belong preferentially to neither the aligned nor the misaligned population. We also compare the average values for the apparent velocities and Doppler beaming factors for the EGRET and non-EGRET blazars, and find no significant differences. We thus find no indication, within the measurement errors, that EGRET blazars are any more strongly beamed than their counterparts which have not been detected in gamma-rays.

VLBI Observations of Supernova 1993J: The First 1000 Days

AbstractWe report on phase-referenced 5 and 8.4 GHz VLBI images of supernova SN 1993J in M81, tracking the evolution of the source from 50 to 1000 days after the explosion.

A Slower Superluminal Velocity for the Quasar 1156+295

As part of an ongoing effort to observe high energy gamma-ray blazars with VLBI, we have produced 8 and 2 GHz VLBI images, at ten epochs spanning the years 1988 to 1996, of the quasar 1156+295. The VLBI data have been taken from the Washington VLBI correlator's geodetic database. We have detected detected four components and have measured their apparent speeds to be 8.8 +/- 2.3, 5.3 +/- 1.1, 5.5 +/- 0.9, and 3.5 +/- 1.2 h^{-1}c from the outermost component inwards. (H_{0}=100h km/(s Mpc), q_{0}=0.5 throughout paper). These velocities contradict a previously published very high superluminal velocity of 26 h^{-1}c for this source.

VLBI Observations of the Gamma‐Ray Blazar 1611+343

As part of an effort to observe high-energy γ-ray blazars with VLBI, we have produced 8 and 2 GHz VLBI images, at eight epochs, of the quasar 1611+343, which has been detected by the EGRET telescope on board the Compton Gamma Ray Observatory. The VLBI data have been taken from the geodetic database of the Washington VLBI correlator. As is expected for a γ-ray quasar, we find that 1611+343 possesses a jet with several compact components moving at apparent superluminal speeds. We have detected five components and have measured speeds for the outer four of 6.7 ± 1.6, 3.8 ± 1.4, 7.6 ± 1.3, and 11.5 ± 2.3 h-1c from the outermost component inward. (H0 = 100 h km s-1 Mpc-1, q0 = 0.5.) By comparing the apparent superluminal expansion speeds with a lower limit to the Doppler beaming factor derived from X-ray observations, we have calculated an upper limit on the angle of the jet to the line of sight of θ ≤ 68 and a lower limit to the Lorentz factor of the components of Γ ≥ 7.1. We have mapped the two-dimensional projected structure of the VLBI jet and clearly see different trajectories for different components at the same distance from the core, which rules out a continuous jet model for this source. A possible connection is found between enhanced levels of γ-ray activity and the ejection of new VLBI components from the core.