Z.-Q. Shen

The VSOP 5 GHz active galactic nucleus survey. IV. The angular size/brightness temperature distribution

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with submilliarcsecond angular resolution, using an orbiting 8 m telescope on board the satellite HALCA with a global Earth-based array of telescopes. A major program is the 5 GHz VSOP Survey Program, which we supplement here with Very Long Baseline Array observations to produce a complete and flux density - limited sample. Using statistical methods of analysis of the observed visibility amplitude versus projected (u, v) spacing, we have determined the angular size and brightness temperature distribution of bright radio emission from active galactic nuclei. On average, the cores have a diameter ( full width, half-power) of 0.20 mas, which contains about 20% of the total source emission, and 14% +/- 6% of the cores are less than 0.04 mas in size. About 20% +/- 5% of the radio cores have a source frame brightness temperature T-b > 1.0 x 10(13) K, and 3% +/- 2% have T-b > 1.0 x 10(14) K. A model of the high brightness temperature tail suggests that the radio cores have brightness temperatures approximate to1 x 10(12) K and are beamed toward the observer with an average bulk motion of beta = 0.993 +/- 0.004.

VLBI Observations of Southern EGRET Identifications. 1; PKS 0208-512, PKS 0521-365 and PKS 0537-441

We present high-resolution very long baseline interferometry images of three southern radio sources that the Energetic Gamma-Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, has identified as greater than 100 MeV gamma-ray sources. These are the first results in a continuing program of VLBI observations of southern EGRET identifications. For two of these sources, PKS 0208-512 (at 4.851 GHz) and PKS 0537-441 (at 4.851 and 8.418 GHz), the images represent first-epoch observations. For the remaining lower redshift object, PKS 0521-365, we present images from three epochs at 4.851 GHz and an image from one further epoch at 8.418 GHz, spanning approximately 1 yr. We discuss the need for further extensive VLBI observations of EGRET-identified radio sources.

The VSOP 5 GHz active galactic nucleus survey. V. Imaging results for the remaining 140 sources

In 1997 February, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Program (VSOP) mission. Approximately 25% of the mission time was dedicated to the VSOP survey of bright compact active galactic nuclei (AGNs) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in the third paper in the series, which contained 102 sources. For most sources, the plots of the (u, v) coverage, the visibility amplitude versus (u, v) distance, and the high-resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed.

The VSOP 5 GHz Active Galactic Nucleus Survey. III. Imaging results for the first 102 sources

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8 m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact active galactic nuclei (AGNs) that are stronger than 1 Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources. We present high-resolution images and plots of visibility amplitude versus projected baseline length. In addition, model-fit parameters to the primary radio components are listed, and from these the angular size and brightness temperature for the radio cores are calculated. For those sources for which we were able to determine the source frame core brightness temperature, a significant fraction (53 out of 98) have a source frame core brightness temperature in excess of 10(12) K. The maximum source frame core brightness temperature we observed was 1.2x10(13) K. Explaining a brightness temperature this high requires an extreme amount of relativistic Doppler beaming. Since the maximum brightness temperature one is able to determine using only ground-based arrays is of the order of 10(12) K, our results confirm the necessity of using space VLBI to explore the extremely high brightness temperature regime.

HIGH-RESOLUTION VLBA OBSERVATIONS OF THREE 7 mm SiO MASERS TOWARD VX Sgr AT FIVE EPOCHS

VX Sgr is a red supergiant at an adopted distance of 1.6 kpc with intense 43 GHz SiO maser emission. In this paper, we present the high-resolution very long baseline interferometry (VLBI) observations of SiO masers toward VX Sgr at five epochs. We used the Very Long Baseline Array to map the J = 1 -> 0 (upsilon = 1, 2) (SiO)-Si-28 masers and confirmed a ring-like structure. In the first two epochs, the upsilon = 1 masers form a ring, but upsilon = 2 maser spots residing only in the southern and northern regions do not form a complete ring. In the third epoch, the two masers are distributed in a ring structure and the upsilon = 2 masers are a bit closer to the central star. In the last two epochs, many new maser spots appear and overlap each other. These overlapping maser spots can be related to the shock waves and reflect the collisional pumping. We compare the observations with the pumping models and speculate that the real pumping mechanism may be complex in VX Sgr and vary with time. The J = 1 -> 0 (upsilon = 0) (SiO)-Si-29 line emission is also detected, but is too weak to produce any VLBI map.

Radio Compact Structure in OQ 208

In light of the newly detected proper motions in two lobes of the gigahertz-peaked spectrum source OQ 208, we introduced the Doppler effect into the fit to the observed spectral turnovers. It shows that two models containing the same beaming parameters but having quite different absorption mechanisms can fit two lobes equally well. In one model, only FFA (free-free absorption) is needed. In the other model, both the SSA (synchrotron self-absorption) in two lobes and an extra FFA toward the southwest lobe are required. Our analysis cannot unambiguously distinguish between two models. Although the FFA process is invoked in both models, the geometry of the absorbing gas could be quite different. However, the beaming effects in both models are eventually the same, with an intrinsically symmetrical advance speed of ≈0.33c for the two-sided lobes and an inclination angle of ≈33° for the radio jet emission to the observers line of sight.

The Milliarcsecond-Scale Jet in the Quasar J1625+4134

We present Very Long Baseline Array observations of the radio source J1625+4134 at 22 and 15 GHz and analyze them in concurrence with other existing VLBI data on this source. The high-resolution images at 15 and 22 GHz show a short and bending jet, which differs by about 270° in position angle with the northern jet detected at lower frequencies. The new high-resolution data, combined with the data available in the literature, allow us to estimate the spectral index of the components and to identify one of the compact components as the VLBI core based on its flat spectrum between 5 and 22 GHz. Relative to this core component, the jet appears to be bidirectional. The proper-motion measurement of the component C2 and the estimate of the Doppler boosting factor suggest that the orientation of the jet is close to the line of sight. The projection effect of an intrinsically sharply bending jet within a few milliarcseconds from the core or the erratic change in the nozzle direction of the jet may account for the uncommon bidirectional structure of the jet in J1625+4134.

The sub-millimeter emission and the visibility of Sagittarius A*

We adopt the ray-tracing algorithm to simulate the emission from Sagittarius (Sgr) A* with three theoretical models, i.e., the radiatively inefficient accretion flow, jet model, and MRI-driven Keplerian accretion flow with highly-magnetized plasma. By compared to the millimeter VLBI imaging and polarization results, our simulation imposes some constraints on the orientation of the central black hole with regard to the accretion flow or jet. These models can further predict black hole shadow images and corresponding visibility profiles along specific sub-millimeter VLBI baselines, providing reasonable explanations for the preliminary data at 1.3mm.

The VSOP Survey: final aggregate results

In February 1997 the Japanese radio astronomy satellite HALCA was launched to provide the space-borne element for the VSOP mission. HALCA provided linear baselines three-times greater than that of ground arrays, thus providing higher resolution and higher AGN brightness temperature measurements and limits. Twenty-five percent of the scientific time of the mission was devoted to the “VSOP survey” of bright, compact, extra-galactic radio sources at 5 GHz. A complete list of 294 survey targets were selected from pre-launch surveys, 91% of which were observed during the satellites lifetime. The major goals of the VSOP Survey are statistical in nature: to determine the brightness temperature and approximate structure, to provide a source list for use with future space VLBI missions, and to compare radio properties with other data throughout the electro-magnetic spectrum. All the data collected have now been analysed and is being prepared for the final image Survey paper. In this paper we present details of the mission, and some statistics of the images and brightness temperatures.

The VSOP 5 GHz AGN Survey II. Data Calibration and Imaging

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsecond angular resolution using an orbiting 8 m telescope, HALCA, and global arrays of Earth-based telescopes. Approximately 25% of the observing time has been devoted to a survey of compact active galactic nuclei (AGNs) at 5 GHz that are stronger than 1 Jy—the VSOP AGN Survey. This paper, the second in a series, describes the data calibration, source detection, self-calibration, imaging, and modeling and gives examples illustrating the problems specific to space VLBI. The VSOP Survey Web site, which contains all results and calibrated data, is described.