Xiaoyu Hong

Same-beam VLBI observations of SELENE for improving lunar gravity field model

The Japanese lunar mission, Selenological and Engineering Explorer (Kaguya), which was successfully launched on 14 September 2007, consists of a main satellite and two small satellites, Rstar and Vstar. Same-beam very long baseline interferometry (VLBI) observations of Rstar and Vstar were performed for 15.4 months from November 2007 to February 2009 using eight VLBI stations. In 2008, S band same-beam VLBI observations totaling 476 h on 179 days were undertaken. The differential phase delays were successfully estimated for most ( about 85%) of the same-beam VLBI observation periods. The high success rate was mainly due to the continuous data series measuring the differential correlation phase between Rstar and Vstar. The intrinsic measurement error in the differential phase delay was less than 1 mm RMS for small separation angles and increased to approximately 2.5 mm RMS for the largest separation angles ( up to 0.56 deg). The long-term atmospheric and ionospheric delays along the line of sight were reduced to a low level ( several tens of milimeters) using the same-beam VLBI observations, and further improved through application of GPS techniques. Combining the eight-station ( four Japanese telescopes of VLBI Exploration of Radio Astrometry and four international telescopes) S band same-beam VLBI data with Doppler and range data, the accuracy of the orbit determination was improved from a level of several tens of meters when only using Doppler and range data to a level of 10 m. As a preliminary test of the technique, the coefficient sigma degree variance of the lunar gravity field was compared with and without 4 months of VLBI data included. A significant reduction below around 10 deg ( especially for the second degree) was observed when the VLBI data were included. These observations confirm that the VLBI data contribute to improvements in the accuracy of the orbit determination and through this to the lunar gravity field model.

Simultaneous multiwavelength observations of sagittarius A

We observed Sgr A* using the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) at multiple centimeter and millimeter wavelengths on 2003 June 17. The measured flux densities of Sgr A*, together with those obtained from the Submillimeter Array (SMA) and the Keck II 10 m telescope on the same date, are used to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 mu m. The simultaneous spectrum shows a spectral break at about 3.6 cm, a possible signature of synchrotron self-absorption of the strong radio outburst that occurred near epoch 2003 July 17. At 90 cm, the flux density of Sgr A* is 0.22 +/- 0.06 Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum at long cm wavelengths appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff similar to 100 cm. This cutoff wavelength appears to be three times longer than that of similar to 30 cm suggested by Davies, Walsh, & Booth based on observations in 1974 and 1975. Our analysis suggests that the flux densities of Sgr A* at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars close to Sgr A*.

VLBI OBSERVATIONS OF 10 COMPACT SYMMETRIC OBJECT CANDIDATES: EXPANSION VELOCITIES OF HOT SPOTS

Observations of 10 Compact Symmetric Object (CSO) candidates have been made with the Very Long Baseline Array (VLBA) at 8.4 GHz in 2005 and with a combined Chinese and European Very Long Baseline Interferometry (VLBI) array at 8.4 GHz in 2009. The 2009 observations incorporate for the first time the two new Chinese telescopes at Miyun and Kunming for international astrophysical observations. The observational data, in combination with archival VLBA data from previous epochs, have been used to derive the proper motions of the VLBI components. Because of the long time baseline of similar to 16 years of the VLBI data sets, the expansion velocities of the hot spots can be measured at an accuracy as high as similar to 1.3 mu as yr(-1). Six of the ten sources are identified as CSOs with a typical double or triple morphology on the basis of both spectral index maps and their mirror symmetry of proper motions of the terminal hot spots. The compact double source J1324+4048 is also identified as a CSO candidate. Among the three remaining sources, J1756+5748 and J2312+3847 are identified as core-jet sources with proper motions of their jet components relating to systemic source expansion. The third source J0017+5312 is likely also a core-jet source, but a robust detection of a core is needed for an unambiguous identification. The kinematic ages of the CSOs derived from proper motions range from 300 to 2500 years. The kinematic age distribution of the CSOs confirm an overabundance of compact young CSOs with ages less than 500 years. CSOs with known kinematic ages may be used to study the dynamical evolution of extragalactic radio sources at early stages.

The Inhomogeneous Jet Parameters in Active Galactic Nuclei

The Konigl inhomogeneous jet model is applied to investigate the properties of the jets in active galactic nuclei (AGNs). A sample of AGNs that includes measurements of the angular size and radio flux density of the VLBI core, proper motion of the components in the jet, and X-ray flux density is collected. The inhomogeneous jet parameters are derived with the same assumptions for all sources. A comparison among the parameters of different types of sources in the sample is presented. It is found that most of EGRET sources have higher Doppler factors delta, larger Lorentz factors gamma, and smaller viewing angles theta as compared to the other sources in the sample. The statistical analyses show that the derived Doppler factor delta is strongly correlated with the observed 22 GHz brightness temperature. Furthermore, there is a correlation between the relative gamma-ray luminosity and the Doppler factor delta. The implications of these results are discussed.

Kinematics of the parsec‐scale radio jet in 3C 48

We present results on the compact steep-spectrum quasar 3C 48 from observations with the Very Long Baseline Array (VLBA), the Multi-Element Radio Linked Interferometer Network (MERLIN) and the European Very long baseline interferometry (VLBI) Network (EVN) at multiple radio frequencies. In the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright knots. The active nucleus is embedded in the southernmost VLBI component A, which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz, respectively. A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly polarized VLBI components are located at component C ~ 0.25 arcsec north of the core, where the jet starts to bend to the north-east. The polarization angles at C show gradual changes across the jet width at all observed frequencies, indicative of a gradient in the emission-weighted intrinsic polarization angle across the jet and possibly a systematic gradient in the rotation measure; moreover, the percentage of polarization increases near the curvature at C, likely consistent with the presence of a local jet–interstellar-medium interaction and/or changing magnetic-field directions. The hot spot B shows a higher rotation measure, and has no detected proper motion. These facts provide some evidence for a stationary shock in the vicinity of B. Comparison of the present VLBI observations with those made 8.43 yr ago suggests a significant northward motion for A2 with an apparent transverse velocity β_(app)= 3.7 ± 0.4c. The apparent superluminal motion suggests that the relativistic jet plasma moves at a velocity of ≳ 0.96c if the jet is viewed at an inclination angle less than 20°. A simple precessing jet model and a hydrodynamical isothermal jet model with helical-mode Kelvin–Helmholtz instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined by the bright knots.

The radio structure of 3C 316, a galaxy with double-peaked narrow optical emission lines

The galaxy 3C 316 is the brightest in the radio band among the optically selected candidates exhibiting double-peaked narrow optical emission lines. Observations with the Very Large Array, Multi-Element Remotely Linked Interferometer Network (e-MERLIN), and the European Very Long Baseline Interferometry Network (EVN) at 5 GHz have been used to study the radio structure of the source in order to determine the nature of the nuclear components and to determine the presence of radio cores. The e-MERLIN image of 3C 316 reveals a collimated coherent east-west emission structure with a total extent of about 3 kpc. The EVN image shows seven discrete compact knots on an S-shaped line. However, none of these knots could be unambiguously identified as an active galactic nucleus (AGN) core. The observations suggest that the majority of the radio structure belongs to a powerful radio AGN, whose physical size and radio spectrum classify it as a compact steep-spectrum source. Given the complex radio structure with radio blobs and knots, the possibility of a kpc-separation dual AGN cannot be excluded if the secondary is either a naked core or radio quiet.

Kinematics of the compact symmetric object OQ 208 revisited

Aims. A long-timeline kinematic study of the archetypal compact symmetric object (CSO) OQ 208 sheds light on the physical properties of the most compact radio sources.

Two Classes of Radio Flares in the Blazar PKS 0420–014

The two 5 GHz very long baseline interferometry (VLBI) observations (1996 June and 1997 November) presented in this Letter, combined with several former VLBI observations at 8.4 and 5 GHz, suggest that the radio flares of the blazar PKS 0420-014 can be divided into two classes according to their geometric origins in 5 or 8.4 GHz VLBI maps and the properties of light curves. One class of flares, which we call core flares, originate from the core. Core flares have large lags between the light curves at different frequencies and will probably lead to the ejection of new jet components. The other class of flares, which we call jet flares, come from jet components. Jet flares vary simultaneously at different wavelengths and may due to the Doppler boosting effect of rotating knots moving along a helical jet. The radio flare in 1991, accompanied by a simultaneous gamma-ray flare, was identified as a core flare.

Radio structure of the blazar 1156 + 295 with sub-pc resolution

Aims. 1156+295 is a flat-spectrum quasar that is loud at both radio and gamma-ray wavelengths. Previous observations of the source revealed a radio morphology on pc to kpc scales consistent with a helical jet model. We carried out VLBA observations at centimeter and millimeter wavelengths to study the structure of the innermost jet and understand the relation between the helical structure and astrophysical processes in the central engine.

Four dual AGN candidates observed with the VLBA

According to hierarchical structure formation models, merging galaxies are expected to be seen in different stages of their coalescence. However, currently there are no straightforward observational methods neither to select nor to confirm a large number of dual active galactic nuclei (AGN) candidates. Most attempts involve the better understanding of double-peaked narrow emission line sources, to distinguish the objects where the emission lines originate from narrow-line kinematics or jet-driven outflows from those which might harbour dual AGN. We observed four such candidate sources with the Very Long Baseline Array (VLBA) at 1.5 GHz with