Oxygen-rich Asymptotic Giant Branch (AGB) stars can be intense emitters of SiO ($v$=1 and 2, J=1$rightarrow$0) and H$_2$O maser lines at 43 and 22 GHz, respectively. VLBI observations of the maser emission provide a unique tool to probe the innermost
layers of the circumstellar envelopes in AGB stars. Nevertheless, the difficulties in achieving astrometrically aligned water and $v$=1 and $v$=2 SiO maser maps have traditionally limited the physical constraints that can be placed on the SiO maser pumping mechanism. We present phase referenced simultaneous spectral-line VLBI images for the SiO $v$=1 and $v$=2, J=1$rightarrow$0, and H$_2$O maser emission around the AGB star R,LMi, obtained from the Korean VLBI Network (KVN). The simultaneous multi-channel receivers of the KVN offer great possibilities for astrometry in the frequency domain. With this facility we have produced images with bona-fide absolute astrometric registration between high frequency maser transitions of different species to provide the positions of the water maser emission, and the centre of the SiO maser emission, and hence reducing the uncertainty in the proper motion for R,LMi by an order of magnitude over that from Hipparcos. This is the first successful demonstration of source frequency phase referencing for mm-VLBI spectral-line observations and also where the ratio between the frequencies is not an integer.
We present our work towards using the Korean VLBI (Very Long Baseline Interferometer) Network (KVN) and VLBI Exploration of Radio Astronomy (VERA) arrays combined into the KVN and VERA Array (KaVA) for observations of radio pulsars at high frequencie
s ($simeq$22-GHz). Pulsar astronomy is generally focused at frequencies approximately 0.3 to several GHz and pulsars are usually discovered and monitored with large, single-dish, radio telescopes. For most pulsars, reduced radio flux is expected at high frequencies due to their steep spectrum, but there are exceptions where high frequency observations can be useful. Moreover, some pulsars are observable at high frequencies only, such as those close to the Galactic Center. The discoveries of a radio-bright magnetar and a few dozen extended Chandra sources within 15 arc-minute of the Galactic Center provide strong motivations to make use of the KaVA frequency band for searching pulsars in this region. Here, we describe the science targets and report progresses made from the KVN test observations for known pulsars. We then discuss why KaVA pulsar observations are compelling.
High precision astrometric Space Very Long Baseline Interferometry (S-VLBI) at the low end of the conventional frequency range, i.e. 20cm, is a requirement for a number of high priority science goals. These are headlined by obtaining trigonometric pa
rallax distances to pulsars in Pulsar--Black Hole pairs and OH masers anywhere in the Milky Way Galaxy and the Magellanic Clouds. We propose a solution for the most difficult technical problems in S-VLBI by the MultiView approach where multiple sources, separated by several degrees on the sky, are observed simultaneously. We simulated a number of challenging S-VLBI configurations, with orbit errors up to 8m in size and with ionospheric atmospheres consistant with poor conditions. In these simulations we performed MultiView analysis to achieve the required science goals. This approach removes the need for beam switching requiring a Control Moment Gyro, and the space and ground infrastructure required for high quality orbit reconstruction of a space-based radio telescope. This will dramatically reduce the complexity of S-VLBI missions which implement the phase-referencing technique.
We have carried out full imaging simulation studies to explore the impact of frequency standards in millimeter and sub-millimeter Very Long Baseline Interferometry (VLBI), focusing on the coherence time and sensitivity. In particular, we compare the
performance of the H-maser, traditionally used in VLBI, to that of ultra-stable cryocooled sapphire oscillators over a range of observing frequencies, weather conditions and analysis strategies. Our simulations show that at the highest frequencies, the losses induced by H-maser instabilities are comparable to those from high quality tropospheric conditions. We find significant benefits in replacing H-masers with cryocooled sapphire oscillator based frequency references in VLBI observations at frequencies above 175 GHz in sites which have the best weather conditions; at 350 GHz we estimate a 20-40% increase in sensitivity, over that obtained when the sites have H-masers, for coherence losses of 20-10%, respectively. Maximum benefits are to be expected by using colocated Water Vapour Radiometers for atmospheric correction. In this case, we estimate a 60-120% increase in sensitivity over the H-maser at 350 GHz.
In February 1997, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Programme (VSOP) mission. Approximately twenty-five percent of the mission time was dedicated to the VSOP S
urvey of bright compact Active Galactic Nuclei (AGN) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in Paper III, which contained 102 sources. For most sources, the plots of the uv-coverage, the visibility amplitude versus uv-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.
