Ground VLBI Research Articles

OmniUV: A Multipurpose Simulation Toolkit for VLBI Observation

We present OmniUV, a multipurpose simulation toolkit for space and ground VLBI observations. It supports various kinds of VLBI stations, including Earth (ground) fixed, Earth orbit, Lunar fixed, Lunar orbit, Moon–Earth, and Earth–Sun Lagrange 1 and 2 points, etc. The main functionalities of this toolkit are: (1) trajectory calculation; (2) baseline uv calculation, by taking the availability of each station into account; (3) visibility simulation for the given uv distribution, source structure, and system noise; (4) image and beam reconstruction. Two scenarios, namely the space VLBI network and the wide-field array, are presented as the application of the toolkit at completely different scales. OmniUV is the acronym of “Omnipotent UV.” We hope it could work as a general framework, in which various kinds of stations could be easily incorporated and the functionalities could be further extended. The toolkit has been made publicly available.

Software Correlator for Radioastron Mission

In this paper, we discuss the characteristics and operation of Astro Space Center (ASC) software FX correlator that is an important component of space–ground interferometer for Radioastron project. This project performs joint observations of compact radio sources using 10[Formula: see text]m space radio telescope (SRT) together with ground radio telescopes at 92, 18, 6 and 1.3 cm wavelengths. In this paper, we describe the main features of space–ground VLBI data processing of Radioastron project using ASC correlator. Quality of implemented fringe search procedure provides positive results without significant losses in correlated amplitude. ASC Correlator has a computational power close to real time operation. The correlator has a number of processing modes: “Continuum”, “Spectral Line”, “Pulsars”, “Giant Pulses”,“Coherent”. Special attention is paid to peculiarities of Radioastron space–ground VLBI data processing. The algorithms of time delay and delay rate calculation are also discussed, which is a matter of principle for data correlation of space–ground interferometers. During five years of Radioastron SRT successful operation, ASC correlator showed high potential of satisfying steady growing needs of current and future ground and space VLBI science. Results of ASC software correlator operation are demonstrated.

RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz

Polarization of radio emission in extragalactic jets at a sub-milliarcsecond angular resolution holds important clues for understanding the structure of the magnetic field in the inner regions of the jets and in close vicinity of the supermassive black holes in the centers of active galaxies. Space VLBI observations provide a unique tool for polarimetric imaging at a sub-milliarcsecond angular resolution and studying the properties of magnetic field in active galactic nuclei on scales of less than 10^4 gravitational radii. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471), made at a wavelength of 18 cm (frequency of 1.6 GHz) as part of the Early Science Programme (ESP) of the RadioAstron} mission, is used here to test the polarimetric performance of the orbiting Space Radio Telescope (SRT) employed by the mission, to establish a methodology for making full Stokes polarimetry with space VLBI at 1.6 GHz, and to study the polarized emission in the target object on sub-milliarcsecond scales. Polarization leakage of the SRT at 18 cm is found to be within 9 percents in amplitude, demonstrating the feasibility of high fidelity polarization imaging with RadioAstron at this wavelength. A polarimetric image of 0642+449 with a resolution of 0.8 mas (signifying an ~4 times improvement over ground VLBI observations at the same wavelength) is obtained. The image shows a compact core-jet structure with low (~2%) polarization and predominantly transverse magnetic field in the nuclear region. The VLBI data also uncover a complex structure of the nuclear region, with two prominent features possibly corresponding to the jet base and a strong recollimation shock. The maximum brightness temperature at the jet base can be as high as 4*10^13 K.

First estimate of the value of the instrumental polarization of the RadioAstron space radio telescope using the results of an early scientific program for observing active galactic nuclei

To interpret radio interferometric observations sensitive to linear polarization it is necessary to take into account the effect of instrumental or “parasitic” polarization. In the case of ground-space very-long-baseline interferometry, this procedure involves a session of polarization-sensitive mapping with the sufficient number of ground stations and sufficiently small projections of ground-space baselines. In the paper, the problem of estimating the value of the instrumental polarization of the Spectr-R space radio telescope of the RadioAstron project using the results of the early scientific program for observing active galactic nuclei is addressed. We use a statistical approach to estimate the value of the instrumental polarization associated with the analysis of already obtained to present time intermediate results of the review of brightness temperatures. Estimates obtained for the frequency bands C and L (95% probability intervals [0.0646, 0.1267] and [0.0945, 0.1736] for 6 and 18 cm, respectively) suggest that the instrumental polarization of the radio telescope does not exceed the values typical for the ground VLBI stations.

ASTROMETRY AND ASTROPHYSICS WITH THE SPACE TELESCOPE RADIOASTRON

It is well-known that gravitational lensing is a powerful tool in the investigation of the distribution of matter, including that of dark matter (DM). Typical angular distances between images and typical time scales depend on the gravitational lens masses. For the case of microlensing, angular distances between images or typical astrometric shifts are about 10-5 - 10-6 arcsec. Such an angular resolution will be reached with the space–ground VLBI interferometer, Radioastron. The basic targets for microlensing searches should be bright point-like radio sources at cosmological distances. In this case, an analysis of their variability and a solid determination of microlensing could lead to an estimation of their cosmological mass density. Moreover, one could not exclude the possibility that non-baryonic dark matter could also form microlenses if the corresponding optical depth were high enough. It is known that in gravitationally lensed systems the probability (the optical depth) of observing microlensing is relatively high. Therefore, for example, gravitationally lensed objects, like the CLASS gravitational lens B1600+434, appear to be most suitable to detect astrometric microlensing, since features of photometric microlensing have been detected in these objects. However, to directly resolve these images and to directly detect the apparent motion of the knots, the Radioastron sensitivity would have to be improved, since the estimated flux density is below the sensitivity threshold. Alternatively, they may be observed by increasing an integration time, assuming that a radio source has a typical core–jet structure and microlensing phenomena are caused by the superluminal apparent motions of knots. In the case of a confirmation (or a negation) of claims about microlensing in gravitational lens systems, one can speculate about the microlens contribution to the gravitational lens mass. Astrometric microlensing due Galactic MACHOs is not very important because of low optical depths and long typical time scales. Therefore, the launch of the space interferometer Radioastron will enable the investigation of microlensing in the radio band, giving rise to the possibility of not only resolving microimages but also of observing astrometric microlensing.

The Quasar 3C 345 at the Highest Resolution with mm-and Space-VLBI

The quasar 3C345 is one of the best examples of extra-galactic jet with superluminal components moving on helical paths. To get better constraints on the jet physics, we have imaged the source at the highest possible resolution, using mm-VLBI and space-VLBI observations. We observed 3C 345 at 5 different epochs so far (1 epoch with 3-mm ground VLBI, and 4 epochs with space VLBI at 6cm and 18 cm). These measurements expand significantly our ongoing monitoring of 3C 345. In this contribution, we discuss the properties of the jet and its moving components in the immediate vicinity of the core (< 3 mas).

Asymmetric Free-Free Absorption towards a Double Lobe of OQ 208

We present 1.66 GHz VSOP (VLBI Space Observatory Programme) observations of a GHz-peaked spectrum (GPS) source, OQ 208. Two compact lobes with a 10 pc separation in the NE-SW direction are identified with those observed with ground VLBI at higher frequencies. We find that the two lobes are highly asymmetric in terms of a flux density ratio NE:SW of ∼ 60:1 at 1.66 GHz, larger than that at higher frequencies. Both lobes show a steeply rising spectrum between 1.66 and 2.32 GHz, which implies free-free absorption by an ambient plasma, rather than synchrotron self-absorption. The absorption features show different peak frequencies, and indicate a larger optical depth towards the fainter SW lobe. This suggests the existence of an external absorber embedding the lobes. Hence, we can estimate the electron temperature, Te, and the electron density, ne, of the absorber. Considering the optical depth and X-ray luminosity, we derive the electron temperature and density in the ranges of 104 < Te < 6 × 107 K and 600 < ne < 7 × 105 cm−3, respectively. These properties coincide with those of NLR (Narrow Line Region). We point out that the NLR plasma affects evolution of radio galaxies, paticularly GPS sources.

VSOP observations of a GHZ-peaked spectrum source OQ 208

We present λ18 cm VSOP observations towards a GHz-peaked spectrum (GPS) source OQ 208. A double lobe seen by high-frequency ground VLBI observations is confirmed, although its brightness is more asymmetric at lower frequency. Comparing with images at higher frequency, we found asymmetric free-free absorption towards the double lobe. We can explain the spectral asymmetry by a slant jet axis against line of sight, which causes different optical depth in the ambient plasma.

JPL contribution to the VSOP mission

Abstract The Jet Propulsion Laboratory (JPL) is a participant in the VSOP Space VLBI mission, an extensive international collaboration led by Japan's Institute of Space and Astronautical Science (ISAS). The JPL effort is funded by the U.S. National Aeronautics and Space Administration (NASA). To obtain data from the orbital element of the VSOP mission, the Japanese HALCA satellite, the Deep Space Network (DSN) of JPL has built a new set of three 11-meter tracking stations in California, Spain, and Australia. These stations have supported over 1000 HALCA passes during the first year of operation, and supply science, telemetry, and Doppler data for the mission. JPL is using the Doppler data to estimate the satellite orbital parameters that are needed by the tracking stations and VLBI data correlators. The DSN also modified their three 70-meter antennas to provide ground VLBI observing support for the mission. In addition to operational support, JPL has had significant involvement in the mission planning and scientific support aspects of the mission, including mission design, international data flow, scientific scheduling, and data analysis during in-orbit checkout. JPL has also aided the scientific community in the use of VSOP by developing a user software package, writing a guide for proposers, and establishing a proposer help desk.

The requirement to the baseline Knowing of a Space VLBI

AbstractIt is shown that the influence of a baseline knowing error on the fringe rate change during coherent averaging time interval is extremely increasing for space VLBI due to more angle velocity of a space baseline rotation comparatively with ground VLBI. It is shown on the example of Radioastron orbit that the reguirement to its orbit for such a short wavelength as 1.35 cm is determined by unknowing of the fringe rate change during coherent averaging time interval rather than by delay ambiguity and this reguirement is equal several meters.