Space VLBI Research Articles

On Utilization of Solar Sails in Triangular Libration Point Missions in the Earth-Moon System

In the real solar system, due to various perturbations, the triangular libration points of the earth-moon system are unstable. However, there are quasi-periodic orbits around them. These orbits show mild instability. Due to the instability, orbit control is necessary for the spacecrafts around these points. In the paper, solar sails were taken to fulfill the control. Numerical simulations were made in the earth-moon system. The results showed that taking the surface of the solar sail as the control parameter can achieve better results. Triangular Libration points in the earth-moon system have fixed configurations with the earth and the moon. This geometrical property is ideal for some space missions. For example, relay stations can be put around them. Spacecrafts can also be sent to these points for deep space observations. Along with the observations on earth, better observation data can be obtained. Besides this, these points are ideal potential candidates for space VLBI observations. In the restricted three-body problem of the earth-moon system, the mass ratio μ is smaller than Routh’s critical value μ1=0.03852…. They are linearly stable in the restricted three-body problem 1) . However, in the real solar system, various perturbations (mainly the gravitational perturbations from the sun) change their dynamical properties and these points are no longer stable anymore. Nevertheless, quasi-periodic orbits staying around them can be found. These orbits show mild instability. Due to their instability, orbit control is necessary for the spacecrafts moving on these orbits. Solar sails can produce long-lasting thrust and do not require any fuels. It’s an ideal choice for space missions. They were taken to fulfill the orbit control in this paper. The magnitude and direction of the thrust produced by the solar sail are coupled, and the force is always directed away from the sun. This is one shortcoming of the solar sails. Due to this property, instead of the tight control strategy, we took a loose one. This strategy needs a pre-determined nominal orbit. A quasi-periodic orbit around the triangular libration points was chosen as the nominal orbit. Two sets of different control variables were used. One set is the surface of the solar sail, and the other one is the normal direction angles of the solar sail. Numerical simulations were done in the earth-moon system. The results showed that taking the surface of the solar sail as the control variable is better. During the lifetime of the spacecraft, it may be shadowed from the sun by the earth or the moon. The solar sail is of no use when the spacecraft is in the shadow. In order to avoid the shadows, the initial epoch of the nominal orbit is carefully chosen. Since the inclination angle between the moon’s and the sun’s orbit planes is small, the z amplitude motion of the nominal orbit is chosen to be large to avoid the shadows too. For a short time mission, these steps can avoid the shadows. For a long time mission, the spacecraft will encounter the shadows anyway. At the end of the paper, a discussion of this case was made. For the shadows in the paper, cylindrical model was considered.

Observations of 44 extragalactic radio sources with the VLBA at 92 cm

<i>Aims. <i/>We have analysed VLBA 92 cm archive data of 44 extragalactic sources in order to identify early targets and potential calibrator sources for the LOFAR radio telescope and the RadioAstron space VLBI mission. Some of these sources will also be suitable as “in-beam” calibrators, permitting deep, wide-field studies of other faint sources in the same field of view.<i>Methods. <i/>All publicly available VLBA 92 cm data observed between 1 January 2003 to December 31, 2006 have been analysed via an automatic pipeline, implemented within AIPS. The vast majority of the data are unpublished.<i>Results. <i/>The sample consists of 44 sources, 34 of which have been detected on at least one VLBA baseline. 30 sources have sufficient data to be successfully imaged. Most of the sources are compact, with a few showing extended structures. Of the 30 sources imaged, 13 are detected on the longest VLBA baselines (~9 M<i>λ<i/>), while all were detected on baselines greater than 2 M<i>λ<i/> (the maximum baseline of LOFAR including the current international baselines).

Identification of Potential Weak Target Radio Quasars for ASTRO-G In-Beam Phase-Referencing

Abstract We apply an efficient selection method to identify potential weak Very Long Baseline Interferometry (VLBI) target quasars simply using optical (SDSS) and low-resolution radio (FIRST) catalogue data. Our search is restricted to within 12 from known compact radio sources that are detectable as phase-reference calibrators for ASTRO-G at 8.4 GHz frequency. These calibrators have estimated correlated flux density $&amp;gt;$20 mJy on the longest ground–space VLBI baselines. The search radius corresponds to the primary beam size of the ASTRO-G antenna. We show that $\sim$20 quasars with at least mJy-level expected flux density can be pre-selected as potential in-beam phase-reference targets for ASTRO-G at 8.4 GHz frequency. Most of them have never been imaged with VLBI. The sample of these dominantly weak sources offers a good opportunity to study their radio structures with unprecedented angular resolution provided by Space VLBI. The method of in-beam phase-referencing is independent from the ability of the orbiting radio telescope to do rapid position-switching manoeuvres between the calibrators and the nearby reference sources, and less sensitive to the satellite orbit determination uncertainties.

VSOP2/ASTRO-G project

We introduce a new space VLBI project, the Second VLBI Space Observatory Program (VSOP2), following the success of the VLBI Space Observatory Program (VSOP1). VSOP2 has 10 times higher angular resolution, up to about 40 micro arcseconds, 10 times higher frequency up to 43 GHz, and 10 times higher sensitivity compared to VSOP1. Then VSOP2 should become a most powerful tool to observe innermost regions of AGN and astronomical masers. ASTRO-G is a spacecraft for VSOP2 project constructing in ISAS/JAXA since July 2007. ASTRO-G will be launched by JAXA H-IIA rocket in fiscal year 2012. ASTRO-G and ground-based facilities are combined as VSOP2. To achieve the good observation performances, we must realize new technologies. They are large precision antenna, fast-position switching capability, new LNAs, and ultra wide-band down link, etc‥ VSOP2 is a huge observation system involving ASTRO-G, ground radio telescopes, tracking stations, and correlators, one institute can not prepare a whole system of VSOP2. Then we must need close international collaboration to get sufficient quality of resultant maps and to give a sufficient quantity of observation time for astronomical community. We formed a new international council to provide guidance on scientific aspects related of VSOP2, currently called the VSOP2 International Science Council (VISC2).

Multi-frequency investigation of the parsec- and kilo-parsec-scale radio structures in high-redshift quasar PKS 1402+044

We investigate the frequency-dependent radio properties of the jet of the luminous high-redshift (z = 3.2) radio quasar PKS 1402+044 (J1405+0415) by means of radio interferometric observations. The observational data were obtained with the VLBI Space Observatory Programme (VSOP) at 1.6 and 5 GHz, supplemented by other multi-frequency observations with the Very Long Baseline Array (VLBA; 2.3, 8.4, and 15 GHz) and the Very Large Array (VLA; 1.4, 5, 15, and 43 GHz). The observations span a period of 7 years. We find that the luminous high-redshift quasar PKS 1402+044 has a pronounced "core-jet" morphology from the parsec to the kilo-parsec scales. The jet shows a steeper spectral index and lower brightness temperature with increasing distance from the jet core. The variation of brightness temperature agrees well with the shock-in-jet model. Assuming that the jet is collimated by the ambient magnetic field, we estimate the mass of the central object as ~10^9 M_sun. The upper limit of the jet proper motion of PKS 1402+044 is 0.03 mas/yr (~3c) in the east-west direction.

The VSOP 5 GHz Active Galactic Nucleus Survey. V. Imaging Results for the Remaining 140 Sources

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 Survey 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.

Design of geodetic SVLBI satellite orbit and its tracking network

SVLBI (space very long baseline interferometry) has some important potential applications in geodesy and geodynamics, for which one of the most difficult tasks is to precisely determine the orbit of an SVLBI satellite. This work studies several technologies that will possibly be able to determine the orbit of a space VLBI satellite. Then, according to the types and characteristics of the satellite and the requirements for geodetic study and the geometry of the GNSS (GPS, GALILEO) satellite to track the space VLBI satellite, the six Keplerian elements of the SVLBI satellite (TEST-SVLBI) are determined. A program is designed to analyze the coverage area of space of different altitudes by the stations of the network, with which the tracking network of TEST-SVLBI is designed. The efficiency of tracking TEST-SVLBI by the network is studied, and the results are presented.

Radiation Testing of Consumer High-Speed LSI Chips for the Next Space VLBI Mission

Abstract We carried out two types of radiation testing of high-speed LSI chips to test their suitability for use in wideband observations by the Japanese next space VLBI mission, VSOP-2. In a total ionization-dose experiment we monitored autocorrelation spectra, which were taken with irradiated LSI chips and the source current at intervals of up to 1000 hr from the ionization dose, but could not detect any change of these features for chips irradiated at the dose rates expected in the VSOP-2 mission. In a single event effect experiment, we monitored the cross-correlation phase and the power spectra between data from radiated and nonradiated devices, and the source current during the irradiation of heavy ions. We observed a few tens of single event upsets as discrete delay jumps for each LSI. We estimated the occurrence rate of single events in space to be between once every few days and once a month. No single event latch-up was seen in any of LSIs. These results show that the tested LSIs have sufficiently high tolerance for the environment for space VLBI observations.

Physical properties of the jet in 0836+710 revealed by its transversal structure

Studying the internal structure of extragalactic jets is crucial for understanding their physics. The Japanese-led space VLBI project VSOP has presented an opportunity for such studies, by reaching baseline lengths of up to 36,000 km and resolving structures down to an angular size of $\approx 0.3$ mas at 5 GHz. VSOP observations of the jet in 0836+710 at 1.6 and 5 GHz have enabled tracing of the radial structure of the flow on scales from 2 mas to 200 mas along the jet and determination of the wavelengths of individual oscillatory modes responsible for the formation of the structure observed. We apply linear stability analysis to identify the oscillatory modes with modes of Kelvin-Helmholtz instability that match the wavelengths of the structures observed. We find that the jet structure in 0836+710 can be reproduced by the helical surface mode and a combination of the helical and elliptic body modes of Kelvin-Helmholtz instability. Our results indicate that the jet is substantially stratified and different modes of the instability grow inside the jet at different distances to the jet axis. The helical surface mode can be driven externally, and we discuss the implications of the driving frequency on the physics of the active nucleus in 0836+710.

Spectral characteristics of radio sources near the North Celestial Pole

We have used the RATAN-600 radio telescope to study the spectral characteristics of a uniform sample of 504 radio sources from the NVSS catalog near the North Celestial Pole at six frequencies from 1.1 to 21.7 GHz, with the aim of selecting sources possessing inverted spectra near 22 GHz, to be included in the program of the Radio Astron future space VLBI mission. We found 17 radio sources with the desired spectral characteristics. Data from spectral studies over a wide wavelength range testify that the spectral behavior of our sample differs from that for a complete sample of sources with the same initial parameters but selected at 20 GHz. We find a 6% deficit of inverted-spectrum sources, which can be explained as an effect of the spectral characteristics of “sub-threshold” sources that were not included in the initial sample at 1.4 GHz.

Verification of the Effectiveness of VSOP-2 Phase Referencing with a Newly Developed Simulation Tool, ARIS

Abstract The next-generation space VLBI mission, VSOP-2, is expected to provide unprecedented spatial resolution at 8.4, 22, and 43 GHz. In this report, phase referencing with VSOP-2 is examined in detail based on a simulation tool called ARIS. The criterion for successful phase referencing was set to keep the phase errors below one radian. Simulations with ARIS reveal that phase referencing achieves good performance at 8.4 GHz, even under poor tropospheric conditions. At 22 and 43 GHz, it is recommended to conduct phase referencing observations under good or typical tropospheric conditions. The satellite is required to have an attitude-switching capability with a one-minute or shorter cycle, and an orbit determination accuracy higher than $\sim$10 cm at apogee; the phase referencing calibrators are required to have a signal-to-noise ratio larger than four for a single scan. The probability to find a suitable phase referencing calibrator was estimated by using VLBI surveys. From the viewpoint of calibrator availability, VSOP-2 phase referencing at 8.4 GHz is promising. However, the chance of finding suitable calibrators at 22 and 43 GHz is significantly reduced; it is important to conduct specific investigations for each target at those frequencies.

The next generation space VLBI project, VSOP-2

AbstractThe first dedicated space-VLBI project, the VLBI Space Observatory Programme (VSOP), commenced with the successful launch of radio-astronomical satellite HALCA in 1997. Plans for a second generation space-VLBI project have been made by a working group over a number of years. This project, VSOP-2, has now been approved by Japan's space agency, JAXA, as the ASTRO-G project. It is planned for the spacecraft to observe in the 8, 22 and 43 GHz bands with cooled receivers for the two higher bands, which include important maser lines. It will have a maximum angular resolution at 43 GHz (7 mm) of about 40 micro-arcseconds. Although the VSOP project mainly observed continuum emission from active galactic nuclei (AGN), VSOP-2/ASTRO-G is expected to enable a variety of high angular resolution maser line observations.

The VSOP Survey: final aggregate results

AbstractIn 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 satellite's 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.

VSOP Monitoring of the Compact BL Lac Object AO 0235+164

Abstract In 1999, the highly compact and variable BL Lac object AO 0235$+$164 was identified as the highest brightness temperature active galactic nucleus observed with the VLBI Space Observatory Programme (VSOP), with ${T_{\mathrm{B}} \gt 5.8 \times10^{13} \,\mathrm{K}} {T_{\mathrm{B}} \gt 5.8 \times10^{13} \,\mathrm{K}}$ (Frey et al. 2000). The sub-milliarcsec radio structure of this source has been studied with dual-frequency (1.6 and 5 GHz), polarization-sensitive VSOP observations during 2001 and 2002. Here we present the results of this monitoring campaign. At the time of these observations, the source was weakly polarized and characterized by a radio core that is clearly resolved on space–ground baselines.

Milliarcsecond-Scale Spectral Properties and Jet Motions in M 87

Abstract We have combined high resolution VLBI Space Observatory Programme (VSOP) data at 1.6 and 4.8 GHz with Very Long Baseline Array (VLBA) data at higher frequencies and with similar resolutions to study the spectral properties of the core of M 87 with milliarcsec resolution. The VSOP data allows a more accurate measurement of the turnover frequency, and hence more reliable determination of associated physical parameters of the source. Comparison of the images with previously published images yields no evidence for significant motions of components in the parsec-scale jet. In addition, the brightness temperatures obtained from modelfits to the core are well below the inverse Compton limit, suggesting that the radio emission we are observing is not strongly Doppler boosted.

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.

Commission 44: Space &amp; High Energy Astrophysics

Division XI was born by merging Commission 44 “Space and High Energy Astrophysics” and Commission 48 “High Energy Astrophysics” by the decision at the IAU General Assembly in The Hague (1994). As the naming of space astronomy is technique oriented, i.e. astronomy from space, it covers quite a wide range of astronomy, almost all branches of astronomy are included by the progress of space observations. Historically, it started from high energy astronomy, UV, X, and gamma rays astronomy, somewhat including cosmic ray physics. However, in these days, space observations have expanded to low energy astronomy, such as optical, infrared, submillimeter and even radio waves(Space VLBI).

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine-Scale Structure

We have examined the compact structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz. With projected baselines out to 440 Mλ, we are able to investigate source structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for short-wavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes. We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron–emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 1011 and 1013 K, but they extend up to 5 × 1013 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.

VSOP Observations of the Gamma-Ray Sources J1733−1304, J1625−2527, and J1015+4926

Abstract We present images from VSOP (VLBI Space Observatory Programme) and VLBA (Very Long Baseline Array) observations of the active galactic nuclei J1733$-$1304, J1625$-$2527, and J1015$+$4926. These three sources have been identified as possible counterparts to gamma-ray sources in the third EGRET catalog. In general, the counterparts to extragalactic EGRET sources have very compact structure, with brightness temperatures ($T_{\mathrm{b}}$) greater than $10^{11} \hbox{--} 10^{12} \,\mathrm{K}$. Evidence for compactness in gamma-ray emitting AGNs is also given by the variability seen from gamma-ray to radio frequencies. In this report, we compare the images of the three sources with previously published images and also examine jet motions and spectral indices. The measured $T_{\mathrm{b}}$ of J1733$-$1304 are an order of magnitude lower than previous $T_{\mathrm{b}}$ measurements, suggesting that the VSOP observations were made while the source was in a relatively quiescent state after the radio outburst of 1995–1997. The relatively low $T_{\mathrm{b}}$ of J1625$-$2527 is also consistent with the highly variable nature of this source. A comparison of the VSOP observation of J1015$+$4926 with a VLBA image made 2.2 yr earlier suggests the existence of stationary jet components in this source.

Spine-sheath polarization structures in four active galactic nuclei jets

We present the results of multifrequency (15 + 8 + 5 GHz) polarization Very Long Baseline Array (VLBA) observations of the three BL Lacertae objects 0745+241, 1418+546 and 1652+398 together with 5-GHz VLBI Space Observatory Programme (VSOP) observations of 1418+546 and 1.6- and 5-GHz VSOP observations of the blazar 1055+018. The jets of all these sources have polarization structure transverse to the jet axis, with the polarization E vectors aligned with the jet along the jet spine and 'sheaths' of orthogonal E vectors at one or both edges of the jet. The presence of polarization aligned with the jet near the 'spine' may indicate that the jets are associated with helical B fields that propogate outward with the jet flow; the presence of orthogonal polarization near the edges of the jet may likewise be a consequence of a helical jet B field, or may be owing to an interaction with the ambient medium on parsec scales. We have tentatively detected interknot polarization in 1055+018 with E aligned with the local jet direction, consistent with the possibility that the jet of this source is associated with a helical B field.