aa r X i v : . [ a s t r o - ph ] S e p VSOP2/
ASTRO-G
Pro ject
Masato Tsuboi Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency(JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510 JapanE-mail: [email protected]
Abstract.
We introduce a new space VLBI project, the Second VLBI Space ObservatoryProgram (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 higherfrequency up to 43 GHz, and 10 times higher sensitivity compared to VSOP1. Then VSOP2should become a most powerful tool to observe innermost regions of AGN and astronomicalmasers.
ASTRO-G is a spacecraft for VSOP2 project constructing in ISAS/JAXA since July2007.
ASTRO-G will be launched by JAXA H-IIA rocket in fiscal year 2012.
ASTRO-G andground-based facilities are combined as VSOP2. To achieve the good observation performances,we must realize new technologies. They are large precision antenna, fast-position switchingcapability, new LNAs, and ultra wide-band down link, etc.. VSOP2 is a huge observation systeminvolving
ASTRO-G , ground radio telescopes, tracking stations, and correlators, one institutecan not prepare a whole system of VSOP2. Then we must need close international collaborationto get sufficient quality of resultant maps and to give a sufficient quantity of observation timefor astronomical community. We formed a new international council to provide guidance onscientific aspects related of VSOP2, currently called the VSOP2 International Science Council(VISC2).
1. Introduction
The pioneering history of radio telescopes is in some ways the history of the quest for higherangular resolution. The angular resolution of a radio interferometer is given by λ/D , where D is baseline length or antenna spacing. The upper limit of D was usually much less than1,000 km for connected interferometers. In order to push this limit, VLBI (Very-Long-BaselineInterferometry) was invented. VLBI is made by connecting existing radio telescopes withmagnetic tapes instead of cables. Then VLBI observes radio sources with relatively highbrightness temperature like AGN and astronomical masers. Superluminal motion of AGN jetwas shown only by very high angular resolution of VLBI. However, ground-based VLBI hasan upper limit of angular resolution because D must not be larger than the diameter of theearth, 13,000 km. In order to push this limit, space VLBI was formulated. Space VLBI is aninterferometer connecting radio telescopes in orbit and ground-base ones. Since the baselinelength of space VLBI can extend beyond the diameter of the earth, angular resolution of spaceVLBI is free from this limit. However, it is very difficult to set a radio telescope in orbit.Space VLBI was realized in 1997 as the first space VLBI project, VSOP1 (VLBI SpaceObservatory Program)[1,2], after some preceding experiments and vast amounts of effort. The Project Scientist,
ASTRO-G project of ISAS, JAXA pacecraft for VSOP1 is known as the first dedicated space VLBI satellite,
HALCA . Thiswas launched by a M-V rocket of Institute of Space and Astronautical Science (ISAS) fromUnchinoura Space Center (USC). The angular resolution of VSOP1 is 3 times better thanthat of ground-based VLBI at 1.6 and 5 GHz (VSOP1 also had 22-GHz band system, whichunfortunately suffered a serious loss of sensitivity at the launch). Seven hundred observationsof AGN jets were made in the mission lifetime of 7 years.Figure 1 shows a schematic display of the comparison between space VLBI and ground-basedVLBI. The Second VLBI Space Observatory Program (VSOP2) is a successor of VSOP1 [3].
ASTRO-G is the satellite for VSOP2, which will be launched by ISAS/JAXA in fiscal year 2012.The construction project of
ASTRO-G has been started since July 2007. This project is a joint-project between ISAS, NAOJ (National Astronomical Observatory, Japan) and the universities.Now, basic design of the satellite and developments of several components for
ASTRO-G areongoing. A review of a basic design of the satellite (PDR: preliminary design review) is scheduledat March of 2009. After decision of the design (CDR: critical design review) in 2009, we willstart the production of the flight model of
ASTRO-G . This satellite will be launched with anH-IIA rocket from Tanegashima Space Center (TNSC) of JAXA.VSOP2 has 10 times higher angular resolution, 10 times higher frequency, and 10 timeshigher sensitivity compared to VSOP1. VSOP2 will be an astronomical tool with unprecedentedangular resolution in all wavelength to explore innermost regions of AGN and astronomicalmasers. The VSOP2 programme foresees a powerful astronomical instrument involving
ASTRO-G , ground radio telescopes, tracking stations, and correlators. For this reason, ISAS alone cannot all different aspects of this complex project, which will be made in collaboration withinternational space and ground-base partners.
Figure 1.
Schematic display of the comparison between space VLBI and ground-based VLBI.
2. Imaging Capability and Scientific Objectives of VSOP2
Here we describe the expected imaging capability of VSOP2. Figure 2 shows an artist’simpression of the
ASTRO-G satellite around perigee. The total size of the
ASTRO-G satellite is igure 2.
Artist’s impression of the
ASTRO-G satellite around perigee.
Table 1.
ASTRO-G parametersorbit parametersapogee 25,000 kmperigee 1,000 kminclination angle 31 ◦ orbital period 7.5 hrfrequency bandX 8.0-8.8 GHzK 20.6-22.6 GHzQ 41.0-45.0 GHzas large as a tennis coat. The observing frequency bands of the ASTRO-G satellite are 8, 22, and43 GHz. There are many large radio telescopes at 8 GHz in the world, being this frequency bandused for down-link satellite communication. The participation of many telescopes in VSOP2increases its imaging capability. The 22 and 43 GHz bands involve H O ( ν = 22 .
235 GHz) andSiO ( ν = 43 .
122 and 42 .
820 GHz) masers, respectively. These frequency bands are summarizedin Table 1.The planned apogee and perigee of the orbit of
ASTRO-G are 25,000 and 1,000 km,respectively. The orbit provides the maximum baseline length of 35,000 km for VSOP2. Theorbital period is about 7.5 hours. VSOP2 will yield an angular resolution of 38 micro arcsecondsat 43 GHz. The inclination angle of the orbit is 31 ◦ , which has a influence on the commonobservation coverage of ground telescopes and the coverage of down link from ASTRO-G todata tracking stations. These are important to realize good quality of the resultant maps. Theparameters are also summarized in Table 1.The expected mission lifetime of
ASTRO-G satellite is 3 years. This limitation will be causedy the following facts:
ASTRO-G has cooled mm-wave receivers by Stirling cycle refrigerator,which will be operated over 3 years. However, the mm-wave receiver has large heat consumptioncompared to IR detectors. The cooling capacity will decrease to the heat consumption in 3years. Furthermore, the radiation condition of
ASTRO-G is as hard as total dose level of 10 rad in 3 years because ASTRO-G will pass the radiation belt 3 times a day. This will havean adverse effect on the surface accuracy of the telescope antenna. The sensitivity of VSOP2will play an important role in determination of the imaging capability. In addition, the imagingcapability also depends considerably on the performances of ground radio telescopes. They willbe summarized after the section of the antenna and LNAs.
The high angular resolution of 38 micro arcsecond at43 GHz will make possible to study the neighborhood of an AGN black hole in unprecedentedquality. Figure 3 shows the comparison among the predicted accretion disk size of the nearestAGN, M 87, and the synthesized beam sizes of VSOP2, VSOP1, and the VLBA. The synthesizedbeam sizes of VSOP2 corresponds to 13 R s , which is smaller than the predicted accretion disk ofM 87. Using VSOP2, we may image the accretion disk of M 87 if it has a sufficient brightness.The resolving an accretion disk disk might solve the following problems about the AGNjet formation. (i) is the origin of AGN jet in the neighboring region of the black hole or theaccretion disk (e.g., [5]). VSOP2 will make clear from where the jets are ejected. (ii) is what isthe role of magnetic field for acceleration of AGN jet (e.g., [6]). VSOP2 can observe polarizationat multi-frequencies. Then VSOP2 will make clear the magnetic field structure in the base ofAGN compensating Faraday effect (cf. [7]). (iii) is acceleration efficiencies of the jet. VSOP2will make clear the energy distribution of the relativistic electrons in the base of AGN frommulti-frequency observations.Blazars are also key sources for exploring the nature and physics of AGN jets. VSOP1 couldimage blazars at 1-pc scale. VSOP1 had provided a great progress in VLBI astronomy of blazars(e.g., [8]). On the other hand, X-ray observations estimate from the timescales of X-ray flaresthe size of the emission region to be about 0.1pc or less. The high-resolution observations ofVSOP2 will resolve the gap between these values. Neighborhood blazars, for example, Mrk421and Mrk501 are nice candidates for this study. Figure 3.
Comparison amongthe predicted accretion disk size ofthe nearest AGN, M 87, and thesynthesized beam sizes of VSOP2(38 µas ), VSOP1 (400 µas ), andVLBA (100 µas ) at selected fre-quencies. .2.2. Young Stellar Objects
Young stellar objects (YSO) are as well promising targets ofVSOP2. The angular resolution of VSOP-2 is comparable to the radius of a YSO. Unfortunately,VSOP2 cannot observe thermal emission from gas in/around YSO. However, motions of H Oand/or SiO maser spots can trace structure and kinematics of such gas, or position andtransversal velocity, and acceleration of such gas (e.g., [9]). Then, VSOP2 will make clearhow such gas accretes into the YSO and how the angular momentum is exchanged.And we already knew that large flares occur around the surface of YSO (see Figure 4). Directimaging of these flares may be a fantastic target of VSOP2. If imaging is possible, we can provea flare scaling law, which is expected widely from solar micro-flares to YSOs (e.g., [10]).
The H O masers of NGC4258 is a famous example of disk masers resolvedby VLBI. Cosmology using disk masers of galaxies may be another scientific target of VSOP2.Acceleration of central maser component, a , is given by a = v /r , where v is rotation velocity ofmaser component and r is the radius of the disk. On the other hand, r is also given by r = dθ ,where d is the distance of disk maser and θ is the angular radius of disk maser. Then d is givenby a simple formula, d = v /aθ . VSOP2 observations will provide the values of a , v , and θ . Wecan determine d based on VSOP2 only. Figure 5 shows a schematic display of the measurementof the distance to maser disk of AGN using VSOP2. VSOP2 can observe several galaxies hostingstrong H O masers. VSOP2 can measure the Hubble constant with high precision, because thesystemic velocities of galaxies are precisely measured with optical observations. Precise values ofHubble constant and its change for redshift will provide additional information about the darkenergy content of the universe.
Figure 4.
Comparison amongthe size, energy, magnetic fieldstrength of flares occured at Sun,stars, and YSO, overlaid on theplane of the emission measure andtemperature of the flares (Shibataand Yokoyama 1999).
3. The Space Radio Telescope of
ASTRO-G
The space radio telescope of
ASTRO-G satellite is an offset Cassegrain-type telescope which hasa large precision antenna, a control momentum gyro for fast-position switching capability, LNAreceivers, and ultra wide-band down link system [11].
Figure 6 shows schematically the space radio telescope of
ASTRO-G satellite. The antennaconsists of a 9.3-m paraboloid main reflector (Large Deployable Reflector, hereafter LDR), ahyperboloid sub-reflector, and three feed horns at 8, 22, 43 GHz. igure 5.
Schematic display of the measurement of the distance to a maser disk of AGN usingVSOP2.
Figure 6.
Schematic display of the space radio telescope of
ASTRO-G satellite.LDR has 7 hexagonal metal mesh-surface modules, which are deployed on the orbit [12].Radial rib-hoop cable structure is newly adopted for the modules to form a surface with accuracyof 0.4 mm-rms. The surface accuracy of
ASTRO-G
LDR is 10 times higher than that of thelarge deployable main reflector of
ETS VIII , which is an engineering satellite of JAXA launchedin 2006. The total weight of LDR is about 200 kg. We will make one-module as an engineeringmodel (EM) up to spring 2009 to evaluate the deployable mechanism and the accuracy. Wecan focus LDR on the orbit using gimbal -focusing mechanism to compensate the large-scaledeformation of LDR.The feed horns at 8, 22, 43 GHz are multi-mode horns for small sizing on the focal plane. Weare now designing the antenna optics with simulation based on physical optics method. Idealantenna efficiencies, depending only on the optics of the LDR, are estimated to be η = 64 − able 2. Expected antenna performances of LDRBand Gain η x-pol. η S exp {− (4 πǫ/λ ) } η A [GHz] [dB] [dB]8 . − . . . − . .
00 0 .
98 0 . . − . . . − . .
00 0 .
88 0 . . − . . . − . .
87 0 .
61 0 . −
10 dB edge level of the main reflector. The antenna optical performances at the presentare listed in Table 2. The beam patterns have good axial symmetry and good cross-polarizationlevel as low as < −
25 dB.The metal mesh surface is a key component for the light weight of the LDR. This is wovenmesh of gold plated molybdenum fiber. The real antenna RF performances of the LDR sufferfrom degradations in the surface accuracy and the reflection efficiency, η S , and the instrumentalpolarization of the metal mesh surface. Because it is difficult to calculate the RF performancesof the metal mesh surface itself, we are measuring the reflection efficiency and the instrumentalpolarization as functions of the tension of the metal mesh surface with a test apparatus. Thesemeasured values are also need for optimum design of LDR back structures.Then, the overall expected antenna efficiency is given by η A = η η S exp {− (4 πǫ/λ ) } , where η is the ideal antenna efficiencies depending only on the illumination pattern and thespillover of the LDR and η S is the reflection efficiency of the metal mesh surface.The values of η A are also listed in Table 2. The last column shows best estimates of theaperture efficiency at present. We hope dramatic improvement even on the orbit compared tothe HALCA antenna. We will be able to achieve the performance at least at the begin of the lifetime. However, the radiation condition of
ASTRO-G is very hard because it will pass radiationbelt over 3000 times during the mission life time. The performances at the end of the life timeare now discussing.
The
ASTRO-G satellite has fast position-switching capability for phase referencing observations.Although a space telescope is free from atmospheric effects, VSOP2 is not completely so becauseVSOP2 is an interferometer between
ASTRO-G and ground based telescopes. We need to slew
ASTRO-G from a target to a calibrator within 3 degrees in 15 seconds to compensate for theatmospheric effects. Then, we would use Control Momentum Gyro (CMG) technology [13],which has been used in the
Space Shuttle . Figure 7 shows the 8, 22, 43 GHz band receivers of
ASTRO-G . The low noise amplifier (LNA)is the most important component of the receiver. LNAs used in space must be ultra-low noiseand un-conditionally stable. The LNAs at 22 and 43 GHz are cooled to 30 K and the dualcircular polarization feed horns are cooled to 100 K by a Stirling-cycle refrigerator. We useGaAs monolithic microwave integrated circuits (GaAs MMIC) technology for these LNAs. Onthe other hand, the LNA at 8 GHz is located at the radiator panel of the receiver system. Thisis cooled passively. The feed horns are dual circular polarization feeds which are juxtaposed onthe Cassegrain focal plane of LDR.The noise temperatures of LNAs at the bread board model (BBM) phase are 60 K at 8GHz, 20 K at 22 GHz, and 35 K at 43 GHz. Figure 8 shows the present status of the noise igure 7.
Schematic display of the 8, 22, and 43 GHz band receivers of the
ASTRO-G satellite.
Figure 8.
Present status of the 22GHz LNA of
ASTRO-G . This LNAis at the BBM phase. The LNAnoise temperature is as low as thetechnical goal demonstrated in theproject book of
ASTRO-G .temperatures of the 22 GHz LNA at the BBM phase. The LNA at 43 GHz does not yet achievethe target values expected in the
ASTRO-G project book. We measured (estimated for somecomponents) loss and noise of the receiver components and estimate system noise temperaturesat the present. We assumed that the system equivalent flux density (
SEF D ) is given by
SEF D = 2 kT sys /A e . Table 3 shows the SEFDs of VSOP2 expected for three receivers. Numbers in parentheses aretarget values at the project book. The present estimated figures do not achieve the target values.Because there are some possibility of overestimation for some parts, they may be the upper limit able 3.
Expected Performances of VSOP2 (Target Values in Brackets)Band resolution SEFD 7- σ detection with VLBA[GHz] [ µ as] [mJy] [mJy]8 205 6100(4080) 32(23)22 75 3600(2200) 72(50)43 38 7550(3170) 188(107)for SEFDs. We will make receivers at the engineering model (EM) phase in autumn 2008; theperformances of the EM receivers will be reported in the beginning of 2009. We can observe simultaneously dual polarizations at each frequency. However, we can notobserve two frequencies at once because different frequency bands use the different feed horns.We can switch two frequencies at least within one minute. The bandwidth and the bit levelsof the analog-to-digital converter (ADC) of
ASTRO-G are 128 MHz and 2 bit, or 256 MHzand 1 bit. Ground-based VLBI radio telescopes record the signal to recording media, tape,disk, etc. In contrast,
ASTRO-G satellite has no recording media. It sends directly the VLBI-formatted data to the tracking stations. The data are sent through a broadband down link of bandwidth ∼
900 MHz at 37.5 GHz with a bit rate of 1 Gbps quadrature phase shift keying(QPSK). Simultaneously, the phase transfer carrier signal to the local oscillator system of the
ASTRO-G is sent through the up-link at 40 GHz. The ultra-wide band down-link may be alsoa technical challenge. Figure 9 shows a block diagram of the observation and data link systemsof the
ASTRO-G satellite. We plan at least 3 tracking stations in the world for obtainingsufficient observation time. However, ISAS has only one dedicated VLBI tracking station atUsuda, Japan. International collaboration is essential for the success of VSOP2. Additionally,a tracking station in southern hemisphere is important for keeping a high observation efficiencyof VSOP2. needs a very high accuracy orbit determination for astrometry. We use both satellitelaser ranging (SLR) and GPS systems for a high accuracy orbit determination. The accuracy ofthe orbit determination is required to be at least 10 cm to get any advantage against ground-base VLBI. The very high accuracy determination using GPS may be also technical challenge.The altitude of
ASTRO-G around the apogee is much higher than that of a GPS constellation.Since the antenna of GPS satellites is usually pointed to earth,
ASTRO-G around the apogeeonly receives the signal from the GPS satellites located on the far side of earth. It is difficultto receive the signals from over four satellites, which are required to determine the orbit of
ASTRO-G directly.
ASTRO-G also has a corner cube reflector for satellite laser ranging at theearth-pointing system of Ka-band link antenna.
4. VSOP2 International Science Council (VISC2) Formation
As mentioned previously, we need a world-wide collaboration for VSOP2 both in space andground telescopes to make resultant maps with sufficient quality and to give a sufficient quantityof observation time for international astronomical community. However, any internationalcollaboration is a complicated issue. VSOP1 formed the VSOP International Science Council(VISC1) as an international body to provide guidance on scientific aspects related of the mission.We are acknowledging that VISC1 functioned successfully in maximizing scientific result withVSOP1. In a similar fashion for VSOP2, we formed VISC2. The primary function of VISC2 will igure 9.
Block diagram of observation and data link systems of
ASTRO-G satellitebe to form an international consensus about issues relevant to science operation of the VSOP2mission. VISC2 expects to have face-to-face meeting approximately twice per year and morefrequently teleconferences.
Acknowledgments
This work is based on cooperative activity of the
ASTRO-G project team.
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