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ISSN 0038 0946, Solar System Research, 2012, Vol. 46, No. 7, pp. 466­475. © Pleiades Publishing, Inc., 2012. Original Russian Text © Yu.A. Alexandrov, V.V. Andreyanov, N.G. Babakin, et al., 2011, published in Vestnik FGUP NPO imeni S.A. Lavochkina, 2011, No. 3, pp. 19­29.

Radioastron (Spectr R Project)--A Radio Telescope Much Larger than the Earth: Ground Segment and Key Science Areas
Yu. A. Alexandrova, V. V. Andreyanova, N. G. Babakina, V. E. Babyshkinb, K. G. Belousova, A. A. Belyaevc, A. V. Biryukova, A. E. Bubnova, A. A. Bykadorov, V. I. Vasil'kova, I. S. Vinogradova, A. S. Gvamichavaa, A. N. Zinovieva, R. V. Komaevb, B. Z. Kanevskiya, N. S. Kardasheva, Yu. A. Kovaleva, Yu. Yu. Kovaleva, A. V. Kovalenkoa, Yu. A. Korneeva, V. I. Kostenkoa, B. B. Kreismana, A. Yu. Kukushkina, M. G. Larionova, S. F. Likhacheva, L. N. Likhachevaa, S. Yu. Medvedevc, M. V. Melekhinb, T. A. Mizyakinaa, N. Ya. Nikolaeva, B. S. Novikova, I. D. Novikova, Yu. K. Pavlenkoc, Yu. N. Ponomareva, M. V. Popova, V. N. Pyshnova, V. M. Rozhkove, B. A. Sakharovc, V. A. Serebrennikovb, A. I. Smirnova, V. A. Stepanyantsd, S. D. Fedorchuka, M. V. Shatskayaa, A. I. Sheikhetb, A. E. Shirshakovb, and V. E. Yakimova
a

Astro Space Center of the Lebedev Physics Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia b NPO Lavochkin, Federal Unitary Enterprise, ul. Leningradskaya 24, Khimki, 141400 Russia c ZAO Vremya Ch, Nizhni Novgorod, Russia d OAO Russian Space Systems, Russia, Moscow e Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, Miusskaya pl. 4, Moscow, 125047 Russia Abstract--The space interferometer Radioastron is working jointly with the largest radio telescopes of the world. Ground tracking stations provide for retrieving the information and determining the orbital parame ters for data processing centers. The project is aimed at systematic studies of images of radio emitting regions, their coordinates, and time dependent variations near super massive black holes in galactic nuclei, stellar mass black holes, neutron and quark stars, regions of star and planet formation in our and other galaxies, the structure of interplanetary and interstellar plasma, and the Earth's gravitational field. Keywords: space interferometer, image synthesis, flow, intensity, polarization, data processing and correlation center DOI: 10.1134/S0038094612070039

INTRODUCTION The launch of the space radio telescope (SRT) pro vides an opportunity to start studying astronomical objects with an angular resolution better by a factor of 30 than the resolution achieved to date. Combined with ground based radio telescopes, the SRT forms an Earth­space interferometer that has a baseline of up to 350 000 km. Table 1 contains the list of all the large radio telescopes with an effective antenna diameter greater than 60 m; the SRT frequency bands are given for which joint interferometric observations are possi ble. More detailed data are available at the website [1]. HIGH INFORMATION SRT RADIO LINK A high information radio link was developed and produced in the process of the Radioastron project implementation. Via this link, scientific data are trans mitted from the spacecraft (SC) to the ground based receiving complex for subsequent processing, and the service information on the SC and SRT conditions is provided for investigators.

The high information radio link includes the onboard high information radio complex (HIRC) (Fig. 1) and the ground based tracking station based on the RT 22 radio telescope at the Pushchino obser vatory of the Astro Space Center of the Lebedev Phys ics Institute (Fig. 2). The HIRC includes the following components: (1) A 15 GHz transmitter to transfer scientific and service data obtained during observations. The trans mitter power is 40 W; for data modulation, double dif ferential phase shift keying (DDPSK) is used. (2) A 8.4 GHz transmitter with an output power of 2 W. (3) A highly sensitive signal receiver at a frequency of 7.2 GHz (the noise temperature is 70 K); (4) The feed system of the high gain antenna with a diameter of 1.5 m and a tracking drive for accurate pointing to ground based tracking stations (GTSs). A ground tracking station (GTS) is designed for the following tasks: (1) Pointing the GTS antenna and tracking the spacecraft during the communication session.

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(2) Receiving the flow of scientific and service data from the SC; recording the data on hard drives. (3) Transmitting a phase stable reference signal synchronized by an H maser of the GTS to the SC. (4) Receiving the response signal that was coher ently transformed on board the SC, measuring the current frequency of a residual Doppler shift and cur rent phase difference between the response and request signals, and their recording with reference to the current time. (5) Receiving the external data required for the GTS work and issuing the information on GTS condi tion to investigators. To solve these problems, the ground based equip ment was designed and fabricated to work at frequen cies corresponding to those on board the HIRC, including the highly sensitive system of receiving the scientific and service data at a frequency of 15 GHz, the transmitter receiver system to work at 7.2/8.4 GHz, the system for recording and primary processing the sci entific and service data, the system of reference fre quencies with an H maser, and the time service with a GPS receiver. The RT 22 antenna was supplied with an especially developed feed system (FS) to work at the quoted frequencies. The onboard HIRC was subjected to a variety of trials and verifications, including tests carried out jointly with the onboard SRT research complex. The GTS complex was tested according to a specific pro gram of ground drills, as well as jointly with the onboard SRT scientific complex. The ability of the ONBOARD­EARTH link was confirmed to reliably receive scientific and service data. THE DATA PROCESSING CENTER OF THE SPACE INTERFEROMETER The results of scientific research carried out at the space interferometer are eventually processed and interpreted at the data processing center of the Astro Space Center of the Lebedev Physics Institute (ASC LPI), as well as at the processing centers of other project members. First, the cross correlation method is used to process the data flows that are recorded at differ ent radio telescopes, including the space segment of the SRT, using the RDR 1 recording system created at the ASC LPI (the recording density is 256 Kb/s). A software FX correlator of the ASC is based on an effective computing cluster with a performance of 1 TFLOP/s and the RAID data storage system with a capacity of up to 220 TB (Fig. 3). The cross correlation function of signals from sep arate interferometers is calculated using the computer realization that includes the following operations: Fourier transform­multiplication­inverse Fourier transform. In contrast to current hardware correlators, this sequence of operations enables us to enhance appre ciably the performance of data processing, to control
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Fig. 1. Onboard high information radio complex.

functionality and multimode properties of this process without additional financial expenses. The software correlator of the ASC LPI accepts data in any VLBI format currently utilized. To syn chronize the data flows within one s, the mission uses precision analytic models of the Radioastron SC motion and actual distant ballistic measurements of its orbital parameters provided by the interpretation cen ter of trajectory measurements of the Institute of Applied Mathematics (Russian Academy of Sciences) (Fig. 4).
Table 1. Large ground based radio telescopes Telescope Arecibo (the United States) GMRT (India) VLA (the United States) GBT (the United States) Effelsberg (Germany) WSRT (the Netherlands) Jodrell Bank (England) DSN Goldstone (the United States) DSN Robledo (Spain) DSN Tidbibilla (Australia) Evpatoriya (Ukraine) Ussuriisk (Russia) Parkes (Australia) Kalyazin (Russia) Usuda (Japan) Sardinia (Italy) Diameter of the SRT bands antenna (m) 300 246 125 100 100 93 76 70 70 70 70 64 64 64 64 P, P P, P, L, P, P, L, L, L, P, P, P, L, L, P, L, C L, C, K L, C, K C, K L, C L, C K K K L, C, L, C, L, C, C C, K L, C,

K K? K

K

Note: Radio telescopes with antenna diameters greater than 60 m that are involved in some observational programs together with the Radioastron radio telescope.


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Table 2. The strongest and most interesting compact extragalactic sources IAU name 1 0420 014 0528+134 0716+714 1055+018 1334 127 1730 130 1741 038 1749+096 2230+114 2255 282 0642+449 0851+202 1226+023 1228+126 1253 055 1508+572 1937 101 2200+420 2251+158 Other designations 2 z 3 0.915 2.07 (0.3) 0.888 0.539 0.902 1.057 0.320 1.037 0.927 3.408 0.306 0.158 0.004 0.538 4.309 3.787 0.069 0.859 S (Jy) 4 7.62 4.21 2.51 4.28 7.17 7.49 4.55 5.13 3.11 5.50 1.67 3.32 7.36 0.73 11.21 (0.1) 0.16 2.78 3.77 max (ms of arc) min (ms of arc) Tb (1013 K) 5 0.09 0.22 0.08 0.23 0.16 0.23 0.16 0.16 0.12 0.13 0.21 0.12 0.13 0.41 0.30 0.30 0.37 0.26 6 <0.02 <0.03 <0.01 <0.02 <0.01 <0.03 <0.02 <0.02 <0.03 <0.02 0.08 <0.05 <0.06 <0.27 <0.05 0.12 <0.03 0.11 7 >5.18 >2.06 >1.85 >1.36 >3.19 >1.50 >2.03 >1.34 >1.27 >2.24 0.43 >0.39 >0.17 >0.007 >0.88 0.015 >0.15 0.105 N
o

N 9 4 3 5 5 4 5 1 4 5 1 4 7 11 5 8 0 10 4

8 6 6 6 8 6 7 3 6 9 2 6 10 15 13 14 2 14 11

4C+01.28 NRAO530 OT 068 4C+09.57 CTA 102

OH471 OJ 287 3C 273 M87 3C279 VSOP BL Lac 3C454.3

Columns: 1--IAU name of the source; 2--other designations; 3--redshift; 4, 5, 6, 7--flux, maximum and minimum size, and bright ness temperature of an unresolved detail at 15 GHz; 8--number of observational epochs; 9--number of observational epochs when an unresolved detail was observed [3].

The operational rate of the ASC cluster makes it possible to receive the data flows from ten stations (including the Radioastron SRT) with a general den sity of 2.56 Gb/s and, accordingly, to process the flows from 45 interferometers formed in the experiment. This occurs almost without decreasing the input rate of observational data incoming in real time. Apart from the correlator, there is a graphic user interface ASL that is supported to implement pro

grams for solving specific astrophysical problems asso ciated with the following objects: (1) the sources with continuous spectra of radia tion; (2) the sources of monochromatic maser radiation (molecular lines OH and H2O); (3) the sources of pulse radiation with a continuous spectrum (pulsars); the aim is to determine the param eters of the medium in the Galaxy where the signals propagate. The cycle of interferometric experiments was car ried out in the SRT (ASC LPI) frequency bands jointly with national and foreign radio telescopes in 2010­ 2011 to verify the hardware and software required for data processing in the Radioastron project. The sensi tivity of the emulated facility and the quality of the results showed that they fully conformed to all the specifications. KEY AREAS OF THE SRT SCIENTIFIC RESEARCH The method of multifrequency synthesis (MFS) imaging is supposed to be implemented for the short est wavelengths (K range) in the Radioastron project [2]. One channel with circular polarization will oper ate at a fixed frequency of 22.232 GHz. The second channel (operating simultaneously) with circular polarization of opposite orientation will have the
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Fig. 2. Ground tracking station based on RT 22 (Pushchino).


Radioastron (SPECTR R PROJECT)--A RADIO TELESCOPE MUCH LARGER (a) (b)

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Fig. 3. Computing cluster with ten servers (performance of 1 TFLOP/s): (a) data storage with capacity of 220 TB; (b) backup device.

FTP Server
FCC TS GRT BC

Job scheduler (work station)

Work station of the CC Correlation Center (CC) Data archive Hard drive arrays

Astronomical data archive Researcher WEB Server Data exchange server

Application Software server correlator Visualization center FCC TS Work station

"Data editor" work station
Fig. 4. Scheme of the data processing center at the Astro Space Center of the Lebedev Physics Institute.

capability to switch in the range 18.392­25.112 GHz, i.e., fmax/fmin = 1.37. This will make it possible to obtain one dimensional images over the time that is determined by the integration time of each channel multiplied by the number of switched frequencies. Two dimensional images can be obtained twice per orbit with the maximum angular resolution, where the filling of an elliptical region in the spatial UV fre quency plane is 1 ­ (fmin/fmax)2 = 46%. It is important to note that this value does not depend on the dis
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tances and other orbital parameters. In general, the channel with the fixed frequency is fully compatible with the ground based radio telescopes. The fre quency tuned channel will be compatible with the same band of the K range of especially prepared ground based radio telescopes. Some advantages of the MFS method are as follows: (1) It is possible to obtain one dimensional images of sources with the highest angular resolution in less than an hour for any part of the orbit;


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40 Declination [µas] V â1E+10 20 0 A B

2 2 1 0 V â1E+10 1 0

­20 ­40 40 20 0 ­20 ­40 Right ascension [as]

­1

­1

­2 2 1 0 U â1E+10 ­2

­2 0 ­1 ­2 U â1E+10 1, NO POL, 22207 MHz Peak flux: Jy/beam. Center at RA19:37:06, DEC 85:12:47 Beam FWHM: 14.47 â 2.53 [as] at ­87.4° 2 1

0.8 Flux, Jy/beam 0.6 Declination [µas] ­40 ­20 1 R [as] 2 40

40 20 0

0.4 0.2 0

­20 ­40 0 40 20 0 ­20 Right ascension [as] ­40

Fig. 5. Simulation of the MFS method applied in Radioastron project [2]. Top left: true image (three point like sources with the fluxes FA = 0.5, FB =1.0, and FC = 0.5, in arbitrary units; AB = 12 s, AC = 15 s, and BC = 9 s. Top center: coverage of the UV plane when switching among eight frequencies of the K channel; bottom left: one dimensional image obtained as a result of this switching. Top right: coverage of the UV plane after five switchings among frequencies as the telescope passes a 5 day path in orbit; bottom right: the corresponding two dimensional image.

(2) Two dimensional images can be obtained dur ing 3­5 days in any part of the orbit or during 0.5­ 1 day near perigee; (3) It is possible to obtain spectra in the K range for different image details; (4) The angular size of an image can be determined as a function of frequency associated with the scatter, absorption, or other physical processes; (5) Maps of linear polarization and Faraday rota tion measures, or a map of circular polarization, can be produced and the degree of polarization can be determined as a function of frequency; (6) Differential coordinates and proper motions can be determined with a very high accuracy; (7) Physical variability of the source structure and/or variability due to interstellar plasma or plasma

in the envelope of the source can be studied as a func tion of frequency. To realize these goals, a corresponding observa tional mode can be chosen that is determined by the central frequency of the tuned channel, namely, 18.392, 19.352, 20.312, 21.272, 22.232, 23.192, 24.152, and 25.112 GHz. The bandwidth for each fre quency is 32 MHz. Figure 5 shows the results of numerical simulation for a one dimensional and two dimensional map of the source consisting of three point like components [5]. One of the main tasks of the scientific research pro gram of the Radioastron space interferometer will be studying the structure and dynamics of central regions of extragalactic radio sources of synchrotron emission, which will possibly enable us to obtain information from regions near the event horizons of supermassive
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Radioastron (SPECTR R PROJECT)--A RADIO TELESCOPE MUCH LARGER 15 GHz 3 3 1 1228 + 126 M87 1995/07/28 Relative decl. (mas) 10 5 0 ­5 VLBA 2 cm Survey 10 5 0 ­5 Relative R.A. (mas)
30 0 ­30 ­60 0

471

3C273 1226 + 023 21/05/99

Sc , Jy

0

100

200 300 ruv, M

400

500

­10

10 Relative decl. (mas) 5 0 ­5 ­10

1228 + 126

21/05/99

­10 +1858

3C273 Relative decl. (mas) 0

1

2

3

4

5

­5
­40 ­60 ­80 ­100 ­120 0 1

VLBA 2 cm Survey 10 5 0 ­5 ­10

­2592
2 3 4 5

20 0 ­20 ­40 0

+174 10 pc
1 2 3 4 5

Relative R.A. (mas)

­5 ­10 Relative R.A. (mas) ­4000­3000 ­2000 ­1000 0 1000 2000 PAD/M/M

Fig. 6. Radio galaxy M87 and quasar 3C273 are the most important extragalactic objects for study in the Radioastron project. Top left: correlated flux density of M87 as a function of baseline projection of the ground based VLBA interferometer at 15 GHz; bottom left: the corresponding image. Top right: image of 3C273 at the same instrument; bottom right: polarization angle as a function of observation frequency.

black holes. Extensive studies of the structure and spectra of extragalactic objects, as well as the detection of components unresolved on Earth, were carried out using many ground based radio telescopes and the VLBA global interferometer at 15 GHz [3]. The stron gest and most interesting compact sources are gath ered in Table 1. Figure 6 (left) shows the image of radio galaxy Virgo A (M87) and the observed flux of the radio emis sion as a function of interferometer baseline, i.e., the angular resolution in observations at a wavelength of 2 cm using the VLBA antenna. The center of M87 contains a supermassive black hole that is one of the largest currently known. Its mass is 6.6 billion solar masses [4]. Figure 6 shows that the central part of the object cannot be resolved even for the largest baselines. It will be possible to study the structure of the innermost parts of this object using the Radioastron interferometer for the first time; maybe, we will be even able to look inside (if it is an entrance to a wormhole rather than a black hole). The mini
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mum fringe width of the Radioastron interferometer is seven arc microseconds; it will be possible to measure the size of a source to within a part of the fringe (depending on the signal to noise ratio), e.g., one tenth of the fringe; this implies that the expected angu lar resolution will be better than one arc microsecond. The expected size of the black hole silhouette, i.e., the diameter of the Schwarzschild sphere around the black hole will be 2RS = 4GM/c2 = 14.6 s. The expected diameter of a circular orbit for light near a nonrotating black hole (a = 0) (108)0.5GM/c2 = 37.8 s. The diam eter of the silhouette of an extremal rotating black hole (a = 1) is 9GM/c2 = 32.8 s and the shift of the image center is (5)0.5 GM/c2 = 8.1 s [4]. Figure 6 (right) shows also the image of the quasar 3C273 at a wavelength of 2 cm [3] and the variations of polarization plane as a function of observational fre quency [5]. This object has one of the strongest radio emission fluxes among radio sources. It is especially interesting because it seems to have a one sided jet of relativistic particles. One possibility for explaining this


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Masers

7 6 5 4 3 2 1 0

0.2 pc

100 200 300 400 500 600 Mega wavelength Freq = 22.2325 GHz, Dw = 16.000 MHz

2 Kpc

Fig. 7. Model for generating the radio lines. Top: model of star formation region with masers; bottom left: observed VLA correlated H2O maser flux (22 MHz) for the region W3(OH) as a function of the interferometer baseline (millions of wavelengths are plotted along the horizontal axis) [10]. Bottom right: H2O megamaser and H line image of galaxy NGC 4258 (distance of 6.4 Mpc) [11].

Time, min 31 23 15 8 0 425.0 Frequency, MHz 427.5 430.0 432.5 435.0

Time, min 36 27 18 9

Time, min 0 52 39 26 13 0

Dec. 22, 1986

Dec. 28, 1986

M

B d1 D d2

Fig. 8. Dynamic spectrum of oscillation observed for the pulsar PSR 1237+25 with the Arecibo telescope at 430 MHz [12]. Top: dynamic spectrum of the pulsar PSR 1237+25 (period 1.4 s; distance 560 pc). Bottom: scheme of a two beam "interstellar" interferometer.

structure is to assume that it is an entrance to a worm hole rather than a supermassive black hole [6]. The most essential question to answer when mea suring the polarization of both objects is whether the

magnetic field around the central object has a dipole or monopole like structure. Perhaps the structure of the magnetic field is more complicated. The dipole character is anticipated if the magnetic field is due to
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Dec. 8, 1986


Radioastron (SPECTR R PROJECT)--A RADIO TELESCOPE MUCH LARGER 8 mJy 6

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4

Radio

2

Optics

0

X ray 0 Flux density, Jy 30 Flux density, Jy 20 10 40 20 0 10 20 30 Time, ms 40 2 4 6 s GHz 8

0 0 200 Time, s 400 600
Fig. 9. Observations of XTE J1810 540. Top right: super giant radio pulse from the Crab pulsar detected with RT 64 at Kalyazin (near Earth flux is 7 MJy at 2.2 GHz, brightness temperature T > 1040 K, electromagnetic field H > 1012 Gs) [15]. Top left: X ray image of the Crab pulsar obtained with the Chandra observatory. Bottom left: model of a rotating neutron star with magnetic field. Center: ordinary pulses from the neutron star in radio, optics, and X ray range over the course of 33 ms (rotation period of the neutron star). Bottom right: emission from the source XTE J1810 540 observed with the GBT telescope at 42 GHz (sometimes it is a radio emitting magnetar, period 5.54 s, magnetic field 2.6 â 1014 Gs) [14].

the accretion disk rotating around a supermassive black hole. The measurements of Faraday rotation near the core of 3C273 suggest that the opposite sides have opposite signs of rotation relative to the central object and, hence, opposite signs of magnetic field. This could be observational evidence for a monopole structure of the magnetic field. If this is the case, it may indicate the existence of an entrance to a wormhole or a black hole with a magnetic charge (former wormhole). Another important research direction is to discover where the cosmic ray sources are located in the Uni verse. Data obtained at the Pierre Auger observatory indicate that cosmic particles with the highest energies observed near the Earth come from the nearest radio galaxy Centaurus A (NGC 5128), from a distance of 3.5 Mpc. The mass of the black hole inside Cen A is 5.5 â 107 solar masses, i.e., approximately one hun
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dredth that of M87. This implies that extragalactic sources of powerful synchrotron radiation generate, at the same time, cosmic rays of the highest energies; if so, relativistic protons in the sources can also produce synchrotron radiation that we can try to detect. The ratio of the limiting brightness temperatures for relativistic protons and electrons (if the intensity is bounded by the Compton scattering of the same radi ation) is Tp/Te = (mp/me)6/5 = (1830)6/5 = 8821, where mp and me are the rest masses of the proton and elec tron, respectively. If the limiting temperature of the synchrotron radiation of relativistic electrons is Te = 1012 K, then the expected limiting radiation tempera ture of relativistic protons is Tp = 8 â 1015 K. For the given radiation flux, F (T/2) T/B2, where T is


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Measurement of anomalous accelerations to within 10­10 m/s2 and construction of the Earth's gravitational potential at large distances; Testing GTR effects (transverse Doppler effect, clock rate, improving the measured values of frequency redshift by two orders of magnitude, verifying the 1/R2 law of the gravitational interaction of bodies).

Fig. 10. Research topics to study the Earth's gravitational field using the data of interferometric observations and high accuracy measurements of the the Radioastron orbit and its evolution.

the brightness temperature, is the solid angle of the source, Be and Bp are the projections of the interfer ometer baseline on the plane of the sky (for the source emitting electrons or protons) that are required to resolve the source with the given flux for the limiting brightness temperatures. Then, Bp/Be = (mp/me)3/5 = 90. Therefore, interferometers with appreciably larger baselines are required to resolve the sources of syn chrotron radiation of protons compared to electrons. The synchrotron radiation of relativistic protons can be detected by very high brightness temperatures that exceed the limit for relativistic electrons. It is important to have in mind, however, that the bright ness temperature can increase due to the motion of the source toward the observer (Doppler enhancement), which can be controlled using the angular velocity measurements of proper motion or expansion of the source. Only the use of high angular resolution will make it possible to establish whether or not a stable source with a temperature of, say, 1015 K is observed. This would suggest that a generator of cosmic rays is really observed and provides the opportunity for us to study all its parameters. Many extragalactic sources show strong variability in observed radio emission flux. Such evidence enables us to give a lower bound for the brightness tempera ture. Thus, the variability of the source 0716+714 on timescales of less than one day implies that its brightness temperature is greater than 1015 K and even 1019 K [9]. Such a brightness temperature must correspond to a Lorentz factor of 90 in the Doppler enhancement model and this can only be verified using the space interferometer. The above sections of the research program were associated with the study of sources of synchrotron radiation that covers many frequency bands. The objects with narrow band maser radiation are another kind of strong source, which are qualitatively different.

Powerful narrow band radiation is observed in the lines of some molecules associated with the regions of young stars and planetary system formation. Figure 7 shows a model for generating this radiation in a star formation region and the observational results obtained for a water vapor line in the source W3(OH) at 22 GHz using the VLBA interferometer [10]. The baseline is measured along a horizontal axis and the correlated flux is plotted on the vertical axis. It can be seen that the source is very compact and cannot be resolved even with the largest baselines. Figure 7 shows also the sources of maser radiation in the same spectral line near the nucleus of the galaxy NGC 4258 [11]. Compact regions that emit in the water vapor line are observed around the central object (possibly a black hole). The regions are shown by dots; red and blue dots denote motions away from and toward us, respectively. This class of observed objects is referred to as megam asers; they are not yet resolved in detail and will be studied in the future. The expected limiting brightness temperatures for them may be up to 1016 K [10]. Together with the structure of star forming complexes and sizes of individual regions, parallaxes and proper motions of maser sources can be determined using the space inter ferometer, which is a very important research direction for constructing a model of our galaxy and other galaxies and for improving cosmological theory. A coherent radiation mechanism can ensure higher brightness temperatures as well. This mechanism is likely to be responsible for the radio emission of pul sars (neutron stars). However, the sizes of these objects (even nearest of them) are too small to be resolved even with the space interferometer. Figure 8 illustrates the method that, as a matter of fact, has not yet been used in radio astronomy. It can be only realized using special observations with the space interferometer. This method proposes observation of scintillations of the correlated signal due to inhomogeneities of the interstellar plasma. Scintillation appears as a result of
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the superposition of the signals that passed through the medium along different paths. This implies that inter ferometers with far greater baselines appear naturally as beams pass through the clouds of interstellar plasma. Figure 8 shows the dynamic spectrum of scin tillation observed for the pulsar PSR1237+25 with the Arecibo telescope at 430 MHz [12]. Time is plotted on the horizontal axis and frequency on the vertical axis. A clear striped pattern corresponds to that expected for two beam signal propagation, i.e., for a two antenna interferometer with a very large space baseline; however, there are no data available so far on the appearance of this pattern in different regions of space near the Earth. Such a distinct interference that corresponds to two beam sig nal propagation is observed rarely. Generally, a more complicated dynamic spectrum can be seen and multi beam signal propagation takes place, but the parameters of the effective interstellar interferometer can be deter mined in those cases as well [13]. This method, combined with the Radioastron, will possibly give resolutions hun dreds or thousands of times greater than those using the Earth­space interferometer. Lately, a new class of pulsars was discovered referred to as magnetars (PSR J1550 5418 with a period of 2.069 s and XTE J1810 5408 with a period of 5.54 s). Magnetars have anomalous flat spectra or even increasing for higher frequencies in the cm and mm bands [14]. It is extremely interesting how coherent radiation can arise at such high frequencies (that is, how tiny charge inhomogeneities can form). At the same time, these objects are transient X ray sources. Their pulsed radio emission appears sporadically too. Figure 9 (bottom right) shows observational data obtained for XTE J1810 540. For some pulsars, individual pulses were detected with amplitudes that exceeded the mean value by many orders of magnitude. In particular, such pulses are observed from the Crab pulsar with a periodicity of approximately one pulse per hour. These pulses have brightness temperatures of 1040 K, which is an absolute record [15]. This is a very interesting research topic, to study phenomena in the magnetospheres of neutron stars responsible for gigantic pulses; moreover, obser vation of these pulses can be of great importance in practice because it can be used to synchronize time all over the Earth provided that the pulse is observed at many sites and on board the Radioastron (Fig. 10). Figure 10 shows also the map of gravitational anomalies constructed using the observations on board the low orbit GRACE satellite [16]. The Radio astron will be able to produce the gravitational field pattern up to very large distances. The Radioastron space observatory is scheduled for launch in the current year. The scientific research pro gram, in accordance with technological capabilities and spacecraft mission life time, is expected to last at least five years. REFERENCES
1. www.asc.rssi.ru/radioastron/index.html
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