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RADIO SPECTRA PROPERTIES OF A COMPLETE SAMPLE
OF SOURCES NEAR THE NORTH CELESTIAL POLE
M.G. Mingaliev 1 , M.G. Larionov 2 , J.V. Sotnikova 1 , N.N. Bursov 1 , N.S. Kardashev 2
1 Special Astrophysical Observatory of RAS, Nizhnij Arkhyz, 369167, Russia;
2 Astro Space Center, Lebedev Physical Institute of RAS, Moscow, Russia
marat@sao.ru
The RATAN­600 radio telescope was used to study spectral properties for a complete sample
of 504 sources from the NVSS catalogue near the North Celestial Pole. The main task of
the work was to determine instantaneous spectra of radio sources with the purpose to
select objects with inverted spectra near the 22 GHz frequency for subsequent investigation
under the space VLBI Project ``RadioAstron''. The high angular resolution of the project
``RadioAstron'' which is to be achieved 10 -6 arcsec imposes strict demands to angular
dimension of sources. These must be super­compact objects with a high value of the
correlated flux. Such objects form a considerable part of objects just with inverse and
flat spectra. At present there are no complete high­frequency catalogues of such objects
up to low flux density levels (0.2 Jy at 22 GHz). The only available data on the North
Celestial Pole are the VLA survey (NVSS) at 1.4 GHz. It is important to obtain spectral
characteristics up to the highest frequency of 22 GHz planned in the work of the space
interferometer. The following criteria were used in selection of sources from the catalogue
NVSS:
1. 00 h
# RA2000 # 24 h
2. +75 o
# DEC2000 # +88 o
3. Flux density: S # [1.4GHz] # 200 mJy from the NVSS catalog
The total number of sources is 504. After data reduction we obtained flux densities of
sources and their spectral characteristics. The sources spectral types were determined: 65%
-- normal, 24% -- steep, 7.3% -- flat, 2.3% -- inverted, and 1.4% -- spectra with a maximum
at centimeter wavelengths (GPS). Eleven sources with inverted spectra were detected. The
statistics of the sources spectra from our sample contrasts with spectral characteristics of
the sample of objects with the same initial parameters but carried out at the frequency
20 GHz by Sadler et al. (2006). We obtained that there is a 25% deficit of sources with
the inverted spectra in our sample. This can be explained by the spectral properties of the
``subliminal'' sources, which did not fall into the initial sample at the frequency of 1.4 GHz.
References
Sadler E.M., Ricci R., Ekers R. D., Ekers J. A., Hancock P.J., Jackson C. A., Kesteven
M.J., Murphy T., Phillips Ch., Reinfrank R. F., Staveley­Smith L., Subrahmanyan R.,
Walker M. A., Wilson W.E., de Zotti, G.: MNRAS, 371, 898.