RATAN-600 radio telescope was used to study the spectral characteristics of a uniform sample of 504 radio sources from the NVSS (NRAO VLA Sky Survey) catalog near the North Celestial Pole at six frequencies, with the aim of selecting sources possessing inverted spectra near 22 GHz, to be included in the program of the RadioAstron future space VLBI mission.

We selected NVSS sources with the following properties for the observations: +75^o +88^o, 00^h RA 24^h, flux density S 200 mJy at the frequency 1.4 GHz. The observations were carried out at 0.9, 2.3, 4.8, 7.7, 11.2, and 21.7 GHz.
The most comprehensive survey of the sources in this region of the sky was carried out at frequency of 1.4 GHz with the use of aperture synthesis VLA (Condon et al., 1998).

We obtained flux densities for 28.6% of the objects at 21.7 GHz and for most of the sample objects at the other frequencies (Mingaliev et al., 2007, 2009), Table 1. We derived the distribution of spectral types for the sources (figure 1). For example, we found for 4.8 GHz (for S ~ , - frequency, - spectral index):

• 65% of the sources have normal spectra (- 1.1 < < - 0.5),
  
• 24% have steep spectra ( < - 1.1),
  
• 7.3% have flat spectra (- 0.5 < < 0),
  
• 2.3% have inverted spectra ( > 0),
  
• 1.4% have spectra with maxima at centimeter wavelengths.
  

Figure 1. Spectral indices distributions for the frequencies between 0.9 and 21.7 GHz.

REFERENCES

• Mingaliev et al., 2009ARep....53..487M
• Mingaliev et al., 2007ARep....51..343M
• Condon et al., 1998AJ....115.1693C