Spectral Characteristics and Variability of Radio Sources near the North Celestial Pole

We present the results of our observations of compact extragalactic radio sources near the north celestial pole (+75 degrees <= delta <= +88 degrees) obtained on the RATAN-600 radio telescope. Our sample consists of 51 radio sources with spectra that are either flat or inverted (growing toward shorter wavelengths) and with flux densities at 1.4 GHz S-nu <= 200 mJy. We observed the sources at 1-21.7 GHz. Multi-frequency instantaneous spectra are presented for 1999-2007. We observed 33 of our sample source daily for 30 days in August 2007. As a result, we revealed 15 objects exhibiting rapid variations on time scales of a day. The multi-frequency instantaneous spectra of these sources indicate that radio flux variations on one-day timescales are characteristic of objects of various spectral types. More than half the sources exhibiting rapid variations demonstrate a growth in the variability amplitude with increasing frequency. For some of the objects, the variability amplitude is virtually independent of frequency.