Multifrequency VLA observations of GHz-peaked-spectrum radio cores

We present simultaneous, multifrequency VLA observations of the cores of 18 suspected GPS sources. 3 of the 18 sources have flat spectra and are probably variable. The remaining 15 are confirmed as true GPS sources with spectral peaks between 0.8 and 6 GHz. The core of the compact double source 0108 + 388 has an optically-thick spectral index of α = +2.14 ± 0.09 which is the most inverted of any compact extragalactic radio source currently known. We combined our data with recent high frequency measurements of four sources with the result that the high frequency spectra of these sources are found to either flatten of fail to steepen out to 90 GHz (230 GHz for 0552 + 398). The flattening of the spectrum indicates that there is very compact structure in two sources in addition to the components which peak near ∼ 1 GHz. The fractional polarization at 22, 18, and 6 cm is remarkably low in the GPS sources. The majority of sources have polarizations less than 0.5% and only one source with an isolated core has a value greater than 1%. At 1.3 cm we obtain upper limits of a few percent for the polarization of isolated cores. Faraday rotation measures are obtained for four GPS sources