We have performed quasi-simultaneous radio Ñux density measurements at 2.7 and 10 GHz for all Palomar-Green (PG) quasars with radio Ñux densities between 4 and 200 mJy. We Ðnd that a large fraction of these sources are variable, Ñat-spectrum quasars. This brings to 40% the total fraction of Ñat-spectrum quasars with a radio-to-optical Ñux ratio R [ 10 in the PG quasar sample. We also Ðnd that the median R-parameter of these Ñat-spectrum quasars is lower than those of steep-spectrum radioloud quasars. This contradicts the predictions of the uniÐed scheme and the idea that all Ñat-spectrum, core-dominated quasars are relativistically boosted lobe-dominated quasars. We show that this discrepancy is due to a population of Ñat-spectrum radio-intermediate quasars with 25 \ R \ 250 that can be explained neither as relativistically boosted radio-loud quasars nor as normal radio-weak quasars. We point out that a natural explanation for the Ñat-spectrum radio-intermediate quasars is relativistic boosting in radio-weak quasars. If the Ñat-spectrum radio-intermediate quasars are considered the boosted counterparts to usual radio-weak quasars, their fraction among radio-weak quasars is roughly 10%, similar to the fraction of boosted radio-loud quasars. This would point toward average Lorentz factors of for radio-loud and radio-weak quasars. The presence of the Ñat-spectrum radio-intermediate c jet \ 2È4 quasars changes the deÐnition of "" radio-loud ÏÏ and can bias some conclusions drawn from optically selected quasar samples, where R^1È10 is used as the dividing line for both Ñat-and steep-spectrum quasars. Instead, one should use separate R-parameters for the dividing line in steep-spectrum (R^25) and Ñat-spectrum (R D 250) quasars.

We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift radio-intermediate PG quasars (RIQ) with flat and variable radio spectrum (III Zw 2, PG 1309+355, PG 2209+184). Their radio-to-optical flux ratio (R) is slightly lower than the average R for steep-spectrum quasars, but their radio spectral properties are those of core-dominated quasars. It was proposed previously that these sources might be relativistically boosted jets in radio-weak quasars. Our VLBI observations now indeed confirm the presence of a high brightness temperature core in all three of these objectstwo of them have lower limits on T B well in excess of 10 10 Kelvin. Moreover, we find no "missing-flux" which means that basically all the flux of these quasars is concentrated in the compact radio core. As the total radio flux is already at the low end for radio-loud quasars, we can place a strong limit on the presence of any extended emission. This limit is consistent with the extended emission in radio-weak quasars, but excludes that the flat-spectrum RIQ reside in typical radio-loud quasars. The observations therefore strongly support the idea that relativistic jets are present in radio-weak quasars and hence that radio-loud and radio-weak quasars have very similar central engines.

Abstract.Using the Effelsberg 100-m telescope, sensitive measurements of the H2O megamaser in NGC 3079 are presented. During 2000-2001, "high velocity" features are seen that are red-shifted up to 225 km s −1 with respect to the systemic velocity of the galaxy (VLSR ∼ 1120 km s −1 ). Symmetrically bracketing the systemic velocity, the H2O emission covers a velocity range of ∼450 km s −1 with only one potential narrow gap (∼20 km s −1 ) near the systemic velocity itself. Velocity drifts of individual components are not convincingly detected. It is shown that the presence of red-shifted emission and the absence of detectable velocity drifts are not inconsistent with the existence of a rotating circumnuclear maser disk at the very center of the galaxy. Significant differences in the overall line profile compared to NGC 4258 and a complex morphology of the radio continuum leave, however, space for scepticism.

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Abstract. Radio observations in the ammonia inversion lines of four comets, C/2001 A2 (LINEAR), 153P/Ikeya-Zhang, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), were performed at the Effelsberg 100-m Radio Telescope during their respective close approaches to Earth. None of the four lowest energy metastable lines (J, K = J), J = 1-4, could be detected in these comets. We derive the following 3σ upper bounds on the NH 3 production rate , and comparing to the corresponding water production rates, percentage NH 3 abundances relative to H 2 O: Q(NH 3 ) < 1.9 × 10