Asymmetric free-free absorption towards a double lobe of OQ 208

We present 1.66 GHz VSOP (VLBI Space Observatory Programme) observations of a GHz-peaked spectrum (GPS) source, OQ 208. Two compact lobes with a 10 pc separation in the NE-SW direction are identified with those observed with ground VLBI at higher frequencies. We find that the two lobes are highly asymmetric in terms of a flux density ratio NE:SW of similar to 60:1 at 1.66 GHz, larger than that at higher frequencies. Both lobes show a steeply rising spectrum between 1.66 and 2.32 GHz, which implies free-free absorption by an ambient plasma, rather than synchrotron self-absorption. The absorption features show different peak frequencies, and indicate a larger optical depth towards the fainter SW lobe. This suggests the existence of an external absorber embedding the lobes. Hence, we can estimate the electron temperature, T-e, and the electron density, n(e), of the absorber. Considering the optical depth and X-ray luminosity, we derive the electron temperature and density in the ranges of 10(4) < T-e < 6 x 10(7) K and 600 < n(e) < 7 x 10(5) cm(-3), respectively. These properties coincide with those of NLR (Narrow Line Region). We point out that the NLR plasma affects evolution of radio galaxies, paticularly GPS sources.