Asymmetric Free-Free Absorption towards a Double Lobe of OQ 208

Abstract

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 ∼ 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, Te, and the electron density, ne, of the absorber. Considering the optical depth and X-ray luminosity, we derive the electron temperature and density in the ranges of 104 < Te < 6 × 107 K and 600 < ne < 7 × 105 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.