Kinematics of the compact symmetric object OQ 208 revisited

Aims. A long-timeline kinematic study of the archetypal compact symmetric object (CSO) OQ 208 sheds light on the physical properties of the most compact radio sources. Methods. Archival data from the Very Long Baseline Array (VLBA) at 15 GHz over a time span of 13.6 yr were used to investigate the kinematics of the radio source. The flux density monitoring data obtained at the Michigan 26-m radio telescope were also used as supplementary information for analyzing the geometry of the radio structure. Results. At 2.3-GHz, the radio emission is dominated by two mini-lobes separated by similar to 10 pc in a northeast-southwest (NE-SW) direction. At 8.4 and 15 GHz, each lobe is further resolved into two subcomponents, which are identified as hotspots. A knotty jet is linked with the NE hotspot and traces back toward the geometric center. The core is too weak to be detected. Significant flux density variation is found in the primary hotspots with a maximum level of 62% (NE1) and 19% (SW1). The flare epoch of NE1 is earlier than that of SW1 by approximately 5.00 yr, suggesting that the northeast lobe is advancing and the southwest lobe is receding. This light travel difference indicates a radial distance difference between the two hotspots of 1.53 pc, which indicates an inclination angle of about 80.8 degrees between the radio jet and the line of sight. The angular separation rate between NE1 and SW1 is 0.027 mas yr(-1), corresponding to a projected speed of 0.133 c. The inner jet knot (J1) moves at 0.047 mas yr(-1) (or 0.230 c), about 3.5 times the hotspot advancing speed. Conclusions. The large viewing angle and the modest jet speed suggest a mildly relativistic jet. The jet axis is close to the plane of the sky. The separation rate and the distance between the two primary hotspots result in a kinematic age of 255 +/- 17 yr, confirming that OQ 208 is indeed a young radio source. In addition to the hotspot advancing motions, sideways motions provide evidence that the lobes are obstructed by the external interstellar medium.