The intraday variable quasar 0917+624: VLBI and X-ray observations

We present the first VLBI images of the extremely rapidly variable radio source 0917+624, at frequencies ranging from 2.3 to 22 GHz. They reveal a compact core containing similar to 75% of the total flux density, and a jet extending to the northwest at position angle -20 degrees. The jet is curved on the sub-milliarcsecond scale, and one of the jet components is elongated perpendicular to the jet direction. This component displays apparent superluminal motion with a speed of beta(app)h = 7.8 +/- 0.8(1)). A first detection of X-rays from 0917+624 with the ROSAT satellite enables us to estimate the magnetic field and the Doppler factor from synchrotron self-Compton theory. The observational results are compared with models for the rapid variability. The orientation of the jet on the sky is consistent with a prediction from two-component models that were developed to explain the polarization variations in 0917+624. The physical parameters derived from the new observations can be accomodated both by intrinsic (shock-in-jet or coherent emission) and extrinsic (interstellar scattering) explanations of the variability.