High frequency very long baseline interferometry observations of active galactic nuclei

Millimeter wavelength very long baseline interferometry (VLBI) provides an excellent tool for the investigation of compact synchrotron sources. The high angular resolution and the decline in synchrotron opacity and lifetime with decreasing wavelength give unprecedented spatial and temporal resolution in active galactic nuclei.; We present here a description of the implementation of millimeter VLBI with the 9-element millimeter array at Hat Creek Radio Observatory. We emphasize the technical details of array phasing, calibration and observation. The Hat Creek array was used successfully in 5 VLBI experiments between 1995 and 1997.; We tested the technique of total power correction for atmospheric phase fluctuations due to water vapor with two epochs of 3mm {dollar}lambda{dollar} VLBI observations with the Hat Creek array and with the National Radio Astronomy Observatory (NRAO) Kitt Peak 12m antenna. We reduced the root-mean-square phase by as much as a factor of two and to as low as one radian. The limiting factor was the gain stability of the receivers.; We present 7mm {dollar}lambda{dollar} VLBI observations made with the NRAO Very Long Baseline Array (VLBA) of the compact, nonthermal radio source Sgr A*. We find that the image of Sgr A* is fully consistent with the hypothesis that it is a cyclo-synchrotron source powered by accretion onto a 10{dollar}sp6Msbodot{dollar} black hole and obscured by a screen of thermal electrons. We find no evidence for any asymmetric structure.; We also present a two year study of the gamma-ray blazar 530 with multiple epochs of VLBI imaging at 3 mm, 7 mm, 1.3 cm and 3.6 cm {dollar}lambda{dollar}. We incorporate flux density monitoring ranging from the radio band to positive correlation over 4 years between the millimeter and gamma-ray flux densities. We also find that a bright millimeter wavelength flare in early 1995 corresponded to the creation of a new parsec-scale jet component. We study the spectra and dynamical evolution of the jet. The jet component appears to decelerate from 7.6 {dollar}pm{dollar} 2.2c to 0.95 {dollar}pm{dollar} 0.16c and we consider models for that deceleration.