Fluctuation of Extragalactic Reference Frame Due to Gravitational Lensing in Our Galaxy.

Abstract

The observed positions of extragalactic objects are shifted due to the gravitational lensing of the matters in our galaxy. Their time variations introduce the aging of the frame. They set a theoretical limit to the accuracy of the extragalactic reference frame. We estimated their expectation values induced by the disk stars and MACHOs in our galaxy. Their typical values are 10 microarcsecond and several microarcsecond/ year, respectively. It is shown that if MACHOs exist, they will lift up the expectation value of induced proper motion, though they contribute little to the positional shift. Also we evaluated the optical depth of such fluctuations. We found that the positional shifts of the 10 microarcsecond level would occur rather rarely. In such case, they are mainly caused by the nearest single source to the line of sight, and therefore, are correctable. In the case of 1 microarcsecond level accuracy, we will face difficulty on the construction of the reference frame. Besides, the detection of these effects will be useful for probing the components of our galaxy.