Is Scintillation the Key to a Better Celestial Reference Frame?

Abstract

Extragalactic radio sources, such as those that define the International Celestial Reference Frame (ICRF), display a variety of structure down to milliarcsecond scales. Further they are all variable on scales of years to weeks. This departure from the point source approximation introduces error in the observable variables (dc-lay and rate). The effect of source structure on Position can be as large as tens of milliarcseconds (e.g. []). Also, as the structure of these sources varies with time, it is important to image them at several epochs in order to define a time dependent source model. Multi-epoch observations, using the Very Long Baseline Array (VLBA) to image northern hemisphere sources and the Australian Long Baseline Array (LBA) to image southern hemisphere sources have been progressing successfully for a few years. However, the need for such extensive monitoring programs could be dramatically reduced if a population of extragalactic radio sources that have little or no milliarcsecond scale structure was available.Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources which exhibit interstellar scintillation suggest that such sources are among the most compact in the sky. In particular, the most variable weak sources, might be the most point-like and, thus, some of the best candidates for densification of the ICRF and consequent improvement in its accuracy. Further, the advent of the MarkV VLBI recording System, with its enhanced sensitivity, will make use of weaker sources easier. We present the evidence for and discuss the viability of this idea which has the potential to revolutionize future Upgrades of the ICRF.