Extending the ICRF to Higher Radio Frequencies -- First Imaging Results

Details Extending the ICRF to Higher Radio Frequencies -- First Imaging Results Extending the ICRF to Higher Radio Frequencies -- First Imaging Results

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aas...201.7611f&link_type=abstract

American Astronomical Society, 201st AAS Meeting, #76.11; Bulletin of the American Astronomical Society, Vol. 34, p.1227

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We present first imaging results and source structure analysis of 65 extragalactic sources observed using the Very Long Baseline Array (VLBA) at 24 GHz and 43 GHz as part of a joint NASA, USNO, NRAO and Bordeaux Observatory program to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies. The long term goals of this program are a) to develop higher frequency reference frames for improved deep space navigation, b) to extend the VLBA calibrator catalog at 24 and 43 GHz, c) to provide the benefit of the ICRF catalog to new applications at these higher frequencies and d) to study source structure variation at 24 and 43 GHz in order to improve the astrometric accuracy. In this paper, we concentrate on the latter goal of evaluating the intrinsic structure of the observed ICRF sources. Preliminary analysis of the observations taken to date shows that the sources are generally more compact as one goes from the ICRF frequency of 8.4 GHz to 24 GHz. This is consistent with the standard theory of extragalactic radio sources in which the emission from quasars and active galactic nuclei is assumed to be powered by a central engine (presumably a black hole) where energetic phenomena occur. The frequency dependent intrinsic structure usually consists of a flat spectrum core with extended emission in the form of multiple steep spectrum jet components which may move superluminally away from the core. The core emission is usually associated with the base of the jet which is presumably very close to the central engine. As one goes higher in observing frequency, the steep spectrum emission will become weaker and be resolved, leaving only the naked core. Since the positions on the sky of the central engines are thought to be stable at the micro-arcsecond level, a reference frame defined at higher radio frequencies should, in general, be more stable than one defined at a lower frequency. However, preliminary analysis of the data suggests that the observed sources may not be more compact at 43 GHz than at 24 GHz as expected but may instead be more extended at 43 GHz. This result suggests that the core emission has become optically thin and that we have started to resolve the core at 43 GHz.

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