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Register a new userAstrometric accuracy of phase-referenced observations with the VLBA and EVN
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Astrometric accuracy of complex modern VLBI arrays cannot be calculated analytically. We study the astrometric accuracy of phase-referenced VLBI observations for the VLBA, EVN and global VLBI array by simulating VLBI data for targets at declinations -25$^circ$, 0$^circ$, 25$^circ$, 50$^circ$, 75$^circ$ and 85$^circ$. The systematic error components considered in this study are calibrator position, station coordinate, Earth orientation and troposphere parameter uncertainties. We provide complete tables of the astrometric accuracies of these arrays for a source separation of 1$^circ$ either along the right ascension axis or along the declination axis. Astrometric accuracy is 50microas at mid declination and is 300microas at low (-25$^circ$) and high (85$^circ$) declinations for the VLBA and EVN. In extending our simulations to source separations of 0.5$^circ$ and 2$^circ$, we establish the formula for the astrometric accuracy of the VLBA: Delta = (Delta_1$^circ$-14)*d+ 14 (microas) where Delta_1$^circ$ is the astrometric accuracy for a separation d=1$^circ$ provided in our tables for various declinations and conditions of the wet troposphere. We argue that this formula is also valid for the astrometric accuracy of the EVN and global VLBI array.
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The recent increase in well-localised fast radio bursts (FRBs) has facilitated in-depth studies of global FRB host properties, the source circumburst medium, and the potential impacts of these environments on the burst properties. The Australian Square Kilometre Array Pathfinder (ASKAP) has localised 11 FRBs with sub-arcsecond to arcsecond precision, leading to sub-galaxy localisation regions in some cases and those covering much of the host galaxy in others. The method used to astrometrically register the FRB image frame for ASKAP, in order to align it with images taken at other wavelengths, is currently limited by the brightness of continuum sources detected in the short-duration (snapshot) voltage data captured by the Commensal Real-Time ASKAP Fast Transients (CRAFT) software correlator, which are used to correct for any frame offsets due to imperfect calibration solutions and estimate the accuracy of any required correction. In this paper, we use dedicated observations of bright, compact radio sources in ASKAPs low- and mid-frequency bands to investigate the typical astrometric accuracy of the positions obtained using this so-called snapshot technique. Having captured these data with both the CRAFT software and ASKAP hardware correlators, we also compare the offset distributions obtained from both data products to estimate a typical offset between the image frames resulting from the differing processing paths, laying the groundwork for future use of the longer-duration, higher signal-to-noise ratio data recorded by the hardware correlator. We find typical offsets between the two frames of $sim 0.6$ and $sim 0.3$ arcsec in the low- and mid-band data, respectively, for both RA and Dec. We also find reasonable agreement between our offset distributions and those of the published FRBs. <Abridged>
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