Abstract
Context. We highlight the capabilities of geodetic VLBI technique to test general relativity in the classical astrometric style, i.e. measuring the deflection of light in the vicinity of the Sun.Aims. In previous studies, the parameterγwas estimated by global analyses of thousands of geodetic VLBI sessions. Here we estimateγfrom a single session where the Sun has approached two strong reference radio sources, 0229+131 and 0235+164, at an elongation angle of 1–3°.Methods. The AUA020 VLBI session of 1 May 2017 was designed to obtain more than 1000 group delays from the two radio sources. The solar corona effect was effectively calibrated with the dual-frequency observations even at small elongation.Results. We obtainedγwith a greater precision (0.9 × 10−4) than has been obtained through global analyses of thousands of standard geodetic sessions over decades. Current results demonstrate that the modern VLBI technology is capable of establishing new limits on observational tests of general relativity.
Highlights
Very long baseline interferometry (VLBI) measures the difference in arrival times of radio waves at two radio telescopes from distant radio sources with a precision of 20–40 ps (Schuh & Behrend 2012)
The accuracy of the parameterized post-Newtonian (PPN) parameter γ obtained from absolute or differential VLBI observations (Fomalont et al 2009; Lambert & Le Poncin-Lafitte 2009, 2011) is lower than the current best limit of (2.1 ± 2.3) × 10−5 based on the Cassini radio science experiments (Bertotti et al 2003) by an order of magnitude
The current paper focuses on radio source approaches at angular distances less than 3◦ from the centre of the Sun in order to measure the light deflection effect at the highest magnitude, and to avoid a possible bias caused by observations at larger elongations
Summary
Very long baseline interferometry (VLBI) measures the difference in arrival times (known as group delay) of radio waves at two radio telescopes from distant radio sources with a precision of 20–40 ps (Schuh & Behrend 2012). The previous attempts to observe reference radio sources close to the Sun used the standard scheduling strategy of geodetic experiments. In this mode all sources around the sky are observed a few times over 24 hours to provide a homogeneous sky coverage for each one-hour time span. The position of the second target 0235+164 is less accurate by a factor of five, but still at the level of the ICRF2 median error and significantly less that 1 mas Both sources are compact and their structure indices measured at the time of the ICRF2 work were 2.4 and 1.3, respectively (Ma et al 2009), ensuring a structure delay lower than 2 ps (Fey & Charlot 1997)
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