Astrometric Apparent Motion of High-redshift Radio Sources

Abstract

Radio-loud quasars at high redshift (z ≥ 4) are rare objects in the universe and rarely observed with Very Long Baseline Interferometry (VLBI). But some of them have flux density sufficiently high for monitoring of their apparent position. The instability of the astrometric positions could be linked to the astrophysical process in the jetted active galactic nuclei in the early universe. Regular observations of the high-redshift quasars are used for estimating their apparent proper motion over several years. We have undertaken regular VLBI observations of several high-redshift quasars at 2.3 GHz (S band) and 8.4 GHz (X band) with a network of five radio telescopes: 40 m Yebes (Spain), 25 m Sheshan (China), and three 32 m telescopes of the Quasar VLBI Network (Russia)—Svetloe, Zelenchukskaya, and Badary. Additional facilities joined this network occasionally. The sources have also been observed in three sessions with the European VLBI Network in 2018–2019 and one Long Baseline Array experiment in 2018. In addition, several experiments conducted with the Very Long Baseline Array in 2017–2018 were used to improve the time sampling and the statistics. Based on these 37 astrometric VLBI experiments between 2017 and 2021, we estimated the apparent proper motions of four quasars: 0901+697, 1428+422, 1508+572, and 2101+600.

J2102+6015: an Intriguing Radio-loud Active Galactic Nucleus in the Early Universe

“…Its high spectroscopic redshift (𝑧 = 4.575) is a tentative measurement and would need independent confirmation. Recent monitoring of its absolute astrometric position suggests a harmonic motion in the sky plane with a period of ∼ 3 yr, but the observations cover an interval only 1.5 times longer than the suspected period [10]. Earlier S/X-band VLBI imaging observations revealed a structure that is dominated by a bright mas-scale feature in the east.…”

“…However, because of the low signal-to-noise ratio in the optical spectrum, the authors labeled this value as marginal. Indeed, the object is rather faint in the optical (𝑔 = 24.9 and 𝑟 = 22.6 [9]) but the 𝑔 − 𝑟 colour difference is not in contradiction with the high redshift [10].…”

“…Most recently, an S/X-band astrometric VLBI study was devoted to measuring the absolute apparent proper motions of a sample of bright high-redshift radio quasars, including J2102+6015 [10]. Interestingly, the densely-sampled position time series covering a time range of about 4.5 yr from 2017 to 2021 hint on a quasi-periodic variation, which is especially prominent in right ascension with an amplitude of ∼ 0.3 mas and a period of about 3 yr.…”

“…Interestingly, the densely-sampled position time series covering a time range of about 4.5 yr from 2017 to 2021 hint on a quasi-periodic variation, which is especially prominent in right ascension with an amplitude of ∼ 0.3 mas and a period of about 3 yr. Certainly, longer-term observations that cover two or more suspected periods would be required to verify if the periodic signal persists [10]. Nevertheless, assuming that the periodicity in the absolute astrometric position is real, one can speculate about its astrophysical origin.…”

“…Nevertheless, assuming that the periodicity in the absolute astrometric position is real, one can speculate about its astrophysical origin. For example, a binary supermassive black hole system with sub-pc separation could produce jet precession that mimics the periodic change in the position of the 'centroid' of the radio emission [10], presumably inside the brightest eastern emission feature.…”