Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network

Abstract

In this work, we studied the Galactic Center supermassive black hole (SMBH), Sagittarius A* (Sgr A*), with the KVN and VERA Array (KaVA)/East Asian VLBI Network (EAVN) monitoring observations. Especially on 13 May 2019, Sgr A* experienced an unprecedented bright near infra-red (NIR) flare; so, we find a possible counterpart at 43 GHz (7 mm). As a result, a large temporal variation of the flux density at the level 15.4%, with the highest flux density of 2.04 Jy, is found on 11 May 2019. Interestingly, the intrinsic sizes are also variable, and the area and major-axis size show a marginal correlation with flux density with >2σ. Thus, we interpret that the emission region at 43 GHz follows the larger-when-brighter relation in 2019. The possible origins are discussed with an emergence of a weak jet/outflow component and the position angle change of the rotation axis of the accretion disk in time.

arXiv comment15 pages, 7 figures, 3 tables, Published in the Galaxies Special Issue "Challenges in Understanding Black Hole Powered Jets with VLBI"

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• Origin of the ring ellipticity in the black hole images of M87*
• The Core-shift of Sagittarius A* as a Discriminant between Disk and Jet Emission Models with millimeter-VLBI