Multi-Epoch VERA Observations of Sagittarius A*: I. Images and Structural Variability

Abstract

We report the results of multi-epoch observations of Sgr A* with VLBI Exploration of Radio Astrometry (VERA) at 43 GHz, carried out from 2004 to 2008. We detected a time variation of flux at 11 % level and intrinsic size at 19 %. In addition, comparisons with previous Very Long Baseline Array (VLBA) results shows that Sgr A* underwent the flaring event at least longer than 10 days in May 2007. The intrinsic size of Sgr A* remained unchanged within 1 $\sigma$ level from the size before/after the flaring event, indicating that the brightness temperature of Sgr A* was increased. The flaring event occurred within 31 d, which is shorter than the refractive time scale. Moreover it is difficult to explain the increase in the spectral index at the flaring event by the simple interstellar scattering model. Hence, the flaring event is most likely associated with the changes in intrinsic properties of Sgr A*. We considered the origin of the brightness temperature variation, and concluded that the flaring event of Sgr A* could be explained by the continuous heating of electrons, such as a standing shock in accretion flow.

13 pages, 7 figures, published in Publications of the Astronomical Society of Japan (PASJ)

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• Wisps in the Galactic center: NIR triggered observations of the radio source Sgr A* at 43 GHz
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• White Paper on East Asian Vision for mm/submm VLBI: Toward Black Hole Astrophysics down to Angular Resolution of 1~R${}_S$
• EVN Observations of HESS J1943+213: Evidence for an Extreme TeV BL Lac Object
• Long-term monitoring of Sgr A* at 7 mm with VERA and KaVA
• Trigonometric parallax and proper motion of Sagittarius A* measured by VERA using the new broad-band back-end system OCTAVE-DAS
• Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network
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• The Core-shift of Sagittarius A* as a Discriminant between Disk and Jet Emission Models with millimeter-VLBI