Trigonometric parallax and proper motion of Sagittarius A* measured by VERA using the new broad-band back-end system OCTAVE-DAS

Abstract

We successfully measured the trigonometric parallax of Sagittarius A* (Sgr A*) to be 117 ± 17 micro-arcseconds ($\, \mu \mbox{as}$) using the VLBI Exploration of Radio Astrometry (VERA) with the newly developed broad-band signal-processing system named “OCTAVE-DAS.” The measured parallax corresponds to a Galactocentric distance at the Sun of $R_0 = 8.5^{+1.5}_{-1.1}\:$kpc. By combining the astrometric results with VERA and the Very Long Baseline Array (VLBA) over a monitoring period of 25 yr, the proper motion of Sgr A* is obtained to be (μα, μδ) = (−3.133 ± 0.003, −5.575 ± 0.005) mas yr−1 in equatorial coordinates, corresponding to (μl, μb) = (−6.391 ± 0.005, −0.230 ± 0.004) mas yr−1 in Galactic coordinates. This gives an angular orbital velocity of the Sun of Ω⊙ = 30.30 ± 0.02 km s−1 kpc−1. We find upper limits to the core wander, Δθ < 0.20 mas (1.6 au), peculiar motion, Δμ < 0.10 mas yr−1 (3.7 km s−1), and acceleration, a < 2.6 $\, \mu \mbox{as} \:$yr−2 (0.10 km s−1 yr−1) for Sgr A*. Thus, we obtained upper mass limits of $\approx 3 \times 10^{4}\, M_{\odot }$ and $\approx 3 \times 10^{3}\, M_{\odot }$ for the supposed intermediate-mass black holes at 0.1 and 0.01 pc from the Galactic center, respectively.