The orbit of the star S2 around SgrA* from VLT and Keck data

TL;DR

This paper combines independent astrometric data sets from VLT and Keck to refine measurements of the Milky Way's central black hole's mass and distance, highlighting the importance of reference frame calibration.

Contribution

It demonstrates that aligning the reference frames of two independent data sets improves the consistency and accuracy of stellar orbit measurements around SgrA*.

Findings

Combined data yields R0 = 8.28 ± 0.15 (stat) ± 0.29 (sys) kpc.

Mass of SgrA* estimated at 4.30 ± 0.20 (stat) ± 0.30 (sys) million solar masses.

Systematic errors dominate the overall measurement uncertainties.

Abstract

Two recent papers (Ghez et al. 2008, Gillessen et al. 2009) have estimated the mass of and the distance to the massive black hole in the center of the Milky Way using stellar orbits. The two astrometric data sets are independent and yielded consistent results, even though the measured positions do not match when simply overplotting the two sets. In this letter we show that the two sets can be brought to excellent agreement with each other when allowing for a small offset in the definition of the reference frame of the two data sets. The required offsets in the coordinates and velocities of the origin of the reference frames are consistent with the uncertainties given in Ghez et al. (2008). The so combined data set allows for a moderate improvement of the statistical errors of mass of and distance to Sgr A*, but the overall accuracies of these numbers are dominated by systematic errors and the long-term calibration of the reference frame. We obtain R0 = 8.28 +- 0.15(stat) +- 0.29(sys) kpc and M(MBH) = 4.30 +- 0.20(stat) +- 0.30(sys) x 10^6 Msun as best estimates from a multi-star fit.