Radial velocity measurements of an orbiting star around Sgr A*

Nishiyama, Shogo; Saida, Hiromi; Takamori, Yohsuke; Takahashi, Masaaki; Schödel, Rainer; Najarro, Francisco; Hamano, Satoshi; Omiya, Masashi; Tamura, Motohide; Takahashi, Mizuki; Gorin, Haruka; Nagatomo, Schun; Nagata, Tetsuya

During the next closest approach of the orbiting star S2/S0-2 to the Galactic supermassive black hole (SMBH), it is estimated that radial velocity (RV) measurements with an uncertainty of a few 10 km s-1 will allow us to detect post-Newtonian effects throughout 2018. To evaluate the best achievable uncertainty in RV and its stability, we carried out near-infrared, high-resolution (R ̃ 20000) spectroscopic monitoring observations of S2 using the Subaru telescope and its near-infrared spectrograph IRCS from 2014 to 2016. Br-γ absorption lines have been detected in the 2015 and 2016 spectra, but have not been clearly detected in the 2014 spectrum. The detected Br-γ absorption lines are used to determine the RVs of S2. The statistical uncertainties are derived using the jackknife analysis, and spectra combined from divided subdata sets. The wavelength calibrations in our three-year monitoring are stable: short-term (hours to days) uncertainties in RVs are ≲ 0.5 km s-1, and the long-term (three years) uncertainty is 1.2 km s-1. We thoroughly analyzed possible sources of systematic uncertainties, such as the incomplete subtraction of OH skylines. The relevant uncertainties are estimated to be less than several km s-1. The final results using the Br-γ line are 877 ± 25 km s-1 in 2015, and 1109 ± 14 km s-1 in 2016. When we use two He I lines at 2.113 μm in addition to Br-γ, the mean RV and its standard error are 1114 km s-1 and 5 km s-1, respectively, in 2016. However, we have found a larger scatter around the expected RV curve with the best-fitting orbiting parameters of S2, implying additional uncertainties not yet considered. The difference between the RVs estimated by Newtonian mechanics and general relativity will reach about 200 km s-1 near the next pericenter passage in 2018. Therefore, in addition to astrometric and spectroscopic data obtained with other telescopes, RV measurements with Subaru in 2018 will form important data sets with which to detect general relativistic effects from the SMBH.

DOI: 10.1093/pasj/psy067