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arXiv · 1709.01598

Radial Velocity Measurements of an Orbiting Star Around Sgr A*

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

During the next closest approach of the orbiting star S2/S0-2 to the Galactic supermassive black hole (SMBH), it is estimated that RV uncertainties of ~ 10 km/s allow us to detect post-Newtonian effects throughout 2018. To evaluate an achievable uncertainty in RV and its stability, we have carried out near-infrared, high resolution (R ~ 20,000) spectroscopic monitoring observations of S2 using the Subaru telescope and the near-infrared spectrograph IRCS from 2014 to 2016. The Br-gamma absorption lines are used to determine the RVs of S2. The RVs we obtained are 497 km/s, 877 km/s, and 1108 km/s in 2014, 2015, and 2016, respectively. The statistical uncertainties are derived using the jackknife analysis. The wavelength calibrations in our three-year monitoring are stable: short-term (hours to days) uncertainties in RVs are < 0.5 km/s, and a long-term (three years) uncertainty is 1.2 km/s. The uncertainties from different smoothing parameter, and from the partial exclusion of the spectra, are found to be a few km/s. The final results using the Br-gamma line are 497 +- 17 (stat.) +- 3 (sys.) km/s in 2014, 877 +- 15 (stat.) +- 4 (sys.) km/s in 2015, and 1108 +- 12 (stat.) +- 4 (sys.) km/s in 2016. When we use two He I lines at 2.113\mum in addition to Br-gamma, the mean RVs are 513 km/s and 1114 km/s for 2014 and 2016, respectively. The standard errors of the mean are 16.2 km/s (2014) and 5.4 km/s (2016), confirming the reliability of our measurements. The difference between the RVs estimated by Newtonian mechanics and general relativity will reach about 200 km/s near the next pericenter passage in 2018. Therefore our RV uncertainties of 13 - 17 km/s with Subaru enable us to detect the general relativistic effects in the RV measurements with more than 10 sigma in 2018.

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Shogo Nishiyama, Hiromi Saida, Yohsuke Takamori, Masaaki Takahashi, Rainer Schoedel, Francisco Najarro, Satoshi Hamano, Masashi Omiya, Motohide Tamura, Mizuki Takahashi, Haruka Gorin, Schun Nagatomo, Tetsuya Nagata. 2017-09-07. Radial Velocity Measurements of an Orbiting Star Around Sgr A*. https://doi.org/10.1093/pasj%2Fpsy067

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