(Proposal ID) S17B-004

(PI) Nishiyama, Shogo

(Proposal Title)
General Relativistic Measurement of Mass of the Galactic SMBH 2

(Abstract)
Stars orbiting the supermassive black hole (SMBH) at the center of our Galaxy 
reach a few % of light speed during their pericenter passages, which makes 
general relativistic (GR) effects potentially detectable. Particularly crucial 
to our understanding of the property of the Galactic SMBH is the time 
evolution of the radial velocity (RV) of an orbiting star S2, because it 
allows us to detect post-Newtonian GR effects, from which we can determine 
the mass of the Galactic SMBH by comparing with GR numerical calculations. 
In the past, no high-precision RV measurement of S2 has been done, and no GR 
effect has been detected. Here we propose the 5th observation of our RV 
monitoring of S2 with Subaru/IRCS. Mainly due to the GR orbital precession of 
S2, the observed RV shows a different time evolution from the Newtonian one. 
The contribution of the GR effects amounts to ~90km/s at the next periapse in 
2018. The RV uncertainty obtained in 2016 with IRCS is ~13km/s, and the 
expected uncertainty in the future observations is ~10km/s. By comparing 
observed RVs and our GR numerical calculations, we will detect the GR effects 
in the RV of S2, and measure the mass of the SMBH with an uncertainty of a few
%. This is the last chance to measure the RV before periapse with Subaru.