(Proposal ID) S16A-038

(PI) Nishiyama, Shogo

(Proposal Title)
Post-Newtonian Effects on Stellar Dynamics near Massive Black Hole 2

(Abstract)
Stars orbiting the supermassive black hole at the center of our Galaxy reach a 
few % of light speed during their pericenter passages, which makes General 
relativistic (GR) perturbations potentially detectable. Particularly crucial 
to our understanding of the theory of gravity is the time evolution of the 
radial velocity (RV) of an orbiting star S2, because it allows us to detect 
post-Newtonian (PN) effects, and to test GR as a theory of gravity in the 
{strongest} relativistic field limit ever probed. In the past, {no} high-
precision RV measurements of S2 have been done, and {no} PN effect has been 
detected. Here we propose the {third} observation of our RV monitoring of S2 
with Subaru/IRCS. Due to the special relativistic transverse Doppler shift and 
the gravitational redshift, the observed RV shows a different time evolution 
from the pure Keplerian one. The contribution of the PN effects amounts to 
~30km/s in 2017, and ~190km/s at the next pericenter passage in 2018. The 
expected RV uncertainty with IRCS is ~12km/s, and thus the PN effects will be 
detectable even in 2017. By comparing RVs observed in 2014 - 2016, which are 
almost the same as the Keplerian prediction, and those in 2017 - 2018, we will 
detect the deviation of RVs due to the PN effects.