(Proposal ID) S15B-027

(PI) Nishiyama, Shogo

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

(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 the orbiting star S2, because it allows us to detect 
post-Newtonian 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 {second} 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 post-Newtonian 
effects amounts to ~30km/s in 2017, and ~190km/s at the next pericenter 
passage in 2018. The expected RV accuracy with IRCS is ~12km/s, and thus the 
post-Newtonian 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 post-Newtonian effects.