(Proposal ID) S17A-025

(PI) Nishiyama, Shogo

(Proposal Title)
General Relativistic Measurement of Mass of the Galactic Massive Black Hole

(Abstract)
Stars orbiting the massive black hole (MBH) 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 MBH 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 MBH by comparing with GR numerical calculations. In the past, 
{no} high-precision RV measurements of S2 have been done, and {no} GR effect 
has been detected. Here we propose the {4th} and {5th} observations of our RV 
monitoring of S2 with Subaru/IRCS. Mainly due to the special relativistic 
transverse Doppler shift, the gravitational redshift, and the GR motion 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 pericenter 
passage in 2018. The expected RV uncertainty with IRCS is ~ 12km/s, and thus 
the GR effects will be detectable even in the late 2017. 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 MBH with an uncertainty of ~3%.