Possibility of Measuring spin precession of the nearest supermassive   black hole by S stars
  The supermassive black hole (SMBH) with 4 millions solar mass inside the radio-source Sgr A* in our Galactic center is the nearest SMBH. Once the S stars with shorter period are observed, the relativistic precessions especially the Lense-Thirring one can be measured by astronomical observations at 10 micro-arcsecond ($\mu$as) level in future. An interesting but so far no attended problem is that the SMBH has not only spin but also spin precession like the other objects in the universe. In this paper, we study the effect of such spin precession on the orbital precessions of orbiting stars. Our results show that the spin precession can produce a periodic oscillation on the precession of the star's orbital plane, but no obvious effect on the periapse shift. For stars with $O(0.1)$yr period, such visible oscillations occur while the SMBH's spin-precession period is about from few tens of years to hundreds of years. And the period of oscillation is the same with the one of the spin precession. In principle, this oscillating orbital plane's precession can be observed and then the spin and spin precession of the nearest supermassive black hole can be determined. 
Wen-Biao Han
astro-ph.GA gr-qc