Orbital precession of stars in the galactic centre

The region around the centre of our Galaxy is very dense of stars. The kinematics of inner moving stars in the Galaxy (the so-called S-stars) has been deeply studied by different research groups leading to the conclusion of the existence of a very compact object (Sgr A*, likely a supermassive black hole) responsible for their high speed. Here, we start from the observational evidence of orbital apsidal line precession for the S2 (also called S0-2) star to investigate on a theoretical side what level of quality in such regime of relatively strong gravitational field is reached in the orbit angular precession determination when using a direct orbital integration of the star motion subjected to an acceleration computed in the post-Newtonian (PN) scheme up to different orders. This approach, although approximated and limited to particle speed not exceeding ~ 0.3c, allows the inclusion of various effects, like that of a possible spin of the central massive object. Our results show that the inclusion of PN terms above the standard 1PN term (the one corresponding to the classic Einstein-Schwarzschild estimate of pericenter advance) is compulsory to determine angular precession at sufficient level of accuracy for those penetrating stars that would allow to pick contemporary the value of the mass and of the spin of a rotating (Kerr-like) supermassive black hole (SMBH). We discuss how future observational data, together with a proper modelization, could allow the determination of both mass and spin of the SMBH of our Galaxy.