Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

Abstract

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A✻ is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU ≈ 1400 Schwarzschild radii, the star has an orbital speed of ≈7650 km s−1, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z = Δλ / λ ≈ 200 km s−1/c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f , with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 ± 0.09|stat ± 0.15|sys. The S2 data are inconsistent with pure Newtonian dynamics.

Classical general relativity effects to second order in mass, spin, and quadrupole moment

“…This result agrees with the result expected from the Schwarzschild metric, up to first order in mass [34]. The gravitational redshift in the orbit of the star S2 agrees with the reported value in literature of 103km s −1 /c near the pericentre [21,22], as it is shown in table 6.…”

“…. This MBH is surrounded by the highly elliptical star S2 whose motion has been an important subject of study in the literature [21,22]. It has been determined that S2 has a semi-major axis a=8122±31 mas and an eccentricity e=0,88466±0,000018 , and so is possible to make an estimate of the contributions of the mass of the MBH to the orbit precession and the gravitational redshift and compare them with the values reported in the literature.…”

Seeing Black Holes: From the Computer to the Telescope

“…In particular, it was expected that the follow-up observations of S2 star closely orbiting Sgr A* might allow us to measure the Schwarzschild-like gravitational field of Sgr A* [31]. Indeed, as the present article was just completed, the observation of the high velocity of S2 star at its passage to pericentre in May 2018 and the associated gravitational redshift (Einstein effect) was announced, confirming once again the validity of General Relativity in the regime of strong gravitational field [32]. In the next few years, the radio interferometer SKA (Square Kilometer Array) [33], built in South Africa and Australia, will be able to follow the orbits of pulsars around the galactic black hole, timing them ultra-precisely to test their properties.…”

Search for a Variation of the Fine Structure Constant around the Supermassive Black Hole in Our Galactic Center

“…The motion of the short-period stars (S-stars) orbiting around the 4 × 10 6 M supermassive black hole (SMBH) at the center of our Galaxy has been monitored for 25 years by two experiments: one carried out at the Keck Observatory [26][27][28] and the other with the New Technology Telescope (NTT) and with the Very Large Telescope (VLT) [29][30][31]. Recently, these measurements have opened a new window to probe fundamental physics around a SMBH.…”

“…Recently, these measurements have opened a new window to probe fundamental physics around a SMBH. Measurements of the short-period star S0-2/S2 have been used to search for a fifth interaction [27,32], to measure the relativistic redshift during its 2018 closest approach [28,30] and to perform a nullredshift test [31].…”