Relativistic redshift of the star S0-2 orbiting the Galactic Center supermassive black hole

Abstract

2General Relativity predicts that a star passing close to a supermassive black hole should exhibit a relativistic redshift. We test this using observations of the Galactic center star S0-2. We combine existing spectroscopic and astrometric measurements from 1995-2017, which cover S0-2's 16-year orbit, with measurements in 2018 March to September which cover three events during its closest approach to the black hole. We detect the combination of special relativistic-and gravitational-redshift, quantified using a redshift parameter, Υ. Our result, Υ = 0.88 ± 0.17, is consistent with General Relativity (Υ = 1) and excludes a Newtonian model (Υ = 0 ) with a statistical significance of 5 σ.General Relativity (GR) has been thoroughly tested in weak gravitational fields in the Solar System (1), with binary pulsars (2) and with measurements of gravitational waves from stellarmass black-hole binaries (3,4). Observations of short-period stars in our Galactic center (GC) (5-8) allow GR to be tested in a different regime (9): the strong field near a supermassive black hole (SMBH) (10,11). The star S0-2 (also known as S2) has a 16 year orbit around Sagittarius A* (Sgr A*), the SMBH at the center of the Milky Way. In 2018 May, it reached its point of closest approach, at a distance of 120 astronomical units (au) with a velocity reaching 2.7% of the speed of light. Within a 6 months interval of that date, the star also passed through its maximum (March) and minimum velocity (September) along the line-of-sight, spanning a range of 6000 km s −1 in radial velocity (RV - Fig. 1). We present observations of all three events and combine them with data from 1995-2017 ( Fig. 2).During 2018, the close proximity of S0-2 to the SMBH causes the relativistic redshift, which is the combination of the transverse Doppler shift from special relativity and the gravitational redshift from GR. This deviation from a Keplerian orbit was predicted to reach 200 km s −1 (Fig. 3) and is detectable with current telescopes. The GRAVITY collaboration (9) previously reported a similar measurement. Our measurements are complementary: i) we present a 3 complete set of independent measurements with 3 additional months of data, doubling the time baseline for the year of closest approach, and including the third turning point (RV minimum) in September 2018, ii) we use three different spectroscopic instruments in 2018, which allows us to probe the presence of instrumental biases, iii) we perform an analysis of the systematic errors that may arise from an experiment spanning over 20 years to test for bias in the result, and iv) we publicly release the stellar measurements and the posterior probability distributions.We use a total of 45 astrometric positional measurements (spanning 24 years) and 115 RVs (18 years) to fit the orbit of S0-2. Of these, 11 are new astrometric measurements of S0-2 from 2016 to 2018 and 28 are new RV measurements from 2017 and 2018 ( Fig 1). Astrometric measurements were obtained at the W. M. Keck Observatory using speckle imaging (a ...

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

“…We infer the radial acceleration experienced by the stars using 25 years of astrometric measurements of the GC. Between 1995 and 2005, speckle imaging data provides astrometric diffraction-limited measurements of the central 5" × 5" of the GC [26,28,49]. Between 2005 and 2018, adaptive optics (AO) imaging provides high-resolution images of the central 10"× 10" of the GC [26,28].…”

“…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].…”

“…The imaging data include both speckle imaging from the Near-Infrared Camera (NIRC) instrument (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) and adaptive optics data from the Near-Infrared Camera 2 (NIRC2) instrument (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). Details of the data reduction are given in [28]. NIR spectra were obtained with the Near-Infrared Integral Field Spectrometer (NIFS) instrument on Gemini North for the stars S0-6, S0-12, S0-13, and S1-5 using the K-band filter (0.95 to 2.40 µm) at a spectral resolution R = λ/∆λ = 5000.…”

Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA

“…Sgr A* is a bright and compact radio source at the Galactic center and is one of the most convincing candidates for supermassive black holes. The mass of Sgr A* has been measured to be 4 × 10 6 M ⊙ through analysis of the orbital parameters of the fast-moving star S2 (e.g., Gravity Collaboration et al 2018a;Do et al 2019a). The radio band of Sgr A* reveals a positive spectral index α (where F ν ∝ ν α ) that peaks around the "submillimeter bump" (Falcke et al 1998), at which point the flux rapidly drops to the infrared regime.…”