1. Probing the close environment of the supermassive black hole at the center of the galaxy with GRAVITY Probing Strong Gravity Prague, February 18, 2010 Perrin

2. Probing the close environment of the supermassive black hole at the center of the galaxy with GRAVITY Probing Strong Gravity Prague, February 18, 2010 Amorim, Araujo-Hauck, Bartko, Baumeister, Berger, Brandner, Carvas, Cassaing, Chapron, Choquet, Clénet, Collin, Dodds-Eden, Eckart, Eisenhauer, Fédou, Fischer, Gendron, Genzel, Gillessen, Gräter, Hamaus, Haubois, Haug, Hippler, Hofmann, Hormuth, Houairi, Ihle, Jocou, Kellner, Kervella, Klein, Kolmeder, Lacour, Lapeyrère, Laun, Lenzen, Lima, Moratschke, Moulin, Naranjo, Neumann, Patru, Paumard, Perraut, Perrin, Pfuhl, Rabien, Ramos, Reess, Rohloff, Rousset, Sevin, Straubmeier, Thiel, Vincent, Wiest, Zanker-Smith, Ziegleder, Ziegler

3. Mini spiral (50’’) S star cluster (12-400 mas) Circumnuclear disk (120’’) 2-disk central cluster (0.5 pc-12.5’’) The environment of Sgr A* Sgr A* 10 µas

4. Orbits of stars around Sgr A* Schödel et al. (2002) Sgr A*

5. Orbits of stars around Sgr A* Schödel et al. (2002) S2S2

6. Two ways of measuring strong GRAVITY effects around Sgr A* 1. Studying the closest star orbits inside the central 60 mas Need to resolve star cluster. Scale ~ 100 R g = 1 mas resolution. (mas) Relativistic precession in Schwarzschild metric

7. Sgr A* blinking Genzel et al. (2003)

8. Two ways of measuring strong GRAVITY effects around Sgr A* 2. Using flaring regions as test particles. Measure flare motion. Scale ~ 1 R g = 10 µ as accuracy Time scale = 10 min Genzel et al. (2003) Hot spot orbiting the ISCO or a more distant orbit. Eckart et al. A&A 500, 935 (2009)

9. Sgr A* is quite dark Long wavelengths are well suited. Stars are bright in the near-infrared (orbits) and instruments sensitive enough to allow for short exposures (flares).

10. How to get to the 1 mas resolution and 10 µas accuracy in the near-infrared ? One of Prague famous astronomers, Tycho Brahe, found the solution: use a large instrument Measurement accuracy scales as the reciprocal of the size of the instrument.

11. Use the 4 VLT in interferometric mode ~ 140 m Resolution: 3 mas @ 2.2 µ m (K band) Build GRAVITY ! (General Relativity viA Vlt InterferomeTrY)

12. Interferometric imaging in the near-infrared works Altair Monnier et al. 2007  Cep Zhao et al. 2008 Betelgeuse Haubois et al. 2009 Mira Perrin et al. in prep  Cyg Lacour et al. 2009

13. Imaging the closest stars with GRAVITY One-night observation image: Paumard et al. (2005) mas Point Spread Function Dirty 6-star imageAfter deconvolution mas

14. Imaging the closest stars with GRAVITY Orbits after 15 months of observation: Paumard et al. (2005) mas 1 mas = 100 R g Schwarzschild advance of pericenter is detected

15. Lens-Thirring and Quadrupole Precession Testing the no-hair theorem Orbital plane precession (precession of the angular momentum vector around the spin of the black hole) Will (2008) Wheeler’s “black holes have no hair” theorem: a BH is fully characterized by only three parameters: Mass M, Spin J, Electric charge In particular Quadrupole Q 2 = -J 2 / M 1 year orbit, e=0.9 Measurement of frame dragging precession may be feasible after a few years for orbits in the radial range between 0.2 mpc and 1 mpc (5 and 25 mas) Merritt et al. (2009)

16. Reference star Sgr A* The measured distance between the two interferograms is:  opd = B.  S Hence:  S =  opd / B A 5 nm accuracy on  opd with a 100 m baseline yields a 10 µas accuracy on  S. SS opd 0  opd = B.  S Narrow angle interferometric astrometry Performance analysis: errors from atmosphere, baseline, noise, pupil position, etc … 23 µas per baseline 13 µas with 6 baselines

17. Muterspaugh et al. (2006): “… the 20μas level has been demonstrated …” 110 m and 87 m baselines 40 cm telescopes The Palomar Testbed Interferometer did it !

18. See Frédéric Vincent’s talk Measuring the last stable orbit Newton primary GR image total image secondary GR image Paumard et al. (2005)

19. Reference sources for GRAVITY near Sgr A* Reference source for adaptive optics Reference sources - IRS 16 - for interferometry (imaging and astrometry)

20. GRAVITY is in the design … Adaptive Optics Wavefront Sensor

21. … and prototyping phase Fibered delay line 4-telescope integrated optics beam combiner Laser metrology system Metrology test on VLT secondary mirror at Paranal

22. Where we are standing and where we are going Preliminary Design Review took place in december 2009 Final Design Review is scheduled for June 2011 First tests at Paranal : 2014 Hopefully first results on Sgr A* in 5 years from now.