We analyze deep near-IR adaptive optics imaging (taken with NAOS/CONICA on the VLT) 1 as well as new proper motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint (H≤ 20, K s ≤ 19) stars peaks within 0.2 ′′ of the black hole candidate SgrA ⋆ . The radial density distribution of this stellar 'cusp' follows a power law of exponent α ∼ 1.3 − 1.4. The K-band luminosity function of the overall nuclear stellar cluster (within 9 ′′ of SgrA ⋆ ) resembles that of the large scale, Galactic bulge, but shows an excess of stars at K s ≤ 14. It fits population synthesis models of an old, metal rich stellar population with a contribution from young, early and late-type stars at the bright end. In contrast, the cusp within ≤ 1.5 ′′ of SgrA ⋆ appears to have a featureless luminosity function, suggesting that old, low mass horizontal branch/red clump stars are lacking. Likewise there appear to be fewer late type giants. The innermost cusp also contains a group of moderately bright, early type stars that are tightly bound to the black hole. We interpret these results as evidence that the stellar properties change significantly from the outer cluster (≥ a few arcsecs) to the dense innermost region around the black hole.We find that most of the massive early type stars at distances 1-10" from SgrA ⋆ are located in two rotating and geometrically thin disks. These disks are inclined at large angles and counter-rotate with respect to each other. Their stellar content is essentially the same, indicating that they formed at the same time. We conclude that of the possible formation scenarios for these massive stars the most probable one is that 5-8 million years ago two clouds fell into the center, collided, were shock compressed and then formed two rotating (accretion) disks orbiting the central black hole. For the OB-stars in the central arcsecond, on the other hand, a stellar merger model is the most appealing explanation. These stars may thus be 'super-blue-stragglers', formed and 'rejuvenated' through mergers of lower mass stars in the very dense (≥ 10 8 M ⊙ pc −3 ) environment of the cusp. The 'collider model' also accounts for the lack of giants within the central few arcseconds.The star closest to SgrA ⋆ in 2002, S2, exhibits a 3.8 µm excess. We propose that the mid-IR emission either comes from the accretion flow around the black hole itself, or from dust in the accretion flow that is heated by the ultra-violet emission of S2.1 Based on observations obtained at the European Southern Observatory, Chile

This paper reports measurements of Sgr A* made with NACO in L ′ -band (3.80 µm), Ks-band (2.12 µm) and H-band (1.66 µm) and with VISIR in N-band (11.88 µm) at the ESO VLT 1 , as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 4 April, 2007, a very bright flare was observed from Sgr A* simultaneously at L ′ -band and X-ray wavelengths. No emission was detected 1 The Very Large Telescope (VLT) at the European Southern Observatory (ESO) on Paranal, Chile: Program IDs 179.B-0261(A) and Program ID: 079.B-0929(A).using VISIR. The resulting SED has a blue slope (β > 0 for νL ν ∝ ν β , consistent with νL ν ∝ ν 0.4 ) between 12 micron and 3.8 micron.For the first time our high quality data allow a detailed comparison of infrared and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak) and prominent substructures in the 3.8 micron light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L ′ -band lightcurve we find that the flaring region must be no more than 1.2 R S in size.The high X-ray to infrared flux ratio, blue νL ν slope MIR to L ′ -band, and the soft νL ν spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.

Direct imaging of exoplanets or circumstellar disk material requires extreme contrast at the 10 -6 to 10 -12 levels at < 100 mas angular separation from the star. Focal-plane mask (FPM) coronagraphic imaging has played a key role in this field, taking advantage of progress in Adaptive Optics on ground-based 8+m class telescopes. However, large telescope entrance pupils usually consist of complex, sometimes segmented, non-ideal apertures, which include a central obstruction for the secondary mirror and its support structure. In practice, this negatively impacts wavefront quality and coronagraphic performance, in terms of achievable contrast and inner working angle. Recent theoretical works on structured darkness have shown that solutions for FPM phase profiles, optimized for non-ideal apertures, can be numerically derived. Here we present and discuss a first experimental validation of this concept, using reflective liquid crystal spatial light modulators as adaptive FPM coronagraphs.

The star S2 orbiting the compact radio source Sgr A* is a precision probe of the gravitational field around the closest massive black hole (candidate). Over the last 2.7 decades we have monitored the star’s radial velocity and motion on the sky, mainly with the SINFONI and NACO adaptive optics (AO) instruments on the ESO VLT, and since 2017, with the four-telescope interferometric beam combiner instrument GRAVITY. In this Letter we report the first detection of the General Relativity (GR) Schwarzschild Precession (SP) in S2’s orbit. Owing to its highly elliptical orbit (e = 0.88), S2’s SP is mainly a kink between the pre-and post-pericentre directions of motion ≈±1 year around pericentre passage, relative to the corresponding Kepler orbit. The superb 2017−2019 astrometry of GRAVITY defines the pericentre passage and outgoing direction. The incoming direction is anchored by 118 NACO-AO measurements of S2’s position in the infrared reference frame, with an additional 75 direct measurements of the S2-Sgr A* separation during bright states (“flares”) of Sgr A*. Our 14-parameter model fits for the distance, central mass, the position and motion of the reference frame of the AO astrometry relative to the mass, the six parameters of the orbit, as well as a dimensionless parameter fSP for the SP (fSP = 0 for Newton and 1 for GR). From data up to the end of 2019 we robustly detect the SP of S2, δϕ ≈ 12′ per orbital period. From posterior fitting and MCMC Bayesian analysis with different weighting schemes and bootstrapping we find fSP = 1.10 ± 0.19. The S2 data are fully consistent with GR. Any extended mass inside S2’s orbit cannot exceed ≈0.1% of the central mass. Any compact third mass inside the central arcsecond must be less than about 1000 M⊙.