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Fig. 1.


Monitoring the S2 orbit around Sgr A with the three VLT(I) instruments NACO (AO-assisted, single UT imaging), GRAVITY (interferometric astrometry-imaging with all four UTs of the VLT) and SINFONI (AO-assisted integral field spectroscopy). Upper left: Deconvolved NACO K-band image of the Galactic centre a few weeks before the 2018 pericentre passage. The source S2 appears slightly elongated because of confusion with Sgr A. Upper right: Nearly simultaneous GRAVITY image of S2 and Sgr A. The image shows the central 150 mas (0.0059 pc ≈ 1.4 × 104RS) after self-calibration and CLEANing with AIPS. The image is reconstructed from 34 integrations of 5 min each from several nights at the end of March 2018. Sgr A was (on average) K = 16.6 mag and the rms noise level of the background after cleaning is ≈20 mag. Both Sgr A and S2 are unresolved. Here and in the other GRAVITY images, the elongation is due to the shape of the interferometric clean beam. Bottom: S2 – Sgr A GRAVITY images (coaddition of several days) from July 2017 (bottom left) and May / June 2018, a few days after pericentre (bottom right). The inset in the bottom left panel shows the instantaneous interferometric beam, which is 2.2 mas × 4.7 mas without Earth rotation. The inset in the middle left shows a co-added SINFONI K-band spectrum of the star S2, taken from Habibi et al. (2017).

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