Bottom left (panel a): time evolution of the east-west (east positive, blue) and north-south (red) position offset of the July 28 (MJD = 58328.0841) centroids from their medians, as well the flux density evolution (right y-axis, black), in units of the flux of S2 (14.0 mag). Error bars are 1σ. For this purpose the total intensity was computed from the sum of the two polarization directions. The data points use the “Pfuhl” analysis (see Appendix B). Bottom right (panel b): projected orbit of the flare centroid on the sky with colours from red to blue marking the time evolution. The orange square indicates the black hole position including long-term astrometric uncertainty. The thin continuous blue curve denotes a simple model of a hot spot on a circular orbit with R = 9 Rg (a = 0, M = 4.14 × 106 M⊙), seen at inclination 135° (clockwise on the sky, as for the data (black curved arrow)) and with the line of nodes at Ω = 120° (same as in Fig. 1), fitted with NERO code (Bauböck et al., in prep. and Yang & Wang 2014; Dexter & Agol 2009). The cyan and violet curves show the orbit in x(t) and y(t). Top (panels c and d): as in (panel a) and (panel b) but for the May 27 flare (MJD 58266.3420). Here the best fitting orbit is face on with R = 7 Rg.