All Figures

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig01.eps} \end{figure}$ Figure 1: Schematic view of the observing schedule. The exact times are listed in Table 1.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig02.eps} \end{figure}$ Figure 2: Light curves of all NIR observations in July 2004. Shown are the dereddened flux densities of S1 (green), Sgr A* (black), and a field free of stars (red). The data were smoothed with a sliding point window comprising five samples in the case of the K- and H-band data, 9 samples in the case of the L'-band data. Here, one sample corresponds to a photometric measurement on an individual image (i.e., DIT $\times$ NDIT integration time, see Table 2). Filters are indicated in the lower left corners, start and stop times on top of the plots. As can be seen, the quality of the data is fairly variable. This is closely related to the seeing and coherence time of the atmosphere during the observations, which result in a variable performance of the AO system. We indicated seeing and atmospheric coherence time in the plots. If there is more than one label in a given plot, then the location of the labels indicates approximately the time window to which the seeing/coherence time values apply. For example, in the L'-band data, seeing and coherence time deteriorated continuously during the observations on July 06/07, while in the July 08 data, only a brief episode of bad seeing occurred near the end of the first third of the time series. Generally, the light curve of S1 and of the background, which are expected to be constant, give a good idea of the data quality and relative uncertainties of the observations. Values for seeing and coherence time were taken from the web site of the ESO Observatories Ambient Conditions Database.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig03.eps} \end{figure}$ Figure 3: Light curve of the K-band observations from July 07 (see Fig. 2). The plot shows the lightcurves of S1 ( top), Sgr A* (middle), and the background flux ( bottom).
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In the text

$\begin{figure} \par\includegraphics[width=12.9cm]{4418fig04.eps}\vspace*{-3mm} \end{figure}$ Figure 4: The 8.6 $\mu$ m ( left) and 19.5 $\mu$ m ( right) image of the central $5.0''\times 4.4''$ as taken with VISIR on 8 May 2004. The emission at the position of Sgr A* is consistent with dust emission similar to what is measured towards other faint components within the central 5-10 arcsec. The cross marks the position of Sgr A* and the positional uncertainty of ${\pm }0.2''$ . As reference sources to determine the relative positioning between the NIR and MIR frame we used the bright and compact sources IRS 3, 7, 21, 10W and, in addition at 8.6 $\mu$ m wavelength, IRS 9, 6E, and 29.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig05.eps}\end{figure}$ Figure 5: The 890 $\mu$ m light curves from July 05-07. The light curves of the quasar calibrators, 1741-038, 1749+096, and 1921-293, are also shown. For each source we plot Stokes I, the total intensity. Due to poorer weather on July 06 and 07, for these days we have averaged two cycles between Sgr A* and the quasar calibrators (see text) for each data point, while the July 05 data is shown at full time resolution.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8.8cm,clip]{4418fig06.ps}\end{figure}$ Figure 6: Chandra light curve for 6/7 July 2005. We also indicate the overlap with the NIR data. Start time is 2004 July 6, 22:35:11.8; Stop time is 2004 July 7, 12:53:44.9.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8.8cm,clip]{4418fig07.ps}\end{figure}$ Figure 7: Chandra light curve for 5/6 July 2005. We also indicate the overlap with the NIR data. Start time is 2004 July 5, 22:38:25.7; Stop time is 2004 July 6, 12:56:58.8.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{4418fig08.eps}\end{figure}$ Figure 8: VLA 43 GHz light curves. The top graphs show the correlated flux density as measured on 6, 7 and 8 of July (see Table 1). The bottom graph shows the excess flux density on 7 July calculated as the difference between the data from this day and the mean of 6 and 8 July.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig09.eps}\end{figure}$ Figure 9: The X-ray and NIR light curves plotted with a common time axis. See text and captions of previous figures. Straight solid lines in the inserted box represent the 0.00, 0.01, and 0.02 counts per second levels. The straight dashed line represent the X-ray IQ-state flux density level.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig10.eps} \end{figure}$ Figure 10: Light curve of the K-band observations from July 07 (see Fig. 2). The plot shows the lightcurves of S1 ( top) and Sgr A* ( bottom). From both plots we have subtracted the background level. The numbers near the light curve of Sgr A* mark the approximate time points for which images are shown in Fig. 11.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig11.eps} \end{figure}$ Figure 11: K-band images of the stellar cluster in the immediate vicinity of Sgr A*. The numbers correspond to the time points indicated in Fig. 10. The images result from the average of five individual exposures, corresponding to 150 s total integration time. A LR deconvolution and restoration with a Gaussian beam was applied. The color scale is linear. North is up, east to the left. The offsets are given with respect to the position of Sgr A*. The white arrows indicate the position of Sgr A*.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig12.eps}\end{figure}$ Figure 12: Cross-correlation between the NIR data (40 s bins; 20 s integration time per image) and the X-ray data (10 min bins) for the three flares $\phi$ 2/I ( top left), $\phi 3$ /III ( top right), and $\phi 4$ /IV ( bottom; see Table 7). We cross-correlated only the flare data that overlap in time i.e. about 40 min before and 20 min after midnight 07 July 0.0 UT for flare $\phi$ 2/I, about 177 min until 223 min after midnight for $\phi 3$ /III, and about 223 min until 256 min after midnight for $\phi 4$ /IV. The error bars indicate an estimate of the SNR derived from the SNR of the individual X-ray bins and the approximate number of bins that were averaged for each of the cross-correlations measurements. At a $\sim$ 2.6, 9, and 3 $\sigma$ level, respectively, the light curves indicate a simultaneous flare event around midnight corresponding to a time delay of less than 10 min (see text). For flares $\phi 3$ /II and $\phi 4$ /IV the entire event was covered. The offsets are given in minutes.
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In the text

$\begin{figure} \par\includegraphics[width=14.5cm,clip]{4418fig13.eps} \end{figure}$ Figure 13: Images of the stellar cluster in the immediate vicinity of the center during the IQ, low NIR flux density state of Sgr A*. The images result from the average of several individual exposures, corresponding to a total integration time of 40 min for H-band, 100 min for K-band, and 80 min for L'-band. LR deconvolution and restoration with a Gaussian beam was applied. The color scale is linear. North is up, east to the left. The field of view shown in each image is $1.3''\times 1.3''$ . The white circles indicate the position of Sgr A*. All images were taken on the 6 July 2004. The positional accuracy of the images is of the order of 0.5 pixels, i.e. $\sim$ 7 mas in the H-band and $\sim$ 14 mas in the K- and L'-bands.
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In the text

$\begin{figure} \par\includegraphics[width=12.3cm,clip]{4418fig14.eps}\end{figure}$ Figure 14: HKL' multi-color image of the central $2.6''\times 2.6''$ as taken with NACO on 6 July 2004. North is up and East is to the left. H is blue, K is green, and L' is red. The white cross marks the position of Sgr A*. The figure is a different representation of the same data shown in Fig. 13 covering a larger field of view. Just west of the position of Sgr A* an extended red emission component D1 is evident.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{4418fig15.eps} % \end{figure}$ Figure 15: The central $0.76''\times 1.06''$ shown in a color and contour map. The contour lines are 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 30, 35, 40, 42 10, 14, 19, 24, 29, 33, 38, 43, 48, 52, 57, 62, 71, 83, 95, 98% of the peak brightness of star S2. Panel a) shows the complete L' band images section. Panel b) shows the images section with scaled PSF subtractions of the sources S2 (L'=12.78 used as a calibrator), S1 ( $L'=13.80\pm 0.12$ ), S17 ( $L'=13.79\pm 0.12$ ), S0-20 ( $L'=13.89\pm 0.13$ ), and Sgr A* ( $L'=14.1\pm 0.2$ ). The dust component D1 ( $L'=12.8\pm 0.20$ ) is clearly visible. Panel c) shows the images with D1 subtracted instead of Sgr A*. In panel d) both Sgr A* and D1 have been subtracted. Panel e) shows the model of the dust component D1 by itself. The background in the central 0.3'' is flat and has a L' brightness of L'=15 per beam with a 3 $\sigma$ deviation of less than $\pm$ 0.42 mag.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{4418fig16.eps}\end{figure}$ Figure 16: The NIR/MIR spectrum of Sgr A* compared to emission models during low flux density states. Red data points indicate the measurements described in this paper. The log( $\nu~ L_{\nu}$ ) values are based on the following data: our new quasi-simultaneous L'-, K-, and H-band flux densities during the low NIR flux IQ-state are shown in red. The 30 $\mu$ m and 24.3 $\mu$ m flux density limits are taken from Telesco et al. (1996 APJ 456, 541) and Serabyn et al. (1997), respectively; at 8.7 $\mu$ m our limit of about 100 mJy is lower than the value given by Stolovy et al. (1996); at 3.8 $\mu$ m our IQ flux density value (19 $\;\pm\;$ 5 mJy) and the IQ flux density values (6.4-17.5 mJy) given by Genzel et al. (2003) as well as the range of flux densities (4-17 mJy, 1.3 $\;\pm\;$ 0.3 mJy) given by Ghez et al. (2004, 2005). For clarity we displaced the latter two ranges slightly in frequency with respect to the center of the L'-band. In addition we plot representative SSC models IQ1-IQ4 with parameters as listed in Table 9.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{4418fig17.eps}\end{figure}$ Figure 17: The NIR/MIR log( $\nu~ L_{\nu}$ ) spectrum of Sgr A* compared to flare emission models for the strong X-ray/NIR flare $\phi~3/III$ . See also caption of Fig. 11. Finally we show the K-band spectra of flares including the slopes (grey shaded area) as measured by Eisenhauer et al. (2005) and Ghez et al. (2005). The thin lines between the grey shaded areas indicate how the slope may change as a function of flare flux as suggested by Ghez et al. (2005). In addition we plot representative SSC flare models F1 and F2 with parameters as listed in Table 7.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{4418fig18.eps}\end{figure}$ Figure 18: Flare amplitude as a function of flare rate for the NIR emission from Sgr A* under the assumption that the characteristic flare duration is of the order of 100 min.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig19.ps}\end{figure}$ Figure 19: Simulation of the Sgr A* flare activity assuming a power spectrum relation between flare amplitude and the number of flares. Here the power-law spectrum is comparable to the value found from our NIR flare data and each bin covers one characteristic flare time.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig01.eps} \end{figure}$	Figure 1: Schematic view of the observing schedule. The exact times are listed in Table 1.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig03.eps} \end{figure}$	Figure 3: Light curve of the K-band observations from July 07 (see Fig. 2). The plot shows the lightcurves of S1 ( top), Sgr A* (middle), and the background flux ( bottom).
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$\begin{figure} \par\includegraphics[angle=270,width=8.8cm,clip]{4418fig06.ps}\end{figure}$	Figure 6: Chandra light curve for 6/7 July 2005. We also indicate the overlap with the NIR data. Start time is 2004 July 6, 22:35:11.8; Stop time is 2004 July 7, 12:53:44.9.
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$\begin{figure} \par\includegraphics[angle=270,width=8.8cm,clip]{4418fig07.ps}\end{figure}$	Figure 7: Chandra light curve for 5/6 July 2005. We also indicate the overlap with the NIR data. Start time is 2004 July 5, 22:38:25.7; Stop time is 2004 July 6, 12:56:58.8.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{4418fig08.eps}\end{figure}$	Figure 8: VLA 43 GHz light curves. The top graphs show the correlated flux density as measured on 6, 7 and 8 of July (see Table 1). The bottom graph shows the excess flux density on 7 July calculated as the difference between the data from this day and the mean of 6 and 8 July.
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$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig09.eps}\end{figure}$	Figure 9: The X-ray and NIR light curves plotted with a common time axis. See text and captions of previous figures. Straight solid lines in the inserted box represent the 0.00, 0.01, and 0.02 counts per second levels. The straight dashed line represent the X-ray IQ-state flux density level.
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$\begin{figure} \par\includegraphics[width=12cm,clip]{4418fig10.eps} \end{figure}$	Figure 10: Light curve of the K-band observations from July 07 (see Fig. 2). The plot shows the lightcurves of S1 ( top) and Sgr A* ( bottom). From both plots we have subtracted the background level. The numbers near the light curve of Sgr A* mark the approximate time points for which images are shown in Fig. 11.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{4418fig18.eps}\end{figure}$	Figure 18: Flare amplitude as a function of flare rate for the NIR emission from Sgr A* under the assumption that the characteristic flare duration is of the order of 100 min.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{4418fig19.ps}\end{figure}$	Figure 19: Simulation of the Sgr A* flare activity assuming a power spectrum relation between flare amplitude and the number of flares. Here the power-law spectrum is comparable to the value found from our NIR flare data and each bin covers one characteristic flare time.
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