Properties of the NIR source Sgr A*

In the past decade, only upper limits for the NIR flux of Sgr A* could be determined and putative detections (e.g. Genzel et al., 1997) could not be reproduced. The observation of a

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NIR Emission from Sagittarius A*

Figure 10.1:

total exposure time) of an area_∼100_×100_{around Sgr A*, observed on May 9, 2003 UT. The} left image was taken at the beginning of the observations, the right image about 40 min later. The flaring source is easily detected in the right image. Its position is offset₋1.4±3.0mas in R.A. and₋0.2±3.0in Dec. from the dynamical position of Sgr A* as it was determined from the orbit of the star S2. The star S2 is marked by a cross, the position of Sgr A* is indicated by a white circle. This Figure corresponds to Figure 1 in Genzel et al. (2003a).

Band

Date

∆RA

∆Dec

Duration

Variability

[UT]

[mas]

[mas]

[min]

L’

2002.66

0(30)

0(30)

_≥15

0.7

H

2003.353

-1.4(3.0)

-0.2(3.0)

30

4.7

K

2003.455

-2.5(4)

3.4(4)

80

3.1

K

2003.457

-6.4(4)

2.5(4)

85

3.2

Table 10.1:

astrometric position of Sgr A* as it was inferred from the focus of the orbit of the star S2. The duration of the flares refers to the full width at zero maximum, the variability factor to the ratio between the excess emission to the ’quiescent’ emission.

NIR source at the position of Sgr A* was particularly difficult since about 1999 because stars such as S2 and S14 approached Sgr A* to projected distances<50−100mas (for comparison, the diffraction limited beam FWHM at the NTT in the K-band is130mas). Sch¨odel et al. (2003) examined ten years of SHARP/NTT data and found no evidence for a varying and/or stationary source at the position of Sgr A*, when taking into account possible time scales of hours to days. They constrained the peak magnitude of possible flares on shorter time scales to less thanK= 13.5.

The flares recently detected with NACO/VLT in 2003 fulfil all these constraints; addition- ally, a ’quiescent’ NIR source at the position if Sgr A* could be detected as well (Genzel et al., 2003a). In summary, the properties of the NIR flares of Sgr A* are:

• Brightness: Peak brightnesses of the flares in K-band of aboutK= 15.0, comparable to the brightness of the star S1 near Sgr A*.

NIR Emission from Sgr A*

101

Figure 10.2:

in 2002 and 2003, observed with NACO/VLT. The L-band flare on August 30, 2002, was only partially covered by observations. Gaps in the time series of the H-band flare on May 10, 2003, and of the K-band flare on June 15, 2003, are due to sky observations and instrument failure, respectively. For comparison, the steady emission of the star S1 near Sgr A* is shown in all the plots (light grey data points). Arrows in the plots of the two K-band flares indicate substructure peaks of the flares. Both K-band flares show very similar quasi-periodicity, although the second flare was observed more than 24 h after the first one and must thus have been an unrelated event. The upper right panel shows the normalised power spectrum of the two K-band flares. Both of them show a significant peak at a frequency corresponding to time scales of16.8± 2.0min. In both cases, the power spectrum of S1 does not show such a frequency. This Figure corresponds to Figure 2 in Genzel et al. (2003a). For a colour version of this Figure see Appendix B.

• Duration: A duration of the flares of_≤85min.

• Time scales: Short rise-and-fall time scale of the flares, of the order 5 min.

• Position: The position of the flares is within a few milli-arc-seconds of the dynamical position of the dark mass as inferred from the orbit of S2 (see Table 10.1). The position of the quiescent source has a larger uncertainty, of the order 10 mas (mainly because of its faintness and confusion with stellar sources), but is still consistent with the dynamical position of the black hole.

• Intrinsic quasi-periodicity of the flares: The two K-band flares, which were com- pletely covered by observations, display a characteristic quasi-periodic substructure, with a period of 16.8±2.0min (see plot of power spectrum in Figure 10.2). The

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NIR Emission from Sagittarius A*

observed ’quiescent’ flux was also variable. There are some indications of quasi- periodicity of the quiescent state of Sgr A*, with periods between 20 and 30 min, but this evidence is still ambiguous.

It is highly improbable that the observed variability was due to a stellar source because of the extremely short time scales of the flares, and because of the astrometric positions of the flares, that were always within< 10mas of Sgr A* at the different epochs: A star close to Sgr A*would have moved by_∼20−50mas during the time interval covered by the four flares (compare with the orbit of S2); a star at greater distances from Sgr A* would have an extremely low probability of being located so close in projection to Sgr A*. The estimated flaring rate is very high: We found 4 flares within a total of 25 hours of observations. Assuming Poisson statistics, we estimate 2 to 6 flares per day. This high frequency and the complex temporal substructure of the light curves rule out the possibility that the flares were related to micro- lensing of cusp stars by the black hole (Alexander & Sternberg, 1999).

As for the relation between the NIR flares and variability at other wavelengths, the dura- tions, rise-and-decay times, and band luminosities of X-ray flares are similar (see e.g. Baganoff et al., 2001 and Figure 10.3). The NIR flare rate, however, was almost twice as high as the X- ray flare rate during Chandra monitoring in 2002 (1.2±0.4flares per day; Baganoff, 2003a). At the moment, it is not clear whether this points to a physically distinct nature of the two kinds of flares or whether this was related to an overall change of the variability of Sgr A* at different epochs. Simultaneous observations are needed to determine the relation between the X-ray and NIR flares. The millimetre/sub-millimetre radiation from Sgr A* was found to be variable on time scales of several days to a few hundred days (Zhao et al., 2003; Miyazaki et al., 2003), but not on shorter time scales, with the exception of a one hour,30%amplitude event seen in March 2000 at a wavelength of 2 mm (Miyazaki et al., 2003).