Intrinsic Near-Infrared Properties of the Variable Source Sagittarius A*

This thesis on observational astronomy focuses on the highly variable near-infrared source Sagittarius A* (Sgr A*) at the center of the Milky Way, associated with the central super-massive black hole. It is divided in two parts: a comprehensive data description of Ks-band measurements of Sgr A*, covering the last seven years of observations with the Very Large Telescope and the state-of-the-art instrument NAOS/CONICA, and an effort in polarimetric instrumentation, the calibration of the instrumental polarization properties of NAOS/CONICA in the Ks-band. In the first part I characterize the statistical properties of the near-infrared variability of Sgr A*, the electromagnetic manifestation of the Galactic Center super-massive black hole, and find the flux density to be power-law distributed. I cannot confirm the evidence of a two state process with different flux density distributions behind the variability, as reported in other publications. I find a linear rms-flux relation for the flux density range up to 12 mJy on a timescale of 24 minutes. This and the power- law flux density distribution imply a phenomenological, formally non-linear statistical variability model with which I can simulate the observed variability and extrapolate its behavior to higher flux levels and longer timescales. I can show that a bright outburst within the last 400 years, that has been discussed as the possible reason for the X- ray emission from massive molecular clouds surrounding the Galactic Center, can be expected as an extreme value of our statistics without the need for a cosmic event. I give arguments, why data with our time support cannot be used to decide on the question whether the power spectral density of the underlying random process shows more structure at timescales below 100 min compared to what is expected from a red noise random process, as discussed in the context of orbiting hot spots in the accretion flow of the black hole. In the second part I report on the results of calibrating and simulating the instrumental polarization properties of the Very Large Telescope adaptive optics camera system NAOS/CONICA (NACO) in the Ks-band. Here my goal was to understand the influence of systematic calibration effects on the time-resolved polarimetric observations of Sgr A*. I used the Stokes/Mueller formalism for metallic reflections to describe the instrumental polarization. The model is compared to standard-star observations and time-resolved observations of bright sources in the Galactic Center. I simulated the differences between calibration methods and tested their influence on three examples of polarimetric Ks-band light curves of Sgr A*. I find the instrumental polarization to be highly dependent on the pointing position of the telescope and about 4% at maximum. I report a polarization angle offset of 13.2° due to a position angle offset of the λ /2-wave plate with respect to the data-header value that affects the calibration of NACO data taken before autumn 2009. With the new model of the instrumental polarization of NACO it is possible to measure the polarization with an accuracy of 1% in polarization degree. The uncertainty of the polarization angle is ≤ 5° for polarization degrees ≥ 4%. For densely sampled polarimetric time series I find that the improved understanding of the polarization properties gives results that are consistent with the previously used method to derive the polarization of Sgr A*. The difference between the derived and the previously employed polarization calibration is well within the statistical uncertainties of the measurements, and for Sgr A* they do not affect the results from our relativistic modeling of the accretion process