Near Infrared Polarimetry: A Tool for Testing Properties of Sagittarius A*

In this thesis I focus on the results of the data modelings and simulations of near-infrared (NIR) observations of the Sagittarius A* (Sgr A*) counterpart associated with the super-massive black hole at the Galactic Center (GC). My goal is to investigate and understand the physical processes behind the variability associated with the NIR flaring emission from Sgr A*. The NIR observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's (ESO's) Very Large Telescope (VLT) and the CIAO NIR camera on the Subaru telescope (13 June 2004, 30 July 2005, 1 June 2006, 15 May 2007, 17 May 2007 and 28 May 2008). I used a model of synchrotron emission from relativistic electrons in the inner parts of an accretion disk. The relativistic simulations have been carried out using the Karas-Yaqoob (KY) ray-tracing code. I also probed the existence of a correlation between the modulations of the observed flux density light curves and changes in polarimetric data. Furthermore, I confirmed that the same correlation is also predicted by the so-called hot spot model. Correlations between intensity and polarimetric parameters of the observed light curves, as well as a comparison of predicted and observed light curve features through a pattern recognition algorithm result in the detection of a signature of orbiting matter under the influence of strong gravity. This pattern is proved to be statistically significant against randomly polarized red noise. The observed correlations between flux modulations and changes in linear polarization degree and angle can be a sign that the NIR flares have properties that are not expected from purely random red-noise. I found that the geometric shape of the emitting region plays a major role in the predictions of the model. From fully relativistic simulations of a spiral shaped emitting region, I concluded that the observed swings in polarization angle during NIR flares support the idea of compact orbiting spots instead of extended patterns. The effects of gravitational shearing, fast synchrotron cooling of the components, and confusion from a variable accretion disk have been taken into account. Furthermore, I discussed the expected results from future observations of VLT interferometry (VLTI) like the GRAVITY experiment. Simulated centroids of NIR images led me to the conclusion that a clear observation of position wander of the center of NIR images with future infrared interferometers will prove the existence of orbiting hot spots in the vicinity of our Galactic super-massive black hole. Finally, I described a novel approach to constrain the physical parameters of the Galactic black hole by using time resolved NIR polarimetric observations. Even though basically the method is developed for Sgr A*, it can be used to test intrinsic properties of several types of compact objects with QPO behavior

Ruppeiner Geometry of RN Black Holes: Flat or Curved?

In some recent studies \cite{aman1, aman2, aman3}, Aman {\it et al.} used the Ruppeiner scalar as a measure of underlying interactions of Reissner-Nordstr\"{o}m black holes, indicating that it is a non-interacting statistical system for which classical thermodynamics could be used at any scale. Here, we show that if we use the complete set of thermodynamic variables, a non-flat state space will be produced. Furthermore, the Ruppeiner curvature diverges at extremal limits, as it would for other types of black holes.Comment: 9 page