Investigating the Physics of Hard X-ray Outbursts from the Galactic Center Supermassive Black Hole Sagittarius A*

The Galactic center supermassive black hole (SMBH) Sagittarius A* (Sgr A*) is the closest such object and thus is an ideal target for investigation of galactic nuclei and their activity cycles. Its remarkable underluminous X-ray state is punctuated by outbursts on different time and energy scales. This thesis presents a study of past, current and possible future X-ray outburst activities from Sgr A*, using the hard X-ray telescope NuSTAR. Indication of substantial past Sgr A* activity, similar to that observed in low-luminosity active Galactic nuclei, has come from the Galactic center molecular clouds (GCMCs). Using these X-ray reflecting GCMCs, I have studied the characters of past Sgr A* X-ray outbursts. The current X-ray quiescence of Sgr A* is punctuated by directly detectable flares. The radiation mechanism and physical process of these X-ray flares are poorly understood. From about 1 Ms NuSTAR observations of Sgr A*, I collected nine bright X-ray flares. I studied their timing behavior and the correlation between flares' strengths and their spectra. Future Sgr A* X-ray activity could increase due to the infall of a gas cloud G2 into this SMBH. Finally, I present the Galactic center cosmic-ray population revealed by non-thermal X-ray filaments and its connection to Sgr A* outbursts.