The supermassive black hole at the centre of our galaxy

Sgr A*, the supermassive black hole at the center of the Galaxy, is variable in radio, sub-millimeter, millimeter, near-IR and X-rays. The flare activity is thought to arise from the innermost regions of the accretion flow, within ten gravitational radii of the black hole. General relativistic effects play therefore a substantial role in determining the flaring properties of Sgr A*. Our goal is to study the processes responsible for the variable emission from Sgr A*, and to analyse the relativistic signatures in the emission and ultimately test General Relativity by constraining the parameters of the black hole. We model the variable emission from a compact source within the accretion disk, orbiting in the equatorial plane close to the event horizon, near the marginally stable orbit of a black hole. Based on our developed scenario, we estimate the black hole spin. We take into account all special and general relativistic effects, and search for relativistic signatures in the resulting light curves and find that they may play a significant role in Sgr A* variability. Motivated by the apparent periodicity in some Sgr A* light curves, and the clear substructure, suggestive of lensing and Doppler beaming effects, together with the presence of time lags at radio wavelengths, we develop a general relativistic time-dependent model that aims to understand the variable emission and the black hole properties. The model incorporates a bubble of synchrotron-emitting electrons orbiting close to the black hole and cooling via synchrotron emission and adiabatic expansion. The model reproduces the observed time delays at radio wavelengths, the shape and structure of the flares, the periodic orbital dynamics and confirms the presence of variability at sub-orbital scales due to lensing and Doppler beaming. We obtain good matches with the observed light curves in NIR/X-Ray and radio. Further analysis of Sgr A* variability at orbital timescales will allow us to test General Relativity and the validity of the Kerr metric.