General Relativistic effects and the NIR variability of Sgr A* II: A systematic approach to temporal asymmetry

A systematic study, based on the third-moment structure function, of Sgr A*'s variability finds an exponential rise time ${\tau }_{1,\mathrm{o}\mathrm{b}\mathrm{s}}=14.8_{-1.5}^{+0.4}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{u}\mathrm{t}\mathrm{e}\mathrm{s}$ and decay time ${\tau }_{2,\mathrm{o}\mathrm{b}\mathrm{s}}=13.1_{-1.4}^{+1.3}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{u}\mathrm{t}\mathrm{e}\mathrm{s}$. This symmetry of the flux-density variability is consistent with earlier work, and we interpret it as caused by the dominance of Doppler boosting, as opposed to gravitational lensing, in Sgr~A*'s light curve. A relativistic, semi-physical model of Sgr~A* confirms an inclination angle $i<45$ degrees. The model also shows that the emission of the intrinsic radiative process can have some asymmetry even though the observed emission does not. The third-moment structure function, which is a measure of the skewness of the light-curve increments, may be a useful summary statistic in other contexts of astronomy because it senses only temporal asymmetry, i.e., it averages to zero for any temporally symmetric signal.