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

TL;DR

This paper investigates the near-infrared variability of Sgr A* using a third-moment structure function, revealing symmetry in flux variability likely caused by Doppler boosting, and constrains the inclination angle of the source.

Contribution

It introduces a systematic approach using the third-moment structure function to analyze temporal asymmetry in Sgr A*'s variability, providing new insights into the physical mechanisms.

Findings

Measured exponential rise and decay times of variability.

Dominance of Doppler boosting over gravitational lensing.

Constraints on the inclination angle of Sgr A*.

Abstract

A systematic study, based on the third-moment structure function, of Sgr A*'s variability finds an exponential rise time $\tau_{1,\rm{obs}}=14.8^{+0.4}_{-1.5}~\mathrm{minutes}$ and decay time $\tau_{2,\rm{obs}}=13.1^{+1.3}_{-1.4}~\mathrm{minutes}$. 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.