Letter to the Editor

General relativistic effects and the near-infrared variability of Sgr A*

II. A systematic approach to temporal asymmetry

¹ Max-Planck-Institute für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² University of Illinois, Urbana-Champagne, USA
³ Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁴ Max Planck Institut for Extraterrestrial Physics, 85748 Garching bei Muenchen, Germany
⁵ Physics and Astronomy Department, University of California, Los Angeles, CA 90095-1547, USA
⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Department of Physics, TUM School of Natural Sciences, Technical University of Munich, 85748 Garching, Germany

Received: 17 June 2024
Accepted: 9 July 2024

Abstract

A systematic study, based on the third-moment structure function, of Sgr A*’s variability finds an exponential rise time, τ_1,obs = 14.8_−1.5^+0.4 minutes, and decay time, τ_2,obs = 13.1_−1.4^+1.3 minutes. This symmetry of the flux-density variability is consistent with earlier work, and we interpret it as being 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 of i ≲ 45°. 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; that is, it averages to zero for any temporally symmetric signal.