Fast Variability and mm/IR flares in GRMHD Models of Sgr A* from Strong-Field Gravitational Lensing

TL;DR

This study uses GPU-accelerated GRMHD simulations to analyze variability and flares in Sgr A*, revealing that disk-dominated models produce short-term variability and IR flares via gravitational lensing, unlike jet models.

Contribution

It demonstrates the capability of high-cadence GRMHD simulations to reproduce observed variability and flares in Sgr A* and highlights the role of gravitational lensing in IR flares.

Findings

Disk-dominated models match observed short-term variability.

Jet-dominated models show only slow variability.

IR flares arise from gravitational lensing of magnetic flux tubes.

Abstract

We explore the variability properties of long, high cadence GRMHD simulations across the electromagnetic spectrum using an efficient, GPU-based radiative transfer algorithm. We focus on both disk- and jet-dominated simulations with parameters that successfully reproduce the time-averaged spectral properties of Sgr A* and the size of its image at 1.3mm. We find that the disk-dominated models produce short timescale variability with amplitudes and power spectra that closely resemble those inferred observationally. In contrast, jet-dominated models generate only slow variability, at lower flux levels. Neither set of models show any X-ray flares, which most likely indicate that additional physics, such as particle acceleration mechanisms, need to be incorporated into the GRMHD simulations to account for them. The disk-dominated models show strong, short-lived mm/IR flares, with short (<~ 1hr) time lags between the mm and IR wavelengths, that arise from strong-field gravitational lensing of magnetic flux tubes near the horizon. Such events provide a natural explanation for the observed IR flares with no X-ray counterparts.