Magnetic flux eruptions at the root of time-lags in low-luminosity AGN

Abstract

Sagittarius A$^\ast$ is a compact radio source at the center of the Milky Way that has not conclusively shown evidence for the presence of a relativistic jet. Nevertheless, indirect methods at radio frequencies do indicate consistent outflow signatures. Temporal shifts between features in frequency bands are known as time lags, associated with flares or outflows of the accretion system. It is possible to gain information on the emission and outflow mechanics by interpreting these time lags. By means of a combined general-relativistic magnetrohydrodynamical and radiative transfer modeling, we study the origin of the time-lags for magnetically arrested disc models at three black hole spins ($a_\ast \in \{ -0.9375, 0, 0.9375 \}$). We exclusively modeled the emission from the source across a frequency range of $\nu$ = 19-47 GHz. Our study also includes a targeted `slow light' study for one of the best-fitting `fast light' windows. We recovered the observational time-lag relations in various windows of our simulated light curves. The theoretical interpretation of these most promising time-lag windows is threefold; i) a magnetic flux eruption perturbs the jet-disc boundary and creates a flux tube, ii) the flux tube orbits and creates a clear emission feature, and iii) the flux tube interacts with the jet-disc boundary. The best-fitting windows have an intermediate (i=30$^\circ$/50$^\circ$) inclination and zero-BH-spin. The targeted `slow light' study did not yield better-fitting time-lag results, which indicates that the fast vs. slow light paradigm is often not intuitively understood and is likely influential in timing-sensitive studies. Sophisticated general-relativistic magnetrohydrodynamical models consistently capture the observational time-lag behavior, which is rooted in the complex dynamic interplay between the flux tube and coupled disk-jet system.