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. Brinkerink et al. (2015) found temporal shifts between frequency bands, called time-lags, which are associated with flares and/or outflows of the accretion system. It is possible to gain information on the emission and potential outflow mechanics by interpreting these time-lags. By means of 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 ($a_\ast$ = 0.9375, 0, -0.9375). The study also includes a targeted `slow light' study for one of the best-fitting `fast light' windows. We were able to recover the time-lags found by Brinkerink et al. (2015) in various windows of our simulated lightcurves. 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 paradign is often not intuitively understood and is likely influential in timing-sensitive studies.Comment: 17 pages, 11 figure