Magnetic flux eruptions at the root of time lags in low-luminosity active galactic nuclei

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
e-mail: jt.vos@astro.ru.nl
² Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
³ Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 550 W 120th St, New York, NY 10027, USA
⁴ Departamento de Matemática da Universidade de Aveiro and Centre for Research and Development in Mathematics and Applications (CIDMA), Campus de Santiago, 3810-193 Aveiro, Portugal

Received: 18 January 2024
Accepted: 2 April 2024

Abstract

Context. Sagittarius A^* is a compact radio source at the center of the Milky Way that has not conclusively shown evidence to support the presence of a relativistic jet. Nevertheless, indirect methods at radio frequencies do indicate consistent outflow signatures.

Aims. 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.

Methods. By means of a combined general-relativistic magnetrohydrodynamical and radiative transfer modeling approach, we studied the origin of the time lags for magnetically arrested disk models with three black hole spins (a_* ∈ { − 0.9375, 0, 0.9375}). We exclusively modeled the emission from the source across a frequency range of ν = 19 − 47 GHz. Our study also includes a targeted “slow light” investigation for one of the best-fitting “fast light” windows.

Results. We were able to recover 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-disk 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-disk boundary. The best-fitting windows have an intermediate (i = 30°/50°) inclination and zero black hole spin. The targeted slow light study did not produce better-fitting time lag results, which indicates that the fast versus slow light paradigm is often not intuitively understood and is likely to be influential in timing-sensitive black hole accretion studies.

Conclusions. While previous studies have sought to interpret time lag properties with spherical or jetted expansion models, we show that this picture is too simplistic. 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.