Flares from plasmoids and current sheets around Sgr A*

¹ Department of Physics, University of Patras, Rio 26504, Greece
² Research Center for Astronomy and Applied Mathematics, Academy of Athens, Athens 11527, Greece
³ Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15784, Athens, Greece
⁴ Institute of Accelerating Systems & Applications, University Campus Zografos, GR 15784, Athens, Greece
⁵ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Strasse 31, 97074 Würzburg, Germany
⁶ Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

^⋆ Corresponding authors; up1098378@upatras.gr, anathanail@academyofathens.gr

Received: 19 July 2024
Accepted: 24 February 2025

Abstract

Context. The supermassive black hole Sgr A* at the center of our galaxy produces repeating near-infrared flares that are observed by ground- and space-based instruments. This activity has been simulated in the past with magnetically arrested disk models that include stable jet formations. We used a different approach, considering a standard and normal evolution (SANE) multi-loop model that lacks a stable jet structure.

Aims. The main objective of this research is to identify regions that contain current sheets and high magnetic turbulence, and to subsequently generate a 2.2 μm light curve from nonthermal particles. These aims required the identification of areas that contain current sheets and high magnetic turbulence, and the averaging of the magnetization in the regions surrounding these areas. Subsequently, particle-in-cell fitting formulas were applied to determine the nonthermal particle distribution and to obtain the sought-after light curve. Additionally, we investigated the properties of the flares, in particular their evolution during flare events, and the similarity of flare characteristics between the generated and observed light curves.

Methods. We employed 2D general relativistic magnetohydrodynamic simulation data from a SANE multi-loop model and introduced thermal radiation to generate a 230 GHz light curve. Physical variables were calibrated to align with the 230 GHz observations. We identified current sheets by analyzing toroidal currents, magnetization, plasma β, density, and dimensionless temperatures. We studied the evolution of current sheets during flare events and calculated higher-energy nonthermal light curves, focusing on the 2.2 μm near-infrared range.

Results. We obtain promising 2.2 μm light curves whose flare duration and spectral index behavior align well with observations. Our findings support the association of flares with particle acceleration and nonthermal emission in current sheet plasmoid chains and at the boundary of the disk inside the funnel above and below the central black hole.