Impact of the nonthermal electron radiation effects on the horizon scale image structure of Sagittarius A*

TL;DR

This study investigates how nonthermal electron radiation influences the horizon-scale image structure of Sagittarius A*, revealing subtle size and brightness changes detectable with current and future observations, especially at higher frequencies.

Contribution

The paper introduces a semi-analytical accretion disk model with nonthermal electrons and quantifies their impact on black hole image features, highlighting effects on crescent brightness, size, and symmetry.

Findings

Nonthermal electrons increase image brightness and symmetry.

Size difference due to nonthermal effects is about 2% at 230 GHz.

Nonthermal effects are more significant at 345 GHz.

Abstract

The Event Horizon Telescope (EHT), with $\sim$20 $\mu$as high angular resolution, recently resolved the millimeter image of the suppermassive black hole in the Galaxy, Sagittarius A*. This opens a new window to study the plasma on horizon scales. The accreting disk probably contains a small fraction of nonthermal electrons and their emissions should contribute to the observed image. We study if such contributions are sufficient to cause structural differences detectable by current and future observational capabilities. We introduce nonthermal electrons in a semi-analytical accretion disk, which considers viscosity-leading heating processes, and adopt a continued hybrid electron energy distribution of thermal distribution and power-law tail. We generate the black hole images and extract the structural features as crescent parameters. We find the existence of nonthermal electron radiation makes the crescent much brighter, slightly larger, moderately thicker, and much more symmetric. When the nonthermal connecting Lorentz factor $\gamma_c=65$, which is equivalent to the nonthermal electrons accounting for $\sim1.5$% of the totals, nonthermal effects cause $\sim2$% size difference at 230 GHz. Comparing with the structural changes caused by other physical factors, including inclination between the system and the observer, black hole spin, and interstellar medium scattering effects, we find that although nonthermal electron radiation takes the most unimportant role at 230 GHz, it becomes more significant at 345 GHz.