Nonthermal hotspot orbiting the supermassive black hole Sgr A*

Abstract

Recent millimeter wavelength observations at 230 GHz by the Event Horizon Telescope have resolved the innermost structure of the Galactic Center, Sgr A*, revealing a ring-like structure consistent with synchrotron emission from a magnetized torus surrounding a supermassive black hole. These observations reveal two key features: three prominent hotspots near the photon ring and low light-curve variability, with a modulation index of $\barσ/\barμ \lesssim 0.1$. State-of-the-art general relativistic magnetohydrodynamic and radiative transfer simulations cannot simultaneously reproduce the three hotspots and low light-curve variability observed in Sgr A*. We propose that an orbiting hot blob, formed in the accretion disk, could account for hotspots. We systematically explore its physical properties, including size, emission amplitude, orbital radius, eccentricity, velocity, and orbital plane orientation. The results -- applying two observational constraints--show that the preferred model corresponds to a black hole with dimensionless spin $a_{\star}=0.5$, a magnetically arrested disk configuration for the magnetic field, and the presence of a hotspot with size $σ_{\rm hs}=2\,M$ and intensity amplitude $A=10$, orbiting circularly at $r_{\rm orb}=6 M$ with Keplerian velocity in the black holes equatorial plane, and an inclination view angle of $i^\circ=20$.