Tracing the hot spot motion using the next generation Event Horizon Telescope (ngEHT)

TL;DR

This paper demonstrates that the next generation Event Horizon Telescope (ngEHT) can effectively trace the dynamical motion of hot spots near SgrA* using a new image reconstruction algorithm, revealing details of accretion processes.

Contribution

The study introduces a novel dynamical image reconstruction algorithm, StarWarps, and evaluates its effectiveness with ngEHT arrays for observing hot spot motions near black holes.

Findings

ngEHT can trace hot spot phases effectively

Additional dishes improve observability of hot spot dynamics

The algorithm is adaptable to various dynamical motions

Abstract

We propose to trace the dynamical motion of a shearing hot spot near the SgrA* source through a dynamical image reconstruction algorithm, StarWarps. Such a hot spot may form as the exhaust of magnetic reconnection in a current sheet near the black hole horizon. A hot spot that is ejected from the current sheet into an orbit in the accretion disk may shear and diffuse due to instabilities at its boundary during its orbit, resulting in a distinct signature. We subdivide the motion to two distinct phases; the first phase refers to the appearance of the hot spot modelled as a bright blob, followed by a subsequent shearing phase simulated as a stretched ellipse. We employ different observational arrays, including EHT(2017,2022) and the next generation event horizon telescope (ngEHTp1, ngEHT) arrays, in which few new additional sites are added to the observational array. We make dynamical image reconstructions for each of these arrays. Subsequently, we infer the hot spot phase in the first phase followed by the axes ratio and the ellipse area in the second phase. We focus on the direct observability of the orbiting hot spot in the sub-mm wavelength. Our analysis demonstrates that newly added dishes may easily trace the first phase as well as part of the second phase, before the flux is reduced substantially. The algorithm used in this work can be extended to any other types of the dynamical motion. Consequently, we conclude that the ngEHT is a key to directly observe the dynamical motions near variable sources, such as SgrA*.