Constraints on horizonless objects after the EHT observation of Sagittarius A*

TL;DR

The paper critiques recent EHT constraints on horizonless objects like Sgr A*, showing that these constraints depend on unrealistic assumptions about energy balance and are invalid for physically plausible absorption coefficients.

Contribution

It demonstrates that the EHT-based radius constraints on horizonless objects are only valid under extremely idealized conditions, challenging previous interpretations.

Findings

Constraints are invalid for absorption coefficients above 10^{-14}

Realistic absorption scenarios weaken the radius bounds

EHT constraints rely on unrealistic energy balance assumptions

Abstract

The images of Sagittarius A$^*$ recently released by the Event Horizon Telescope collaboration have been accompanied [Ap.J.Lett.\,{\bf 930\,\#2}\,(2022)\,L17] by an analysis of the constraints on the possible absence of a trapping horizon, i.e.~on the possibility that the object at the center of our galaxy is an ultra-compact object with a surface re-emitting incident radiation. Indeed, using the observed image size and the broadband spectrum of Sgr A$^*$, it is claimed that the radius of any such thermal surface is strongly bounded from above by these latest observations. Herein, we discuss how the reported constraint relies on the extremely strong assumption of perfect balance in the energy exchange between the accretion disk and the central object, and show that this is violated whenever the surface is endowed with any non-zero absorption coefficient. We discuss in detail the upper-bound constraints that can be cast on the radius and dimensionless absorption coefficient of the surface. We show that the conclusions of the analysis presented by the EHT collaboration hold only for unnaturally small values of the absorption coefficient (i.e. much lower than $10^{-14}$), and thus have to be significantly revised in scenarios with physical significance.