Photon Ring and Observational Appearance of a Hairy Black Hole

TL;DR

This paper explores how a hairy black hole's unique photon potential structure can produce a wide, bright photon ring, significantly affecting the observed image compared to traditional black holes.

Contribution

It demonstrates that hairy black holes can have a wide, bright photon ring due to specific photon potential features, unlike the narrow ring in Schwarzschild black holes.

Findings

Wide and bright photon ring observed in certain parameter regimes.

Multiple concentric bright rings with different luminosities.

Significant contribution of photon ring to the total flux.

Abstract

Recently, the image of a Schwarzschild black hole with an accretion disk has been revisited, and it showed that the "photon ring", defined as highly bent light rays that intersect the disk plane more than twice, is extremely narrow and makes a negligible contribution to the total brightness. In this paper, we investigate the observational appearance of an optically and geometrically thin accretion disk around a hairy black hole in an Einstein-Maxwell-scalar model. Intriguingly, we find that in a certain parameter regime, due to an extra maximum or an "ankle-like" structure in the effective potential for photons, the photon ring can be remarkably wide, thus making a notable contribution to the flux of the observed image. In particular, there appears a wide and bright annulus, which comprises multiple concentric bright thin rings with different luminosity, in the high resolution image.