Conic Sections on the Sky: Shadows of Linearly Superrotated Black Holes

TL;DR

This paper investigates how linearly superrotated black holes, characterized by soft hairs, influence the shape of black hole shadows, revealing that such hairs can deform shadows into ellipses, unlike supertranslations.

Contribution

It analytically constructs photon orbits around linearly superrotated black holes and shows these hairs deform the shadow shape, highlighting observable effects of infrared structures in gravity.

Findings

Superrotation hairs deform the black hole shadow into an ellipse.

Supertranslated hairs shift the shadow's position without shape change.

Linearly superrotated black holes are free of near-zone pathologies.

Abstract

Soft hairs are an intrinsic infrared feature of a black hole, which may also affect near-horizon physics. In this work, we study some of the subtleties surrounding one of the primary observables with which we can study their effects in the context of Einstein's gravity: the black hole shadow. First, we clarify the singular pathology associated with black holes with soft hairs and demonstrate that the metrics of linearly superrotated black holes are free of near-zone pathologies due to appropriate asymptotic falloff conditions being imposed on the event horizon. We then analytically construct the photon orbits around such black holes, derive the shadow equation for near-zone observers, and find that the linear superrotation hairs deform the circular shadow of a bald Schwarzchild black hole into an ellipse. This is in sharp contrast to their supertranslated counterparts, which only shift the position of the center of the circular shadow but do not change its shape. Our results suggest a richness to the observable effects due to the infrared structures of Einstein's gravity.