Observational signatures of hot spots orbiting horizonless objects

TL;DR

This paper explores how electromagnetic observations of hot spots can distinguish black holes from horizonless objects like boson and Proca stars, revealing unique lensing signatures due to the absence of a horizon.

Contribution

It demonstrates that horizonless objects produce a distinctive lensing feature—an extra photon image—potentially observable with current or future telescopes.

Findings

Horizonless objects create an additional photon image due to photons passing through the central region.

This lensing signature is likely universal for matter weakly coupled to the standard model.

The study provides a method to test the existence of horizons in supermassive compact objects.

Abstract

Pushed by a number of advances, electromagnetic observatories have now reached the horizon scale of supermassive black holes. The existence and properties of horizons in our universe is one of the outstanding fundamental issues that can now be addressed. Here we investigate the ability to discriminate between black holes and compact, horizonless objects, focusing on the lensing of hot spots around compact objects. We work in particular with boson and Proca stars as central objects, and show that the absence of a horizon gives rise to a characteristic feature -- photons that plough through the central object and produce an extra image. This feature should be universal for central objects made of matter weakly coupled to the standard model.