Appearance of an Infalling Star in Black Holes with Multiple Photon Spheres

TL;DR

This paper studies how a star falling into hairy black holes with one or two photon spheres appears to distant observers, revealing unique luminosity patterns and light flashes caused by photon trapping and orbiting.

Contribution

It provides the first detailed analysis of the observational signatures of infalling stars in hairy black holes with multiple photon spheres, highlighting distinct luminosity behaviors.

Findings

Exponential decay of luminosity in single photon sphere black holes.

Presence of luminosity peaks and additional light flashes in double photon sphere cases.

Photon trapping causes observable differences in light signals from infalling stars.

Abstract

Photon spheres play a pivotal role in the imaging of luminous objects near black holes. In this paper, we examine observational appearances of a star freely falling in hairy black holes, which can possess one or two photon spheres outside the event horizon. When there exists a single photon sphere, the total luminosity measured by distant observers decreases exponentially with time at late times. Due to successive arrivals of photons orbiting around the photon sphere different times, a specific observer would see a series of light flashes with decreasing intensity, which share a similar frequency content. Whereas in the case with two photon spheres, photons temporarily trapped between the photon spheres can cause a peak of the total luminosity, which is followed by a slow exponential decay, at late times. In addition, these photons lead to one more series of light flashes seen by the specific observer.