Photon emissions from Kerr equatorial geodesic orbits

TL;DR

This paper investigates the observability of photon emissions from emitters moving along equatorial geodesics near Kerr black holes, analyzing photon escape probabilities and blueshifts to understand potential astrophysical signals.

Contribution

It provides a detailed numerical analysis of photon escape probabilities and blueshifts for emitters on various geodesics, highlighting the observability of plunging emitters near high-spin black holes.

Findings

Plunging emitters near unstable orbits have high observability.

High-energy emitters near high-spin black holes show significant observational features.

Photon escape probability varies with emitter trajectory and black hole spin.

Abstract

We consider the light emitters moving freely along the geodesics on the equatorial plane near a Kerr black hole and study the observability of these emitters. To do so, we assume these emitters emit the photons isotropically and monochromatically, and we compute the photon escaping probability (PEP) and the maximum observable blueshift (MOB) of the photons that reach infinity. We obtain numerical results of PEP and MOB for the emitters along various geodesic orbits, which exhibit distinct features for the trajectories of different classes. We exhaustively investigate the effects of the emitters' motion on the PEP and MOB. In particular, we find that the plunging emitters approaching the unstable circular orbits could have very good observability, before fading away suddenly. This interesting observational feature becomes more significant for the high-energy emitters near a high-spin black hole. As the radiatively-inefficient accretion flow may consist of such plunging emitters, the present work could be of great relevance to the astrophysical observations.