Distinguishing black holes and naked singularities with iron line spectroscopy

TL;DR

This study explores whether iron line spectroscopy can differentiate between black holes and naked singularities, focusing on Janis-Newman-Winicour metrics, and finds that certain naked singularities cannot mimic fast-rotating black holes at specific angles.

Contribution

It introduces a method to distinguish naked singularities from black holes using iron line spectra, providing observational constraints on their existence.

Findings

Janis-Newman-Winicour singularities cannot mimic fast-rotating Kerr black holes at low/moderate inclination angles.

Iron line shapes show potential to differentiate between black holes and naked singularities.

Observations can constrain the presence of naked singularities in the universe.

Abstract

It is commonly thought that the final product of gravitational collapse is a black hole. Nevertheless, theoretical studies have not yet provided a final answer to the question whether black holes are the only possible outcome or whether naked singularities are also allowed. Observational tests may thus represent a complementary approach. In the present paper, we consider the Janis-Newman-Winicour metric, which describes a rotating source with a surface-like naked singularity. We calculate iron line shapes in the reflection spectrum of a putative disk around a Janis-Newman-Winicour singularity and we compare our results with the iron line shapes expected in the spectrum of a Kerr black hole. While it is difficult to distinguish the two spacetimes from the iron line shape in general, it seems that Janis-Newman-Winicour singularities cannot mimic fast-rotating Kerr black holes observed at a low or moderate inclination angle. Our analysis thus suggests that available observations of specific sources can already constrain the possible existence of Janis-Newman-Winicour singularities in the Universe.