# Iron K$\alpha$ line of Kerr black holes with Proca hair

## TL;DR

This study examines whether current X-ray observations can differentiate Kerr black holes with Proca hair from standard Kerr black holes, finding that present data cannot distinguish them, but future missions might detect Proca hair with better theoretical models.

## Contribution

It extends previous work by analyzing Kerr black holes with Proca hair and assessing their X-ray reflection signatures, highlighting the challenge in distinguishing them with current observations.

## Key findings

- Current X-ray missions cannot distinguish hairy black holes from Kerr black holes.
- Even highly hairy black holes can mimic standard Kerr black hole iron line profiles.
- Future X-ray missions may detect Proca hair with improved theoretical intensity profiles.

## Abstract

We continue our study on the capabilities of present and future X-ray missions to test the nature of astrophysical black hole candidates via X-ray reflection spectroscopy and distinguish Kerr black holes from other solutions of 4-dimensional Einstein's gravity in the presence of a matter field. Here we investigate the case of Kerr black holes with Proca hair [1]. The analysis of a sample of these configurations suggests that even extremely hairy black holes can mimic the iron line profile of the standard Kerr black holes, and, at least for the configurations of our study, we find that current X-ray missions cannot distinguish these objects from Kerr black holes. This contrasts with our previous findings for the case of Kerr black holes with scalar (rather than Proca) hair [2], even though such comparison may be biased by the limited sample. Future X-ray missions can detect the presence of Proca hair, but a theoretical knowledge of the expected intensity profile (currently missing) can be crucial to obtain strong constraints.