Iron K$\alpha$ line of boson stars

TL;DR

This paper investigates the iron Kα line signatures of rotating boson stars, comparing them to Kerr black holes, and assesses whether current X-ray observations can distinguish between these exotic objects and black holes.

Contribution

It extends previous work by analyzing the limiting case of boson stars and evaluates their observational signatures in X-ray spectra.

Findings

Some boson star solutions can be ruled out with current data.

Certain boson stars are indistinguishable from Kerr black holes with current X-ray facilities.

The analysis helps constrain the nature of compact objects in astrophysics.

Abstract

The present paper is a sequel to our previous work [Y. Ni et al., JCAP 1607, 049 (2016)] in which we studied the iron K$\alpha$ line expected in the reflection spectrum of Kerr black holes with scalar hair. These metrics are solutions of Einstein's gravity minimally coupled to a massive, complex scalar field. They form a continuous bridge between a subset of Kerr black holes and a family of rotating boson stars depending on one extra parameter, the dimensionless scalar hair parameter $q$, ranging from 0 (Kerr black holes) to 1 (boson stars). Here we study the limiting case $q=1$, corresponding to rotating boson stars. For comparison, spherical boson stars are also considered. We simulate observations with XIS/Suzaku. Using the fact that current observations are well fit by the Kerr solution and thus requiring that acceptable alternative compact objects must be compatible with a Kerr fit, we find that some boson star solutions are relatively easy to rule out as potential candidates to explain astrophysical black holes, while other solutions, which are neither too dilute nor too compact are more elusive and we argue that they cannot be distinguished from Kerr black holes by the analysis of the iron line with current X-ray facilities.