**Publication date:** Dec 2014

**Abstract:**

We have previously applied several models of high-frequency quasi-

periodic oscillations (HF QPOs) to estimate the spin of the central Kerr

black hole in the three Galactic microquasars, GRS 1915+105, GRO

J1655-40, and XTE J1550-564. Here we explore the alternative possibility

that the central compact body is a super-spinning object (or a naked

singularity) with the external space-time described by Kerr geometry

with a dimensionless spin parameter a ≡ cJ/GM^{2}> 1. We

calculate the relevant spin intervals for a subset of HF QPO models

considered in the previous study. Our analysis indicates that for all

but one of the considered models there exists at least one interval of

a> 1 that is compatible with constraints given by the ranges of the

central compact object mass independently estimated for the three

sources. For most of the models, the inferred values of a are several

times higher than the extreme Kerr black hole value a = 1. These values

may be too high since the spin of superspinars is often assumed to

rapidly decrease due to accretion when a ≫ 1. In this context, we

conclude that only the epicyclic and the Keplerian resonance model

provides estimates that are compatible with the expectation of just a

small deviation from a = 1.

**Authors:**

Kotrlová, Andrea; Török, Gabriel; Šrámková, Eva; Stuchlík, Zdeněk;