**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/GM2> 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;