Super-spinning compact objects and models of high-frequency quasi-periodic oscillations observed in Galactic microquasars

TL;DR

This paper investigates the possibility that the central objects in certain microquasars are super-spinning entities with spin parameters exceeding the Kerr black hole limit, analyzing their implications for high-frequency QPO models.

Contribution

It explores super-spinning compact objects as alternatives to black holes in explaining HF QPOs, extending previous models to spin parameters greater than 1.

Findings

Most models allow for a > 1 consistent with mass estimates.

Estimated spins are often several times higher than the Kerr limit.

Only epicyclic and Keplerian resonance models align with small deviations from a=1.

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.