Publication date: Dec 2012
Twin peak quasi-periodic oscillations (QPOs) appear in the X-ray power-
density spectra of several accreting low-mass neutron star (NS)
binaries. Observations of the peculiar Z-source Circinus X-1 display
unusually low QPO frequencies. Using these observations, we have
previously considered the relativistic precession (RP) twin peak QPO
model to estimate the mass of the central NS in Circinus X-1. We have
shown that such an estimate results in a specific mass-angular-momentum
(M – j) relation rather than a single preferred combination of M and j.
Here we confront our previous results with another binary, the atoll
source 4U 1636-53 that displays the twin peak QPOs at very high
frequencies, and extend the consideration to various twin peak QPO
models. In analogy to the RP model, we find that these imply their own
specific M – j relations. We explore these relations for both sources
and note differences in the χ2 behavior that represent a
dichotomy between high- and low-frequency sources. Based on the RP
model, we demonstrate that this dichotomy is related to a strong
variability of the model predictive power across the frequency plane.
This variability naturally comes from the radial dependence of
characteristic frequencies of orbital motion. As a consequence, the
restrictions on the models resulting from observations of low-frequency
sources are weaker than those in the case of high-frequency sources.
Finally we also discuss the need for a correction to the RP model and
consider the removing of M – j degeneracies, based on the twin peak QPO-
independent angular momentum estimates.
Török, Gabriel; Bakala, Pavel; Šrámková, Eva; Stuchlík, Zdeněk; Urbanec, Martin; Goluchová, Kateřina;