Gabriel Torok (SU Opava, Czech Republic)

Properties of neutron stars implied by models of twin-peak QPOs

Twin-peak quasiperiodic oscillations (QPOs) are observed in the X-ray power-density spectra of several
accreting low-mass neutron star (NS) binaries including both bright Z- and faint atoll- sources. They consist
from the so called lower and upper QPO, two distinct peaks with frequencies NUL and NUU. The peculiar Zsource
Circinus X-1 displays unusually low QPO frequencies. Its observations reveal NUL = 50-250Hz and
NUU =200-500Hz. Using these observations we have previously considered the relativistic precession (RP)
QPO model to estimate the mass of NS in Circinus X-1. We have shown that such estimate results in a
specific mass--angular-momentum (M-j) relation rather than a single preferred combination of M and j. In the
present work we confront our previous results to another binary, atoll source 4U 1636-53 that displays the
twin-peak QPOs at very high frequencies, NUL=550-1000Hz and NUU=800-1250Hz, and extend the
consideration to several other twin-peak QPO models. We find that, in analogy to the RP model, these
models imply their own specific M-j relations. Our findings that include evaluation of individual M-j relations
enable one to make comparison between predictions of the considered orbital twin-peak QPO models for
two very different systems. Finally, we discuss differences in the chi-square behaviour between high- and
low- frequency sources and the requirement of a (likely non-geodesic) correction to the RP model. We
demonstrate that the differences in the chi-square behaviour can be related to the variability of the model
predictive power across the frequency plane. Consequently, the same correction can be required for both
classes of sources. We show that for a particular toy-model of such correction the data of 4U 1636-53 are
well matched assuming the angular momentum inferred from the X-ray burst measurements. We also show
that a consideration of low frequency QPOs and the Lense-Thirring precession mechanism within the RP
model can be finally crucial for fixing the value of j and challenging for application of the concrete NS
equations of state. 
This issue however requires a further detailed future work joining both additional data analysis
and modelling the influence of the neutron star equation of state.