Studying Frequency Relations of kHz QPOs for 4U 1636-53 and Sco X-1: Observations Confront Theories

TL;DR

This study tests various QPO models against observed frequency data from two neutron star sources, finding none fit well, indicating the need for model improvements.

Contribution

It provides a comprehensive comparison of all current QPO models against observational data, highlighting their limitations and guiding future model development.

Findings

No model satisfactorily fits the data with plausible neutron star parameters.

Data significantly deviate from linear frequency relationships.

Some models perform relatively better but still have issues.

Abstract

By fitting the frequencies of simultaneous lower and upper kilohertz Quasi-Periodic Oscillations (kHz QPOs) in two prototype neutron star QPO sources (4U~1636-536 and Sco X-1), we test the predictive power of all currently proposed QPO models. Models predict either a linear, power-law or any other relationship between the two frequencies. We found that for plausible neutron star parameters (mass and angular momentum), no model can satisfactorily reproduce the data, leading to very large chi-squared values in our fittings. Both for 4U~1636-53 and Sco X-1, this is largely due to the fact that the data significantly differ from a linear relationship. Some models perform relatively better but still have their own problems. Such a detailed comparison of data with models shall enable to identify routes for improving those models further.