Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?

Abstract

There is a general consensus that the frequencies of the kilohertz Quasi-Periodic Oscillations (kHz QPOs) in neutron-star low-mass X-ray binaries are directly linked to the spin of the neutron star. The root of this idea is the apparent clustering of the ratio of the frequency difference of the kHz QPOs and the neutron-star spin frequency, $\Delta\nu/\nu_s$, at around 0.5 and 1 in ten systems for which these two quantities have been measured. Here we reexamine all available data of sources for which there exist measurements of two simultaneous kHz QPOs and spin frequencies, and we advance the possibility that $\Delta\nu$ and $\nu_s$ are not related to each other. We discuss ways in which this possibility could be tested with current and future observations.