  Detection of paired kilohertz quasi-periodic oscillations (kHz QPOs) in the
X-ray emission of a compact object is compelling evidence that the object is an
accreting neutron star. In many neutron stars, the stellar spin rate is equal
or roughly equal to Delta-nu, the frequency separation of the QPO pair, or to
2Delta-nu. Hence, if the mechanism that produces the kilohertz QPOs is similar
in all stars, measurement of Delta-nu can provide an estimate of the star's
spin rate. The involvement of the stellar spin in producing Delta-nu indicates
that the magnetic fields of these stars are dynamically important.
  We focus here on the implications of the paired kHz QPOs recently discovered
in the low-mass X-ray binary (LMXB) system Cir X-1 (Boutloukos et al. 2006).
The kHz QPOs discovered in Cir X-1 are generally similar to those seen in other
stars, establishing that the compact object in the Cir X-1 system is a neutron
star. However, the frequency nu-u of its upper kHz QPO is up to a factor of
three smaller than is typical, and Delta-nu varies by about a factor 2 (167 Hz,
the largest variation so far observed). Periodic oscillations have not yet been
detected from Cir X-1, so its spin rate has not yet been measured directly. The
low values of nu-u and the large variation of Delta-nu challenge current models
of the generation of kHz QPOs. Improving our understanding of Cir X-1 will
improve our knowledge of the spin rates and magnetic fields of all neutron
stars.
