Simultaneous observations of a pair of kilohertz QPOs and a plausible
  1860 Hz QPO from an accreting neutron star system
  We report an indication (3.22 sigma) of ~ 1860 Hz quasi-periodic oscillations
from a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this will
be by far the highest frequency feature observed from an accreting neutron star
system, and hence could be very useful to understand such systems. This
plausible timing feature was observed simultaneously with lower (~ 585 Hz) and
upper (~ 904 Hz) kilohertz quasi-periodic oscillations. The two kilohertz
quasi-periodic oscillation frequencies had the ratio of ~ 1.5, and the
frequency of the alleged ~ 1860 Hz feature was close to the triple and the
double of these frequencies. This can be useful to constrain the models of all
the three features. In particular, the ~ 1860 Hz feature could be (1) from a
new and heretofore unknown class of quasi-periodic oscillations, or (2) the
first observed overtone of lower or upper kilohertz quasi-periodic
oscillations. Finally we note that, although the relatively low significance of
the ~ 1860 Hz feature argues for caution, even a 3.22 sigma feature at such a
uniquely high frequency should be interesting enough to spur a systematic
search in the archival data, as well as to scientifically motivate sufficiently
large timing instruments for the next generation X-ray missions.
