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We report an indication (3.22 sigma) of ~ 1860 Hz quasi-periodic oscillationsfrom a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this willbe by far the highest frequency feature observed from an accreting neutron starsystem, and hence could be very useful to understand such systems. Thisplausible timing feature was observed simultaneously with lower (~ 585 Hz) andupper (~ 904 Hz) kilohertz quasi-periodic oscillations. The two kilohertzquasi-periodic oscillation frequencies had the ratio of ~ 1.5, and thefrequency of the alleged ~ 1860 Hz feature was close to the triple and thedouble of these frequencies. This can be useful to constrain the models of allthe three features. In particular, the ~ 1860 Hz feature could be (1) from anew and heretofore unknown class of quasi-periodic oscillations, or (2) thefirst observed overtone of lower or upper kilohertz quasi-periodicoscillations. Finally we note that, although the relatively low significance ofthe ~ 1860 Hz feature argues for caution, even a 3.22 sigma feature at such auniquely high frequency should be interesting enough to spur a systematicsearch in the archival data, as well as to scientifically motivate sufficientlylarge timing instruments for the next generation X-ray missions.

Simultaneous observations of a pair of kilohertz QPOs and a plausible 1860 Hz QPO from an accreting neutron star system