We report an indication(3.22 σ)of ≈ 1860Hz 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 in understanding such systems.This plausible timing feature was observed simultaneously with lower(≈ 585Hz)and upper(≈ 904Hz)kilohertz quasi-periodic oscillations.The two kilohertz quasi-periodic oscillation frequencies had a ratio of ≈ 1.5,and the frequency of the alleged ≈ 1860Hz feature was close to the triple and the double values of these frequencies.This can be useful for constraining the models of all the three features.In particular,the ≈ 1860Hz 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 ≈ 1860Hz feature argues for caution,even a 3.22 σ 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.
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