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

TL;DR

This paper reports a tentative high-frequency quasi-periodic oscillation at 1860 Hz from a neutron star system, which, if confirmed, could provide new insights into neutron star accretion physics and QPO models.

Contribution

It presents the first potential observation of a ~1860 Hz QPO, possibly representing a new class or overtone of known QPOs, and discusses its implications for neutron star models.

Findings

Indication of a 1860 Hz QPO at 3.22 sigma significance.

Simultaneous detection of lower and upper kilohertz QPOs with a 1.5 ratio.

The 1860 Hz feature is near the harmonic of the kilohertz QPOs.

Abstract

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.