The stochastic X-ray variability of the accreting millisecond pulsar MAXI J0911-655

Abstract

In this work I report on the stochastic X-ray variability of the 340 Hz accreting millisecond pulsar MAXI J0911-655. Analyzing pointed observations of the XMM-Newton and NuSTAR observatories I find that the source shows broad band-limited stochastic variability in the $0.01-10$ Hz range, with a total fractional variability of $\sim24\%$ rms in the $0.4-3$ keV energy band, which increases to $\sim40\%$ rms in the $3-10$ keV band. Additionally a pair of harmonically related quasi-periodic oscillations are discovered. The fundamental frequency of this harmonic pair is observed between frequencies of $62$ mHz and $146$ mHz. Like the band-limited noise, the amplitude of the QPOs show a steep increase as a function of energy, suggesting they share a similar origin, which is likely the inner accretion flow. Based on their energy dependence and their frequency relation with respect to the noise terms, the QPOs are identified as Low-Frequency oscillations, and discussed in terms of Lense-Thirring precession model.