Characterizing the Intermittency of the 4 Hz Quasi-periodic Oscillation in XTE J1550-564 via Hilbert-Huang Transform
Yi-Hao Su1*, Yi Chou1, Chin-Ping Hu1, Ting-Chang Yang1
1Graduate Institute of Astronomy, National Central University, Jhongli, Taiwan
* presenting author:Yi-Hao Su,
We present the time-frequency analysis results from Hilbert-Huang transform (HHT) for the evolution of a 4 Hz low-frequency quasi-periodic oscillation (LFQPO) around the black hole X-ray binary XTE J1550-564. The reason why and how black hole X-ray binaries commonly exhibit low-frequency, finite-width peaks, which are named as LFQPOs, in their Fourier power spectra is still under debate. To understand the cause of the broadening of the peak, we utilize a recently developed time-frequency analysis method, HHT, to track the evolution of the 4 Hz LFQPO from XTE J1550-564. Through adaptively decomposing the ~4 Hz oscillation component from the light curve and acquiring its instantaneous frequency, the HHT analysis results illustrate the LFQPO is composed of a series of intermittent oscillations appearing occasionally between 3 Hz and 5 Hz. We further characterized this intermittency via the confidence limit of the instantaneous amplitudes of the intermittent oscillations and constructed both of the oscillation lifetimes and break times distributions. The mean lifetime is 1.45 s and 90% of the oscillation segments have lifetimes less than 3.1 s whereas the mean break time is 0.42 s and 90% of break times are less than 0.73 s. In addition, these intermittent oscillations show a correlation between oscillation amplitude and mean count rate. This correlation derived from HHT could be an analogy to the linear rms-flux relation found in the 4 Hz LFQPO through Fourier analysis. Finally, we conclude that the LFQPO peak in power spectrum is broadened by intermittent, frequency-changing oscillations, which could be explained by Lense-Thirring precession model. Moreover, while comparing to the other fields HHT is not quite popular in astronomy yet, our results demonstrate the successful capability of HHT and it can be an alternative time-frequency analysis tool for astronomical data in the future.

Keywords: accretion disks, black hole physics, data analysis, X-ray binaries, XTE J1550-564