Low-frequency oscillations in XTE J1550-564

TL;DR

This study analyzes low-frequency QPOs in XTE J1550-564, revealing their correlation with accretion disk emission, the presence of a new harmonic component, and energy-dependent timing features during outburst states.

Contribution

It introduces the detection of a broad 1.5× harmonic component and discusses the fundamental nature of the sub-harmonic peak, advancing understanding of QPO harmonic sequences.

Findings

QPO frequency varies with count rate and accretion disk emission.

A new broad 1.5× harmonic component was detected.

Energy dependence of timing features shows different behaviors for continuum components.

Abstract

We present the results of timing analysis of the low-frequency Quasi-Periodic Oscillation (QPO) in the Rossi X-Ray Timing Explorer data of the black hole binary XTE J1550--564 during its 1998 outburst. The QPO frequency is observed to vary on timescales between $\sim$100 s and days, correlated with the count rate contribution from the optically thick accretion disk: we studied this correlation and discuss its influence on the QPO width. In all observations, the quality factors ($\nu_0$/FWHM) of the fundamental and second harmonic peaks were observed to be consistent, suggesting that the quasi-periodic nature of the oscillation is due to frequency modulation. In addition to the QPO and its harmonic peaks, a new 1.5$\nu$ component was detected in the power spectra. This component is broad, with a quality factor of $\sim$0.6. From this, we argue what the peak observed at half the QPO frequency, usually referred to as "sub-harmonic" could be the fundamental frequency, leading to the sequence 1:2:3:4. We also studied the energy dependence of the timing features and conclude that the two continuum components observed in the power spectrum, although both more intense at high energies, show a different dependence on energy. At low energies, the lowest-frequency component dominates, while at high energies the higher-frequency one has a higher fractional rms. An interplay between these two components was also observed as a function of their characteristic frequency. In this source, the transition between low/hard state and hard-intermediate state appears to be a smooth process.