The energy dependence of the centroid frequency of the low-frequency quasi-periodic oscillations in XTE J1550-564

In this paper, we investigate the energy dependence of the centroid frequency of the low-frequency quasi-periodic oscillations (QPOs; 0.08-20 Hz) in XTE J1550-564. We have used the observational data from the Rossi X-ray Timing Explorer during the first half part of the 1998-1999 outburst. We have found that the centroid frequency of the fundamental QPO did not vary with photon energy when its QPO frequency was less than similar to 0.4-0.8 Hz and it did not increase much when its frequency was between similar to 0.8 and similar to 3.3 Hz. However, it clearly increased with photon energy when it was larger than similar to 3.3 Hz. The most interesting point is that it increased much more quickly with photon energy when its centroid frequency was between similar to 6 and similar to 8.5 Hz. Subsequently, its rising rate dropped sharply when its frequency was larger than similar to 8.5 Hz. We have also investigated the dependence of the first-harmonic and subharmonic QPO frequency on photon energy. We have found that for more than 70 per cent of the subharmonic QPO observations, the frequency increased with the photon energy, while the rest did not change obviously. Meanwhile, similar to 60 per cent of the first-harmonic QPOs exhibited a negative correlation between their frequencies and photon energy, while the remaining correlations were irregular. For the amplitudes of the QPOs, complicated dependences on photon energy were also found. The amplitudes of the first-harmonic and subharmonic QPOs have shown a similar correlation with photon energy (i.e. their amplitudes increased with photon energy until similar to 6-7 keV, beyond which the amplitudes decreased). By contrast, the fundamental QPO amplitude increased until similar to 10 keV and became saturated toward higher energies. These newly discovered patterns of the dependence of QPOs on photon energy cannot be easily explained in the currently existing frames of QPO models. Therefore, they provide an important insight into our understanding of QPO physics.