XTE J1550-564 quasi-periodic oscillations

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 ~0.4-0.8Hz and it did not increase much when its frequency was between ~0.8 and ~3.3Hz.

However, it clearly increased with photon energy when it was larger than ~3.3Hz. The most interesting point is that it increased much more quickly with photon energy when its centroid frequency was between ~6 and ~8.5Hz. Subsequently, its rising rate dropped sharply when its frequency was larger than ~8.5Hz.