A study of the low-frequency quasi-periodic oscillations in the X-ray   light curves of the black hole candidate XTE J1859+226
  We present the results of an extensive timing analysis of the 1999 outburst of the soft X-ray transient and black hole candidate XTE J1859+226 as observed with the Rossi X-Ray Timing Explorer. Three main different types of low frequency (1-9 Hz) quasi-periodic oscillations (QPOs) were observed and classified, strenghtening the general picture that is emerging for the variability properties of black hole X-ray binaires. Rapid transitions between different power spectral shapes were observed and their link with the count rate was studied. Furthermore, we show that a frequency of ~6 Hz seems to hold a particular place: one of the three QPO types we found was very stable when at this frequency, as it happens in Z sources as well. The coherence of its subharmonic peak was higher when the fundamental was close to 6 Hz, thus suggesting the presence of some resonance at this frequency. 
Piergiorgio Casella (INAF - Oss. Roma; Univ. Roma Tre), Tomaso Belloni   (INAF Oss. Brera), Jeroen Homan (MIT) and Luigi Ste...