A study of the low-frequency quasi-periodic oscillations in the X-ray light curves of the black hole candidate XTE J1859+226

¹ INAF - Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Roma, Italy e-mail: casella@mporzio.astro.it
² Physics Department, Università degli Studi “Roma Tre”, Via della Vasca Navale 84, 00146 Roma, Italy
³ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ Center for Space Researh, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

Received: 5 May 2004
Accepted: 6 July 2004

Abstract

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, strengthening 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.