The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE

Abstract

Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed 156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this source. We investigated the fast-time variability as a function of position on the Z track. During these observations, Sco X-2 traced out the most extensive Z track ever reported from this object, making this the most comprehensive study thus far. We found the broad peaked flaring branch noise that is typical of Sco X-2, with a centroid frequency in the range 3.3--5.8 Hz. We also discovered low frequency noise, and a new peaked noise feature, with centroid frequencies in the range 5.4--7.6 Hz and 11--54 Hz, respectively. We discuss the phenomenology of these features, their relationship with the power spectral components found in other low-mass X-ray binaries, and the implications for current models. In particular, the low frequency noise we observed was strongest at intermediate energies, in contrast to the low frequency noise seen in other Z sources. We also detected very low frequency noise, and have calculated complex cross spectra between intensity and hardness. We found that the very low frequency noise is not entirely due to motion along the Z track.