W. Zhang et al., DISCOVERY OF 2 SIMULTANEOUS KILOHERTZ QUASI-PERIODIC OSCILLATIONS IN THE PERSISTENT FLUX OF GX-349+2, The Astrophysical journal, 500(2), 1998, pp. 167-169
We report the discovery of two simultaneous quasi-periodic oscillation
s in the persistent flux of GX 349+2 at frequencies 712 +/- 9 and 978
+/- 9 Hz, with rms amplitudes 1.25% +/- 0.34% and 1.34% +/- 0.32%, res
pectively. During our 152 ks observation with the Rossi X-ray Timing E
xplorer, GX 342 + 2 was in either the normal branch or the flaring bra
nch with count rates in the nominal 2-60 keV RXTE-PCA band ranging fro
m a low of 8000 counts s(-1) to a high of 15,000 counts s(-1). The kHz
QPOs were observed only when the source was at the top of the normal
branch, when the count rate was about 8200 counts s(-1) corresponding
to a flux of 1.4 x 10(-8) ergs cm(-1) s(-1) in the 2-10 keV band. With
this report, now kHz QPOs have been observed in all six Z sources.