Timing properties of XB 1254-690

Abstract

We analyze archival Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) data of the low mass X-ray binary (LMXB) system XB 1254-690. We calculate colour-colour diagram, hardness-intensity diagram and power spectra of this source, associate its broad low-frequency timing features with the portions of the colour-colour diagram, and establish that XB 1254-690 is an atoll source. This will be important to correlate the timing and spectral features of this dipping LMXB with its various states, which may be useful to understand LMXBs in general. We find the source always in the high intensity banana state, which may explain why ~ 1 Hz "dipper QPOs" have never been observed from XB 1254-690. We also report a suggestive evidence of millisecond period brightness oscillations at the frequency ~ 95 Hz during a thermonuclear X-ray burst for the first time from this source.