Timing Studies on X-Per and Discovery of its Transient QPO Feature

Abstract

We present timing analysis of {\emph{RXTE}}-PCA and {\emph{INTEGRAL}}-ISGRI observations of X Per between 1998 and 2010. All pulse arrival times obtained from the {\emph{RXTE}}-PCA observations are phase connected and a timing solution is obtained using these arrival times. We update the long-term pulse frequency history of the source by measuring its pulse frequencies using {\emph{RXTE}}-PCA and {\emph{INTEGRAL}}-ISGRI data. From the {\emph{RXTE}}-PCA data, the relation between frequency derivative and X-ray flux suggests accretion via the companion's stellar wind. On the other hand, detection of the transient QPO feature peaking at $\sim 0.2$ Hz suggests the existence of an accretion disc. We find that double break models fit the average power spectra well, which suggests that the source has at least two different accretion flow components dominating the overall flow. From the power spectrum of frequency derivatives, we measure a power law index of $\sim -1$ which implies that on short time scales disc accretion dominates over noise, while on time scales longer than the viscous time scales the noise dominates. From pulse profiles, we find a correlation between pulse fraction and count rate of the source.