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

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 companions 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.

RXTE and SWIFT Observations of SWIFT J1729.9-3437

We analyze emph{RXTE} and emph{Swift} observations of SWIFT J1729.9$-$3437 after its outburst from 2010 July 20 to 2010 August 12. We calculate a spin frequency and spin frequency derivative of $1.8734(8) times 10^{-3}$ Hz and $6.42(6) times 10^{-12}$ Hz/s respectively from the quadratic fit of pulse arrival times. The quadratic fit residuals fit well to a circular orbital model with a period of $15.3(2)$ days and a mass function of about $1.3M_{odot}$, but they can also be explained by a torque noise strength of $6.8 times 10^{-18}$ Hz sec$^{-2}$. Pulse profiles switches from double-peaked to single-peaked as the source flux continues to decrease. We find that the pulse shape generally shows no strong energy dependence. The hardness ratios reveal that the source becomes softer with decreasing flux. We construct a single spectrum from all the available RXTE and Swift observations. We find that adding an emph{Fe} line complex feature around 6.51 keV slightly improves the spectral fit and that this feature is more likely to originate from the source rather than the Galactic ridge. From the pulse phase spectral analysis, it is shown that that photon index and folding energy of the high energy cut-off vary with varying pulse phase.

Analysis on RXTE, INTEGRAL and ROTSE IIId observations of the X-ray Pulsar 4U 1907+09

In this paper we present our recent timing and spectral analysis of the X-ray pulsar 4U 1907+09. Our X-ray data consist of an extended set of RXTE & INTEGRAL observations that were analyzed before ({c{S}}ahiner et al. 2012). From the X-ray observations we extend the pulse period history of the source and obtain a revised orbital distribution of the X-ray dips. Using ROTSE IIId optical observations, we present the long term optical light curve of the source to have an understanding of long term optical behaviour.

A Comprehensive Study on RXTE & INTEGRAL Observations of the X-ray Pulsar 4U 1907+09

We analyse emph{INTEGRAL} (between 2005 October and 2007 November) and emph{RXTE} (between 2007 June and 2011 March) observations of the accretion powered pulsar 4U 1907+09. From emph{INTEGRAL} IBIS-ISGRI and emph{RXTE}-PCA observations, we update pulse period history of the source. We construct power spectrum density of pulse frequencies and find that fluctuations in the pulse frequency derivatives are consistent with the random walk model with a noise strength of $1.27times10^{-21}$ Hz s$^{-2}$. From the X-ray spectral analysis of emph{RXTE}-PCA observations, we find that Hydrogen column density is variable over the binary orbit, tending to increase just after the periastron passage. We also show that the X-ray spectrum gets hardened with decreasing X-ray flux. We discuss pulse-to-pulse variability of the source near dipping ingress and egress. We find that the source more likely undergoes in dipping states after apastron until periastron when the accretion from clumpy wind might dominate so that occasional transitions to temporary propeller state might occur.

The Orbital Period of Swift J1626.6-5156

We present the discovery of the orbital period of Swift J1626.6-5156. Since its discovery in 2005, the source has been monitored with Rossi X-ray Timing Explorer, especially during the early stage of the outburst and into the X-ray modulating episode. Using a data span of $sim$700 days, we obtain the orbital period of the system as 132.9 days. We find that the orbit is close to a circular shape with an eccentricity 0.08, that is one of the smallest among Be/X-ray binary systems. Moreover, we find that the timescale of the X-ray modulations varied, which led to earlier suggestions of orbital periods at about a third and half of the orbital period of Swift J1626.6-5156.

Analysis of RXTE-PCA Observations of SMC X-1

We present timing and spectral analysis of RXTE-PCA observations of SMC X-1 between January 1996 and December 2003. From observations around 30 August 1996 with a time span of $sim 6$ days, we obtain a precise timing solution for the source and resolve the eccentricity as 0.00089(6). We find an orbital decay rate of $dot P_{orb}/P_{orb} =-3.402(7) times 10^{-6}$ yr$^{-1}$ which is close to the previous results. Using our timing analysis and the previous studies, we construct a $sim 30$ year long pulse period history of the source. We show that frequency derivative shows long (i.e. more than a few years) and short (i.e. order of days) term fluctuations. From the spectral analysis, we found that all spectral parameters except Hydrogen column density showed no significant variation with time and X-ray flux. Hydrogen column density is found to be higher as X-ray flux gets lower. This may be due to the increase in soft absorption when the pulsar is partially obscured as in Her X-1 or may just be an artifact of the tail of a soft excess in energy spectrum.

RXTE-PCA Observations of XMMU J054134.7-682550

We analyzed RXTE-PCA observations of a recent outburst of the X-ray pulsar XMMU J054134.7$-$682550. We calculated the pulse frequency history of the source. We found no sign of a binary companion. The source spins up when the X-ray flux is higher, with a correlation between the spin-up rate and X-ray flux, which may be interpreted as a sign of an accretion disk. On the other hand, the source was found to have an almost constant spin frequency when the X-ray flux is lower without any clear sign of a spin-down episode. The decrease in pulsed fraction with decreasing X-ray flux was intrepreted as a sign of accretion geometry change, but we did not find any evidence of a transition from accretor to propeller regimes. The source was found to have variable pulse profiles. Two peaks in pulse profiles were usually observed. We studied the X-ray spectral evolution of the source throughout the observation. Pulse phase resolved analysis does not provide any further evidence for a cyclotron line, but may suggest a slight variation of intensity and width of the 6.4 keV iron line with phase.