S. Ç. İnam

Timing studies on RXTE observations of SAX J2103.5+4545

SAX J2103.5+4545 has been continuously monitored for ∼900 d by the Rossi X-ray Timing Explorer (RXTE) since its outburst in 2002 July. Using these observations and previous archival RXTE observations of SAX J2103.5+4545, we refined the binary orbital parameters and find the new orbital period as P = (12.665 36 ± 0.000 88) d and the eccentricity as 0.4055 ± 0.0032. With these new orbital parameters, we constructed the pulse frequency and pulse frequency derivative histories of the pulsar and confirmed the correlation between X-ray flux and pulse frequency derivative presented by Baykal, Stark & Swank. We constructed the power spectra for the fluctuations of pulse frequency derivatives and found that the power-law index of the noise spectra is 2.13 ± 0.6. The power-law index is consistent with random walk in pulse frequency derivative and is the steepest among the HMXRBs. X-ray spectra analysis confirmed the inverse correlation trend between power-law index and X-ray flux found by Baykal et al.

A comprehensive study of RXTE and INTEGRAL observations of the X-ray pulsar 4U 1907+09

We analyse observations of the accretion-powered pulsar 4U 1907+09 from the International Gamma-Ray Astrophysics Laboratory (INTEGRAL; between 2005 October and 2007 November) and from the Rossi X-ray Timing Explorer (RXTE; between 2007 June and 2011 March). Using observations from the Imager on-board the INTEGRAL satellite (IBIS), the INTEGRAL Soft Gamma-Ray Imager (ISGRI) and the RXTE Proportional Counter Array (PCA), we update the pulse period history of the source. We construct power spectrum density of pulse frequencies, and we find that fluctuations in the pulse frequency derivatives are consistent with the random walk model with a noise strength of 1.27 × 10−21 Hz s−2. From the X-ray spectral analysis of RXTE/PCA observations, we find that the hydrogen column density is variable over the binary orbit, and that it tends to increase just after the periastron passage. We also show that the X-ray spectrum becomes hardened with decreasing X-ray flux. We discuss the pulse-to-pulse variability of the source near dipping ingress and egress. We find that the source is more likely to undergo dipping states after apastron until periastron when the accretion from clumpy wind might dominate. Thus, occasional transitions to a temporary propeller state might occur.

Evidence of a change in the long-term spin-down rate of the X-ray pulsar 4U 1907+09

We analysed RXTE archival observations of 4U 1907 +09 between 1996 February 17 and 2002 March 6. The pulse timing analysis showed that the source stayed at almost constant period around 1998 August and then started to spin-down at a rate of (-1.887 ∓ 0.042) × 10 -14 Hz s -1 which is ∼0.60 times lower than the long-term (∼15 yr) spin-down rate. Our pulse-frequency measurements for the first time resolved significant spin-down rate variations since the discovery of the source. We also presented orbital phase resolved X-ray spectra during two stable spin-down episodes during 1996 November-1997 December and 2001 March-2002 March. The source has been known to have two orbitally locked flares. We found that X-ray flux and spectral parameters except hydrogen column density agreed with each other during the flares. We interpreted the similar values of X-ray fluxes as an indication of the fact that the source accretes not only via transient retrograde accretion disc but also via the stellar wind of the companion, so that the variation of the accretion rate from the disc does not cause significant variation in the observed X-ray flux. Lack of significant change in spectral parameters except hydrogen column density was interpreted as a sign of the fact that the change in the spin-down rate of the source was not accompanied by a significant variation in the accretion geometry.

Timing and X-ray spectral features of Swift J1626.6-5156

In this paper, we extend the timing analysis by Baykal et al. of Swift J1626.6―5156 using RXTE―PCA observations between MJD 53724 and MJD 55113 together with a Chandra-ACIS observation on MJD 54897 with a 20-ks exposure. We also present the X-ray spectral analysis of these RXTE and Chandra observations. We find that the spin-up rate of the source is correlated with the X-ray flux. Using this correlation, we estimate the distance and surface magnetic field of the source as ∼15 kpc and ∼9 x 10 11 G, respectively. From the spectral analysis, we find that the power-law index increases and hydrogen column density decreases with a decreasing flux.

X-ray spectral evolution of Her X-1 in a low state and the following short high state

We analysed spectral variations of ∼8.5-d long Rossi X-ray Timing Explorer (RXTE) monitoring observations of Her X-1 in 2001 December. This set of observations enables, for the first time, frequent continuous monitoring (111 pointings in ∼8.5 d) of the source with RXTE including a ∼1.7-d-long low state part and the following ∼6.8-d-long short high state part. We used an absorbed power-law model with an iron line energy complex modelled as a Gaussian to fit both the 3-60 keV Proportional Counter Array-High Energy X-ray Timing Experiment (PCA-HEXTE) overall short high state spectrum and 3-20 keV individual PCA spectra. An additional partial cold absorber model was used for both cases. Using 3-20 keV individual PCA spectra, absorption in anomalous dips (ADs) and pre-eclipse dips (PDs) in the short high state was compared. Decreasing the ratio of unabsorbed to absorbed flux with increasing unabsorbed flux in ADs and PDs was interpreted as evidence of the fact that the regions causing opaque obscuration and soft absorption are not geometrically far away from each other. Higher iron line peak energies in the low state and the short high state (∼6.6-6.9 keV) were interpreted as a clue of the presence of iron line components other than the Kα emission line.

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 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 .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∼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 . Subject headings: pulsars: individual (Swift J1626.6−5156 ) − X−rays: stars − stars: oscillations

Recent Torque Reversal of 4U 1907+09

We present timing and spectral analysis of RXTE-PCA (Proportional Counter Array) observations of the accretion powered pulsar 4U 1907+09 between 2007 June and 2008 August. 4U 1907+09 had been in a spin-down episode with a spin-down rate of -3.54 × 10 -14 Hz s -1 before 1999. From RXTE observations after 2001 March, the source showed a ∼60 per cent decrease in spin-down magnitude, and INTEGRAL observations after 2003 March showed that source started to spin-up. We found that the source recently entered into a new spin-down episode with a spin-down rate of -3.59 x 10 -14 Hz s -1 . This spin-down rate is pretty close to the previous long-term spin-down rate of the source measured before 1999. From the spectral analysis, we showed that hydrogen column density varies with the orbital phase.

Analysis of RXTE-PCA Observations of SMC X-1

We present timing and spectral analysis of Rossi X-ray Timing Explorer-Proportional Counter Array observations of SMC X-1 between 1996 January and 2003 December. From observations around 1996 August 30 with a time-span of ~6 d, we obtain a precise timing solution for the source and resolve the eccentricity as 0.00089(6). We find an orbital decay rate of P orb /P orb = -3.402(7) × 10 -6 yr -1 which is close to the previous results. Using our timing analysis and the previous studies, we construct a ~30 yr long pulse period history of the source. We show that frequency derivative shows long- (i.e. more than a few years) and short-term (i.e. order of days) 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 artefact of the tail of a soft excess in energy spectrum.

Discovery of a glitch in the accretion-powered pulsar SXP 1062

We present timing analysis of the accretion powered pulsar SXP 1062, based on the observations of \textit{Swift}, \textit{XMM-Newton} and \textit{Chandra} satellites covering a time span of about 2 years. We obtain a phase coherent timing solution which shows that SXP 1062 has been steadily spinning down with a rate

Comprehensive timing and X-ray spectral analysis of GX 1+4

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