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

Authors


E-mail: seyda@astroa.physics.metu.edu.tr (ŞŞ); inam@baskent.edu.tr (SÇİ); altan@astroa.physics.metu.edu.tr (AB)

ABSTRACT

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.

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