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

TL;DR

This study analyzes RXTE and INTEGRAL observations of the X-ray pulsar 4U 1907+09, revealing its pulse period evolution, spectral variability, and accretion behavior over its binary orbit.

Contribution

It provides an updated pulse period history, characterizes spectral variability, and discusses accretion dynamics and dipping states in 4U 1907+09.

Findings

Pulse frequency fluctuations follow a random walk model.

Hydrogen column density varies with orbital phase.

X-ray spectrum hardens as flux decreases.

Abstract

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 $1.27\times10^{-21}$ 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.