Recent Torque Reversal of 4U 1907+09

TL;DR

This paper analyzes RXTE-PCA observations of the pulsar 4U 1907+09, revealing a recent torque reversal from spin-up to spin-down and examining spectral variations with orbital phase.

Contribution

It provides the first detailed timing analysis of the recent torque reversal and characterizes spectral changes, offering new insights into the pulsar's accretion dynamics.

Findings

The pulsar recently entered a new spin-down episode with a rate similar to pre-1999 levels.

Hydrogen column density varies with orbital phase.

The source transitioned from spin-up to spin-down after a period of spin-down.

Abstract

We present timing and spectral analysis of RXTE-PCA observations of the accretion powered pulsar 4U 1907+09 between June 2007 and August 2008. 4U 1907+09 had been in a spin-down episode with a spin-down rate of $-3.54\times10^{-14}$ Hz s$^{-1}$ before 1999. From RXTE observations after March 2001, the source showed a $\sim 60$% decrease in spin-down magnitude and INTEGRAL observations after March 2003 showed that source started to spin-up. We found that the source recently entered a new spin-down episode with a spin-down rate of $-3.59 \times 10^{-14}$ 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.