New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT

TL;DR

This paper reports the first detailed long-term timing and spectral analysis of the accretion-powered pulsar 4U 1626-67, revealing a significant torque reversal after 18 years of spin-down, accompanied by increased flux.

Contribution

It provides the first comprehensive timing and spectral study of the recent torque reversal in 4U 1626-67 using Fermi/GBM and Swift/BAT data.

Findings

Detected a torque reversal near MJD 54500 in 2008.

Observed a transition from spin-down to steady spin-up.

Noted a ~2.5 times increase in flux during the reversal.

Abstract

After about 18 years of steadily spinning down, the accretion-powered pulsar 4U 1626-67, experienced a torque reversal at the beginning of 2008. For the present study we have used all available Fermi/GBM data since its launch in 2008 June 11 and over 5 yr of hard X-ray Swift/BAT observations (starting from 2004 October up to the present time). This second detected torque reversal is centered near MJD 54500 (2008 Feb 4) and it lasts approximately 150 days. From 2004 up to the end of 2007 4U 1626-67 the spin-down rate decreased at a mean rate of ~ -5.5E-13 Hz s-1 until the source reversed torque again. Since then it has been following a steady spin-up at a mean rate of ~ 5E-13 Hz s-1. In addition, 4U 1626-67 increased its flux simultaneously (a ~2.5 factor). We present detailed long-term timing analysis of this source and a long term spectral hardness ratio study in order to see whether there are spectral changes around this new observed torque reversal.