2010ApJ...708.1500C

After about 18 yr of steadily spinning down, the accretion-powered pulsar 4U 1626-67 experienced a new torque reversal at the beginning of 2008. For the present study, we have used all available Fermi/Gamma-ray Burst Monitor data since its launch in 2008 June 11 and over 5 yr of hard X-ray Swift/Burst Alert Telescope observations (starting from 2004 October up to the present time). From 2004 up to the end of 2007 the spin-down rate averaged at a mean rate of {dot}ν=-4.8 ×10^{-13} Hz/s until the torque reversal reported here. This second detected torque reversal was centered near MJD 54500 (2008 February 4) and it lasted approximately 150 days. During the reversal, the source also underwent an increase in flux by a fraction of ∼2.5. Since then it has been following a steady spin-up at a mean rate of {dot}ν=4 ×10^{-13} Hz/s. We present a 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.