We compiled X-ray and Optical observations of the accreting X-ray binary system A 0535+262 since its discovery in 1975, that will allow us to shed light on the unpredictable behavior of this binary system. We present the data in terms of the Be-disk
interaction with the neutron star companion. In addition, we show recent results from the continuous monitoring of this source by the Gamma-ray Burst Monitor(GBM), on board the Fermi observatory, since its launch in 2008 June 11.
Recent X-ray observations by Fermi/GBM discovered a new torque reversal of 4U 1626-67 after 18 years of steady spinning down. Using Swift/BAT observations we were able to center this new torque reversal on Feb 4 2008, lasting approximately 150 days.
From 2004 up to the end of 2007, the spin-down rate averaged at a mean rate of ~dnu/dt=-4.8e-13 Hz s-1 until the torque reversal reported here. Since then it has been following a steady spin-up at a mean rate of ~dnu/dt= 4e-13 Hz s-1. The properties of this torque reversal, as well as the lack of correlation between the X-ray flux and the torque applied to the neutron star before this transition, challenges our understanding of the physical mechanisms operating in this system.
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.
