Changes in the pulse phase dependence of X-ray emission lines in 4U 1626-67 with a torque reversal.

Abstract

We report results from an observation with the XMM–Newton observatory of a unique X-ray pulsar 4U 1626−67. European Photon Imaging Camera-pn data during the current spin-up phase of 4U 1626−67 have been used to study pulse phase dependence of low-energy emission lines. We found strong variability of low-energy emission line at 0.915 keV with the pulse phase, varying by a factor of 2, much stronger than the continuum variability. Another interesting observation is that behaviour of one of the low-energy emission lines across the pulse phase is quite different from that observed during the spin-down phase. This indicates that the structures in the accretion disc that produce pulse phase dependence of emission features have changed from spin-down to spin-up phase. This is well supported by the differences in the timing characteristics (like pulse profiles, quasi periodic oscillations, etc.) between spin-down and spin-up phases. We have also found that during the current spin-up phase of 4U 1626−67, the X-ray pulse profile below 2 keV is different compared to the spin-down phase. The X-ray light curve also shows flares which produce a feature around 3 mHz in power density spectrum of 4U 1626−67. Since flares are dominant at lower energies, the feature around 3 mHz is prominent at low energies.