Torque reversals and pulse profile of the pulsar 4U 1626-67

摘要

We review the pulse profile evolution of the unique accretion powered X-raypulsar 4U 1626-67 over the last 40 years since its discovery. This pulsarshowed two distinct eras of steady spin-up separated by a steady spin-downepisode for about 18 years. In the present work, using data from differentobservatories active during each phase of spin-up and spin-down we establish aclear correlation between the accretion torque acting on this pulsar and itspulse profile. The energy resolved pulse profiles are identical in both thespin-up eras and quite different in the spin-down era, especially in the lowenergy band. This correlation, along with the already known feature of strongQuasi Periodic Oscillations (QPO) that was present only in the spin-down era,clearly establish two different accretion modes onto the neutron star whichproduce different pulse profiles and only one of which produces the QPOs.