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

We review the pulse profile evolution of the unique accretion powered X-ray pulsar 4U 1626-67 over the last 40 yr since its discovery. This pulsar showed two distinct eras of steady spin-up separated by a steady spin-down episode for about 18 yr. In this work, using data from different observatories active during each phase of spin-up and spin-down, we establish a clear correlation between the accretion torque acting on this pulsar and its pulse profile. The energy-resolved pulse profiles are identical in both the spin-up eras and quite different in the spin-down era, especially in the low-energy band. This correlation, along with the already known feature of strong quasi-periodic oscillations (QPO) that was present only in the spin-down era, clearly establishes two different accretion modes on to the neutron star which produce different pulse profiles and only one of which produces the QPOs.