Abstract:
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.
