We report here a study of the long term properties of Quasi Periodic
Oscillations (QPO) in an unusual accreting X-ray pulsar, 4U 1626--67. This is a
unique accretion powered X-ray pulsar in which we have found the QPOs to be
present during all sufficiently long X-ray observations with a wide range of
X-ray observatories. In the present spin-down era of this source, the QPO
central frequency is found to be decreasing. In the earlier spin-up era of this
source, there are only two reports of QPO detections, in 1983 with EXOSAT and
1988 with GINGA with an increasing trend. The QPO frequency evolution in 4U
1626--67 during the last 22 years changed from a positive to a negative trend,
somewhat coincident with the torque reversal in this source. In the accretion
powered X-ray pulsars, the QPO frequency is directly related to the inner
radius of the accretion disk, as per Keplerian Frequency Model (KFM) and Beat
Frequency Model (BFM). A gradual depletion of accretion disk is reported
earlier from the X-ray spectral, flux and pulse profile measurements. The
present QPO frequency evolution study shows that X-ray flux and mass accretion
rate may not change by the same factor, hence the simple KFM and BFM are not
able to explain the QPO evolution in this source. This is the only X-ray pulsar
to show persistent QPOs and is also the first accreting X-ray pulsar in which
the QPO history is reported for a long time scale relating it with the long
term evolution of the accretion disk.Comment: 14 pages, 3 figures. Accepted for publication in Ap
