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We report here a study of the long term properties of Quasi PeriodicOscillations (QPO) in an unusual accreting X-ray pulsar, 4U 1626--67. This is aunique accretion powered X-ray pulsar in which we have found the QPOs to bepresent during all sufficiently long X-ray observations with a wide range ofX-ray observatories. In the present spin-down era of this source, the QPOcentral frequency is found to be decreasing. In the earlier spin-up era of thissource, there are only two reports of QPO detections, in 1983 with EXOSAT and1988 with GINGA with an increasing trend. The QPO frequency evolution in 4U1626--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 accretionpowered X-ray pulsars, the QPO frequency is directly related to the innerradius of the accretion disk, as per Keplerian Frequency Model (KFM) and BeatFrequency Model (BFM). A gradual depletion of accretion disk is reportedearlier from the X-ray spectral, flux and pulse profile measurements. Thepresent QPO frequency evolution study shows that X-ray flux and mass accretionrate may not change by the same factor, hence the simple KFM and BFM are notable to explain the QPO evolution in this source. This is the only X-ray pulsarto show persistent QPOs and is also the first accreting X-ray pulsar in whichthe QPO history is reported for a long time scale relating it with the longterm evolution of the accretion disk.

A study of the long term evolution of quasi periodic oscillations in the accretion powered X-ray pulsar 4U 1626-67