Abstract:
We report here a study of the long-term properties of quasiperiodic oscillations (QPOs) in an unusual accreting X-ray
pulsar, 4U 1626À67. 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 1626À67 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 the Keplerian frequency model (KFM) and the beat frequency mdel
(BFM). A gradual depletion of accretion disk is reported earlier from the X-ray spectral, flux, and pulse profile mea-
surements. 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 timescale relating it with the long-term evolution of the accretion disk.
