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

Abstract

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.

arXiv comment14 pages, 3 figures. Accepted for publication in ApJ

References in corpus (7)

• Observations of Accreting Pulsars
• High-Speed Optical Photometry of the Ultracompact X-ray Binary 4U 1626-67
• Double-Peaked X-Ray Lines from the Oxygen/Neon-Rich Accretion Disk in 4U1626-67
• High-resolution X-ray spectroscopy of the ultracompact LMXB pulsar 4U 1626-67
• Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67
• Millihertz Optical/UV Oscillations in 4U 1626-67: Evidence for a Warped Accretion Disk
• Quasi periodic oscillations in XTE J0111.2--7317, highest frequency among the HMXB pulsars

Cited by in corpus (21)

• New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT
• Can disk-magnetosphere interaction models and beat frequency models for quasi-periodic oscillation in accreting X-ray pulsars be reconciled?
• Discovery of a 0.02 Hz QPO feature in the Transient X-ray Pulsar KS 1947+300
• Timing and spectral studies of the transient X-ray pulsar GX 304-1 during an outburst
• Quasiperiodic oscillations in Cen X-3 and the long term intensity variations
• Torque reversals and pulse profile of the pulsar 4U 1626-67
• Spectral and temporal changes associated with flux enhancement in 4U 1626-67
• RXTE-PCA observations of 1A 1118--61: timing and spectral studies during an outburst
• Broadband mHz QPOs and spectral study of LMC X$-$4 with AstroSat
• 4U 1626-67 Returns to Spin-Down: Timing features toe the line
• Flares, Broadening of the Pulse frequency peak, and Quasi Periodic Oscillations in the Transient X-ray Pulsar 4U 1901+03
• High energy Millihertz quasi-periodic oscillations in 1A 0535+262 with Insight-HXMT challenge current models
• Neutron Stars in X-ray Binaries and their Environments
• Constraints on the disc-magnetosphere interaction in accreting pulsar 4U 1626--67
• SALT observation of X-ray pulse reprocessing in 4U 1626-67
• Changes in the pulse phase dependence of X-ray emission lines in 4U 1626-67 with a torque reversal
• A diagnosis on torque reversals in 4U 1626-67
• Sidebands to mHz QPOs in 4U 1626$-$67 in the second spin-down state
• The ongoing spin-down episode of 4U 1626-67
• On the minimum spin period of accreting pulsars
• Timing and Spectral Analysis of 4U 1626-67 Using AstroSat/LAXPC