SALT observation of X-ray pulse reprocessing in 4U 1626-67

Abstract

We investigate optical reprocessing of X-rays in the LMXB pulsar 4U 1626-67 in its current spin-up phase using observations with Southern African Large Telescope (SALT), near-simultaneous observations with Swift-XRT and non-simultaneous RXTE-PCA observations and present the results of timing analysis. Using SALT observations carried out on 5th and 6th March, 2014, we detect some interesting reprocessing signatures. We detect a weak optical Quasi Periodic Oscillation (QPO) in the power density spectrum on March 5th at 48 mHz with a fractional rms of 3.3% in spite of the fact that source shows no corresponding X-ray QPO in the spin-up phase. In the light curve obtained on March 5th, we detect a coherent pulsation at the spin period of ~7.677 s. A previously known, slightly down-shifted side-band is also detected at 129.92 mHz. The frequency spacing between main pulse and this side-band is different from earlier observations, though the statistical significance of the difference is limited. The light curve of 6th March displays short time-scale variability in the form of flares on timescales of a few minutes. Folded pulse profiles resulting from data of this night show an interesting trend of pulse peak drifting. This drift could be due to i) rapid changes in the reprocessing agent, like orbital motion of an accretion disk warp around the neutron star or ii) intrinsic pulse phase changes in X-rays. We also examine some X-ray light curves obtained with RXTE-PCA during 2008-2010 for pulse shape changes in short time scales during X-ray flares.

arXiv commentAccepted for publication in MNRAS

References in corpus (17)

• Observations of Accreting Pulsars
• First science with SALT: peering at the accreting polar caps of the eclipsing polar SDSS J015543.40+002807.2
• Rapid optical and X-ray timing observations of GX 339-4: multi-component optical variability in the low/hard state
• High-Speed Optical Photometry of the Ultracompact X-ray Binary 4U 1626-67
• BeppoSAX observation of 4U1626-67: Discovery of an absorption cyclotron resonance feature
• New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT
• Multiwavelength Observations of EXO 0748-676 - I. Reprocessing of X-ray Bursts
• Magnetically Driven Precession of Warped Disks and Milli-Hertz Variabilities in Accreting X-Ray Pulsars
• Large X-ray Flares from LMC X-4: Discovery of Milli-hertz Quasi-periodic Oscillations and QPO-modulated Pulsations
• Millihertz Optical/UV Oscillations in 4U 1626-67: Evidence for a Warped Accretion Disk
• Torque reversals and pulse profile of the pulsar 4U 1626-67
• The absence of sub-minute periodicity in classical T Tauri stars
• A study of the long term evolution of quasi periodic oscillations in the accretion powered X-ray pulsar 4U 1626-67
• Spectral and temporal changes associated with flux enhancement in 4U 1626-67
• Long term flux variations in Cen X-3: clues from flux dependent orbital modulation and pulsed fraction
• Discovery of optical pulsations in V2116 Ophiuchi/GX 1+4
• Discovery of mHz UV Quasiperiodic Oscillations in Hercules X-1

Cited by in corpus (6)

• Detection of Very Low-Frequency Quasi-Periodic Oscillations in the 2015 Outburst of V404 Cygni
• AstroSat detection of a mHz QPO and cyclotron line in IGR J19294+1816 during the 2019 outburst
• Neutron Stars in X-ray Binaries and their Environments
• On the peculiar torque reversals and the X-ray luminosity history of the accretion-powered X-ray pulsar 4U 1626--67
• Changes in the pulse phase dependence of X-ray emission lines in 4U 1626-67 with a torque reversal
• Timing and Spectral Analysis of 4U 1626-67 Using AstroSat/LAXPC