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), nearsimultaneous observations with Swift-XRT and non-simultaneous RXTE-PCA observations and present the results of timing analysis. Using SALT observations carried out on 2014, March 5 and 6, we detect some interesting reprocessing signatures. We detect a weak optical Quasi Periodic Oscillation (QPO) in the power density spectrum on March 5 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 5, 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 March 6 displays short time-scale variability in the form of flares on time-scales 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 disc 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.

Changes in the pulse phase dependence of X-ray emission lines in 4U 1626−67 with a torque reversal

“…Duration of flares is few hundred of seconds. Recurrence time-scales of these flares varies between 300 to 1000 seconds and these time-scales are consistent with the previous reports (see Joss et al 1978;Li et al 1980;Raman et al 2016) Unlike other flaring sources like LMC X-4, SMC X-1, where persistent emission begins just after the end of flares, it is interesting to notice a sharp dip in the light curve near the decay of bright flares at 18000, 23000 seconds (second panel) and 51000 seconds (fourth panel) of Figure 1. This feature has never been reported before in 4U 1626-67.…”

Collisional Ionization in the X-ray Spectrum of the Ultracompact Binary 4U 1626-67

“…measurements (Levine et al 1988;Shinoda et al 1990;Chakrabarty et al 1997;Jain et al 2007). A persistent lower sideband to the pulsation peak in the optical power spectrum is thought to arise from reprocessing in a binary companion with a 42-minute orbital period (Middleditch et al 1981;Chakrabarty 1998;Chakrabarty et al 2001;Raman et al 2016). Combined with the X-ray timing limits, this indicates that 4U 1626−67 is an ultracompact binary with an extremely low-mass companion (Levine et al 1988;Verbunt et al 1990;Chakrabarty 1998).…”

“…Double-peaked emission lines in the X-ray (Schulz et al 2001) and the ultraviolet (Homer et al 2002) are indicative of Keplerian disk motion. Quasi-periodic oscillations (QPOs) in both X-rays (Shinoda et al 1990;Owens et al 1997;Kommers et al 1998;Kaur et al 2008) and the optical band (Chakrabarty 1998;Chakrabarty et al 2001;Raman et al 2016) are also thought to arise in the accretion disk.…”

On the peculiar torque reversals and the X-ray luminosity history of the accretion-powered X-ray pulsar 4U 1626–67

“…During the spin-down phase of 4U 1626-67, Chakrabarty (1998) detected 48 mHz quasi-periodic oscillation (QPO) in both X-ray and optical bands. Raman et al (2016) analysed the 2014-data of the source and detected a weak ∼ 48 mHz optical QPO but could not detect any QPO in the corresponding X-ray data. The QPO frequency is thought to be associated with the angular velocity of the pulsar and Keplerian angular velocity at the inner disc as such in the beat frequency model, ω = Ω K (r in ) − Ω * (Alpar & Shaham 1985).…”