Monitoring the Superorbital Period Variation and the Spin Period Evolution of SMC X-1

Abstract

The X-ray pulsar SMC X-1 shows a superorbital modulation with an unstable cycle length in the X-ray band. We present its timing behaviors, including the spin, orbital, and superorbital modulations, beyond the end of the Rossi X-ray Timing Explorer mission. The time-frequency maps derived by the wavelet Z-transform and the Hilbert-Huang transform suggest that a new superorbital period excursion event occurred in ~MJD 57100 (2015 March). This indicates the excursion is recurrent and probably (quasi)periodic. The hardness ratio obtained with the Monitor of All-sky X-ray Image (MAXI) suggests increased absorption during the transition from the high to the low state in the superorbital cycle. Compared to the regular epochs, the superorbital profile during the excursion epochs has a shallower and narrower valley, likely caused by a flatter warp. By tracking the spin period evolution with the MAXI Gas Slit Camera in 2-20 keV, we derive an averaged spin-up rate of $\dot{\nu }=2.515(3)\times 10^{-11}$ s${}^{-2}$ during the period between MJD 55141 (2009 November) and 58526 (2019 February). We obtain no positive correlation between the spin frequency residual and the superorbital frequency, but a torque change accompanying the superorbital period excursion is possible. We suggest that the accretion torque on the neutron star could be changed by various mechanisms, including the change of mass accretion rate and the warp angle. We update the value of the orbital decay as ${\dot{P}}_{\mathrm{orb}}/P_{\mathrm{orb}}=-3.380(6)\times 10^{-6}$ yr${}^{-1}$. Finally, we reconfirm the detection of the superorbital modulation in the optical band and its coherence in phase with the X-ray modulation.

25 pages, 11 figures, 7 tables, ApJ in press (with minor proofreading correction)

References in corpus (9)

• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs
• Determination of the mass of the neutron star in SMC X-1, LMC X-4 and Cen X-3 with VLT/UVES
• Tracking the Orbital and Super-orbital Periods of SMC X-1
• Two ~35 day clocks in Her X-1: evidence for neutron star free precession
• Tracking the Evolution of QPO in RE J1034+396 Using the Hilbert-Huang Transform
• Evolution of Spin, Orbital, and Superorbital Modulations of 4U 0114+650
• Observing the transient pulsations of SMC X-1 with NuSTAR
• Reproductions of super-orbital X-ray light-curves with the precessing accretion ring model and implications on accretion flows through accretion rings

Cited by in corpus (11)

• Investigation of X-ray timing and spectral properties of ESO 243-49 HLX-1 with long-term Swift Monitoring
• Probing the polarized emission from SMC X-1: the brightest X-ray pulsar observed by IXPE
• A Comprehensive Analysis of the Gravitational Wave Events with the Hilbert-Huang Transform: From Compact Binary Coalescence to Supernova
• Evolution of the spin, spectrum and super-orbital period of the ultraluminous X-ray pulsar M51 ULX7
• Extending the baseline for SMC X-1's spin and orbital behavior with NuSTAR stray light
• Investigating the superorbital modulations in 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 with Swift XRT, BAT and NuSTAR observations
• Monitoring Observations of SMC X-1's Excursions (MOOSE) I: Program Description and Initial High-State Spectral Results
• Monitoring observations of SMC X-1's excursions (MOOSE)-II: A new excursion accompanies spin-up acceleration
• Constraining the evolution of the unstable accretion disk in SMC X-1 with NICER
• Optical and X-ray Follow-Up to a Globular Cluster Ultraluminous X-ray Source in NGC 4472
• Monte-Carlo simulations on possible collimation effects of outflows to fan-beamed emission of ultraluminous accreting X-ray pulsars