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.

arXiv comment25 pages, 11 figures, 7 tables, ApJ in press (with minor proofreading correction)

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• Monitoring observations of SMC X-1's excursions (MOOSE)-II: A new excursion accompanies spin-up acceleration
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• Optical and X-ray Follow-Up to a Globular Cluster Ultraluminous X-ray Source in NGC 4472
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