Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Monitoring the Superorbital Period Variation and the Spin Period
  Evolution of SMC X-1   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
$\dot{\nu}=2.515(3)\times10^{-11}$ 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
$\dot{P}_{\rm{orb}}/P_{\rm{orb}}=-3.380(6)\times10^{-6}$ 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.

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The categories of this paper are: math
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The categories of this paper are: plasm-ph
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The categories of this paper are: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: stat
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The categories of this paper are: acc-phys
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The categories of this paper are: cs
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The categories of this paper are: alg-geom
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The categories of this paper are: nlin
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The categories of this paper are: chem-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: dg-ga
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The categories of this paper are: quant-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: q-bio