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名稱Title
2019天文年會
國立自然科學博物館
5月17日至19日

論文摘要

On the Connection between Spin Period Evolution and Superorbital Period Excursion of SMC X-1

[ Oral ]

Chin-Ping Hu (Kyoto University, JSPS International Research Fellow); Tatehiro Mihara (RIKEN); Mutsumi Sugizaki (Tokyo Institute of Technology); Teruaki Enoto (Kyoto University); Yoshihiro Ueda (Kyoto University)

SMC X-1 is an X-ray pulsar residing in an eclipsing high-mass X-ray binary system. It shows a superorbital modulation with a highly variable cycle length. We present its timing behaviors, including the spin, orbital, and superorbital modulations, beyond the end of the Rossi X-ray Timing Explorer mission in 2012. The time-frequency map derived by wavelet Z-transform and the Hilbert-Huang transform suggest that a new excursion event of the superorbital modulation period occurred in MJD 57100. The hardness ratio obtained with Monitor of All-sky X-ray Image (MAXI) suggests an absorption feature during the superorbital transition states. In contrast to the regular epochs, the superorbital profile during the excursion epochs has a shallower and narrower valley, likely caused by a flatter warp of the disk or a ring with a lower opacity. Thanks for the excellent temporal resolution of MAXI, we are able to track the detailed spin period evolution of SMC X-1 and derive an averaged spin-up rate as nu_dot=2.513(3) x 10^{-11} s^{-2} during MJD 55000 and 58600. We obtain no significant positive correlation between the spin period residual and the superorbital frequency but observe a hint of anti-correlation. We suggest that the accretion torque could be changed by various origins, including the change of mass accretion rate and the warp angle. We further update the value of the orbital decay as P_dot/P = -3.393(7) x 10^{-6} yr^{-1}. Finally, we reconfirm the detection of the superorbital modulation in the optical band. The coherence in phase suggest that the X-ray and optical modulations have the same origin.