发表机构
School of Information Engineering, Sanming University（三明学院信息工程学院）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
该研究通过二十年X射线监测发现，Hercules X-1在2025–2026年延长低态中，其35天超轨道时钟并非停止而是衰减，且转矩与吸积通量保持关联，脉冲轮廓不变，无需自由进动解释。
AI 中文摘要
我们利用费米/GBM、Swift/BAT和MAXI/GSC对食双星吸积脉冲星Hercules X-1进行了长达二十年的X射线监测，重点聚焦于2025年5月开始的延长低态（ELS），这是22年来的首次此类事件，截至撰写本文时该低态仍在持续，自其开始以来已约480天。35天的超轨道时钟在ELS中并未简单停止，而是逐渐衰减。存留的调制信号在主线高态（35天周期的亮相）通量衰减至其正常水平的百分之几之前，始终与ELS之前的时钟保持相位锁定。随后，同相相干性在2026年1月至3月间丧失。我们将ELS之前的脉冲频率历史划分为三种转矩状态：自旋加速、自旋减速和平坦。自旋导数与主线高硬X射线通量呈正相关（皮尔逊相关系数r≈0.7），而ELS期间记录了持续最长的自旋减速，在费米/GBM覆盖的约380天以上时间内，自旋变化率约为-4.7×10^-13 Hz/s。在固定的超轨道相位下，8–50 keV脉冲轮廓形态在不同转矩状态间在统计上无显著差异，且形状参数在18年的记录中未显示长期漂移。这些结果表明，进动的翘曲吸积盘保持其节律，而脉冲星从直接视野中逐渐消失：时钟保持其相位，转矩持续追踪吸积通量，波束保持其形状。当前的监测数据无需中子星自由进动来解释。
英文摘要
We present two decades of X-ray monitoring of the eclipsing accreting pulsar Hercules X-1 with \textit{Fermi}/GBM, \textit{Swift}/BAT and \textit{MAXI}/GSC, centred on the extended low state (ELS) that began in 2025 May (the first such event in 22 years) and is still ongoing at the time of writing, $\sim$480~d after its onset. The 35-d superorbital clock does not simply stop in the ELS; it fades. The surviving modulation stays phase-locked to the pre-ELS clock until the flux of the main high state (the bright phase of the 35-d cycle) has decayed to a few per cent of its normal level. The in-phase coherence is then lost, around 2026 January--March. We classify the pre-ELS pulse-frequency history into three torque states: spin-up, spin-down and flat. The spin derivative correlates positively with the main-high hard X-ray flux (Pearson $r\simeq0.7$), and the ELS hosts the longest sustained spin-down of the record, $\dotν\approx-4.7\times10^{-13}$~Hz~s$^{-1}$ over the $\gtrsim$380~d of \fer/GBM coverage. The 8--50 keV pulse-profile morphology is statistically indistinguishable across the torque states at fixed superorbital phase, and the shape parameters show no secular drift over the 18-yr record. These results show that the precessing warped accretion disc keeps its rhythm while the pulsar fades from direct view: the clock keeps its phase, the torque keeps tracking the accretion flux, and the beam keeps its shape. No free precession of the neutron star is required by the current monitoring data.
Comments13 pages, 8 figures. Submitted to MNRAS; under revision following peer review
