游荡自旋中子星连续引力波的隐马尔可夫模型跟踪
Hidden Markov model tracking of continuous gravitational waves from a neutron star with wandering spin
摘要：
当恒星的自旋频率从电磁观测中先验未知并且在内部（例如超流体或磁层）或外部（例如吸积）扭矩的作用下随机游荡时，引力波搜索来自中子星的连续波信号尤其具有挑战性。它示出了隐马尔可夫模型（HMM）方法的频率跟踪可以与现有的极大似然相干匹配滤波器，如F-统计量相结合，以克服自旋漂移所提出的一些挑战。具体地说，它被发现，对于一个孤立的，双轴转子的自旋频率随机走动，HMM跟踪的F-统计输出的相干段持续时间T_drift = 10 d的总观察时间T_obs = 1 yr可以检测信号波应变h 0> 2 e-26在噪声水平的先进激光干涉仪引力波天文台（Advanced LIGO）的特性。对于在双星轨道中具有随机行走自旋的双轴转子，其轨道周期和半长轴从电磁观测中近似已知，对贝塞尔加权F统计输出的HMM跟踪可以检测h 0> 8 e-26的信号。一个高效的，递归的，HMM求解器的基础上维特比算法，这需要约10^3 CPU小时的典型，宽带（0.5 kHz）搜索低质量的X射线二进制天蝎座X-1，包括相关的F统计输入的生成。在一个“现实”的观测场景中，维特比跟踪成功地检测到了50个合成信号中的41个，而在LIGO科学合作组织召集的Scorpius X-1模拟数据挑战赛的第一阶段中没有自旋漂移，波应变为h 0 = 1.1e-25，恢复频率的均方根精度为<= 4.3e-3 Hz。
英文摘要：
Gravitational wave searches for continuous-wave signals from neutron stars are especially challenging when the star's spin frequency is unknown a priori from electromagnetic observations and wanders stochastically under the action of internal (e.g. superfluid or magnetospheric) or external (e.g. accretion) torques. It is shown that frequency tracking by hidden Markov model (HMM) methods can be combined with existing maximum likelihood coherent matched filters like the F-statistic to surmount some of the challenges raised by spin wandering. Specifically it is found that, for an isolated, biaxial rotor whose spin frequency walks randomly, HMM tracking of the F-statistic output from coherent segments with duration T_drift = 10d over a total observation time of T_obs = 1yr can detect signals with wave strains h0 > 2e-26 at a noise level characteristic of the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO). For a biaxial rotor with randomly walking spin in a binary orbit, whose orbital period and semi-major axis are known approximately from electromagnetic observations, HMM tracking of the Bessel-weighted F-statistic output can detect signals with h0 > 8e-26. An efficient, recursive, HMM solver based on the Viterbi algorithm is demonstrated, which requires ~10^3 CPU-hours for a typical, broadband (0.5-kHz) search for the low-mass X-ray binary Scorpius X-1, including generation of the relevant F-statistic input. In a "realistic" observational scenario, Viterbi tracking successfully detects 41 out of 50 synthetic signals without spin wandering in Stage I of the Scorpius X-1 Mock Data Challenge convened by the LIGO Scientific Collaboration down to a wave strain of h0 = 1.1e-25, recovering the frequency with a root-mean-square accuracy of <= 4.3e-3 Hz.
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