射电脉冲星的自转行为与磁场演化

Pulsar timing has a great application potential. The origin of pulsar timing noise, which still remains to be understood completely, is one of the hotspots of current research in the field. This project is targeted at the interpretation of rotational behaviors and the origin of the timing noise by the statistics and analysis on the observational data as well as building a physical model. The main contents are as follows: (1) the correlation between the timing noise and the changes in the pulse shape strongly suggests that the timing noise is linked and caused by changes in the pulsar’s magnetosphere. Therefore, we propose to test the models of magnetic field evolution, and to improve and extend the phenomenological model we constructed previously, through the most extensive study of the long term timing irregularities of radio pulsars; (2) approximate symmetry has been found between the frequency’s second derivatives of the pulsar spins with positive and negative signs. However, the symmetry might be lowered to some extent by the long-term monotonic decay of the effective magnetic fields of these pulsars. Thus, the decay timescale may be determined by measuring the asymmetry between the frequency’s second derivatives of the pulsar spins with positive and negative signs; (3) “slow glitches” are characterized by a gradual increase in frequency. It is worth studying whether they are a unique phenomenon, or caused by the same process as the timing noise pulsars; (4) the diffusive motion of the magnetic fields perturbs the background dipole magnetic fields at the base of the NS crust. Such perturbations propagate as circularly polarized “Hall waves” along the background field lines upward into the lower density regions in the crusts. The Hall waves can strain the crust with a wave period that roughly equal to the period of quasi-periodical oscillations in timing noise. We will compute and simulate the specific contribution of these waves on pulsar timing noise. Our preliminary work demonstrated the feasibility of our proposed research work.

脉冲星计时研究具有巨大的应用潜力，理解计时噪音的起源是当前脉冲星领域的研究热点之一。 本项目紧扣射电观测数据，通过对数据全面和细致的分析，以理解脉冲星的自转行为和噪音的起源为目的，尝试建立高精度的噪音模型。其主要内容包括：(1) 脉冲星的计时噪音与脉冲宽度的协同相关性表明，计时噪音很可能起源于脉冲星的磁层活动。本项目将运用长时段大样本的计时观测数据，检验脉冲星磁场演化的各种方式，进一步完善和扩展我们之前建立的磁场短期振荡调制长期幂律衰减的唯象模型； (2) 研究脉冲星的磁场长期演化对其自转周期二阶导数的正负值分布的对称性的影响； (3) 探讨脉冲周期的缓变与准周期性计时噪音的在观测和物理起源上的异同；(4) 研究内壳层的Hall效应对脉冲星极区的晶格和磁场的影响，模拟该影响对噪音的贡献，同时对中子星内部物理参数给出限制。

射电脉冲星是带有强磁场的快速自转的中子星，其最为显著的特点就是自转的稳定性，正是基于这种稳定性，脉冲星计时研究可以应用于检验广义相对论和探测引力波辐射，用作可以和原子钟精度相比拟的时间基准，以及运用到空间探测器的自主定位和导航等领域。不过脉冲星的计时数据中依然存在当前模型无法预计的准周期性质的计时噪音。本项目针对计时噪音主要开展两方面的研究，基本内容如下：.1. 磁场是影响中子星演化和各类观测特征的重要参量之一。当前测量射电脉冲星等效磁场的首要方法是基于磁偶极辐射理论测量星体的自转参量。同时，自转参量也被广泛用于磁场衰减的的研究，但在衰减时标的估计上存在较大分歧。其主要原因在于绝大部分脉冲星的真实年龄未知且经典年龄与真实年龄可能存在较大差异。射电脉冲星的磁场衰减的统计研究中，我们分析脉冲星自转频率的二阶导数的分布和对称性的改变,测量了脉冲星偶极磁场的衰减时标。该方法给出对年老脉冲星磁场衰减时标超过1亿年，这是目前最紧致的限制。.2. 在中子星壳层的磁场活动对星体自转的影响的研究中，我们构建了完整自洽的理论模型，计算了内壳层Hall波对星体自转参量的影响并模拟了该效应对应的计时噪音。发现该成分可能作为基本组分普遍存在于脉冲星中。同时，我们利用观测的计时噪音的频率和振幅特征，限制中子星内壳层的温度与磁场演化参量。