Pulsar radiation mechanisms

Gamma rays produced by electrons accelerated in the strong magnetic fields of neutron stars annihilate to electron-positron pairs. This leads to a two-stream situation, which results in bunching and coherent radio emission.     

Pulsar observations in China

The development history and achievement of pulsar observation in China were introduced. The history is very short, only ten years. The cooperation program about pulsar timing observation at 18 cm using a 25-m radio telescope of UAO between China, Australia and Great Britain has been carried out since 1997. Some scientific results such as glitch, model changing, interstellar scintillation and the spectra of pulsar have been obtained. The plans of development of the pulsar observation were also introduced using Chinese radio telescopes.     

差分X射线脉冲星自主导航研究

X射线脉冲星自主导航(XNAV)中,时间测量与转换误差会对导航精度产生不可忽视的影响. 本文在对XNAV算法研究的基础上,进一步研究了利用差分抑制时间测量与转换误差对导航精度的影响. 研究结果表明:双差能有效抑制时间测量误差对导航精度的影响,但是无法抑制时间转换误差对导航精度的影响;三差既能有效抑制时间测量误差对导航精度的影响,也能有效抑制时间转换误差对导航精度的影响.      

Pulsar sub-pulses and the emission process

This article describes another regular phenomenon in the sub-pulse behaviour of pulsars. The drift speed of successive sub-pulses follows a sawtooth pattern with a linearly increasing drift speed and sudden jumps as the speed drops to repeat the cycle.     

一类无粘性项BBM-Burgers方程解的整体存在性和衰减估计

本文通过构造柯西逼近序列证明了一类无粘BBM-Burgers方程解的局部存在性和衰减估计,应用常数变易公式结合基本解的衰减估计证明了小初值条件下解的整体存在性,并得到了衰减估计.     

Coherent spin manipulation without magnetic fields in strained semiconductors

A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin-orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing and spintronics, forming the basis of various device proposals. For example, the concept of spin field-effect transistors is based on spin precession due to the spin-orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to approximately 30 MHz.     

基于压缩感知的脉冲星轮廓构建算法

利用有限的观测时间获得高精度的累积脉冲轮廓,对于X射线脉冲星导航具有重要意义.提出一种基于压缩感知的脉冲星累积脉冲轮廓重构算法.首先,研究脉冲轮廓,获取先验知识,在此基础上构建波形匹配冗余词典,得到脉冲轮廓的稀疏表示;然后,设计观测矩阵,利用原始观测数据得到具有较少点数的非自适应随机测量值;最后,采用匹配追踪方法从随机测量值中恢复脉冲轮廓.利用罗希X射线探测器的实测脉冲星数据进行实验,结果表明：利用相同时段的观测数据,该算法得到的累积脉冲轮廓的信噪比远高于周期叠加方法得到的累积脉冲轮廓.     

中子星壳层中强磁场与β衰变

 基于原子核壳层模型,分析了在强磁场作用下中子星壳层中的β衰变率,并以反应⁶³Co(β^-)⁶³Ni为例作详细计算.结果表明:弱磁场对中子星壳层中的β衰变几乎没有影响,强磁场(B>10⁸T)使β衰变率显著增大;同时在相同的磁场强度下,基态的β衰变比激发态的β衰变所受影响更大.这些结论对进一步研究中子星晚期演化和双中子星系统快中子俘获过程的核合成是有意义的.     

基于计算机集群的脉冲星相干消色散系统研究

来自脉冲星的射电辐射在传播过程中会与星际间介质中的自由电子相互作用,发生色散现象.脉冲星相干消色散就是通过计算,将采样后的脉冲星信号进行快速傅里叶(FFT)变换,在频域同chirp函数相乘,从而实现消色散的过程.介绍了基于计算机集群的相干消色散的原理以及数据计算流程,同时分析了计算过程中参数的选取及其与集群性能的关系,并对于脉冲星相干消色散集群系统的发展进行了介绍.     

脉冲星信号的模糊阈值小波降噪算法

为了提高脉冲星辐射脉冲信号的信噪比,提出了基于模糊阈值的小波降噪算法.该算法的核心思想为:引入模糊理论,通过建立隶属度函数,计算出每个采样点信号幅值的隶属度,再利用小波阈值消噪的方法设定门限来分析隶属度,将隶属度大于门限的采样值划归为信号,反之为噪声,从而达到压抑噪声、保留信号的目的.实验结果表明,与基于极大极小原理和Stein无偏似然原理的降噪方法相比,基于模糊阈值的小波降噪算法将脉冲信号的信噪比提高了大约5~6 dB,同时可以保留信号的细节.新算法为脉冲星信号处理与应用提供了一个行之有效的算法.     

改进UKF在X射线脉冲星导航中的应用研究

为了提高标准Unscented卡尔曼滤波(UKF)计算效率,本文对标准UKF采样点计算方法进行改进,使状态更新过程中的多次积分过程简化为一次积分过程,并证明了标准UKF和改进UKF的等效性.计算机仿真结果表明,在计算精度一致(位置确定精度约为70m,速度确定精度约为0.005m/s)的情况下,改进UKF的计算效率是标准UKF的5倍.     

Pulsar spins from an instability in the accretion shock of supernovae

Rotation-powered radio pulsars are born with inferred initial rotation periods of order 300 ms (some as short as 20 ms) in core-collapse supernovae. In the traditional picture, this fast rotation is the result of conservation of angular momentum during the collapse of a rotating stellar core. This leads to the inevitable conclusion that pulsar spin is directly correlated with the rotation of the progenitor star. So far, however, stellar theory has not been able to explain the distribution of pulsar spins, suggesting that the birth rotation is either too slow or too fast. Here we report a robust instability of the stalled accretion shock in core-collapse supernovae that is able to generate a strong rotational flow in the vicinity of the accreting proto-neutron star. Sufficient angular momentum is deposited on the proto-neutron star to generate a final spin period consistent with observations, even beginning with spherically symmetrical initial conditions. This provides a new mechanism for the generation of neutron star spin and weakens, if not breaks, the assumed correlation between the rotational periods of supernova progenitor cores and pulsar spin.