毫秒脉冲星轮廓变化的研究

发布时间：2018-04-30 08:36

本文选题：脉冲星 + 脉冲轮廓　；参考：《西南大学》2016年硕士论文

【摘要】：脉冲星是高速自转并具有超强磁场的中子星。可根据脉冲星稳定的周期性,将一定频率范围内的几百个甚至几千个单脉冲按周期进行叠加,会得到一个非常稳定的累积脉冲轮廓。脉冲轮廓是可以直接观测的量,轮廓通常呈现出单峰、双峰以及多峰等多种结构,已有不同类型的脉冲星辐射束模型来解释脉冲轮廓出现多种形态的原因。研究脉冲星累积脉冲轮廓的稳定性是一个重要的课题,可以为脉冲星辐射机制和辐射区结构的研究提供基础,还有助于提高脉冲星计时精度。前人研究发现一些脉冲星的累积脉冲轮廓展现出了不稳定性。脉冲轮廓会发生两种重要的变化现象,都影响着脉冲星的多种观测特征。其中一种变化是模式变换,即脉冲星具有两个不同形状的稳定累积脉冲轮廓,分别称为正常模式和反常模式,两种模式之间相互转换。模式变换现象通常与脉冲强度的突然变化有关。另外一种变化称为消零现象,即脉冲星的射电辐射在连续的多个周期内会突然停止,导致暂时探测不到脉冲星的脉冲信号,一段时间后脉冲星又恢复到正常的辐射状态。通常在消零期结束后,能观测到单脉冲的强度和中心相位有明显的变化。脉冲星累积脉冲轮廓的变化还表现在脉冲宽度以及各脉冲成分之间的相对强度等随频率的不同而发生的变化。正常脉冲星和毫秒脉冲星的脉冲轮廓都会随着频率的不同而发生变化。但是毫秒脉冲星脉冲轮廓的变化相对更加复杂,脉冲成分的宽度和间距可能随着频率的增加而增大,也可能随着频率的增加而减小。脉冲星辐射强度发生变化的原因,可能是脉冲星自身辐射强度的变化,也可能发生在射电波穿过星际空间的过程中,受到星际介质中电子密度不均匀性闪烁引起的不同频率上流量密度的变化,即脉冲星累积脉冲轮廓的稳定性会受星际闪烁的干扰。本文研究了毫秒脉冲星PSR J1022+1001累积脉冲轮廓的稳定性问题,该脉冲星呈现双峰结构,在1400MHz观测频率附近,具有两种典型的脉冲轮廓,其峰值比分别是小于1和大于1。研究发现当把总带宽分成八个子带宽后,每个子带宽的脉冲轮廓会随着频率的不同而发生变化,其峰值比随着频率的增加而增大。同时由于星际闪烁的影响,各个频率上的流量密度也会发生变化。当把各个频率上的脉冲轮廓按照流量密度的大小加权叠加后,就会形成峰值比小于1和大于1的两种总轮廓形态。所以该脉冲星总轮廓呈现变化的主要原因在于脉冲轮廓会随观测频率发生形态改变,同时星际闪烁引起各频率上流量密度变化造成的。但研究还发现,有少数情况中即使在同一个子频率上,脉冲星的轮廓也已经发生了改变,因此认为部分累积脉冲轮廓表现出的不稳定性也可能是脉冲星内禀或是其他因素引起的。
[Abstract]:Pulsars are neutron stars with high speed rotation and super magnetic field. According to the stable periodicity of pulsars, a very stable cumulative pulse profile can be obtained by stacking hundreds or even thousands of monopulse pulses in a certain frequency range. Pulse contour is a direct observable quantity, and the contour usually presents a single peak, two peaks and multiple peaks. Different types of pulsar radiation beam models have been developed to explain the reasons for the appearance of various shapes of the pulse profile. It is an important subject to study the stability of pulsar cumulative pulse profile, which can provide a basis for the study of the radiation mechanism and radiation region structure of pulsars, and also help to improve the timing accuracy of pulsars. Previous studies have found that the cumulative pulse profiles of some pulsars exhibit instability. There are two important changes in pulse profile, both of which affect the observation characteristics of pulsars. One of the variations is mode transformation, in which pulsars have two stable cumulative pulse profiles with different shapes, which are called normal mode and anomalous mode respectively, and the two modes are converted to each other. The phenomenon of mode transformation is usually related to the sudden change of pulse intensity. The other change is called zero elimination, that is, the radio radiation of pulsars will stop suddenly for several consecutive periods, which results in the pulsars being unable to detect the pulsars' pulse signals temporarily, and the pulsars return to the normal radiation state after a period of time. At the end of zero elimination period, the intensity and central phase of monopulse can be observed. The variation of pulsar cumulative pulse profile is also reflected in the variation of pulse width and the relative strength of each pulse component with different frequencies. The pulse profiles of both normal pulsars and millisecond pulsars vary with different frequencies. But the variation of pulse profile of millisecond pulsars is more complex. The width and spacing of pulse components may increase with the increase of frequency or decrease with the increase of frequency. The pulsar's radiation intensity may have changed because of the pulsar's own radiation intensity, or in the process of radio waves passing through interstellar space. The variation of flux density at different frequencies caused by the inhomogeneity of electron density in interstellar medium means that the stability of pulsar cumulative pulse profile will be disturbed by interstellar scintillation. The stability of the cumulative pulse profile of millisecond pulsar PSR J1022 1001 is studied in this paper. The pulsar has a bimodal structure. There are two typical pulse profiles near the 1400MHz observation frequency, the peak ratio of which is less than 1 and greater than 1, respectively. It is found that when the total bandwidth is divided into eight sub-bandwidth, the pulse profile of each sub-bandwidth changes with the frequency, and the peak ratio increases with the increase of the frequency. At the same time, due to the influence of interstellar flicker, the flux density on each frequency will also change. When the pulse profiles on each frequency are weighted according to the size of the flow density, two kinds of total contour shapes with peak ratio less than 1 and greater than 1 will be formed. Therefore, the main reason for the variation of the total profile of the pulsar is that the shape of the pulse profile changes with the observed frequency, and the flux density changes at various frequencies caused by interstellar scintillation. But the study also found that in a few cases, even at the same subfrequency, the profile of the pulsar has changed. It is suggested that the instability of partial cumulative pulse profile may also be caused by intrinsic pulsar or other factors.
【学位授予单位】：西南大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：P145.6

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