云南大学中国西南天文研究所研究团组提出快速射电暴可能源于双中子星并合形成的大质量中子星的活动
Fast radio bursts may originate from activities of newly born massive neutron stars resulted from binary neutron star mergers
近日，南京大学天文与空间科学学院王发印教授与云南大学中国西南天文研究所杨元培助理教授及其合作者，提出快速射电暴可能起源于双中子星并合产生的大质量中子星的活动过程。
第一个重复快速射电暴FRB 121102被发现位于一个恒星形成率较高的矮星系中。该类星系也常常是超亮超新星及长伽马射线暴的宿主星系，表明快速射电暴有可能源于核塌缩产生的年轻中子星。然而，不同于FRB 121102，新近被定位的快速射电暴FRB 180924和FRB 190523 均出现在恒星形成率较低的大质量星系中，且离星系中心有较大距离。这些性质与短伽马射线暴的宿主星系相似，而短伽马射电暴通常认为起源于双中子星或中子星-黑洞并合过程。
在本研究中，研究人员通过计算双中子星并合事件率和并合时间，并基于星族合成方法预测出双中子星在不同类型宿主星系中最有可能发生并合的位置，发现近期几个被定位的快速射电暴在宿主星系中的位置均与双中子星并合通道的预测一致（图1）。因快速射电暴事件率高于双中子星并合事件率，这意味着目前观测到的快速射电暴中有很大一部分来自并合后形成的大质量中子星的多次爆发过程。通过分析中子星磁层中相干曲率辐射和快速射电暴在星云中的传播，研究人员还预测了来自双中子星并合产生的快速射电暴的可观测信号，包括流量、能谱、色散量和法拉第旋转量等。该项研究工作发表在国际天文学专业学术期刊《天体物理学杂志》（The Astrophysical Journal）上（文章链接：https://iopscience.iop.org/article/10.3847/1538-4357/ab74d0）。
图1. 椭圆星系中双中子星并合产生的大质量中子星偏离星系中心位置的累积分布，以及观测到的短伽马射电暴以及快速射电暴的偏移度分布。The cumulative distributions of offsets for neutron star–neutron star mergers in elliptical galaxies that may produce the observed distributions of short gamma-ray bursts and of fast radio bursts.
The first repeating fast radio burst, FRB 121102, was localized in a star-forming dwarf galaxy similar to the host galaxies of superluminous supernovae and long gamma-ray bursts. Thus, young neutron stars born in core-collapse explosions were proposed to be candidates for the central engines of fast radio bursts. However, recent observations have showed that FRB 180924 and FRB 190523 are actually located in massive galaxies of low star formation rates, and the offsets between the FRBs and the host centers are relatively large. These properties are similar to those of the host galaxies of short gamma-ray bursts, which are produced by binary neutron star or neutron star–black hole mergers.
In this work, the researchers have calculated the binary neutron star merger rates and times, and predicted the most likely merger locations for different types of host galaxies using the population synthesis method. They find that the binary neutron star merger channel is consistent with the recently reported offsets of FRB 180924 and FRB 190523, as shown in Figure 1. The event rate of fast radio bursts is larger than those of binary neutron star mergers, and requires that a significant fraction of the merger products emit multiple number of bursts. By analyzing the coherent curvature radiation in a neutron star magnetospheres and its propagation in the surrounding nebula, they predict the observational properties of fast radio bursts resulted from binary neutron star mergers, including the flux, spectrum, dispersion and rotation measures etc. The paper has been published in The Astrophysical Journal (article link: https://iopscience.iop.org/article/10.3847/1538-4357/ab74d0).
