The Galactic Neutron Star Population - II. Systemic Velocities and Merger Locations of Binary Neutron Stars

Nicola Gaspari,Andrew J. Levan，Ashley A. Chrimes,Gijs Nelemans

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY（2024）

Cited 1|Views15

Abstract

ABSTRACT The merger locations of binary neutron stars (BNSs) encode their galactic kinematics and provide insights into their connection to short gamma-ray bursts (SGRBs). In this work, we use the sample of Galactic BNSs with measured proper motions to investigate their kinematics and predict their merger locations. Using a synthetic image of the Milky Way and its Galactic potential we analyse the BNS mergers as seen from an extragalactic viewpoint and compare them to the location of SGRBs on and around their host galaxies. We find that the Galactocentric transverse velocities of the BNSs are similar in magnitude and direction to those of their Local Standards of Rest, which implies that the present-day systemic velocities are not isotropically oriented and the peculiar velocities might be as low as those of BNS progenitors. Both systemic and peculiar velocities fit a lognormal distribution, with the peculiar velocities being as low as ∼22–157 km s−1. We also find that the observed BNS sample is not representative of the whole Galactic population, but rather of systems born around the Sun’s location with small peculiar velocities. When comparing the predicted BNS merger locations to SGRBs, we find that they cover the same range of projected offsets, host-normalized offsets, and fractional light. Therefore, the spread in SGRB locations can be reproduced by mergers of BNSs born in the Galactic disc with small peculiar velocities, although the median offset match is likely a coincidence due to the biased BNS sample.

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Key words

Compact Binary Mergers,Gamma-Ray Bursts

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【要点】：该论文通过分析具有测量 proper motions 的银河系双中子星样本，研究其动力学并预测其合并位置，发现中子星的合并位置与其银河动力学有关，并可提供与短伽马射线暴(SGRBs)联系的见解。

【方法】：采用合成银河系图像及其银河势，从外银河视角分析中子星合并，并与宿主星系上的SGRB位置进行比较。

【实验】：研究使用具有类似 22-157 km s(-1) 低奇特速度的低磁黄道带标准样本（Local Standards of Rest），发现观测到的双中子星样本并不代表整个银河系中子星人口，而是更接近太阳周围区域，具有小奇特速度的系统。预测的双中子星合并位置与SGRB位置相似，支持了SGRB位置的分散可能由在银河盘中具有小奇特速度出生的双中子星合并所致。

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