Multi-messenger Study Of The Kilonova Remnant Candidate G4.8+6.2

A kilonova is a transient astronomical event,which is a violent explosion phe-nomenon that occurs when two compact objects merge,including binary neutron stars（BNSs）or neutron star-black hole binaries.The merging will eject some fraction of matter,and the total kinetic energy of ejecta is about 10⁵⁰erg.These merger ejecta continue to expand outward and interact with the interstellar medium,forming an ex-tended source known as a“kilonova remnant”（i.e.the remnant of a kilonova explosion,KNR）.The multi-messenger observations of the gravitational wave（GW）event GW170817have played a crucial role in the study of BNS mergers,providing a theoretical ba-sis for investigating its remnants.Chapter 1 will systematically introduce the basic concepts and research background of BNS mergers.Based on the Galactic event rate of BNS mergers,we expect one or more KNRs in the Milky Way,while none has yet been confirmed.This motivated us to search for KNRs.In addition,the formation mechanism of BNS systems also affects the spatial distribution of KNRs.Generally speaking,BNS systems tend to form far from star-forming regions and merge at high Galactic latitudes.As a new field of astronomy,the observation properties of KNRs remain unclear.How to find a KNR is an open question.So Chapter 2 makes bold predictions about its physical properties,providing a theoretical basis for the subsequent research.We expect that the physical properties of KNRs are similar to those of supernova remnants（SNRs）,so we use analogical methods to study its morphology,dynamical evolution,and emission mechanisms.In addition,we also explore the possibility that neutron stars（NSs）originate from BNS mergers and use mass to determine its physical origins.Chapter 3 introduces a feasible search strategy for KNRs,which is to look for the ancient records of guest stars with kilonova features,and then look for the associated KNR at the same sky position（i.e.,they have a common physical origin）.This is because the guest star can help us not only narrow the search area,but also identify KNRs and find those that are young.Through comparison between the catalogues of guest stars and SNRs,we found a pair of potential sources:the guest star AD1163and KNR G4.8+6.2.Based on historical records from ancient Korea,we believe that the guest star AD1163 is a kilonova explosion,and the associated G4.8+6.2 is an860-year-olds KNR.Due to the reliability of historical records being questionable,further analysis of G4.8+6.2 is needed.So Chapter 4 and 5 respectively performed multi-messenger ob-servation analysis and a directed search for continuous gravitational waves（CWs）from G4.8+6.2.Currently,G4.8+6.2 has only radio and high-energy gamma-ray imaging observations.The sky position,morphology,and polarization properties of the rem-nant all indicate that G4.8+6.2 may be born in a uniform and low density environment,which is consistent with the model of KNRs.Unfortunately,no valid observations were found to measure the distance and age of G4.8+6.2.In addition,BNS mergers may leave behind a massive isolated millisecond magnetar at the center of a KNR,making it an ideal directed search target for CWs.However,the central compact objects of G4.8+6.2 is currently invisible to electromagnetic.We carried out a search for CWs from the putative NS using LIGO O3 data,with a frequency range of 20-1500 Hz.While no signal was found in our search.With improving the detector sensitivities,the search algorithms and longer observation times,CWs will provide the opportunity to see whether a NS is harbored inside G4.8+6.2.Detection of such signals would allow us to probe the association between the NS,the KNR G4.8+6.2,and the guest star AD1163.To further study the physical properties of KNRs,numerical relativistic magne-tohydrodynamic simulations are necessary.Chapter 6 first use Lorene to solve the initial data of the BNS system,and then use Einstein Toolkit for dynamical evolution to extract information such as dynamical ejecta and GW.Detailed software usage and specific parameter settings are provided in the appendix.Finally,Chapter 7 will con-clude with a discussion.