Data Reduction And Research Of Gamma-ray Burst Optical Afterglow And Kilonovae

Gamma-ray bursts(GRBs)are the most violent events in the universe.Since the discovery of the first GRB over half a century ago,the research of GRBs has made rapid progress.Based on their duration,GRBs can be divided into two classes:long bursts with durations longer than 2 seconds and short bursts with durations shorter than 2 seconds.Long bursts are generally believed to originate from the death of massive stars,while short bursts are thought to result from the merger of neutron star-neutron star or neutron star-black hole binaries.Observations of multiwavelength afterglows from GRBs have greatly deepened our understanding of these phenomena.In particular,detailed studies of the various emission components inside the jet and their evolution in the early stages of the burst strongly constrain the theoretical model of GRBs.Optical data from GRB afterglows can be used to investigate the internal physics of GRBs,their relativistic jet properties,and their interaction with the environment.The neutron-rich matter ejected from the neutron star merger(neutron star-neutron star merger or neutron star-black hole merger)is one of the important sites for the synthesis of r-process elements.Theoretical studies over the years have suggested that the energy released by the decay of these r-process elements will form optical/near-infrared emission after heating,and this optical/near-infrared transient phenomenon is called"kilonova".Kilonova is one of the most promising gravitational wave electromagnetic counterparts,and studying it can help us test theories of short GRBs progenitor stars and better understand the process of compact star mergers.Analyzing the historical optical data of gamma-ray bursts is also an important means of searching for kilonova signals.This thesis mainly introduces the analysis of multi-wavelength afterglows of gamma-ray bursts(GRBs)through optical data reduction,aiming to search for kilonova signals in the afterglows and constrain the physical parameters of GRB jets.Chapter 1 provides an introduction to GRBs and kilonovae.The section on GRBs primarily covers the historical background,observational characteristics,and theoretical models.The fireball model and the radiation mechanism of afterglow are specifically described.The introduction to kilonovae includes the development history and kilonova research and its physical model,followed by an introduction to the double neutron star merger event GW170817/AT2017gfo and its significance.Chapter 2 describes in detail the method and process of optical data reduction.Using several GRBs’data as examples,the chapter introduces the preprocessing of images,photometry,calculation methods for magnitude errors and limiting magnitude,and the processing method of spectral images.Software used includes IRAF,GAIA-starlink,and others.Chapter 3 focuses on the data processing and analysis of GRB 070707.The light curve of this burst is similar to that of AT2017gfo,so we reduced and measured the optical data taken by the Very Large Telescope(VLT)and the Hubble Space Telescope(HST)using software such as IRAF to obtain the magnitudes of its R-band and J-band observations.We assume that a galaxy with a redshift of 0.2394 nearby is its host galaxy,with a probability of chance coincidence of Pcc=0.046.Then,by constructing a physical model and fitting the optical data of GRB 070707,we found that neither forward shock emission nor energy injection can well explain the afterglow behavior,but the model of the afterglow plus a kilonova can fit the data well,indicating that there may be kilonova components in the afterglow of GRB 070707.In addition,we also reduced the spectrum of the host galaxy of the burst and analyzed the properties of the galaxy.The high metallicity,low star formation rate,and older stellar population of this burst’s host galaxy are similar to other host galaxies of kilonova-associated GRBs.GRB 070707 was the first short burst discovered by INTEGRAL satellite,the identification of a kilonova signature with it strongly indicates that kilonovae are rather common among the neutron star merger events.Chapter 4 describes the data reduction and analysis of GRB 19122 1B.This burst was previously reported to have an optical plateau in its afterglow light curve,which is rather unusual for gamma-ray bursts.We reduced the image data from the Swift satellite and other ground-based telescopes using software such as Heasoft to obtain the light curve of the multi-band afterglow of GRB 19122 1B.Through analysis of the optical,ultraviolet,X-ray,and radio data of this gamma-ray burst,we showed that it can be explained by a "two-component jet" structure,which includes a narrow jet(θj=0.025 rad)with a high initial Lorentz factor(Γ0=400)and a wider(θj=0.048 rad)but slower(Γ0=25)jet.And the narrow jet dominates the early decay,and the wider jet causes the optical plateau and dominates late decay.We also explained the inconsistency in the decay indices of the UVOT and Rc-band data around 2000 seconds using reverse shock emission.The final chapter summarizes the work of this thesis and prospects the observation and research of electromagnetic emission from future neutron star mergers.