Optical Follow-Up Observations Of Gamma-Ray Bursts:the Study Of Early Afterglow And High-redshift Bursts

Gamma-ray bursts（GRBs）are the most violent explosions in the Universe except for the Big Bang,which can release as much energy in seconds as the entire lifetime of the sun.Lots of breakthroughs have been made in the study of gamma-ray bursts since they were first discovered in the 1970s.Gamma-ray bursts can be divided into long-soft bursts and shorthard bursts according to their duration and energy spectrum hardness.Long soft bursts with T90>2s have been shown to originate from the collapse of massive stars associated with broad-lined Type Ic supernovae.Short hard bursts with T90<2 s have also been confirmed that the merger of neutron stars can indeed produce short bursts accompanied with a bright near-infrared transient called kilonovae in 2017.At the same time,we also found some special cases,such as merger-origin long-duration GRB s and collapsar-origin short-duration GRBs.BeppoSAX mission has ushered in the era of afterglow observation.So far,thousands of GRBs have been detected,and nearly 600 GRBs with redshift measured.But there are two major issues with the current gamma-ray burst study:First,early optical observations of GRBs can significantly contribute to the study of the central engine and physics therein.However,of the thousands observed so far,still only a few have data at optical wavelengths in the first minutes after the onset of the prompt emission.Second,although hundreds of GRB redshifts have been identified,most of them are less than 3.High-redshift GRBs provide a powerful tool to probe the early universe,but still for relatively few do we have good observations of the afterglow.Therefore,early observations and high redshift gamma-ray bursts are the focus of our observations.This paper mainly includes the following aspects:In Chapter 1,we introduce the necessary background of GRBs.We firstly introduce the history of observations from their discovery,.Then we introduce the classification of GRBs and the progenitor stars.Finally,we briefly introduce the current model and emission mechanism of GRBs and discuss the current status of optical afterglow observation of GRBs.In Chapter 2,we discuss the challenges associated with ground-based follow-up observation and briefly introduce our observation groups and optical data processing.We provide a detailed overview of our telescopes and instruments,such as NEXT,HMT,CNEOST,Xinglong 2.16m,NOT and VLT.Despite the challenges posed by the unpredictable of GRB s,our group has made significant breakthroughs in optical observation,such as identified 3 bursts with redshift larger than 4,observed 1 burst associated with supernovae,reported 143 GCNs,published or submitted 8 papers.Efforts on the very early observation and high-redshift of GRBs increase the understanding of prompt phase and the Universe.In Chapter 3,we report on GRB 190106A,whose afterglow was observed in optical bands just 36 s after the Swift/BAT trigger,i.e.,during the prompt emission phase.The early optical afterglow exhibits a twin-peak structure followed by a normal decay,with a faster decay after～T0+1 day.We present optical photometric and spectroscopic observations of GRB 190106A.We derive the redshift z=1.861±0.002 via metal absorption lines from Xinglong 2.16m/BFOSC spectroscopic observations.The double-peak optical light curve is a significant feature predicted by the reverse-forward external-shock model.The shallow decay followed by a normal decay in both the X-ray and optical light curves is well explained with the standard forward-shock model with late-time energy inj ection.Therefore,GRB 190106A offers a case study for GRBs emission from both reverse and forward shocks.In Chapter 4,we report the optical and near-infrared observations of the afterglow of a relatively high-redshift event,GRB 220101 A,triggered on New Year’s Day of 2022.With the optical spectra obtained at Xinglong 2.16m/BFOSC and NOT/ALFOSC,we determine the redshift of the burst at z=4.615±0.001.Based on our optical and near-infrared data,combined with the X-ray data,we perform multiband fit with the python package afterglowpy.A jet-break is found to constrain the opening angle of the jet as～3.4 degree.We also determine circumburst density of n₁=0.15 cm^-3 as well as kinetic energy E_K,iso=3.5×10⁵⁴ erg.The optical afterglow is among the most luminous ever detected.We also find a "mirror" feature in the light curve during the prompt phase of the burst from 80 s to 120 s.The physical origin of such mirror feature is unclear.Finally,we summarize the work during the doctoral research.We also discuss the limitations of current observations,the future missions for GRBs science on board the satellite of China and the prospect of future work.