The Study Of High Energy Bursts Related To Neutron Stars And Black Holes

Neutron stars(NSs)and black holes(BHs)are extremely dense celestial objects,which have very strong gravitational and magnetic fields.The sources usually show intense activities during their evolution,and are accompanied by high energy bursts.The ob-servation and study of such phenomena plays an important role in exploring the fun-damental physics in extreme relativistic conditions.Observations of binary neutron star systems have indirectly proved the existence of gravitational wave radiation that is predicted by the general relativity,while gravitational waves from a few binary black hole systems are directly recorded by the LIGO collaboration.The association between gravitational wave event and short Gamma ray burst(GW170817/GRB 170817A)di-rectly confirms that at least some short GRBs originate from binary neutron stars.Gamma ray bursts(GRBs)and fast radio bursts(FRBs)are extremely violent burst-ing phenomena in the universe.Observations and theoretical researches of those bursts indicate that they are associated with NS and BH systems.With the revamping and advancing of gravitational wave detectors,and the continuous progress of multi-band joint observations,more and more NS-NS mergers,BH-BH mergers and even BH-NS mergers will be detected in the future.In this PhD thesis,we mainly study the repeat-ing fast radio burst FRB 121102 statistically,and suggest a few explanations for the continuous brightening of the multi-band afterglow of GW170817/GRB 170817A.The thesis is organized as follows.We briefly introduce the high energy bursts of GRBs and FRBs that are related to BHs and NSs in the first chapter.The second chapter is a detailed overview of GRBs,including observational and theoretical progresses of GRB prompt emission and afterglow emission.We also briefly describe the progenitors and central engines of GRBs,and the standard fireball-shock model for the afterglow emission.Then we review the observations and theories concerning FRBs in Chapter 3,including the observational characteristics,radiation mechanisms,and related physics.The fourth chapter presents one of our study on FRBs.In this chapter,the ob-servational characteristics and research progresses of FRB 121 102 are first introduced.Then many important observational data of this repeating burst are collected and de-scribed.We carry out statistical analyses on the nearly 180 repeating outbursts of FRB 121 102.Especially,we analyze the characteristics of the waiting times between bursts.It is found that there is an obvious bimodal distribution for the waiting times.Although most waiting times are clustered at hundreds of seconds(peaking at?170s),a small portion(?6%)of the waiting times are in the range of 2-40 millisecond.The cumu-lative distribution of the waiting times is analyzed.We also analyze the correlation between the waiting time and other parameters of the source.It is found that the wait-ing time does not correlate with the burst intensity,either for the preceding burst or the subsequent burst.Thus it seems that the source does not need time to accumulate ener-gy for producing the next burst.It strongly indicates that the repeating FRBs should be generated by some external mechanisms rather than internal mechanisms.For exam-ple,the models which involves collisions between small celestial bodies and neutron stars could be a competitive mechanism.In the fifth chapter,we first describe the observations about the multi-band af-terglow of GW170817/GRB 170817A.We then propose a delayed energy injection model to explain the brightening of the event.We consider the merger remnant in two cases,i.e.,a BH or a NS:(1)If the merger remnant of GW170817/GRB 170817A is a black hole with a mass of 2.7M?,it can continuously accrete some fall-back ejecta.The gravitational energy released is transformed into Poynting-flux and injected into the jet,which leads to the continuous brightening of the afterglow.Assuming the ener-gy transform efficiency as 0.1,only 0.006M? is needed.This value is smaller than the ejecta mass during the double neutron star merging process(10-3-10-2M?).Our late-time energy injection model can well fit the multi-band afterglow of GW170817/GRB 170817A,with the chi-square of the fit being X2?42.3 for 22 degrees of freedom.(2)If the central compact object is a fast-rotating magnetar,it would eject an electron-positron-pair(e+/e-)wind.We find that the interaction between the e+/e-wind and the jet can also produce a long-lived reverse shock and inject energy to generate a bright-ening afterglow for approximately 150 days.We consider the spin down of the NS due to two mechanisms:the gravitational wave radiation and the magnetic dipole radiation.It is found that the observed data can be well fitted by the model.A braking NS due to gravitational wave radiation,with an ellipticity of?4.4×10-5 and a polar magnetic field of 1012 Gaussian is more preferred.In addition,we have also discussed the effect of a double-sided structured jet on the GW170817/GRB 170817A afterglow emission.Finally,in the last chapter,we summarize our study and give some prospects on future studies.Recently,in China,with the operation of various large astronomical instruments such as the Hard X-ray Modulation Telescope(Insight-HXMT)and the Five-hundred-meter Aperture Spherical Telescope(FAST),our observational ability of high energy busts related to NSs and BHs will be greatly enhanced.Furthermore,other large astronomical equipments,such as GECAM and QTT,are also being developed in China.They will bring more opportunities for GRB and FRB studies.