Multi-wavelength Emissions From Gamma-ray Binaries And Accretion-Induced Collapse Of White Dwarfs

More than half of the stars in the universe are observed in binary systems,and isolated stars(like the sun)account for only a small proportion.The initial masses of component stars and the matter transfer in the binary system will profoundly affect the evolution of each star and lead to different evolution products.In this thesis,we focus on the high-energy emissions from gamma-ray binaries that consist of a neutron star orbiting around a massive star,and we also investigate the multi-wavelength emissions from the binary white dwarfs mergers and the accretion induced collapse of white dwarfs into neutron stars.A brief review on the research of gamma-ray binaries are given in chapter 1.Firstly,we introduce some of the main equipments in gamma-ray astronomy,including the gamma-ray telescope in the space and the Cherenkov telescope on the ground.Then we summarize the observing characteristics of seven high-mass gamma-ray binaries that have been detected so far.Finally,we present the details of the wind interaction model of gamma-ray binaries,including the main properties of pulsars,massive stars and their outflows,the cooling process of accelerated electrons and radiation mechanism in the shock.In chapter 2,we studied the multi wavelength emissions from the gamma-ray binary 1FGL J1018.6-5856 under the pulsar wind shock model.Firstly,we report the analysis results of the Fermi/LAT using 9.75 years of data.The phase-resolved spectrum indicate that the GeV emission contains a steady com-ponent which is likely be produced by magnetospheric emission of pulsar,and a modulated component which shows flux maximum around inferior-conjunction.Furthermore,the ke V/TeV light curves of 1FGL J1018.6-5856 also exhibit a sharp peak around inferior-conjunction,and they are attributed to the boost-ed emission from the shock,while another broad sinusoidal modulations are likely originating from the defected shock tail at a larger distance.The modulations of GeV flux are likely caused by the boosted synchrotron emission from the shock and the inverse-Compton(IC)emission in the cold pulsar wind.Finally,we discuss the similarities and differences of 1FGL J 1018.6-5856 and other gamma-ray binaries(like LS 5039?LMC P3)and the origins of high-energy emissions from these systems.In chapter 3,we investigate the interactions between the pulsar wind and stellar outflows,especially with the presence of the disc,and present a multiwavelength modelling of the emission from this system.We show that the double-peak profiles of X-ray and TeV gamma-ray light curves are caused by the enhancements of the magnetic field and soft photons at the shock during the disc passages.As the pulsar is passing through the equatorial disc,the additional pressure of the disc pushes the shock surface closer to the pulsar,which causes the enhancement of magnetic field in the shock,and thus increases the synchrotron luminosity.The TeV gamma-rays due to the IC scattering of shocked electrons with seed photons from the star are expected to peak around periastron,which is inconsistent with observations.However,the shock heating of the stellar disc could provide additional seed photons for IC scattering during the disc passages,and thus produces the double-peak profiles as observed in the TeV gamma-ray light curve.Our model can possibly be examined and applied to other similar gamma-ray binaries,such as PSR J2032+4127/MT91 213,HESS J0632+057,and LS I+61°303.Generally,neutron stars are formed by the collapses of progenitor stars with mass in the range of 8-25M?.Besides,the merger remnant of white dwarf binary and the accretion of white dwarf from its companion star can also collapse into neutron stars.In chapter 4,we investigate the optical and radio transients after the collapse of super-chandrasekhar white dwarf merger remnants.Super-Chandrasekhar remnants of double white dwarf mergers could sometimes collapse into a rapidly rotating neutron star(NS),accompanying with a mass ejection of a few times 0.01?M.Bright optical transient emission can be produced by the ejecta due to heating by radioactivities and particularly by energy injection from the NS.Since the merger remnants before collapse resemble a star evolving from the asymptotic giant branch phase to the planetary nebula phase,an intense dusty wind is considered to be driven about several thousand years ago before the collapse and surround the remnant at large radii.Therefore,the optical transient emission can be somewhat absorbed and scattered by the dusty wind,which can suppress the peak emission and cause a scattering plateau in optical light curves.Several years later,as the ejecta finally catches up with the wind material,the shock interaction between them can further give rise to a detectable radio transient emission on a timescale of tens of days.Discovery of and observations to such dust-affected optical transients and shock-driven radio transients can help to explore the nature of super-Chandrasekhar merger remnants and as well as the density and type ratios of double white dwarf systems,which is beneficial in assessing their gravitational wave contributions.In chapter 5,we investigate the X-ray and optical transients from the accretion-induced collapse of white dwarfs.The accretion-induced collapse(AIC)of a white dwarf in a binary with a non-degenerate companion can sometimes lead to the formation of a rapidly-rotating and highly magnetized neutron star(NS).The spin-down of this NS can drive a powerful pulsar wind(PW)and bring out some detectable multi-wavelength emissions.On the one hand,the PW can evaporate the companion in a few days to form a torus surrounding the NS.Then,due to the blockage of the PW by the torus,a reverse shock can be formed in the wind to generate intense hard X-rays.This emission component will disappear in a few weeks,after the torus is broken down at its inner boundary and scoured into a very thin disk.On the other hand,the interaction between the PW with an AIC ejecta can leads to a termination shock of the wind,which can produce a long-lasting soft X-ray emission component.In any case,the high-energy emissions from deep inside the system can be detected only after the AIC ejecta becomes transparent for X-rays.Meanwhile,by absorbing the X-rays,the AIC ejecta can be heated effectively and generate a fast-evolving and luminous thermal optical transient.Therefore,the predicted hard and soft X-ray emissions,associated by a supernova-like transient,can provide a clear observational signature for identifying AIC events in current and future observations(e.g.,AT 2018cow).Finally,we present some discussions and outlooks of gamma-ray binaries,double white dwarf merg-ers,and the accretion induced collapse processes of white dwarf in chapter 6.