Multiwavelength Study Of Be/X-ray Binaries

A Be/X-ray binary is a system which contains a Be star and a compact star.Be stars’ luminosity class is Ⅲ-Ⅴ,and at some point of their lives they have shown spectral lines in emission.Be stars have circumstellar disks on their equator planes.When the compact stars pass through the periastrons,they can accrete materials from circumstellar disks.The materials fall into the gravitational potential well,at the same time,their gravitational energy turns into the internal energy,and emit outward in X-ray.In optical and infrared bands,Be stars have two main observational characteristics,spectral emission lines and an excess of IR emission.Both emission line and IR excess originate from the circumstellar disk of the Be star.The optical emission variation of Be/X-ray binaries is dominated by Be star’s disk.Our aim is to study the multiwavelength long-term variability of Swift J0243.6+6124,1A 0535+262,and KS 1947+300 before and after giant X-ray outbursts.We have obtained optical spectroscopy and photometry data before,during,and after the X-ray outbursts.The long-term photometric light curve and the equivalent widths of the Hα and He I λ6678 lines were used to monitor the state of the Be star’s circumstellar disk.The Ha line profiles qualitatively show evidence for V/R variability that was accounted for by fitting the Ha spectral line profile with two Gaussian functions.We divided our data into different phases according to the intensity of the X-ray,optical,and infrared emission to more specifically study changes in the sources.In chapter one,we introduced the classification of X-ray binaries,the two kinds of outbursts of transient Be/X-ray binaries,and the optical and infrared characters of Be stars and Be/X-ray binaries.Chapter two introduces the telescopes we use,the databases we utilize,and the observation strategies and data processing.In chapter three,we divide our observations into three phases based on the changes in the observed parameters of Swift J0243.6+6124,bounded by MJD 58180 and MJD 58530.For Swift J0243.6+6124,phase I covers the rise and decay of the giant X-ray outburst that took place in October-November 2017.We interpret phase Ⅱ as the dissipation of the Be star’s equatorial disk and phase Ⅲ as its recovery.The timescale of a complete formation and dissipation process is about 1250 days.The epoch when the dissipation process stopped and the reformation period began is estimated to be around MJD 58530.We find a delay of～100-200 days between the minimum of the optical or infrared intensity and the strength of the Ha line after the X-ray outburst,which may indicate that the dissipation of the disk begins from the inner parts.The motion of the density perturbation inside the disk is prograde,with a V/R quasi-period of about four years.The source shows a positive correlation in the(B-V)color index versus Vband magnitude diagram,which implies that the system is seen at a small or moderate inclination angle.In chapter four,we carry out long-term optical spectroscopic and photometric observations on the Be/X-ray binary 1A 0535+262 from 2010 to 2021.Our observations cover the giant X-ray outburst in 2020 and one month and one year before it.We find that the V-band brightness,the IR-band brightness and the emission intensities of Ha lines change one by one,which possibly is the result of the viscous diffusion of the materials on the Be disk.The motion of the density perturbation inside the disk is retrograde,and the density perturbations in the inner and outer regions of the circumstellar disk are inconsistent in 2021.Periods Ⅰ→Ⅱ→Ⅲ→Ⅳ constitute a cycle in the(B-V)color index vs.Ⅴ-band magnitude diagram.In chapter five,from 2013 to 2021,we monitored KS 1947+300 for eight years with the intermediate dispersion spectrometer,revealing the evolution of the Be star’s disk and its relation with the multiwavelength activities of the source.From 2000 to 2021,the long-term change trend of KS 1947+300 can be divided into two periods,which are demarcated by October 2013.The two groups of type I outbursts at KS 1947+300 generally have no preferred orbital phase,but adjacent outbursts often occur in close phases.According to the time interval of the type I outbursts,it can be concluded that Pburst<N×Porb,so we can infer that the precession sense of its stellar disk is opposite to the sense of the neutron star’s revolution(retrograde).We find two periodic signals,198 days and 193 days,in the light curves of ZTF-g and r bands,respectively,which probably originate from the circumstellar disk of Be star or the orbital motion of the matter around the binaries.The relationship between the(B-V)color index and the V-band magnitude shows that the inclination of KS 1947+300 is small or medium;moreover,the structure of the 2013-2021 circumstellar disk is different from that of the 1990-1991 disk.Chapter six concludes the whole thesis and gives an outlook for future work.