Study On Quasi Periodic Oscillations In The X-ray Radiation Of Active Galaxy Nucleus

Active Galactic Nuclei(AGN)is a special kind of galaxies with intense emission characteristics in the central region.It has important observational properties including the radiation of supermassive black hole accretion,time-dependent light curves and strong emission lines,among them the light curves change is a very common phenomenon in AGN.AGN produces a wide range of radiation,which can cover from radio to X-ray,but as the X-ray radiation originates from the region at the edge of the black hole,the light curve information obtained in the X-ray band is the most important.Quasi-periodic oscillations(QPO)in the X-ray band are possible periodic variations in the intensity of radiation in this spectral band over time.Although the exact physical mechanism of this phenomenon is not known,it is still an effective way to probe the physical formation of accretion of black holes and related compact objects in the Universe.This paper focuses on the X-ray emission from a special class of ultrasoft active galactic nucleus(RX J1301.9+2747),and have obtained innovative results.The specific research contents are as follows:Chapter 1 briefly introduces the basic concepts and observational features of AGNs,the X-ray emission properties in them and the basic models of quasi-periodic oscillations,outlines the current research status of research on QPOs in the X-ray band light curves of AGNs,and highlights the significant research implications of the search for QPO signals in them.Chapter 2 introduces three main analytical methods for extracting periodic signals based on light curves,including the Fast Fourier Transform,the Lomb-Scargle periodogram and the WWZ wavelet transform method,and analyzed the advantages and disadvantages of them.Based on the research and analysis of different algorithms,we can evaluate the data in various aspects,and then select a set of suitable algorithms to analyse the time series according to its own characteristics.Chapter 3 analyses the active galaxies Mrk 766 and RE J1034+396 according to Lomb-Scargle periodograms and the WWZ wavelet transform method,reproducing the results of studies on QPO detection from the literature,not only verifying the accuracy of our data processing methods for QPO signal detection,but also confirming that the combination of Lomb-Scargle periodogram and the WWZ method can give a more reliable detection of QPO.Chapter 4 is the most important work in this paper.RX J1301.9+2747 is the first AGN reported to have X-ray flare discoveries,with an extremely soft X-ray energy spectrum.We have collected observations of RX J1301.9+2747 at X-ray band intervals of about 20 years and applied a combination of Fast Fourier Transform,Lomb-Scargle periodogram and WWZ wavelet transform methods to carefully analyse the light curves.A significant QPO signal is detected in the power spectrum of two independent X-ray observations at 6.68 x 10-4Hz,corresponding to a period of about 1500s.A Monte Carlo simulation of the light curve gives a confidence level of 99.893%for the detection of the QPO signal,which obeys the theoretical prediction of the 3:2 dual-frequency disk resonance model in terms of frequency and mass.According to this model we put new constraints on the spin of the black hole of RX J1301.9+2747.Our research finds that the detections of QPO in AGN are not only related to the black hole mass and accretion rate,but also have a strong correlation with the shape of the soft X-ray spectrum,giving the prospect of detecting QPO in ultrasoft AGN.This chapter concludes with a summary of the importance and innovation of this work and elaborates on plans for more in-depth studies and exploration of X-ray radiation bursts and quasi-periodic oscillation phenomena in ultrasoft AGN.In Chapter 5,other work during my postgraduate is introduced.In addition to mastering the data processing methods of X-ray variability,I also had a preliminary study of radio astronomy during my master’s degree.I have a certain understanding of the radiation mechanism of radio sources and the radio data processing process.The results of related interferometric continuum spectral imaging and spectral line analysis studies are shown.Finally,I summarized all the work in the end of this chapter.