本論文旨在研究GRS 1915+105於長時間尺度下的變化，其中GRS 1915+105是由一顆規則恆星的伴星和黑洞所組成的X射線雙星系統。該系統於1992年8月15日第一次被全天監視天文望遠鏡Granat所觀測到，同時成為第一個銀河系內已知的超光速噴流。我們透過MAXI天文望遠鏡分析自2009年到2020年底對GRS 1915+105的觀測資料，顯示出其在X射線波段長時間穩定的爆發，然而2018年開始GRS 1915+105進入了相對低通量，且較為混亂的高硬度狀態。藉由光譜圖和硬度圖下的變化可以區分為5段不同的表現；同時我們比對過往的觀察，發現了GRS 1915+105過去不曾被討論過，處於相較之下低通量但低硬度的異常狀態，稱為非常低硬度態(VSS)。在長時間尺度的觀測下，我們主要利用單一參數模型來解釋這些狀態的物理意義，及模擬吸積盤或周圍物質可能發生的幾何變化。藉以更進一步了解GRS 1915+105系統的演化，並有助於在X射線天文學領域中進行具有類似行為活躍星系核的後續研究。

This thesis aims to research the X-ray variability on a long-term scale of the X-ray binary system GRS 1915+105 which is composed of a regular star and a black hole. This system was first discovered by the all-sky monitor telescope Granat on August 15, 1992, and became the first known astronomical object that ejects material with apparent superluminal motion velocities in the Milky Way. We analyzed the observational data of GRS 1915+105 from 2009 to the end of 2020 through the MAXI telescope, demonstrating several transient outbursts persistently occurred in the X-ray energy band over a long timescale. GRS 1915+105 entered a relatively low flux and rather chaotic hard state since 2018, called obscured state. We can divide the behavior of GRS 1915+105 into 5 different parts via the spectra and the hardness-intensity diagram. In addition, we compared the literature and found three unprecedentedly soft but relatively low flux states that GRS 1915+105 had not been discussed in the past. We called them the very soft states. In this thesis, we mainly adopt simple spectral models to explain the physics behind these states and model the geometric transition of the accretion disk or surrounding materials. This study provides insight into the evolution of GRS 1915+105 and will motivate similar studies of long-term X-ray evolution of active galactic nuclei.

§ Chapter1: Introduction 1
1.1 A Brief History of X-ray Astronomy 1
1.2 Introduction of X-ray binaries 2
1.3 Hardness Ratios 6
1.3.1 Color-Color Diagram 6
1.3.2 Hardness-Intensity Diagram 8
1.4 X-ray Long-term Variability 9
1.5 Introduction of X-ray Telescopes 11
1.5.1 Swift’s Burst Alert Telescopes (Swift/BAT) 12
1.5.2 Rossi X-ray Timing Explorer (RXTE) 14
1.5.3 Monitor of All-sky X-ray Image (MAXI) 16

§ Chapter2: A Brief History of GRS 1915+105 19
2.1 Model-Independent Analysis of Variability 20
2.1.1 Spectral States and Variability Classifications of GRS 1915+105 20
2.1.2 The Disk-Jet Coupling of GRS 1915+105 24
2.2 Research Outline 26

§ Chapter3: Observations and Data Analysis 27
3.1 Observations 27
3.2 Hurst Exponent 28
3.3 Hilbert-Huang Transform 29
3.3.1 Empirical Mode Decomposition 30
3.3.2 Orthogonality 31

§ Chapter4: Results 33
4.1 Overall Behavior 33
4.2 Hardness Ratios Analysis 37
4.2.1 Color-Color Diagram (CD) 37
4.2.2 Hardness-Intensity Diagram (HID) 39
4.3 Empirical Mode Decomposition Analysis 41
4.4 Spectral Analysis 47

§ Chapter5: Discussion and Summary 55
5.1 Discussion 55
5.1.1 Physical Mechanism of the Obscured and VSS states 55
5.1.2 Long-term Variability Analysis 60
5.1.3 Classification of GRS 1915+105 in long-term timescales 61
5.2 Summary and Conclusions 63

§ Reference 64

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