低质量X射线双星的长期监测和吸积物理

Monitoring Observations of Low Mass X-ray Binary and Accretion Physics

本论文基于X射线空间卫星的多波段监测数据,研究明亮的低质量X射线双星的X射线能谱的演化和态跃迁过程,暂现源在爆发过程中的一些性质以及不同波段的光度的演化和相关关系。暂现源的爆发所涉及的光度范围很大,这对于研究吸积物理具有不可比拟的重要性,可以研究不同的吸积率下的吸积状态以及随着吸积率的演化过程和因果关系。通过这些研究增进我们对吸积物理的理解,尤其是暂现源爆发过程中X射线能谱演化的驱动机制,喷流的性质和X射线能谱态的关系,不同能谱态下的辐射起源等一些尚未完全解决的问题。

In this thesis, we have performed the multi-wavelength monitoring observations of Galactic low-mass X-ray binaries and studied the X-ray spectral evolution and spectral state transitions in bright X-ray binaries, the properties of transient sources during the out- bursts and the evolution of multi-wavelength emission during the outburst and the relation between different wavelengths. The large dynamical range of X-ray luminosity is invaluable for studying the accretion physics, since we can watch the evolution of the system through the full range of accretion rates and follow causal sequences between them. The results of our study improve our understanding of accretion physics, especially in some unresolved problems, such as the key role in determining the X-ray spectral transitions, the relation between jet and X-ray spectral states and the origin of emission in different X-ray spectral states. Firstly, we systematically studied the different spectral states and state transitions in the bright Galactic X-ray binaries during the past more than ten years. We could study the long term evolution in different energy bands by using the soft （2 12 keV） and hard （15-50 keV） X-ray monitoring light curves. The hardnessratio of the two energy band can be used to describe the X-ray spectral evolution and define the spectral states and state transitions. We confirmed that the luminosity of hard-to-soft state transition positively correlates with the peak luminosity of following soft state, and also found positively correlation between the luminosity of hard-to-soft state transition and the rate-of-increase of luminosity. These two empirical correlations indicate that the brightest hard state and the hard-to-soft state transition are determined by the non-stationary accretion which is characterised by the rate-of-increase of mass accretion rate. Both correlations do not show any saturation or cut-off in the high luminosity end, which implies that the brighter outbursts and brighter hard states are permitted by physics and can be observed in the Galactic X-ray binaries, and these phenomena might have been observed in ultra-luminous X-ray sources in nearby galaxies. We then have statistically studied the rise timescales of outbursts in low-mass X-ray binary transients during the past more than ten years. The results shows the rise timescales are nearly constant with large scatter in different outbursts, which supports the positive correlation between the rate-of-increase of luminosity and peak luminosity. We also found that the mean value of rise timescale is about several days and weakly correlates with the orbital period. Our results indicate that the rise timescale may correspond to the viscous timescale at some outer radius in the accretion disc, and somehow correlates with the size of the accretion disc. We have also performed the analysis of the multi-wavelength observations （including ultraviolet, optical and X-ray） cross the state transition during the 2010 outburst of GX 339-4 by Swift. We found that the UV flux positively correlates with the 0.6-10 keV X-ray in a form of fur ∝ fx^0.52＋0.04, and 10 days before the hard-to-soft state transition, UV flux shows a rapid drop, during which the X-ray flux still increases. We argued that the UV emission was dominated by jet during this outburst, and the optically thick spectrum can extend to higher frequency in a more powerful jet in a hard state. The drop in UV flux indicates the jet starts to switch off before hard-to-soft state transition, and could be used to predict the occurrence of the hard-to-soft transition.