Probing the Inner Accretion Flow Properties Around Black Holes with X-ray Observations

Citation

Xu, Yanjun (2021) Probing the Inner Accretion Flow Properties Around Black Holes with X-ray Observations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9kkg-4b58. https://resolver.caltech.edu/CaltechTHESIS:05272021-232230617

Abstract

Accretion, governed by gravity, is a fundamental source of energy in the universe. Accretion is important to the growth and evolution of black holes, as well as the structure in the universe on larger scales. Accretion disks around supermassive black holes are likely to have produced most of the ionizing radiation in the universe since the epoch of reionization. Outflows launched from the accretion disk of black holes, either in the form of disk winds or jets, provide a feedback mechanism that plays an important role in the co-evolution of black holes and their host galaxies. It has been decades since the fundamental theories about black hole accretion were established. Black hole accretion has been widely studied in active galactic nuclei (AGNs) and black hole binaries ever since the early days of X-ray astronomy. However, important questions still remain regarding the fundamental physical properties of black holes, the structure and geometry of accretion disks and coronae, and the nature of disk winds and jets.

In this thesis, I report results from recent X-ray observations of an ultra-luminous infrared galaxy with a central AGN, IRAS 05189–2524, and several black hole X-ray binaries (or black hole candidates for those currently lacking dynamical mass determinations), IGR J17091–3624, MAXI J1535–571, Swift J1658.2–4242, MAXI J1631–479, and MAXI J1820+070. Most of the black hole X-ray binaries studied in this thesis were uncatalogued sources and were discovered as bright Galactic X-ray transients over the past few years, offering great opportunities for investigating black hole accretion with high quality datasets. The launch of the NuSTAR telescope in 2012 has brought the advanced capabilities of performing high sensitivity observations in the hard X-ray band and remaining free from pile-up for bright Galactic sources, providing new angles for the detailed study of observational phenomena around both supermassive and stellar-mass accreting black holes. I study the inner accretion flow properties around black holes in the above objects by conducting spectral and timing analyses of the NuSTAR observations and the simultaneous soft X-ray band data from the Swift or XMM-Newton telescope. I have searched for and analyzed various accretion related observational features in these systems (disk reflection spectra, ionized absorption caused by disk winds, quasi periodic oscillations (QPOs) in the X-ray light curves, and unusual aperiodic flux variations or accretion state changes), and interpret the results in terms of the physical properties about the inner accretion flows and the central black holes.

By modeling the relativistic disk reflection spectra, I have found that the inner edge of the optically-thick accretion disk is truncated in IGR J17091–3624 during the bright hard state, in MAXI J1631–479 during the very high state, and in MAXI J1820+070 during the faint hard state, whereas the inner accretion disk is consistent with extending down to the ISCO in MAXI J1535–571 and Swift J1658.2–4242 during their bright hard states, and in MAXI J1631–479 during its soft state. Based on all the observational evidence gathered in this thesis, the general picture about the accretion flow geometry at different accretion states seems to be more complicated than that from previously well accepted theoretical predictions. In addition, I have measured the black holes spins and inner accretion disk inclinations for IRAS 05189–2524, MAXI J1535–571, Swift J1658.2–4242, and MAXI J1631–479 using the disk reflection modeling method, where the central black holes are all found to be rapidly spinning. There are also evidence for strong disk winds detected in the X-ray spectra of some of the objects studied in this thesis with interesting implications. I summarize my studies of these individual objects at the end of this thesis in a more broader context of the characteristic behaviors of the population and their general physical implications, and discuss about the possibilities of extending my research in this direction with the upcoming new X-ray missions.

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