Persistent Link:
http://hdl.handle.net/10150/289825
Title:
Accretion processes around supermassive black holes
Author:
Liu, Siming
Issue Date:
2002
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
Active Galactic Nuclei (AGNs) are believed to be powered by accretion onto supermassive black holes (BHs). With the development in high resolution observations over a broad frequency range, it is now tenable to study the corresponding physical processes in detail. We find that the emission from the closest supermassive BH candidate, Sagittarius A*, a compact radio source presumably accreting from stellar winds prevailing at the Galactic Center, can be explained as due to a quasi-spherical accretion flow, which circularizes to form a small magnetized accretion disk near the BH's event horizon. The mm/sub-mm and X-ray emissions are produced via thermal synchrotron processes and their self-Comptonization, respectively, in the inner ten Schwarzschild radii of the resultant Keplerian structure. The cm radio emission, however, appears to be produced by non-thermal synchrotron processes in the circularization zone. The recently detected X-ray flare seems to indicate a transient enhancement of mass accretion rate through the inner accretion disk. The 106-day cycle seen at 2.0 cm and 1.3 cm, on the other hand, suggests that the disk is precessing around a spinning BH, whose spin may be determined by timing observation of Sgr A* at mm/sub-mm wavelengths. Our tentative observational result is consistent with this magnetized disk model. The supermassive BH M31*, a compact radio source in the nucleus of M31, has many features in common with Sgr A*, yet their differences are significant. We show that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the capture radius. Sgr A* is presumably a 'hot' BH. While M31* seems to be a member of the 'cold' BH family. The study of the nuclei in radio galaxies reveals many new characteristics of the large scale accretion flows. In NGC 4261, we show that a turbulence-dominated disk, illuminated by its AGN, can not only account for the observed sub-parsec scale radio gap in the core, but also produce the optical broad lines emitted from the region. However, the prominent radio jets distinguish such BHs from those in the compact radio sources. The relativistic jets are probably driven by the action of supermassive, fast spinning BHs. Our study on NGC 6251* indicates that the initial ejection of matter can be associated with the thermal expansion of the accreted gas, which is heated by a spinning BH near its even horizon.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Physics, Astronomy and Astrophysics.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Physics
Degree Grantor:
University of Arizona
Advisor:
Melia, Fulvio

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleAccretion processes around supermassive black holesen_US
dc.creatorLiu, Simingen_US
dc.contributor.authorLiu, Simingen_US
dc.date.issued2002en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractActive Galactic Nuclei (AGNs) are believed to be powered by accretion onto supermassive black holes (BHs). With the development in high resolution observations over a broad frequency range, it is now tenable to study the corresponding physical processes in detail. We find that the emission from the closest supermassive BH candidate, Sagittarius A*, a compact radio source presumably accreting from stellar winds prevailing at the Galactic Center, can be explained as due to a quasi-spherical accretion flow, which circularizes to form a small magnetized accretion disk near the BH's event horizon. The mm/sub-mm and X-ray emissions are produced via thermal synchrotron processes and their self-Comptonization, respectively, in the inner ten Schwarzschild radii of the resultant Keplerian structure. The cm radio emission, however, appears to be produced by non-thermal synchrotron processes in the circularization zone. The recently detected X-ray flare seems to indicate a transient enhancement of mass accretion rate through the inner accretion disk. The 106-day cycle seen at 2.0 cm and 1.3 cm, on the other hand, suggests that the disk is precessing around a spinning BH, whose spin may be determined by timing observation of Sgr A* at mm/sub-mm wavelengths. Our tentative observational result is consistent with this magnetized disk model. The supermassive BH M31*, a compact radio source in the nucleus of M31, has many features in common with Sgr A*, yet their differences are significant. We show that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the capture radius. Sgr A* is presumably a 'hot' BH. While M31* seems to be a member of the 'cold' BH family. The study of the nuclei in radio galaxies reveals many new characteristics of the large scale accretion flows. In NGC 4261, we show that a turbulence-dominated disk, illuminated by its AGN, can not only account for the observed sub-parsec scale radio gap in the core, but also produce the optical broad lines emitted from the region. However, the prominent radio jets distinguish such BHs from those in the compact radio sources. The relativistic jets are probably driven by the action of supermassive, fast spinning BHs. Our study on NGC 6251* indicates that the initial ejection of matter can be associated with the thermal expansion of the accreted gas, which is heated by a spinning BH near its even horizon.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectPhysics, Astronomy and Astrophysics.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplinePhysicsen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorMelia, Fulvioen_US
dc.identifier.proquest3060979en_US
dc.identifier.bibrecord.b43041747en_US
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