More than ten years ago, the discovery of ultraluminous X-ray sources (ULXs) has opened up an entirely new field in astrophysics. Many ideas were developed to explain the nature of these sources, like their emission mechanism, mass, and origin, without any strong conclusions. Their discovery boosted the fields of X-ray binaries, accretion physics, stellar evolution, cosmology, black hole formation and growth, due to the concept of intermediate-mass black holes (IMBHs). Since their discovery is related to the domain of X-ray astrophysics, there have been very few 1 studies made in other wavelengths. This thesis focuses on the multiwavelength nature of ultraluminous X-ray sources and intermediate-mass black holes from various aspects, which help to overcome some difficulties we face today. First, I investigated the accretion signatures of a putative intermediate-mass black hole in a particular globular cluster. To this purpose, 1 I characterized the nature of the innermost X-ray sources in the cluster. Then I calculated an upper limit on the mass of the black hole by studying | possible accretion efficiencies and rates based on the dedicated X-ray and radio observations. The accreting properties of the source was described with standard spherical accretion and in the context of inefficient accretion. Secondly, I attempted to dynamically measure the mass of the black hole in a particular ULX via optical spectroscopy. I discovered that a certain emission line has a broad component that markedly shifts in wavelength. I investigated the possibility whether this line originates in the accretion disk, and thus might trace the orbital motion of the binary System. I also characterized the parameters of the binary system, such as the mass function, possible orbital separation, the size of the line emitting region, and an upper limit on the mass of the black hole. Then I studied the environment of a number of ULXs that are associated with large- scale optical and radio nebulae. 1 use these nebulae as a calorimeters to infer the total intrinsic power of these ULXs, characterize the feedback on the environment, and com- pare to Galactic sources. I I also investigated the power mechanism of inflating a bubble by jets. Then I discuss the current status of radio observations on ULXs, including ongoing radio surveys. Finally, I studied HLX-1, which is likely the first bona fide IMBH. This involves radio observations and analysis of the radio data that led to a discovery of radio flares from this source during an X-ray state transition. From the strength of the flares, I provided additional constraints on the mass of the black hole and its accretion rate.

This book is based on the lecture notes of a one-semester course on black hole astrophysics given by the author and is aimed at advanced undergraduate and graduate students with an interest in astrophysics. The material included goes beyond that found in classic textbooks and presents details on astrophysical manifestations of black holes. In particular, jet physics and detailed accounts of objects like microquasars, active galactic nuclei, gamma-ray bursts, and ultra-luminous X-ray sources are covered, as well as advanced topics like black holes in alternative theories of gravity. The author avoids unnecessary technicalities and to some degree the book is self-contained. The reader will find some basic general relativity tools in Chapter 1. The appendices provide some additional mathematical details that will be useful for further study, and a guide to the bibliography on the subject.

This volume brings together contributions from many of the world's leading authorities on black hole accretion. The papers within represent part of a new movement to make use of the relative advantages of studying stellar mass and supermassive black holes, and to bring together the knowledge gained from the two approaches. The topics discussed include black hole observational and theoretical work-variability, spectroscopy, disk-jet connections, and multi-wavelength campaigns on black holes.

Until recently, black holes were often considered as exotic objects of dubious existence. In the last decade, observations have provided overwhelming evidence in favour of the presence of supermassive black holes at the centre of galaxies, including the Milky Way; of stellar-mass black holes in binary stellar systems; and, possibly, of intermediate-mass black holes in ultraluminous X-ray sources in nearby galaxies. Black holes are now widely accepted as real physical entities, playing an important role in modern astrophysics. IAU S238 brought together observers and theoreticians working in black hole astrophysics - from stellar-mass black holes to supermassive ones residing at the centre of galaxies - with the aim of highlighting and discussing similarities in the physics involved. Leading researchers review the subject and report on recent results on accretion discs, relativistic jets, spectroscopy in different spectral bands - from X-rays to radio - and other aspects of black hole astrophysics.

For the last 40 years astronomers have observed sources of intense X-ray radiation from beyond our Solar System. These have been associated with remarkable objects, such as neutron stars and black holes. Current satellite observatories have enabled us to extend the studies of these objects from the Milky Way and Magellanic Clouds to individual X-ray sources in other galaxies. At the same time, imaging facilities at gamma-rays and TeV energies have improved enormously, leading to many further discoveries within our own galaxy. This Symposium presents an overview of these new developments, covering detailed studies of individual sources within the Milky Way, global descriptions of X-ray source populations in other galaxies, and less resolved relatives at high redshift. Galaxy nuclei and diffuse emission components are included, as are new discoveries at gamma-rays and TeV energies. IAU S230 is a valuable resource for astronomers and graduates working in high-energy astrophysics.