Satellite observations of thought experiments close to a black hole

Since black holes are `black', methods of their identification must necessarily be indirect. Due to very special boundary condition on the horizon, the advective flow behaves in a particular way, which includes formation of centrifugal pressure dominated boundary layer or CENBOL where much of the infall energy is released and outflows are generated. The observational aspects of black holes must depend on the steady and time-dependent properties of this boundary layer. Several observational results are written down in this review which seem to support the predictions of thought experiments based on this advective accretion/outflow model. In future, when gravitational waves are detected, some other predictions of this model could be tested as well.Comment: Published in Classical and Quantum Gravity, v. 17, No. 12, p. 2427, 200

Scalar and Spinor Perturbation to the Kerr-NUT Spacetime

We study the scalar and spinor perturbation, namely the Klein-Gordan and Dirac equations, in the Kerr-NUT space-time. The metric is invariant under the duality transformation involving the exchange of mass and NUT parameters on one hand and radial and angle coordinates on the other. We show that this invariance is also shared by the scalar and spinor perturbation equations. Further, by the duality transformation, one can go from the Kerr to the dual Kerr solution, and vice versa, and the same applies to the perturbation equations. In particular, it turns out that the potential barriers felt by the incoming scalar and spinor fields are higher for the dual Kerr than that for the Kerr. We also comment on existence of horizon and singularity.Comment: 31 pages including 20 figures, RevTeX style: Final version to appear in Classical and Quantum Gravit

Accretion Disks Around Black Holes: Twenty Five Years Later

We study the progress of the theory of accretion disks around black holes in last twenty five years and explain why advective disks are the best bet in explaining varied stationary and non-stationary observations from black hole candidates. We show also that the recently proposed advection dominated flows are incorrect.Comment: 30 Latex pages including figures. Kluwer Style files included. Appearing in `Observational Evidence for Black Holes in the Universe', ed. Sandip K. Chakrabarti, Kluwer Academic Publishers (DORDRECHT: Holland

Soliton-Like Solutions of the Grad-Shafranov Equation

A new class of soliton-like solutions is derived for the Grad-Shafranov (GS) equations. A mathematical analogy between the GS equation for MHD equilibria and the cubic Schr\"odinger (CS) equation for non-linear wave propagation forms the basis to derive the new class of solutions. The soliton-like solutions are considered for their possible relevance to astrophysics and solar physics problems. We discuss how a soliton-like solution can be generated by a repetitive process of magnetic arcade stretching and plasmoid formation induced by the differential rotation of the solar photosphere or of an accretion disk.Comment: Accepted for publication on Physical Review Letter

Accretion Disc Theory: From the Standard Model Until Advection

Accretion disc theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disc. One of the most important new invention of this theory was a phenomenological treatment of the turbulent viscosity, known as ''alpha'' prescription, when the (r$\phi$) component of the stress tensor was approximated by ($\alpha$ P) with a unknown constant $\alpha$. This prescription played the role in the accretion disc theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disc are discussed, including nonlinear hydrodynamical turbulence, convection and magnetic field role. In parallel to the optically thick geometrically thin accretion disc models, a new branch of the optically thin accretion disc models was discovered, with a larger thickness for the same total luminosity. The choice between these solutions should be done of the base of a stability analysis. The ideas underlying the necessity to include advection into the accretion disc theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disc model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed.Comment: Roceeding of the Int. Workshop "Observational Evidence for Black Holes in the Universe". Calcutta, 11-17 January 1998. Kluwer Acad. Pu

General relativistic radiative transfer: formulation and emission from structured tori around black holes

We construct a general relativistic radiative transfer (RT) formulation, applicable to particles with or without mass in astrophysical settings. Derived from first principles, the formulation is manifestly covariant. Absorption and emission, as well as relativistic, geometrical and optical depth effects are treated self-consistently. The RT formulation can handle 3D geometrical settings and structured objects with variations and gradients in the optical depths across the objects and along the line-of-sight. The presence of mass causes the intensity variation along the particle bundle ray to be reduced by an aberration factor. We apply the formulation and demonstrate RT calculations for emission from accretion tori around rotating black holes, considering two cases: idealised optically thick tori that have a sharply defined emission boundary surface, and structured tori that allow variations in the absorption coefficient and emissivity within the tori. Intensity images and emission spectra of these tori are calculated. Geometrical effects, such as lensing-induced self-occulation and multiple-image contribution are far more significant in accretion tori than geometrically thin accretion disks. Optically thin accretion tori emission line profiles are distinguishable from the profiles of lines from optically thick accretion tori and optically thick geometrically thin accretion disks. Line profiles of optically thin accretion tori have a weaker dependence on viewing inclination angle than those of the optically thick accretion tori or accretion disks, especially at high viewing inclination angles. Limb effects are present in accretion tori with finite optical depths. Finally, in accretion flows onto relativistic compact objects, gravitationally induced line resonance can occur. This resonance occurs easily in 3D flows, but not in 2D flows, such as a thin accretion disk around a black hole.Comment: 13 pages, 10 figures, Accepted for publication in Astronomy and Astrophysic

Summary of the Activities of the Working Group I on High Energy and Collider Physics

This is a summary of the projects undertaken by the Working Group I on High Energy Collider Physics at the Eighth Workshop on High Energy Physics Phenomenology (WHEPP8) held at the Indian Institute of Technology, Mumbai, January 5-16, 2004. The topics covered are (i) Higgs searches (ii) supersymmetry searches (iii) extra dimensions and (iv) linear collider.Comment: summary of Working Group I at the Eighth Workshop on High Energy Physics Phenomenology (WHEPP8), I.I.T., Mumbai, January 5-16, 200

Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy

(abridged) We present near-infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope (SALT) of a luminous infrared galaxy (LIRG) IRAS 19115-2124. These data are combined with archival HST imaging and Spitzer imaging and spectroscopy, allowing us to study this disturbed interacting/merging galaxy, dubbed the Bird, in extraordinary detail. In particular, the data reveal a triple system where the LIRG phenomenon is dominated by the smallest of the components. One nucleus is a regular barred spiral with significant rotation, while another is highly disturbed with a surface brightness distribution intermediate to that of disk and bulge systems, and hints of remaining arm/bar structure. We derive dynamical masses in the range 3-7x10^10 M_solar for both. The third component appears to be a 1-2x10^10 M_solar mass irregular galaxy. The total system exhibits HII galaxy-like optical line ratios and strengths, and no evidence for AGN activity is found from optical or mid-infrared data. The star formation rate is estimated to be 190 M_solar/yr. We search for SNe, super star clusters, and detect 100-300 km/s outflowing gas from the Bird. Overall, the Bird shows kinematic, dynamical, and emission line properties typical for cool ultra luminous IR galaxies. However, the interesting features setting it apart for future studies are its triple merger nature, and the location of its star formation peak - the strongest star formation does not come from the two major K-band nuclei, but from the third irregular component. Aided by simulations, we discuss scenarios where the irregular component is on its first high-speed encounter with the more massive components.Comment: 24 pages, 16 figures. Accepted MNRAS version, minor corrections only, references added. Higher resolution version (1.3MB) is available from http://www.saao.ac.za/~petri/bird/ulirg_bird_highres_vaisanen_v2.pd