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Register a new userStudy of Accretion processes Around Black Holes becomes Science: Tell Tale Observational Signatures of Two Component Advective Flows
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Sandip K. Chakrabarti
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An accretion flow around a black hole has a saddle type sonic point just outside the event horizon to guarantee that the flow enters the black hole supersonically. This feature exclusively present in strong gravity limit makes its marks in every observation of black hole candidates. Another physical sonic point is present (as in a Bondi flow) even in weak gravity. Every aspect of spectral or temporal properties of every black hole can be understood using this transonic or advective flow having more than one saddle type points. This most well known and generalized solution with viscosity and radiative transfer has been verified by numerical simulations also. Spectra, computed for various combinations of the standard Keplerian, and advective sub-Keplerian components match accurately with those from satellite observations. Standing, oscillating and propagatory oscillating shocks are produced due to centrifugal barrier of the advective component. The post-shock region acts as the Compton cloud producing the power-law spectra. Jets and outflows are also produced from this post-shock region, commonly known as the CENtrifugal barrier supported BOundary Layer or CENBOL. In soft states, the CENBOL is cooled down by soft photons from the Keplerian disk, and thus the outflow is absent. Type-C and Type-B QPOs are generated respectively due to strong and weak resonance oscillations of the CENBOL. Away from resonance, oscillation may be triggered when Rankine-Hugoniot conditions are not satisfied and Type-A QPOs could be seen.
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The fundamental difference between accretion around black holes and neutron stars is the inner boundary condition, which affects the behavior of matter very close to the compact objects. This leads to formation of additional shocks and boundary layers for neutron stars. Previous studies on the formation of such boundary layers focused on Keplerian flows that reached the surface of the star, either directly or through the formation of a transition layer. However, behavior of sub-Keplerian matter near the surface of a neutron star has not been studied in detail. Here, we study the effect of viscosity, in presence of cooling, on the sub-Keplerian flows around neutron stars, using Smoothed Particle Hydrodynamics. Our time-dependent study shows that multiple shocks, transition and boundary layers form in such type of accretion, when viscosity is significant, and one or more layers are absent when the viscosity is moderate. These flows are particularly of interest for the wind dominated systems such as Cir X-1. We also report the formation of a generalized flow configuration, Two-Component Advective Flow, for the first time.
Ultra-luminous X-ray sources (ULXs) have been puzzling us with a debate whether they consist of an intermediate mass black hole or super-Eddington accretion by a stellar mass black hole. Here we suggest that in the presence of large scale strong magnetic fields and non-negligible vertical motion, the luminosity of ULXs, particularly in their hard states, can be explained with sub-Eddington accretion by stellar mass black holes. In this framework of 2.5D magnetized advective accretion flows, magnetic tension plays the role of transporting matter (equivalent to viscous shear via turbulent viscosity) and we neither require to invoke an intermediate mass black hole nor super-Eddington accretion. Our model explains the sources, like, NGC 1365 X1/X2, M82 X42.3+59, M99 X1 etc. which are in their hard power-law dominated states.
We present our view of the immediate environment of an astrophysical black hole. We discuss the origin of the electron-positron and electron-proton jet components, and the origin of the large-scale magnetic field. We show how the aberrated radiation field viewed by the rotating plasma generates an azimuthal induction electric field, and how the curl of this electric field steadily grows the magnetic field to equipartition over a few hundred million dynamical times. That timescale corresponds to weeks for stellar mass black holes, and million years for super-massive black holes. We conclude with numerical and observational confirmation of the action of a Cosmic Battery in accretion flows around astrophysical black holes.
These notes resulted from a series of lectures at the IAC winter school. They are designed to help students, especially those just starting in subject, to get hold of the fundamental tools used to study accretion powered sources. As such, the references give a place to start reading, rather than representing a complete survey of work done in the field. I outline Compton scattering and blackbody radiation as the two predominant radiation mechanisms for accreting black holes, producing the hard X-ray tail and disc spectral components, respectively. The interaction of this radiation with matter can result in photo-electric absorption and/or reflection. While the basic processes can be found in any textbook, here I focus on how these can be used as a toolkit to interpret the spectra and variability of black hole binaries (hereafter BHB) and Active Galactic Nuclei (AGN). I also discuss how to use these to physically interpret real data using the publicly available XSPEC spectral fitting package (Arnaud et al 1996), and how this has led to current models (and controversies) of the accretion flow in both BHB and AGN.
Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height $H$ to cylindrical radius $R$ ratio of $|H/R|sim 0.2--1$) accretion flows around BHs with various dimensionless spins ($a/M$, with BH mass $M$) and with initially toroidally-dominated ($phi$-directed) and poloidally-dominated ($R-z$ directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough $|a/M|$, coherent large-scale (i.e. $gg H$) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. For sufficiently high $|a/M|$ or low $|H/R|$ the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric magnetically choked accretion flow (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with $gtrsim 100$% efficiency for $|a/M|gtrsim 0.9$. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has spherical harmonic $|m|=1$ mode period of $tausim 70GM/c^3$ for $a/Msim 0.9$ with coherence quality factors $Qgtrsim 10$. [abridged]
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