Modes of disc accretion onto black holes

Exploring the physical nature of the spectral transition in galactic black hole binaries

X-ray binaries in our galaxies, either of transient or persistent nature, are commonly observed undergoing so-called transitions, i.e. dramatic changes in their spectral and variability properties. The main goal of accretion theory is to map as closely as possible the observed spectral states onto the different possible theoretical  solutions of the accretion problem, or accretion modes. In order to do so, we need first to associate each different observed spectral  state to a physical model, and then to single out the main physical parameters within each model that drive the transitions from one state to another.

My research has recently led to the definition of a self-consistent physical picture that describes the coupled accretion disc-corona
system in magnetically viscous plasmas. Under the only assumptions that (1) Magneto-Rotational  instability (MRI) generates the turbulence that produces the anomalous viscosity needed for accretion to proceed, and that (2) the magnetic field amplified by the instability saturates due to buoyant vertical escape, I was able to self-consistently solve the disc structure equations including the fraction of power that is
carried off by vertical Poynting flux.  Such a treatment allows a simple analytic study of the main parameters driving observable changes of, for example, X-ray spectral indices,  disc to corona flux ratios, magnetic compactness of the X-ray emitting gas. Also, it predicts a relationship between magnetic viscosity and these same observables, which can be used as a guideline for numerical MHD simulations of magnetically driven turbulent discs. In a follow-up project, together with S. Nayakshin (MPA),  I plan to incorporate this physical coupling between disc viscosity and coronal properties into time dependent accretion disc simulations, in order to explore the mechanisms of state transition itself.

Black holes at high accretion rates, and the nature of Ultra Luminous X-ray source

In my recent research work, I have also shown that,  within the framework of coupled thin disc-corona solutions, a new stable, radiation pressure dominated solution for high viscosity discs is allowed, characterized by powerful coronae and appearing only above a critical accretion rate.
This newly discovered thin disc solutions, possibly accompanied by powerful, magnetically dominated coronae and outflows, should be relevant for models of black holes accreting close to (or above) the Eddington rate, and can open up the way to a new understanding of the most extreme black hole-powered sources, as galactic Microquasars  or Radio Loud QSOs.

Among these extreme objects,  are also the so-called Ultra-luminous X-ray sources (ULX), found in large numbers in nearby galaxies, with luminosities comparable to, or even larger than, the Eddington luminosity of a stellar mass black hole. These sources are clearly accretion powered, as their rapid  variability and spectral properties indicate. However, it is still matter of a fierce debate whether they are `ordinary'
stellar mass black holes accreting at extremely high rates and whose radiation is beamed into our line of sight, or they represent a new class of intermediate mass (~ 10^3-10^4 M_sun) black holes. The fundamental plane relationship can be used to test the different scenarios, as it predicts very different levels of radio luminosity for different black hole masses, at any given (observed) X-ray luminosity.

With R. Fender and E. Gallo (University of Amsterdam) and M. Rupen (NRAO) we have been awarded VLA time for next year  to observe the radio  emission from the ULX J1216.9+3743 in the nearby galaxy NGC 4244. In case of detection, we will be able to derive a stringent lower limit
to the black hole mass, as well as demonstrate the feasibility of this method as an alternative way to pin down the properties of ULXs as a

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These pages are maintained by Andrea Merloni; last update: December 2003