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