7,907 research outputs found
Accretion Discs in Blazars
The characteristic properties of blazars (rapid variability, strong
polarization, high brightness) are widely attributed to a powerful relativistic
jet oriented close to our line of sight. Despite the spectral energy
distributions (SEDs) being strongly jet-dominated, a "big blue bump" has been
recently detected in sources known as flat spectrum radio quasars (FSRQs).
These new data provide a unique opportunity to observationally test coupled
jet-disc accretion models in these extreme sources. In particular, as energy
and angular momentum can be extracted by a jet magnetically coupled to the
accretion disc, the thermal disc emission spectrum may be modified from that
predicted by the standard model for disc accretion. We compare the
theoretically predicted jet-modified accretion disc spectra against the new
observations of the "big blue bump" in FSRQs. We find mass accretion rates that
are higher, typically by a factor of two, than predicted by standard accretion
disc theory. Furthermore, our results predict that the high redshift blazars
PKS 0836+710, PKS 2149-307, B2 0743+25 and PKS 0537-286 may be predominantly
powered by a low or moderate spin (a < 0.6) black hole with high mass accretion
rates mdot_a ~ 50 - 200 msol/yr, while 3C 273 harbours a rapidly spinning black
hole (a = 0.97) with mdot_a ~ 20 msol/yr. We also find that the black hole
masses in these high redshift sources must be > 5 * 10^9 msol.Comment: Accepted for publication (17 August 2009) in MNRA
Black hole accretion discs
This is an introduction to models of accretion discs around black holes.
After a presentation of the non-relativistic equations describing the structure
and evolution of geometrically thin accretion discs we discuss their
steady-state solutions and compare them to observation. Next we describe in
detail the thermal-viscous disc instability model and its application to dwarf
novae for which it was designed and its X-ray irradiated-disc version which
explains the soft X--ray transients, i.e. outbursting black-hole low-mass X-ray
binaries. We then turn to the role of advection in accretion flows onto black
holes illustrating its action and importance with a toy model describing both
ADAFs and slim discs. We conclude with a presentation of the
general-relativistic formalism describing accretion discs in the Kerr
space-time.Comment: 41 pages, 11 figures, Chapter 1 in: "Astrophysics of Black Holes -
From fundamental aspects to latest developments", Ed. Cosimo Bambi, Springer:
Astrophysics and Space Science Library 440 (2016). Minor errors correcte
Influence of Ohmic and ambipolar heating on thermal structure of accretion discs
We investigate dynamics of accretion discs of young stars with fossil
large-scale magnetic field. Our magneto-gas-dynamic (MHD) model of the
accretion discs includes equations of Shakura and Sunyaev, induction equation,
equations of thermal and collisional ionization. Induction equation takes into
account Ohmic and magnetic ambipolar diffusion, magnetic buoyancy. We also
consider the influence of Ohmic and ambipolar heating on thermal structure of
the accretion discs. We analyse the influence of considered dissipative MHD
effects on the temperature of the accretion discs around classical T Tauri
star. The simulations show that Ohmic and ambipolar heating operate near the
borders of the region with low ionization fraction (`dead' zone). Temperature
grows by K near the inner boundary of the `dead' zone, au, and by K near its outer boundary,
au.Comment: 8 pages, 3 figures, The Third Russian Conference on
Magnetohydrodynamics, accepted for publication in a Special Issue of the
Magnetohydrodynamics Journa
Dynamics of warped accretion discs
Accretion discs are present around both stellar-mass black holes in X-ray
binaries and supermassive black holes in active galactic nuclei. A wide variety
of circumstantial evidence implies that many of these discs are warped. The
standard Bardeen--Petterson model attributes the shape of the warp to the
competition between Lense--Thirring torque from the central black hole and
viscous angular-momentum transport within the disc. We show that this
description is incomplete, and that torques from the companion star (for X-ray
binaries) or the self-gravity of the disc (for active galactic nuclei) can play
a major role in determining the properties of the warped disc. Including these
effects leads to a rich set of new phenomena. For example, (i) when a companion
star is present and the warp arises from a misalignment between the companion's
orbital axis and the black hole's spin axis, there is no steady-state solution
of the Pringle--Ogilvie equations for a thin warped disc when the viscosity
falls below a critical value; (ii) in AGN accretion discs, the warp can excite
short-wavelength bending waves that propagate inward with growing amplitude
until they are damped by the disc viscosity. We show that both phenomena can
occur for plausible values of the black hole and disc parameters, and briefly
discuss their observational implications.Comment: 28 pages, 11 figure
Accretion Discs Trapped Near Corotation
We show that discs accreting onto the magnetosphere of a rotating star can
end up in a 'trapped' state, in which the inner edge of the disc stays near the
corotation radius, even at low and varying accretion rates. The accretion in
these trapped states can be steady or cyclic; we explore these states over wide
range of parameter space. We find two distinct regions of instability, one
related to the buildup and release of mass in the disk outside corotation, the
other to mass storage within the transition region near corotation. With a set
of calculations over long time scales we show how trapped states evolve from
both nonaccreting and fully accreting initial conditions, and also calculate
the effects of cyclic accretion on the spin evolution of the star. Observations
of cycles such as found here would provide important clues on the physics of
magnetospheric accretion. Recent observations of cyclic and other unusual
variability in T Tauri stars (EXors) and X-ray binaries are discussed in this
context.Comment: 14 pages, 10 figures, accepted to MNRA
X-ray irradiation in low mass binary systems
We calculate self-consistent models of X-ray irradiated accretion discs in
close binary systems. We show that a point X-ray source powered by accretion
and located in the disc plane cannot modify the disc structure, mainly because
of the self-screening by the disc of its outer regions. Since observations show
that the emission of the outer disc regions in low mass X-ray binaries is
dominated by the reprocessed X-ray flux, accretion discs in these systems must
be either warped or irradiated by a source above the disc plane, or both. We
analyse the thermal-viscous stability of irradiated accretion discs and derive
the stability criteria of such systems. We find that, contrary to the usual
assumptions, the critical accretion rate below which a disc is unstable is
rather uncertain since the correct formula describing irradiation is not well
known.Comment: to be published in MNRAS, uses epsfig.st
Viscous accretion discs around rotating black holes
The stationary hydrodynamic equations for transonic viscous accretion discs
in Kerr geometry are derived. The consistent formulation is given for the
viscous angular momentum transport and the boundary conditions on the horizon
of a central black hole. An expression for the thickness of the disc is
obtained from the vertical Euler equation for general accretion flows with
vanishing vertical velocity. Different solution topologies are identified,
characterized by a sonic transition close to or far from the marginally stable
orbit. A numerical method is presented that allows to integrate the structure
equations of transonic accretion flows. Global polytropic solutions for the
disc structure are calculated, covering each topology and a wide range of
physical conditions. These solutions generally possess a sub-Keplerian angular
momentum distribution and have maximum temperatures in the range
K. Accretion discs around rotating black holes are hotter and
deposit less angular momentum on the central object than accretion discs around
Schwarzschild black holes.Comment: 15 pages LateX, requires mn.sty, accepted for publication in the
MNRAS, figures available at http://www.lsw.uni-heidelberg.de/~jpeitz/PV
Bending Instabilities in Magnetized Accretion Discs
We study the global bending modes of a thin annular disc subject to both an
internally generated magnetic field and a magnetic field due to a dipole
embedded in the central star with axis aligned with the disc rotation axis.
When there is a significant inner region of the disc corotating with the star,
we find spectra of unstable bending modes. These may lead to elevation of the
disc above the original symmetry plane facilitating accretion along the
magnetospheric field lines. The resulting non-axisymmetric disc configuration
may result in the creation of hot spots on the stellar surface and the periodic
photometric variations observed in many classical T Tauri stars (CTTS).
Time-dependent behaviour may occur including the shadowing of the central
source in magnetic accretors even when the dipole and rotation axes are
aligned.Comment: Accepted by MNRAS. 18 pages, 11 figures. LaTeX2e in the MN style.
PostScript and HTML files are also available from
http://www-star.qmw.ac.uk/~va/ or by e-mail: [email protected]
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