201 research outputs found
Fuelling Active Galactic Nuclei
We suggest that most nearby active galactic nuclei are fed by a series of
small--scale, randomly--oriented accretion events. Outside a certain radius
these events promote rapid star formation, while within it they fuel the
supermassive black hole. We show that the events have a characteristic time
evolution. This picture agrees with several observational facts. The expected
luminosity function is broadly in agreement with that observed for
moderate--mass black holes. The spin of the black hole is low, and aligns with
the inner disc in each individual feeding event. This implies radio jets
aligned with the axis of the obscuring torus, and uncorrelated with the
large--scale structure of the host galaxy. The ring of young stars observed
about the Galactic Centre are close to where our picture predicts that star
formation should occur.Comment: MNRAS, in pres
Retrograde Accretion and Merging Supermassive Black Holes
We investigate whether a circumbinary gas disc can coalesce a supermassive
black hole binary system in the centre of a galaxy. This is known to be
problematic for a prograde disc. We show that in contrast, interaction with a
retrograde circumbinary disc is considerably more effective in shrinking the
binary because there are no orbital resonances. The binary directly absorbs
negative angular momentum from the circumbinary disc by capturing gas into a
disc around the secondary black hole, or discs around both holes if the binary
mass ratio is close to unity. In many cases the binary orbit becomes eccentric,
shortening the pericentre distance as the eccentricity grows. In all cases the
binary coalesces once it has absorbed the angular momentum of a gas mass
comparable to that of the secondary black hole. Importantly, this conclusion is
unaffected even if the gas inflow rate through the disc is formally
super--Eddington for either hole. The coalescence timescale is therefore always
, where is the secondary black hole mass and
the inflow rate through the circumbinary disc.Comment: 8 pages, 4 figures. Accepted for publication in MNRAS. Movies of the
simulations can be found at:
http://www.astro.le.ac.uk/users/cjn12/RetroBinaryMovies.htm
Stellar disruption by a supermassive black hole: is the light curve really proportional to ?
In this paper we revisit the arguments for the basis of the time evolution of
the flares expected to arise when a star is disrupted by a supermassive black
hole. We present a simple analytic model relating the lightcurve to the
internal density structure of the star. We thus show that the standard
lightcurve proportional to only holds at late times. Close to the
peak luminosity the lightcurve is shallower, deviating more strongly from
for more centrally concentrated (e.g. solar--type) stars. We test
our model numerically by simulating the tidal disruption of several stellar
models, described by simple polytropic spheres with index . The
simulations agree with the analytical model given two considerations. First,
the stars are somewhat inflated on reaching pericentre because of the effective
reduction of gravity in the tidal field of the black hole. This is well
described by a homologous expansion by a factor which becomes smaller as the
polytropic index becomes larger. Second, for large polytropic indices wings
appear in the tails of the energy distribution, indicating that some material
is pushed further away from parabolic orbits by shocks in the tidal tails. In
all our simulations, the lightcurve is achieved only at late stages.
In particular we predict that for solar type stars, this happens only after the
luminosity has dropped by at least two magnitudes from the peak. We discuss our
results in the light of recent observations of flares in otherwise quiescent
galaxies and note the dependence of these results on further parameters, such
as the star/hole mass ratio and the stellar orbit.Comment: 10 pages, 10 figures, MNRAS accepte
The Nature of SS433 and the Ultraluminous X-ray Sources
The periodic precession (162--day) and nodding (6.3--day) motions of the jets
in SS433 are driven in the outer regions of the disc, whereas the jets
themselves, being relativistic, are launched near the black hole at the disc
centre. Given that the nutation period is comparable to the dynamical timescale
in the outer regions of the disc, it seems unlikely that these periods can be
communicated efficiently to the disc centre. Here we propose that the massive
outflow observed in SS433 is launched at large radii in the disc, about 1/10 of
the outer disc edge, and that it is this outflow which responds to the
oscillations of the outer disc and determines the direction of the jets. The
massive outflow is launched at large radius because the mass transfer rate is
hyper-Eddington. This implies not only that the total luminosity of SS433
exceeds by a considerable factor, but also that the radiative output is
collimated along the outflow. We thus suggest that SS433 is an ultraluminous
X-ray source (ULX) viewed `from the side'. We also suggest that the obscured
{\it INTEGRAL} sources may be SS433--like objects, but with slightly lower mass
transfer rates.Comment: MNRAS, in press. 7 pages, no figures. Comment on Kaaret et al (2006)
corrected, reference to Katz (1986) adde
RS Ophiuchi: Thermonuclear Explosion or Disc Instability?
Sokoloski et al (2008) have recently reported evidence that the recurrent
nova RS Ophiuchi produced a pair of highly collimated radio jets within days of
its 2006 outburst. This suggests that an accretion disc must be present during
the outburst. However in the standard picture of recurrent novae as
thermonuclear events, any such disc must be expelled from the white dwarf
vicinity, as the nuclear energy yield greatly exceeds its binding energy. We
suggest instead that the outbursts of RS Oph are thermal--viscous instabilities
in a disc irradiated by the central accreting white dwarf. The distinctive
feature of RS Oph is the very large size of its accretion disc. Given this, it
fits naturally into a consistent picture of systems with unstable accretion
discs. This picture explains the presence and speed of the jets, the brightness
and duration of the outburst, and its rise time and linear decay, as well as
the faintness of the quiescence. By contrast, the hitherto standard picture of
recurrent thermonuclear explosions has a number of severe difficulties. These
include the presence of jets, the faintness of quiescence, and the fact the the
accretion disc must be unstable unless it is far smaller than any reasonable
estimate.Comment: MNRAS, in pres
Aligning spinning black holes and accretion discs
We consider the alignment torque between a spinning black hole and an
accretion disc whose angular momenta are misaligned. This situation must hold
initially in almost all gas accretion events on to supermassive black holes,
and may occur in binaries where the black hole receives a natal supernova kick.
We show that the torque always acts to align the hole's spin with the total
angular momentum without changing its magnitude. The torque acts dissipatively
on the disc, reducing its angular momentum, and aligning it with the hole if
and only if the angle theta between the angular momenta J_d of the disc and J_h
of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this
condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc
counteraligns.Comment: MNRAS, in pres
Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution
Magnetic connections to the plunging region can exert stresses on the inner
edge of an accretion disk around a black hole. We recompute the relativistic
corrections to the thin-disk dynamics equations when these stresses take the
form of a time-steady torque on the inner edge of the disk. The additional
dissipation associated with these stresses is concentrated relatively close
outside the marginally stable orbit, scaling as r to the -7/2 at large radius.
As a result of these additional stresses: spin-up of the central black hole is
retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs
at a/M=0.94; the disk spectrum is extended toward higher frequencies; line
profiles (such as Fe K-alpha) are broadened if the line emissivity scales with
local flux; limb-brightening, especially at the higher frequencies, is
enhanced; and the returning radiation fraction is substantially increased, up
to 58%. This last effect creates possible explanations for both synchronized
continuum fluctuations in AGN, and polarization rises shortward of the Lyman
edge in quasars. We show that no matter what additional stresses occur, when
a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound
on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Disc instability in RS Ophiuchi: a path to Type Ia supernovae?
We study the stability of disc accretion in the recurrent nova RS Ophiuchi.
We construct a one-dimensional time-dependent model of the binary-disc system,
which includes viscous heating and radiative cooling and a self-consistent
treatment of the binary potential. We find that the extended accretion disc in
this system is always unstable to the thermal-viscous instability, and
undergoes repeated disc outbursts on ~10-20yr time-scales. This is similar to
the recurrence time-scale of observed outbursts in the RS Oph system, but we
show that the disc's accretion luminosity during outburst is insufficient to
explain the observed outbursts. We explore a range of models, and find that in
most cases the accretion rate during outbursts reaches or exceeds the critical
accretion rate for stable nuclear burning on the white dwarf surface.
Consequently we suggest that a surface nuclear burning triggered by disc
instability may be responsible for the observed outbursts. This allows the
white dwarf mass to grow over time, and we suggest that disc instability in RS
Oph and similar systems may represent a path to Type Ia supernovae.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
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