1,825 research outputs found
Alignment Timescale of the Microquasar GRO J1655-40
The microquasar GRO J1655-40 has a black hole with spin angular momentum
apparently misaligned to the orbital plane of its companion star. We
analytically model the system with a steady state disc warped by Lense-Thirring
precession and find the timescale for the alignment of the black hole with the
binary orbit. We make detailed stellar evolution models so as to estimate the
accretion rate and the lifetime of the system in this state. The secondary can
be evolving at the end of the main sequence or across the Hertzsprung gap. The
mass-transfer rate is typically fifty times higher in the latter case but we
find that, in both cases, the lifetime of the mass transfer state is at most a
few times the alignment timescale. The fact that the black hole has not yet
aligned with the orbital plane is therefore consistent with either model. We
conclude that the system may or may not have been counter-aligned after its
supernova kick but that it is most likely to be close to alignment rather than
counteralignment now.Comment: Accepted for publication in MNRA
The alignment of disk and black hole spins in active galactic nuclei
The inner parts of an accretion disk around a spinning black hole are forced
to align with the spin of the hole by the Bardeen-Petterson effect. Assuming
that any jet produced by such a system is aligned with the angular momentum of
either the hole or the inner disk, this can, in principle provide a mechanism
for producing steady jets in AGN whose direction is independent of the angular
momentum of the accreted material. However, the torque which aligns the inner
disk with the hole, also, by Newton's third law, tends to align the spin of the
hole with the outer accretion disk. In this letter, we calculate this alignment
timescale for a black hole powering an AGN, and show that it is relatively
short. This timescale is typically much less than the derived ages for jets in
radio loud AGN, and implies that the jet directions are not in general
controlled by the spin of the black hole. We speculate that the jet directions
are most likely controlled either by the angular momentum of the accreted
material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter
Warped discs and the directional stability of jets in Active Galactic Nuclei
Warped accretion discs in Active Galactic Nuclei (AGN) exert a torque on the
black hole that tends to align the rotation axis with the angular momentum of
the outer disc. We compute the magnitude of this torque by solving numerically
for the steady state shape of the warped disc, and verify that the analytic
solution of Scheuer and Feiler (1996) provides an excellent approximation. We
generalise these results for discs with strong warps and arbitrary surface
density profiles, and calculate the timescale on which the black hole becomes
aligned with the angular momentum in the outer disc. For massive holes and
accretion rates of the order of the Eddington limit the alignment timescale is
always short (less than a Myr), so that jets accelerated from the inner disc
region provide a prompt tracer of the angular momentum of gas at large radii in
the disc. Longer timescales are predicted for low luminosity systems, depending
on the degree of anisotropy in the disc's hydrodynamic response to shear and
warp, and for the final decay of modest warps at large radii in the disc that
are potentially observable via VLBI. We discuss the implications of this for
the inferred accretion history of those Active Galactic Nuclei whose jet
directions appear to be stable over long timescales. The large energy
deposition rate at modest disc radii during rapid realignment episodes should
make such objects transiently bright at optical and infrared wavelengths.Comment: MNRAS, in press. Revised to match accepted version, with one new
figure showing alignment timescale as a function of black hole mas
Massive planet migration: Theoretical predictions and comparison with observations
We quantify the utility of large radial velocity surveys for constraining
theoretical models of Type II migration and protoplanetary disk physics. We
describe a theoretical model for the expected radial distribution of extrasolar
planets that combines an analytic description of migration with an empirically
calibrated disk model. The disk model includes viscous evolution and mass loss
via photoevaporation. Comparing the predicted distribution to a uniformly
selected subsample of planets from the Lick / Keck / AAT planet search
programs, we find that a simple model in which planets form in the outer disk
at a uniform rate, migrate inward according to a standard Type II prescription,
and become stranded when the gas disk is dispersed, is consistent with the
radial distribution of planets for orbital radii 0.1 AU < a < 2.5 AU and planet
masses greater than 1.65 Jupiter masses. Some variant models are disfavored by
existing data, but the significance is limited (~95%) due to the small sample
of planets suitable for statistical analysis. We show that the favored model
predicts that the planetary mass function should be almost independent of
orbital radius at distances where migration dominates the massive planet
population. We also study how the radial distribution of planets depends upon
the adopted disk model. We find that the distribution can constrain not only
changes in the power-law index of the disk viscosity, but also sharp jumps in
the efficiency of angular momentum transport that might occur at small radii.Comment: ApJ, in press. References updated to match published versio
Lense-Thirring precession of accretion disks around compact objects
Misaligned accretion disks surrounding rotating compact objects experience a
torque due to the Lense-Thirring effect, which leads to precession of the inner
disk. It has been suggested that this effect could be responsible for some low
frequency Quasi-Periodic Oscillations observed in the X-ray lightcurves of
neutron star and galactic black hole systems. We investigate this possibility
via time-dependent calculations of the response of the inner disk to impulsive
perturbations for both Newtonian point mass and Paczynski-Wiita potentials, and
compare the results to the predictions of the linearized twisted accretion disk
equations. For most of a wide range of disk models that we have considered, the
combination of differential precession and viscosity causes the warps to decay
extremely rapidly. Moreover, at least for relatively slowly rotating objects,
linear calculations in a Newtonian point mass potential provide a good measure
of the damping rate, provided only that the timescale for precession is much
shorter than the viscous time in the inner disk. The typically rapid decay
rates suggest that coherent precession of a fluid disk would not be observable,
though it remains possible that the damping rate of warp in the disk could be
low enough to permit weakly coherent signals from Lense-Thirring precession.Comment: ApJ, in press. Minor revisions to match accepted version. Animations
showing warp evolution are available at
http://www.cita.utoronto.ca/~armitage/lense_thirring.htm
Megamaser Disks in Active Galactic Nuclei
Recent spectroscopic and VLBI-imaging observations of bright extragalactic
water maser sources have revealed that the megamaser emission often originates
in thin circumnuclear disks near the centers of active galactic nuclei (AGNs).
Using general radiative and kinematic considerations and taking account of the
observed flux variability, we argue that the maser emission regions are clumpy,
a conclusion that is independent of the detailed mechanism (X-ray heating,
shocks, etc.) driving the collisionally pumped masers. We examine scenarios in
which the clumps represent discrete gas condensations (i.e., clouds) and do not
merely correspond to velocity irregularities in the disk. We show that even two
clouds that overlap within the velocity coherence length along the line of
sight could account (through self-amplification) for the entire maser flux of a
high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and
we suggest that cloud self-amplification likely contributes also to the flux of
the background-amplifying ``systemic'' features in these objects. Analogous
interpretations have previously been proposed for water maser sources in
Galactic star-forming regions. We argue that this picture provides a natural
explanation of the time-variability characteristics of extragalactic megamaser
sources and of their apparent association with Seyfert 2-like galaxies. We also
show that the requisite cloud space densities and internal densities are
consistent with the typical values of nuclear (broad emission-line region-type)
clouds.Comment: 55 pages, 7 figures, AASTeX4.0, to appear in The Astrophysical
Journal (1999 March 1 issue
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Constraining uncertainty in aerosol direct forcing
The uncertainty in present-day anthropogenic forcing is dominated by uncertainty in the strength of the contribution from aerosol. Much of the uncertainty in the direct aerosol forcing can be attributed to uncertainty in the anthropogenic fraction of aerosol in the present-day atmosphere, due to a lack of historical observations. Here we present a robust relationship between total present-day aerosol optical depth and the anthropogenic contribution across three multi-model ensembles and a large single-model perturbed parameter ensemble. Using observations of aerosol optical depth, we determine a reduced likely range of the anthropogenic component and hence a reduced uncertainty in the direct forcing of aerosol
A Note on Bimodal Accretion Disks
The existence of bimodal disks is investigated. Following a simple argument
based on energetic considerations we show that stationary, bimodal accretion
disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an
advection dominated accretion flow (ADAF) at smaller radii are never possible
using the standard slim disk approach, unless some extra energy flux is
present. The same argument, however, predicts the possibility of a transition
from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE
disk to a SSD. Both types of solutions have been found.Comment: 9 pages including 9 figures, accepted for publication in The
Astrophysical Journa
The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
We investigate the relationship between age and chromospheric activity for
139 M dwarf stars in wide binary systems with white dwarf companions. The age
of each system is determined from the cooling age of its white dwarf component.
The current limit for activity-age relations found for M dwarfs in open
clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for
the exploration of this relationship at ages older than 4 Gyr. The general
trend of stars remaining active for a longer time at later spectral type is
confirmed. However, our larger sample and greater age range reveals additional
complexity in assigning age based on activity alone. We find that M dwarfs in
wide binaries older than 4 Gyr depart from the log-linear relation for clusters
and are found to have activity at magnitudes, colors and masses which are
brighter, bluer and more massive than predicted by the cluster relation. In
addition to our activity-age results, we present the measured radial velocities
and complete space motions for 161 white dwarf stars in wide binaries.Comment: 22 pages including 9 figures and 5 tables. Accepted for publication
in The Astronomical Journa
Evidence for Frame-Dragging Around Spinning Black Holes in X-Ray Binaries
In the context of black hole spin in X-ray binaries, we propose that certain
type of quasi-period oscillations (QPOs) observed in the light curves of black
hole binaries (BHBs) are produced by X-ray modulation at the precession
frequency of accretion disks, due to relativistic dragging of inertial frames
around spinning black holes. These QPOs tend to be relatively stable in their
centroid frequencies. They have been observed in the frequency range of a few
to a few hundred Hz for several black holes with dynamically determined masses.
By comparing the computed disk precession frequency with that of the observed
QPO, we can derive the black hole angular momentum, given its mass. When
applying this model to GRO J1655-40, GRS 1915+105, Cyg X-1, and GS 1124-68, we
found that the black holes in GRO J1655-40 and GRS 1915+105, the only known
BHBs that occasionally produce superluminal radio jets, spin at a rate close to
the maximum limit, while Cyg X-1 and GS 1124-68, typical (persistent and
transient) BHBs, contain only moderately rotating ones. Extending the model to
the general population of black hole candidates, the fact that only
low-frequency QPOs have been detected is consistent with the presence of only
slowly spinning black holes in these systems. Our results are in good agreement
with those derived from spectral data, thus strongly support the classification
scheme that we proposed previously for BHBs.Comment: new title, minor revisions; change title to conform to ApJL rules;
replaced with the updated version to avoid confusio
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