2,540 research outputs found
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
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
Evaluation of a global aerosol microphysics model against size-resolved particle statistics in the marine atmosphere
A statistical synthesis of marine aerosol measurements from experiments in four different oceans is used to evaluate a global aerosol microphysics model (GLOMAP). We compare the model against observed size resolved particle concentrations, probability distributions, and the temporal persistence of different size particles. We attempt to explain the observed sub-micrometre size distributions in terms of sulfate and sea spray and quantify the possible contributions of anthropogenic sulfate and carbonaceous material to the number and mass distribution. The model predicts a bimodal size distribution that agrees well with observations as a grand average over all regions, but there are large regional differences. Notably, observed Aitken mode number concentrations are more than a factor 10 higher than in the model for the N Atlantic but a factor 7 lower than the model in the NW Pacific. We also find that modelled Aitken mode and accumulation mode geometric mean diameters are generally smaller in the model by 10–30%. Comparison with observed free tropospheric Aitken mode distributions suggests that the model underpredicts growth of these particles during descent to the marine boundary layer (MBL). Recent observations of a substantial organic component of free tropospheric aerosol could explain this discrepancy. We find that anthropogenic continental material makes a substantial contribution to N Atlantic MBL aerosol, with typically 60–90% of sulfate across the particle size range coming from anthropogenic sources, even if we analyse air that has spent an average of >120 h away from land. However, anthropogenic primary black carbon and organic carbon particles (at the emission size and quantity assumed here) do not explain the large discrepancies in Aitken mode number. Several explanations for the discrepancy are suggested. The lack of lower atmospheric particle formation in the model may explain low N Atlantic particle concentrations. However, the observed and modelled particle persistence at Cape Grim in the Southern Ocean, does not reveal a diurnal cycle consistent with a photochemically driven local particle source. We also show that a physically based cloud drop activation scheme better explains the observed change in accumulation mode geometric mean diameter with particle number
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
The technique of bladder implantation: further results and an assessment.
JuLL (1951) suggested that the surgical introduction of a pellet containing a test chemical into the lumen of the mouse bladder might be useful for routine testing for carcinogenic activity. The method, it was thought, would possess the following advantages: (i) the chemical would be slowly eluted from the pellet and would therefore remain in contact with the bladder epithelium for a prolonged period; (ii) the metabolic processes of the liver, etc., would be by-passed, and (iii) the bladder would function under approximately normal conditions. Bladder implantation has been used successfully in Leeds (Bonser, Clayso
Evolution of accretion disks around massive black holes: constraints from the demography of active galactic nuclei
Observations have shown that the Eddington ratios (the ratio of the
bolometric luminosity to the Eddington luminosity) in QSOs/active galactic
nuclei (AGNs) cover a wide range. In this paper we connect the demography of
AGNs obtained by the Sloan Digital Sky Survey with the accretion physics around
massive black holes and propose that the diversity in the Eddington ratios is a
natural result of the long-term evolution of accretion disks in AGNs. The
observed accretion rate distribution of AGNs (with host galaxy velocity
dispersion sigma~70-200 km/s) in the nearby universe (z<0.3) is consistent with
the predictions of simple theoretical models in which the accretion rates
evolve in a self-similar way. We also discuss the implications of the results
for the issues related to self-gravitating disks, coevolution of galaxies and
QSOs/AGNs, and the unification picture of AGNs.Comment: 18 pages, 2 figures; revised, main conclusions not changed; to appear
in ApJ, Oct., 200
Theoretical Modeling of the Thermal State of Accreting White Dwarfs Undergoing Classical Novae
White dwarfs experience a thermal renaissance when they receive mass from a
stellar companion in a binary. For accretion rates < 10^-8 Msun/yr, the freshly
accumulated hydrogen/helium envelope ignites in a thermally unstable manner
that results in a classical novae (CN) outburst and ejection of material. We
have undertaken a theoretical study of the impact of the accumulating envelope
on the thermal state of the underlying white dwarf (WD). This has allowed us to
find the equilibrium WD core temperatures (T_c), the classical nova ignition
masses (M_ign) and the thermal luminosities for WDs accreting at rates of
10^-11 - 10^-8 Msun/yr. These accretion rates are most appropriate to WDs in
cataclysmic variables (CVs) of P_orb <~ 7 hr, many of which accrete
sporadically as dwarf novae. We have included ^3He in the accreted material at
levels appropriate for CVs and find that it significantly modifies the CN
ignition mass. We compare our results with several others from the CN
literature and find that the inclusion of ^3He leads to lower M_ign for
>~ 10^-10 Msun/yr, and that for below this the particular author's
assumption concerning T_c, which we calculate consistently, is a determining
factor. Initial comparisons of our CN ignition masses with measured ejected
masses find reasonable agreement and point to ejection of material comparable
to that accreted.Comment: 14 pages, 11 figures; uses emulateapj; accepted by the Astrophysical
Journal; revised for clarity, added short discussion of diffusio
Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231
The VLBA and the VLA have been used to image the continuum radio emission
from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local
universe. The smallest scales reveal a double source less than 2 pc in extent.
The components of this central source have minimum brightness temperatures of
10^9 to 10^{10} K, spectral turnovers between 2 and 10 GHz, and appear to
define the galaxy nucleus plus the inner regions of a jet. The components may
be free-free absorbed or synchtrotron self-absorbed.
On larger scales, the images confirm a previously known north-south triple
source extending 40 pc and elongated perpendicular to a 350-pc starburst disk.
Both lobes show evidence for free-free absorption near 2 GHz, probably due to
ionized gas with a density of 1-2 X 10^3 cm^{-3} in the innermost parts of the
starburst disk. The absorbing gas may be ionized by the active nucleus or by
local regions of enhanced star formation. The elongation of the 40-pc triple
differs by 65 deg from that of the 2-pc source. The different symmetry axes on
different scales imply strong curvature in the inner part of the radio jet.
The radio continuum from the 350-pc disk has a spectral index near -0.4 above
1.4 GHz and is plausibly energized by a massive burst of star formation. On VLA
scales, asymmetric and diffuse emission extends for more than 25 kpc. This
emission has a steep spectrum, linear polarization exceeding 50% at some
locations, and shares the symmetry axis of the 40-pc triple. The diffuse radio
source is probably generated by energy deposition from a slow-moving nuclear
jet, which conceivably could help energize the off-nuclear starburst as well.Comment: 34 pages, 7 Postscript figures, LaTeX file in AASTeX format, accepted
in ApJ, Vol. 516, May 1, 199
Can Thin Disks Produce Anomalous X-Ray Pulsars?
We investigate whether young neutron stars with fall-back disks can produce
Anomalous X-Ray Pulsars (AXPs) within timescales indicated by the ages of
associated supernova remnants. The system passes through a propeller stage
before emerging as an AXP or a radio pulsar. The evolution of the disk is
described by a diffusion equation which has self-similar solutions with either
angular momentum or total mass of the disk conserved. We associate these two
types of solutions with accretor and propeller regimes, respectively. Our
numerical calculations of thin disk models with changing inner radius take into
account the super-critical accretion at the early stages, and electron
scattering and bound-free opacities with rich metal content. Our results show
that, assuming a fraction of the mass inflow is accreted onto the neutron star,
the fall-back disk scenario can produce AXPs for acceptable parameters.Comment: 16 pages, 4 Figures, to be published in Astrophysical Journal Vol.
599, Dec. 1
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