452 research outputs found
Induced automorphisms on irreducible symplectic manifolds
We introduce the notion of induced automorphisms in order to state a criterion to determine whether a given automorphism on a manifold of K3[n]-type is, in fact, induced by an automorphism of a K3 surface, and the manifold is a moduli space of stable objects on the K3. This criterion is applied to the classification of non-symplectic prime order automorphisms on manifolds of K3[2]-type, and we prove that almost all cases are covered. Variations of this notion and the above criterion are introduced and discussed for the other known deformation types of irreducible symplectic manifolds. Furthermore, we provide a description of the picard lattice of several irreducible symplectic manifolds having a lagrangian fibration
On the Baldwin Effect in Active Galactic Nuclei: I. The Continuum-Spectrum - Mass Relationship
We suggest that the Baldwin Effect is a result of the spectral dependence of
the line-driving ionizing continuum on the black hole mass. We derive a
relationship between the mass of the central black hole and the broad emission
line luminosity in active galactic nuclei (AGN). Assuming the UV spectrum of
AGN is emitted from an optically thick medium we find an expression for the
characteristic energy of the ``UV bump'' in terms of the observable luminosity
and emission-line width. We show empirically and analytically that the bump
energy is anti-correlated with the black-hole mass and with the continuum
luminosity. Our model reproduces the observed inverse correlation between
equivalent width and continuum luminosity, yielding an explanation of the
Baldwin effect from first principles. The model gives a good fit to the Baldwin
Effect of the CIV line for a mean quasar EUV spectrum (Zheng et al. 1997) and
for several model spectra. The model also predicts a correlation between the
strength of the Baldwin Effect (the slope of the equivalent width as a function
of luminosity) and the ionization potential, consistent with recent data.Comment: 19 pages Latex, 2 figures. Accepted for publication in the
Astrophysical Journa
An absorption event in the X-ray lightcurve of NGC 3227
We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing
Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed
an unusual hardening of the 2-10 keV X-ray spectrum which lasted several
months. The spectral hardening was not accompanied by any correlated variation
in flux above 8 keV. We therefore interpret the spectral change as transient
absorption by a gas cloud of column density 2.6 10^23 cm^-2 crossing the line
of sight to the X-ray source. A spectrum obtained by XMM-Newton during an early
phase of the hard-spectrum event confirms the obscuration model and shows that
the absorbing cloud is only weakly ionised. The XMM-Newton spectrum also shows
that ~10% of the X-ray flux is not obscured, but this unabsorbed component is
not significantly variable and may be scattered radiation from a large-scale
scattering medium. Applying the spectral constraints on cloud ionisation
parameter and assuming that the cloud follows a Keplerian orbit, we constrain
the location of the cloud to be R~10-100 light-days from the central X-ray
source, and its density to be n_H~10^8cm^-3, implying that we have witnessed
the eclipse of the X-ray source by a broad line region cloud.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter
Intention to Change Dietary Habits, and Weight Loss Among Norwegian-Pakistani Women Participating in a Culturally Adapted Intervention
The aim was to explore the relationships between degree of participation in a culturally adapted lifestyle intervention and stages of change for healthy eating and weight loss among Pakistani immigrant women in Norway. The intervention lasted 7 months and included 198 women, randomized into control and intervention groups. The odds of losing weight from baseline to follow-up, and being in action stages of change (compared to pre-action stages) with regard to intake of amount and type of fat, sugar and white flour at follow-up, increased significantly with number of group sessions attended. Those in action stage of reducing intake of fat and increasing intake of vegetables, as well as of reducing weight, were significantly more likely than others to have experienced weight loss at follow-up. Participation in the culturally adapted intervention was related to increase in intentions to change dietary behaviours and to weight loss
Cosmic ray production in supernova remnants including reacceleration: the secondary to primary ratio
We study the production of cosmic rays (CRs) in supernova remnants (SNRs),
including the reacceleration of background galactic cosmic rays (GCRs) - thus
refining the early considerations by Blandford & Ostriker (1980) and Wandel et
al. (1987) - and the effects of the nuclear spallation inside the sources (the
SNRs). This combines for the first time nuclear spallation inside CR sources
and in the diffuse interstellar medium, as well as reacceleration, with the
injection and subsequent acceleration of suprathermal particles from the
postshock thermal pool. Selfconsistent CR spectra are calculated on the basis
of the nonlinear kinetic model. It is shown that GCR reacceleration and CR
spallation produce a measurable effect at high energies, especially in the
secondary to primary (s/p) ratio, making its energy-dependence substantially
flatter than predicted by the standard model. Quantitatively, the effect
depends strongly upon the density of the surrounding circumstellar matter. GCR
reacceleration dominates secondary CR production for a low circumstellar
density. It increases the expected s/p ratio substantially and flattens its
spectrum to an almost energy-independent form for energies larger than 100
GeV/n if the supernovae explode on average into a hot dilute medium with
hydrogen number density cm. The contribution of CR spallation
inside SNRs to the s/p ratio increases with increasing circumstellar density
and becomes dominant for N_H\gsim 1 cm, leading at high energies to a
flat s/p ratio which is only by a factor of three lower than in the case of the
hot medium. Measurements of the boron to carbon ratio at energies above 100
GeV/n could be used in comparison with the values predicted here as a
consistency test for the supernova origin of the GCRs.Comment: 11 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
A non-hydrodynamical model for acceleration of line-driven winds in Active Galactic Nuclei
We present a study of the acceleration phase of line-driven winds in AGNs, in
order to examine the physical conditions for the existence of such winds for a
wide variety of initial conditions. We built a simple and fast non-hydrodynamic
model, QWIND, where we assume that a wind is launched from the accretion disc
at supersonic velocities of the order of a few 10^2 km/s and we concentrate on
the subsequent supersonic phase. We show that this model can produce a wind
with terminal velocities of the order of 10^4 km/s. There are three zones in
the wind, only the middle one of which can launch a wind: in the inner zone the
wind is too ionized and so experiences only the Compton radiation force which
is not effective in accelerating gas. This inner failed wind however plays an
important role in shielding the next zone, lowering the ionization parameter
there. In the middle zone the lower ionization of the gas leads to a much
larger radiation force and the gas achieves escape velocity This middle zone is
quite thin (about 100 gravitational radii). The outer, third, zone is shielded
from the UV radiation by the central wind zone and so does not achieve a high
enough acceleration to reach escape velocity. We also describe a simple
analytic approximation of our model, based on neglecting the effects of gravity
during the acceleration phase. This analytic approach is in agreement with the
results of the numerical code, and is a powerful way to check whether a
radiation driven wind can be accelerated with a given set of initial
parameters. Our analytical analysis and the fast QWIND model are in agreement
with more complex hydrodynamical models, and allow an exploration of the
dependence of the wind properties for a wide set of initial parameters: black
hole mass, Eddington ratio, initial density profile, X-ray to UV ratio.Comment: 15 pages, 9 figures. Accepted for publication in Astronomy &
Astrophysic
A possible bias on the estimate of Lbol/Ledd in AGN as a function of luminosity and redshift
The BH mass (and the related Eddington ratio) in broad line AGN is usually
evaluated by combining estimates (often indirect) of the BLR radius and of the
FWHM of the broad lines, under the assumption that the BLR clouds are in
Keplerian motion around the BH. Such an evaluation depends on the geometry of
the BLR. There are two major options for the BLR configuration: spherically
symmetric or ``flattened''. In the latter case the inclination to the line of
sight becomes a relevant parameter. This paper is devoted to evaluate the bias
on the estimate of the Eddington ratio when a spherical geometry is assumed
(more generally when inclination effects are ignored), while the actual
configuration is ``flattened'', as some evidence suggests. This is done as a
function of luminosity and redshift, on the basis of recent results which show
the existence of a correlation between the fraction of obscured AGN and these
two parameters up to at least z=2.5. The assumed BLR velocity field is akin to
the ``generalized thick disk'' proposed by Collin et al. (2006). Assuming an
isotropic orientation in the sky, the mean value of the bias is calculated as a
function of luminosity and redshift. It is demonstrated that, on average, the
Eddington ratio obtained assuming a spherical geometry is underestimated for
high luminosities, and overestimated for low luminosities. This bias converges
for all luminosities at z about 2.7, while nothing can be said on this bias at
larger redshifts due to the lack of data. The effects of the bias, averaged
over the luminosity function of broad line AGN, have been calculated. The
results imply that the bias associated with the a-sphericity of the BLR make
even worse the discrepancy between the observations and the predictions of
evolutionary models.Comment: 6 pages, 3 figures, accepted for publication in A&
Balmer Emission Line Profiles and the Complex Properties of Broad Line Regions in Active Galactic Nuclei
In this work we analyze a sample of AGN spectra, selected from the 6th Data
Release of the Sloan Digital Sky Survey, exploiting a generalized technique of
line profile analysis, designed to take into account the whole profiles of
their broad emission lines. We find that the line profile broadening functions
result from a complex structure, but we may be able to infer some constraints
about the role of the geometrical factor, thus improving our ability to
estimate AGN properties and their relation with the host galaxy. Our results
suggest that flattening and inclination within the structure of the Broad Line
Region (BLR) must be taken into account. We detect low inclinations of the BLR
motion plane with respect to our line of sight, typically i < 20 degrees, with
a geometrical effect which generally decreases as the line profile becomes
broader.Comment: 13 pages, 11 figures, updated reference list. ApJ accepte
The Intrinsically X-ray Weak Quasar PHL 1811. I. X-ray Observations and Spectral Energy Distribution
This is the first of two papers reporting observations and analysis of the
unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line
quasar PHL 1811, focusing on the X-ray properties and the spectral energy
distribution. Two Chandra observations reveal a weak X-ray source with a steep
spectrum. Variability by a factor of 4 between the two observations separated
by 12 days suggest that the X-rays are not scattered emission. The XMM-Newton
spectra are modelled in the 0.3--5 keV band by a steep power law with \Gamma =
2.3\pm 0.1, and the upper limit on intrinsic absorption is 8.7 x 10^{20}
cm^{-2}. The spectral slopes are consistent with power law indices commonly
observed in NLS1s, and it appears that we observe the central engine X-rays
directly. Including two recent Swift ToO snapshots, a factor of ~5 variability
was observed among the five X-ray observations reported here. In contrast, the
UV photometry obtained by the XMM-Newton OM and Swift UVOT, and the HST
spectrum reveal no significant UV variability. The \alpha_{ox} inferred from
the Chandra and contemporaneous HST spectrum is -2.3 \pm 0.1, significantly
steeper than observed from other quasars of the same optical luminosity. The
steep, canonical X-ray spectra, lack of absorption, and significant X-ray
variability lead us to conclude that PHL 1811 is intrinsically X-ray weak. We
also discuss an accretion disk model, and the host galaxy of PHL 1811.Comment: 45 pages, 9 figures, accepted for publication in Ap
Fractional Models of Cosmic Ray Acceleration in the Galaxy
Possible formulations of the problem of cosmic rays acceleration in the
interstellar galactic medium are considered with the use of fractional
differential equations. The applied technique has been physically justified. A
Fermi result has been generalized to the case of the acceleration of particles
in shock waves in the supernovae remnants fractally distributed in the Galaxy.Comment: 10 page
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