2,695 research outputs found
Clues to Quasar Broad Line Region Geometry and Kinematics
We present evidence that the high-velocity CIV lambda 1549 emission line gas
of radio-loud quasars may originate in a disk-like configuration, in close
proximity to the accretion disk often assumed to emit the low-ionization lines.
For a sample of 36 radio-loud z~2 quasars we find the 20--30% peak width to
show significant inverse correlations with the fractional radio core-flux
density, R, the radio axis inclination indicator. Highly inclined systems have
broader line wings, consistent with a high-velocity field perpendicular to the
radio axis. By contrast, the narrow line-core shows no such relation with R, so
the lowest velocity CIV-emitting gas has an inclination independent velocity
field. We propose that this low-velocity gas is located at higher
disk-altitudes than the high-velocity gas. A planar origin of the high-velocity
CIV-emission is consistent with the current results and with an accretion
disk-wind emitting the broad lines. A spherical distribution of randomly
orbiting broad-line clouds and a polar high-ionization outflow are ruled out.Comment: 5 Latex pages, 1 figure, accepted for publication in ApJ Letter
Thin Animals
Lattice animals provide a discretized model for the theta transition
displayed by branched polymers in solvent. Exact graph enumeration studies have
given some indications that the phase diagram of such lattice animals may
contain two collapsed phases as well as an extended phase. This has not been
confirmed by studies using other means. We use the exact correspondence between
the q --> 1 limit of an extended Potts model and lattice animals to investigate
the phase diagram of lattice animals on phi-cubed random graphs of arbitrary
topology (``thin'' random graphs). We find that only a two phase structure
exists -- there is no sign of a second collapsed phase.
The random graph model is solved in the thermodynamic limit by saddle point
methods. We observe that the ratio of these saddle point equations give
precisely the fixed points of the recursion relations that appear in the
solution of the model on the Bethe lattice by Henkel and Seno. This explains
the equality of non-universal quantities such as the critical lines for the
Bethe lattice and random graph ensembles.Comment: Latex, 10 pages plus 6 ps/eps figure
HST Observations of the Double-Peaked Emission Lines in the Seyfert Galaxy Markarian 78: Mass Outflows from a Single AGN
Previous ground based observations of the Seyfert 2 galaxy Mrk 78 revealed a
double set of emission lines, similar to those seen in several AGN from recent
surveys. Are the double lines due to two AGN with different radial velocities
in the same galaxy, or are they due to mass outflows from a single AGN?We
present a study of the outflowing ionized gas in the resolved narrow-line
region (NLR) of Mrk 78 using observations from Space Telescope Imaging
Spectrograph (STIS) and Faint Object Camera (FOC) aboard the Hubble Space
Telescope(HST) as part of an ongoing project to determine the kinematics and
geometries of active galactic nuclei (AGN) outflows. From the spectroscopic
information, we deter- mined the fundamental geometry of the outflow via our
kinematics modeling program by recreating radial velocities to fit those seen
in four different STIS slit positions. We determined that the double emission
lines seen in ground-based spectra are due to an asymmetric distribution of
outflowing gas in the NLR. By successfully fitting a model for a single AGN to
Mrk 78, we show that it is possible to explain double emission lines with
radial velocity offsets seen in AGN similar to Mrk 78 without requiring dual
supermassive black holes.Comment: 22 pages, 7 figures (2 color), accepted for publication in The
Astrophysical Journa
The Yang Lee Edge Singularity on Feynman Diagrams
We investigate the Yang-Lee edge singularity on non-planar random graphs,
which we consider as the Feynman Diagrams of various d=0 field theories, in
order to determine the value of the edge exponent.
We consider the hard dimer model on phi3 and phi4 random graphs to test the
universality of the exponent with respect to coordination number, and the Ising
model in an external field to test its temperature independence. The results
here for generic (``thin'') random graphs provide an interesting counterpoint
to the discussion by Staudacher of these models on planar random graphs.Comment: LaTeX, 6 pages + 3 figure
A Potts/Ising Correspondence on Thin Graphs
We note that it is possible to construct a bond vertex model that displays
q-state Potts criticality on an ensemble of phi3 random graphs of arbitrary
topology, which we denote as ``thin'' random graphs in contrast to the fat
graphs of the planar diagram expansion.
Since the four vertex model in question also serves to describe the critical
behaviour of the Ising model in field, the formulation reveals an isomorphism
between the Potts and Ising models on thin random graphs. On planar graphs a
similar correspondence is present only for q=1, the value associated with
percolation.Comment: 6 pages, 5 figure
Probing the Ionizing Continuum of Narrow-Line Seyfert 1 Galaxies. I.Observational Results
We present optical spectra and emission-line ratios of 12 Narrow-Line Seyfert
1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A
common feature in the EUV continuum of active galactic nuclei is the big blue
bump (BBB), generally associated with thermal accretion disk emission. While
Galactic absorption prevents direct access to the EUV range, it can be mapped
by measuring the strength of a variety of forbidden optical emission lines that
respond to different EUV continuum regions. We find that narrow emission-line
ratios involving [OII]3727, Hbeta, [OIII]5007, [OI]6300, Halpha,[NII]6583, and
[SII]6716,6731 indicate no significant difference between NLS1s and Broad-Line
Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy
distributions of their ionizing EUV - soft X-ray continua are similar. The
relative strength of important forbidden high ionization lines like [NeV]3426
compared to HeII4686 and the relative strength of [FeX]6374 appear to show the
same range as in BLS1 galaxies. However, a trend of weaker
F([OI]6300)/F(Halpha) emission-line ratios is indicated for NLS1s compared to
BLS1s. To recover the broad emission-line profiles we used Gaussian components.
This approach indicates that the broad Hbeta profile can be well described with
a broad component (FWHM = 3275 +- 800 km/s) and an intermediate broad component
(FWHM = 1200 +- 300 km/s). The width of the broad component is in the typical
range of normal BLS1s. The emission-line flux that is associated with the broad
component in these NLS1s amounts to at least 60% of the total flux. Thus it
dominates the total line flux, similar to BLS1 galaxies.Comment: 34 pages, 9 figures. accepted for publication in the
Astrophys.Journa
The origin of the Narrow Line Region of Mrk 3: an overpressured jet cocoon
We have obtained HST FOC long-slit optical spectroscopy of the Narrow Line
Region of the Seyfert 2 galaxy Mrk 3. In the region cospatial with the
radio-jet the velocity field is highly perturbed and shows two velocity systems
separated by as much as 1700 km/s. We interpret this to be the consequence of
the rapid expansion of a cocoon of hot gas, shocked and heated by the
radio-emitting outflow, which compresses and accelerates the ambient gas. The
NLR itself is essentially a cylindrical shell expanding supersonically. From
the size and velocity of the expanding region, we derive an upper limit to the
radio-source age, ~ 2
E42 erg/s required to inflate the cocoon and estimate that the jet minimum
advance speed is 3 E-3 pc per year. The total kinetic energy of the high
velocity NLR gas can be estimated as ~6 E54 erg, comparable to the total energy
carried by the jet over its lifetime and this quantitatively supports the idea
that the NLR gas is accelerated by the jet. If the advance speed of Mrk 3 is
representative of the Seyfert population then these sources must also be short
lived and probably recurrent. The jet kinetic luminosity of Mrk 3 is between 2
and 3 orders of magnitude smaller than that derived for radio-loud AGNs with
similar emission-line luminosity. On the other hand, the fraction of jet power
dissipated in radio-emission is similar. We speculate that the main distinction
between radio-quiet and radio-loud AGN is ascribed to a difference in jet power
rather than to a different efficiency in synchrotron emission production.Comment: 13 pages, 8 figures, Astrophysical Journal in pres
Multivariate Granger Causality and Generalized Variance
Granger causality analysis is a popular method for inference on directed
interactions in complex systems of many variables. A shortcoming of the
standard framework for Granger causality is that it only allows for examination
of interactions between single (univariate) variables within a system, perhaps
conditioned on other variables. However, interactions do not necessarily take
place between single variables, but may occur among groups, or "ensembles", of
variables. In this study we establish a principled framework for Granger
causality in the context of causal interactions among two or more multivariate
sets of variables. Building on Geweke's seminal 1982 work, we offer new
justifications for one particular form of multivariate Granger causality based
on the generalized variances of residual errors. Taken together, our results
support a comprehensive and theoretically consistent extension of Granger
causality to the multivariate case. Treated individually, they highlight
several specific advantages of the generalized variance measure, which we
illustrate using applications in neuroscience as an example. We further show
how the measure can be used to define "partial" Granger causality in the
multivariate context and we also motivate reformulations of "causal density"
and "Granger autonomy". Our results are directly applicable to experimental
data and promise to reveal new types of functional relations in complex
systems, neural and otherwise.Comment: added 1 reference, minor change to discussion, typos corrected; 28
pages, 3 figures, 1 table, LaTe
Optimal control of a linear system subject to partially specified input noise
One of the most basic problems in control theory is that of controlling a discrete-time linear system subject to uncertain noise with the objective of minimising the expectation of a quadratic cost. If one assumes the noise to be white, then solving this problem is relatively straightforward. However, white noise is arguably unrealistic: noise is not necessarily independent and one does not always precisely know its expectation. We first recall the optimal control policy without assuming independence, and show that in this case computing the optimal control inputs becomes infeasible. In a next step, we assume only knowledge of lower and upper bounds on the conditional expectation of the noise, and prove that this approach leads to tight lower and upper bounds on the optimal control inputs. The analytical expressions that determine these bounds are strikingly similar to the usual expressions for the case of white noise
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