748 research outputs found
An Ontology for Submarine Feature Representation on Charts
A landform is a subjective individuation of a part of a terrain. Landform recognition is a difficult task because its definition usually relies on a qualitative and fuzzy description. Achieving automatic recognition of landforms requires a formal definition of the landforms properties and their modelling. In the maritime domain, the International Hydrographic Organisation published a standard terminology of undersea feature names which formalises a set of definition mainly for naming and communication purpose. This terminology is here used as a starting point for the definition of an ontology of undersea features and their automatic classification from a terrain model. First, an ontology of undersea features is built. The ontology is composed of an application domain ontology describing the main properties and relationships between features and a representation ontology deals with representation on a chart where features are portrayed by soundings and isobaths. A database model was generated from the ontology. Geometrical properties describing the feature shape are computed from soundings and isobaths and are used for feature classification. An example of automatic classification on a nautical chart is presented and results and on-going research are discussed
Iron K Lines from Gamma Ray Bursts
We present models for reprocessing of an intense flux of X-rays and gamma
rays expected in the vicinity of gamma ray burst sources. We consider the
transfer and reprocessing of the energetic photons into observable features in
the X-ray band, notably the K lines of iron. Our models are based on the
assumption that the gas is sufficiently dense to allow the microphysical
processes to be in a steady state, thus allowing efficient line emission with
modest reprocessing mass and elemental abundances ranging from solar to
moderately enriched. We show that the reprocessing is enhanced by
down-Comptonization of photons whose energy would otherwise be too high to
absorb on iron, and that pair production can have an effect on enhancing the
line production. Both "distant" reprocessors such as supernova or wind remnants
and "nearby" reprocessors such as outer stellar envelopes can reproduce the
observed line fluxes with Fe abundances 30-100 times above solar, depending on
the incidence angle. The high incidence angles required arise naturally only in
nearby models, which for plausible values can reach Fe line to continuum ratios
close to the reported values.Comment: 37 pages, 10 figures. Ap. J in pres
An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?
We report on a short XMM-Newton observation of the radio-quiet Narrow Line
Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong
excess of counts between 6-9 keV in the rest frame. This feature can be modeled
by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity
of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV
appears blue-shifted with respect to the iron Kalpha emission band between
6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar
rest frame. The line profile can be fitted by reflection from the inner
accretion disk, but an inclination angle of >60 deg is required to model the
extreme blue-wing of the line. Furthermore the extreme strength of the line
requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2
keV is dominated by the pure-reflection component from the disk, while little
or none of the direct hard power-law is observed. Alternatively the spectrum
above 2 keV may instead be explained by an ionized absorber, if the column
density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is
ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV.
This absorber could originate in a large column density, high velocity outflow,
perhaps similar to those which appear to be observed in several other high
accretion rate AGN. Further observations, especially at higher spectral
resolution, are required to distinguish between the accretion disk reflection
or outflow scenarios.Comment: Accepted for publication in ApJ (18 pages, 5 figures, 1 table
Can there be neutrino oscillation in Gamma-Ray Bursts fireball ?
The central engine which powers the Gamma-Ray Burst (GRB) fireball, produces
neutrinos in the energy range of about 5-20 MeV. Fractions of these neutrinos
may propagate through the fireball which is far away from the central engine.
We have studied the propagation of these neutrinos through the fireball which
is contaminated by baryons and have shown that, resonant conversion of
neutrinos are possible for the oscillations of nu_e to nu_{mu,tau}, nu_e to
nu_s and anti-nu_(mu,tou) to anti-nu_s if the neutrino mass square difference
and mixing angle are in the atmospheric and/or LSND range. On the other hand it
is probably difficult for neutrinos to have resonant oscillation if the
neutrino parameters are in the solar neutrino range. From the resonance
condition we have estimated the fireball temperature and the baryon load in it.Comment: 4 pages, two column text, To be published in Phys. Rev.
EXITE2 Observation of the SIGMA Source GRS 1227+025
We report the EXITE2 hard X-ray imaging of the sky around 3C273. A 2h
observation on May 8, 1997, shows a 260 mCrab source detected at
in each of two bands (50-70 and 70-93 keV) and located 30'
from 3C273 and consistent in position with the SIGMA source GRS1227+025. The
EXITE2 spectrum is consistent with a power law with photon index 3 and large
low energy absorption, as indicated by the GRANAT/SIGMA results. No source was
detected in more sensitive followup EXITE2 observations in 2000 and 2001 with
3 upper limits of 190 and 65 mCrab, respectively. Comparison with the
flux detected by SIGMA shows the source to be highly variable, suggesting it
may be non-thermal and beamed and thus the first example of a ``type 2''
(absorbed) Blazar. Alternatively it might be (an unprecedented) very highly
absorbed binary system undergoing accretion disk instability outbursts,
possibly either a magnetic CV, or a black hole X-ray nova.Comment: 12 pages, 4 figures, accepted for publication in Ap
Discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an historical low X-ray flux state
We report the discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an
extremely low X-ray state. A comparison of Swift observations obtained in May
and June/July 2007 with all previous X-ray observations between 1971 to 2006
show the AGN to have diminished in flux by a factor of more than 30, the lowest
X-ray flux Mkn 335 has ever been observed in. The Swift observations show an
extremely hard X-ray spectrum at energies above 2 keV. Possible interpretations
include partial covering absorption or X-ray reflection from the disk. In this
letter we consider the partial covering interpretation. The Swift observations
can be well fit by a strong partial covering absorber with varying absorption
column density N_H= 1-4 x 10^{23} cm-2 and a covering fraction f_c=0.9 - 1.
When corrected for intrinsic absorption, the X-ray flux of Mkn 335 varies by
only factors of 4-6. In the UV Mkn 335 shows variability in the order of 0.2
mag. We discuss the similarity of Mkn 335 with the highly variable NLS1
WPVS007, and speculate about a possible link between NLS1 galaxies and
broad-absorption line quasars.Comment: ApJ Letter accepted; 8 pages, 2 figures; The new version has three
more sentences in the introduction and three references added to the
discussio
The Formation of Broad Line Clouds in the Accretion Shocks of Active Galactic Nuclei
Recent work on the gas dynamics in the Galactic Center has improved our
understanding of the accretion processes in galactic nuclei, particularly with
regard to properties such as the specific angular momentum distribution,
density, and temperature of the inflowing plasma. This information can be
valuable in trying to determine the origin of the Broad Line Region (BLR) in
Active Galactic Nuclei (AGNs). In this paper, we explore various scenarios for
the cloud formation based on the underlying principle that the source of plasma
is ultimately that portion of the gas trapped by the central black hole from
the interstellar medium. Based on what we know about the Galactic Center, it is
likely that in highly dynamic environments such as this, the supply of matter
is due mostly to stellar winds from the central cluster. Winds accreting onto a
central black hole are subjected to several disturbances capable of producing
shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and
turbulence. Shocked gas is initially compressed and heated out of thermal
equilibrium with the ambient radiation field; a cooling instability sets in as
the gas is cooled via inverse-Compton and bremsstrahlung processes. If the
cooling time is less than the dynamical flow time through the shock region, the
gas may clump to form the clouds responsible for broad line emission seen in
many AGN spectra. Clouds produced by this process display the correct range of
densities and velocity fields seen in broad emission lines. Very importantly,
the cloud distribution agrees with the results of reverberation studies, in
which it is seen that the central line peak responds slower to continuum
changes than the line wings.Comment: 22 pages, 5 figure
Self-Organized Criticality in Compact Plasmas
Compact plasmas, that exist near black-hole candidates and in gamma ray burst
sources, commonly exhibit self-organized non-linear behavior. A model that
simulates the non-linear behavior of compact radiative plasmas is constructed
directly from the observed luminosity and variability. The simulation shows
that such plasmas self organize, and that the degree of non-linearity as well
as the slope of the power density spectrum increase with compactness. The
simulation is based on a cellular automaton table that includes the properties
of the hot (relativistic) plasmas, and the magnitude of the energy
perturbations. The plasmas cool or heat up, depending on whether they release
more or less than the energy of a single perturbation. The energy release
depends on the plasmas densities and temperatures, and the perturbations
energy. Strong perturbations may cool the previously heated plasma through
shocks and/or pair creation.
New observations of some active galactic nuclei and gamma ray bursters are
consistent with the simulationComment: 9 pages, 5 figures, AASTeX, Submitted to ApJ
The Correlation between X-ray spectral slope and FeKalpha line energy in radio-quiet active galactic nuclei
A significant correlation between FeKalpha line energy and X-ray spectral
slope has been discovered among radio-quiet active galactic nuclei. The
ionization stage of the bulk of the FeKalpha emitting material is not the same
in all active galactic nuclei and is related to the shape of the X-ray
continua. Active galactic nuclei with a steep X-ray spectrum tend to have a
fluorescence FeKalpha line from highly ionized material. In the narrow-line
Seyfert 1 galaxies with steeper X-ray spectrum (Gamma_X > 2.1), the FeKalpha
line originates from highly ionized material. In the Seyfert 1 galaxies and
quasars with flatter X-ray spectrum (Gamma_X < 2.1), bulk of the FeKalpha
emission arises from near neutral or weakly ionized material. The correlation
is an important observational characteristic related to the accretion process
in radio quiet active galactic nuclei and is driven by a fundamental physical
parameter which is likely to be the accretion rate relative to the Eddington
rate.Comment: 4 pages, To apear in ApJ Letter
New constraints on the continuum-emission mechanism of AGN: Intensive monitoring of NGC 7469 in the X-ray and ultraviolet
We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC
7469 in the X-ray with RXTE over a ~30d baseline. The source shows strong
variability with a root-mean-square (rms) amplitude of ~16 per cent, and
peak-to-peak variations of a factor of order 2. Simultaneous data over this
period were obtained in the ultraviolet (UV) using IUE, making this the most
intensive X-ray UV/X-ray variability campaign performed for any active galaxy.
Comparison of the continuum light curves reveals very similar amplitudes of
variability, but different variability characteristics, with the X-rays showing
much more rapid variations. The data are not strongly correlated at zero lag.
The largest absolute value of the correlation coefficient occurs for an
anticorrelation between the two bands, with the X-ray variations leading the UV
by ~4d. The largest positive correlation is for the ultraviolet to lead the
X-rays by ~4d. Neither option appears to be compatible with any simple
interband transfer function. The peak positive correlation at ~4d occurs
because the more prominent peaks in the UV light curve appear to lead those in
the X-rays by this amount. However, the minima of the light curves are
near-simultaneous. These observations provide new constraints on theoretical
models of the central regions of active galactic nuclei. Models in which the
observed UV emission is produced solely by re-radiation of absorber X-rays are
ruled out by our data, as are those in which the X-rays are produced solely by
Compton upscattering of the observed UV component by a constant distribution of
particles.Comment: 33 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journal. Also available via
http://lheawww.gsfc.nasa.gov/users/nandra/pubs/7469/abstract.htm
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