289 research outputs found
Computational investigation of structure, dynamics and nucleation kinetics of a family of modified Stillinger-Weber model fluids in bulk and free-standing thin films
In recent decades, computer simulations have found increasingly widespread
use as powerful tools of studying phase transitions in wide variety of systems.
In the particular and very important case of aqueous systems, the commonly used
force-fields tend to offer quite different predictions with respect to a wide
range of thermodynamic and kinetic properties, including the ease of ice
nucleation, the propensity to freeze at a vapor-liquid interface, and the
existence of a liquid-liquid phase transition. It is thus of fundamental and
practical interest to understand how different features of a given water model
affect its thermodynamic and kinetic properties. In this work, we use the
forward-flux sampling technique to study the crystallization kinetics of a
family of modified Stillinger-Weber (SW) potentials with energy ()
and length () scales taken from the monoatomic water (mW) model, but
with different tetrahedrality parameters (). By increasing
from 21 to 24, we observe the nucleation rate increases by 48 orders of
magnitude at a supercooling of . Using classical
nucleation theory, we are able to demonstrate that this change can largely be
accounted for by the increase in , the thermodynamic driving
force. We also perform rate calculations in freestanding thin films of the
supercooled liquid, and observe a crossover from a surface-enhanced
crystallization at to a bulk-dominated crystallization for
.Comment: 10 pages, 9 figures, five table
The rise of an ionized wind in the Narrow Line Seyfert 1 Galaxy Mrk 335 observed by XMM-Newton and HST
We present the discovery of an outflowing ionized wind in the Seyfert 1
Galaxy Mrk 335. Despite having been extensively observed by most of the largest
X-ray observatories in the last decade, this bright source was not known to
host warm absorber gas until recent XMM-Newton observations in combination with
a long-term Swift monitoring program have shown extreme flux and spectral
variability. High resolution spectra obtained by the XMM-Newton RGS detector
reveal that the wind consists of three distinct ionization components, all
outflowing at a velocity of 5000 km/s. This wind is clearly revealed when the
source is observed at an intermediate flux state (2-5e-12 ergs cm^-2 s^-1). The
analysis of multi-epoch RGS spectra allowed us to compare the absorber
properties at three very different flux states of the source. No correlation
between the warm absorber variability and the X-ray flux has been determined.
The two higher ionization components of the gas may be consistent with
photoionization equilibrium, but we can exclude this for the only ionization
component that is consistently present in all flux states (log(xi)~1.8). We
have included archival, non-simultaneous UV data from HST (FOS, STIS, COS) with
the aim of searching for any signature of absorption in this source that so far
was known for being absorption-free in the UV band. In the COS spectra obtained
a few months after the X-ray observations we found broad absorption in CIV
lines intrinsic to the AGN and blueshifted by a velocity roughly comparable to
the X-ray outflow. The global behavior of the gas in both bands can be
explained by variation of the covering factor and/or column density, possibly
due to transverse motion of absorbing clouds moving out of the line of sight at
Broad Line Region scale.Comment: 24 pages, 12 figures, ApJ accepte
The properties of the clumpy torus and BLR in the polar-scattered Seyfert 1 galaxy ESO 323-G77 through X-ray absorption variability
We report results from multi-epoch X-ray observations of the polar-scattered
Seyfert 1 galaxy ESO 323-G77. The source exhibits remarkable spectral
variability from months to years timescales. The observed spectral variability
is entirely due to variations of the column density of a neutral absorber
towards the intrinsic nuclear continuum. The column density is generally
Compton-thin ranging from a few times 10 cm to a few times
10 cm. However, one observation reveals a Compton-thick state
with column density of the order of 1.5 10 cm. The
observed variability offers a rare opportunity to study the properties of the
X-ray absorber(s) in an active galaxy. We identify variable X-ray absorption
from two different components, namely (i) a clumpy torus whose individual
clumps have a density of 1.7 10 cm and an average
column density of 4 10 cm, and (ii) the broad
line region (BLR), comprising individual clouds with density of 0.1-8
10 cm and column density of 10-10 cm. The
derived properties of the clumpy torus can also be used to estimate the torus
half-opening angle, which is of the order of 47 . We also confirm the
previously reported detection of two highly ionized warm absorbers with outflow
velocities of 1000-4000 km s. The observed outflow velocities are
consistent with the Keplerian/escape velocity at the BLR. Hence, the warm
absorbers may be tentatively identified with the warm/hot inter-cloud medium
which ensures that the BLR clouds are in pressure equilibrium with their
surroundings. The BLR line-emitting clouds may well be the cold, dense clumps
of this outflow, whose warm/hot phase is likely more homogeneous, as suggested
by the lack of strong variability of the warm absorber(s) properties during our
monitoring.Comment: 15 pages, 4 tables, and 9 figures. Accepted for publication in MNRA
High resolution X-ray spectroscopy of the Seyfert 1 Mrk841: insights into the warm absorber and warm emitter
The Seyfert 1 galaxy Mrk841 was observed five times between 2001 and 2005 by
the XMM-Newton X-ray observatory. The source is well known for showing spectral
complexity in the variable iron line and in the soft X-ray excess. This paper
reports on the first study of Mrk841 soft X-ray spectrum at high spectral
resolution. The availability of multiple exposures obtained by the Reflection
Grating Spectrometer (RGS) cameras allows a thorough study of the complex
absorption and emission spectral features in the soft X-ray band.The three
combined exposures obtained in January 2001 and the two obtained in January and
July 2005 were analysed using the SPEX software. We detect a two-phase warm
absorber: a medium ionisation component (logxi~1.5-2.2 ergs s cm^{-1}) is
responsible for a deep absorption feature in the Unresolved Transition Array of
the Fe M-shell and for several absorption lines in the OVI-VIII band; a higher
ionisation phase with logxi~3 ergs s cm^{-1} is required to fit absorption in
the NeIX-X band. The ionisation state and the column density of the gas present
moderate variation from 2001 to 2005 for both phases. The high ionisation
component of the warm absorber has no effect in the Fe K band. No significant
velocity shift of the absorption lines is measured in the RGS data. Remarkably,
the 2005 spectra show emission features consistent with photoionisation in a
high density (n_e>10^{11} cm^{-3}) gas: a prominent OVII line triplet is
clearly observed in January 2005 and narrow Radiative Recombination Continua
(RRC) of OVII and CVI are observed in both 2005 data sets. A broad Gaussian
line around 21.7 Angstrom is also required to fit all the data sets. The
derived radial distance for the emission lines seems to suggest that the
photoionisation takes place within the optical Broad Line Region of the source.Comment: In press on A&A, replaced version includes language editing and typo
on velocities corrected in Table
Unveiling the broad band X-ray continuum and iron line complex in Mkr 841
Mkr 841 is a bright Seyfert 1 galaxy known to harbor a strong soft excess and
a variable K iron line. It has been observed during 3 different periods
by XMM for a total cumulated exposure time of 108 ks. We present in this
paper a broad band spectral analysis of the complete EPIC-pn data sets. We were
able to test two different models for the soft excess, a relativistically
blurred photoionized reflection (\r model) and a relativistically smeared
ionized absorption (\a model). The continuum is modeled by a simple cut-off
power law and we also add a neutral reflection. These observations reveal the
extreme and puzzling spectral and temporal behaviors of the soft excess and
iron line. The 0.5-3 keV soft X-ray flux decreases by a factor 3 between 2001
and 2005 and the line shape appears to be a mixture of broad and narrow
components. We succeed in describing this complex broad-band 0.5-10 keV
spectral variability using either \r or \a to fit the soft excess. Both models
give statistically equivalent results even including simultaneous BeppoSAX data
up to 200 keV. Both models are consistent with the presence of remote
reflection characterized by a constant narrow component in the data. However
they differ in the presence of a broad line component present in \r but not
needed in \a. This study also reveals the sporadic presence of relativistically
redshifted narrow iron lines.Comment: Accepted in A&A. 17 pages and 21 figure
Extreme Warm Absorber variability in the Seyfert Galaxy Mrk 704
In about half of Seyfert galaxies, the X-ray emission is absorbed by an
optically thin, ionized medium, the so-called "Warm Absorber", whose origin and
location is still a matter of debate. The aims of this paper is to put more
constraints on the warm absorber by studying its variability. We analyzed the
X-ray spectra of a Seyfert 1 galaxy, Mrk 704, which was observed twice, three
years apart, by XMM-Newton. The spectra were well fitted with a two zones
absorber, possibly covering only partially the source. The parameters of the
absorbing matter - column density, ionization state, covering factor - changed
significantly between the two observations. Possible explanations for the more
ionized absorber are a torus wind (the source is a polar scattering one) or, in
the partial covering scenario, an accretion disk wind. The less ionized
absorber may be composed of orbiting clouds in the surroundings of the nucleus,
similarly to what already found in other sources, most notably NGC 1365.Comment: 10 pages. Accepted for publication in Astronomy & Astrophysic
X-ray narrow line region variability as a geometry probe: The case of NGC 5548
We study the long time scale variability of the gas responsible for the X-ray
narrow emission lines in the Seyfert 1 galaxy NGC 5548, in order to constrain
the location and geometry of the emitting gas. Using X-ray spectra taken with
the Chandra-LETGS and HETGS instruments and with XMM-Newton RGS and combining
them with long-term monitoring observations of the Rossi X-ray Timing Explorer
(RXTE), we perform a correlation analysis in order to try constrain the time
scale on which the narrow line emitting gas responds to variations of the
continuum flux. With the inclusion of the 2007 Chandra-LETGS observation we
have an additional observation at an historically low flux level. We conclude
that the NLR in NGC 5548 is in the form of an ionization cone, compact in size,
and located between 1 and 15 pc from the central source, depending on the exact
geometry of the NLR.Comment: 6 pages, 4 figures, accepted for publication in A&
The origin of the strong soft excess and puzzling iron line complex in Mkn 841
Mkn 841 has been observed during 3 different periods (January 2001, January
2005 and July 2005) by XMM-Newton for a total cumulated exposure time of ~108
ks. We present in this paper a broad band spectral analysis of the complete
EPIC-pn data sets. These observations confirm the presence of the strong soft
excess and complex iron line profile known to be present in this source since a
long time. They also reveal their extreme and puzzling spectral and temporal
behaviors. Indeed, the 0.5-2 keV soft X-ray flux decreases by a factor 3
between 2001 and 2005 and the line shape appears to be a mixed of broad and
narrow components, both variable but on differen timescales. The broad-band
0.5-10 keV spectra are well described by a model including a primary power law
continuum, a blurred photoionized reflection and a narrow iron line, the
blurred reflection fitting self-consistently the soft excess and the broad line
component. The origin and nature of the narrow component is unclear.Comment: 4 pages, 6 Figures. Accepted for publication in Astronomische
Nachrichten, proceedings of the ESAC Workshop "Variable and Broad Iron Lines
around Black Holes
The nature of the torus in the heavily obscured AGN Markarian 3: an X-ray study
In this paper we report the results of an X-ray monitoring campaign on the
heavily obscured Seyfert galaxy Markarian 3 carried out between the fall of
2014 and the spring of 2015 with NuSTAR, Suzaku and XMM-Newton. The hard X-ray
spectrum of Markarian 3 is variable on all the time scales probed by our
campaign, down to a few days. The observed continuum variability is due to an
intrinsically variable primary continuum seen in transmission through a large,
but still Compton-thin column density (N_H~0.8-1.110 cm).
If arranged in a spherical-toroidal geometry, the Compton scattering matter has
an opening angle ~66 degrees and is seen at a grazing angle through its upper
rim (inclination angle ~70 degrees). We report a possible occultation event
during the 2014 campaign. If the torus is constituted by a system of clouds
sharing the same column density, this event allows us to constrain their number
(175) and individual column density, [~(4.91.5)10
cm]. The comparison of IR and X-ray spectroscopic results with
state-of-the art "torus" models suggests that at least two thirds of the X-ray
obscuring gas volume might be located within the dust sublimation radius. We
report also the discovery of an ionized absorber, characterised by variable
resonant absorption lines due to He- and H-like iron. This discovery lends
support to the idea that moderate column density absorbers could be due to
clouds evaporated at the outer surface of the torus, possibly accelerated by
the radiation pressure due to the central AGN emission leaking through the
patchy absorber.Comment: Accepted for publication in MNRAS, 17 pages, 11 figures, 5 table
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