2,851 research outputs found
The absorption-dominated model for the X-ray spectra of type I active galaxies: MCG-6-30-15
MCG-6-30-15 is the archetypal example of a type I active galaxy showing broad
"red-wing" emission in its X-ray spectrum at energies below the 6.4 keV Fe
K-alpha emission line and a continuum excess above 20 keV. Miller et al. (2008)
showed that these spectral features could be caused by clumpy absorbing
material, but Reynolds et al. (2009) have argued that the observed Fe K-alpha
line luminosity is inconsistent with this explanation unless the global
covering factor of the absorber(s) is very low. However, the Reynolds et al.
calculation effectively considers the only source of opacity to be the Fe K
bound-free transition and neglects the opacity at the line energy: correction
to realistic opacity decreases the predicted line flux by a large factor. We
also discuss the interpretation of the covering factor and the possible effect
of occultation by the accretion disk. Finally, we consider a model for
MCG-6-30-15 dominated by clumpy absorption, which is consistent with global
covering factor 0.45, although models that include the effects of Compton
scattering are required to reach a full understanding. Variations in covering
fraction may dominate the observed X-ray spectral variability.Comment: Accepted for publication in MNRAS letter
An absorption origin for the X-ray spectral variability of MCG-6-30-15
The Seyfert I galaxy MCG-6-30-15 shows one of the best examples of a broad
"red wing" of emission in its X-ray spectrum at energies 2 < E < 6.4 keV,
commonly interpreted as being caused by relativistically-blurred reflection
close to the event horizon of the black hole. We aim to test an alternative
model in which absorption creates the observed spectral shape, explains the
puzzling lack of variability of the red wing and reduces the high reflection
albedo, substantially greater than unity, that is otherwise inferred at
energies E > 20 keV. We compiled all the available long-exposure, high-quality
data for MCG-6-30-15: 522 ks of Chandra HETGS, 282 ks of XMM-Newton pn/RGS and
253 ks of Suzaku XIS/PIN data. This is the first analysis of this full dataset.
We investigated the spectral variability on timescales >20 ks using principal
components analysis and fitted spectral models to "flux state" and mean spectra
over the energy range 0.5-45 keV (depending on detector). The absorber model
was based on the zones previously identified in the high-resolution grating
data. Joint fits were carried out to any data that were simultaneous. Multiple
absorbing zones covering a wide range of ionisation are required by the grating
data, including a highly ionised outflowing zone. A variable partial-covering
zone plus absorbed low-ionisation reflection, distant from the source, provides
a complete description of the variable X-ray spectrum. A single model fits all
the data. We conclude that these zones are responsible for the red wing, its
apparent lack of variability, the absorption structure around the Fe K-alpha
line, the soft-band "excess" and the high flux seen in the hard X-ray band. A
relativistically-blurred Fe line is not required in this model. We suggest the
partial covering zone is a clumpy wind from the accretion disk.Comment: Accepted for publication in Astronomy and Astrophysics. Corrected
versio
The variable X-ray spectrum of Markarian 766 - I. Principal components analysis
Aims: We analyse a long XMM-Newton spectrum of the narrow-line Seyfert 1
galaxy Mrk 766, using the marked spectral variability on timescales >20ks to
separate components in the X-ray spectrum. Methods: Principal components
analysis is used to identify distinct emission components in the X-ray
spectrum, possible alternative physical models for those components are then
compared statistically. Results: The source spectral variability is
well-explained by additive variations, with smaller extra contributions most
likely arising from variable absorption. The principal varying component,
eigenvector one, is found to have a steep (photon index 2.4) power-law shape,
affected by a low column of ionised absorption that leads to the appearance of
a soft excess. Eigenvector one varies by a factor 10 in amplitude on
time-scales of days and appears to have broad ionised Fe K-alpha emission
associated with it: the width of the ionised line is consistent with an origin
at about 100 gravitational radii. There is also a strong component of
near-constant emission that dominates in the low state, whose spectrum is
extremely hard above 1 keV, with a soft excess at lower energies, and with a
strong edge at Fe K but remarkably little Fe K-alpha emission. Although this
component may be explained as relativistically-blurred reflection from the
inner accretion disc, we suggest that its spectrum and lack of variability may
alternatively be explained as either (i) ionised reflection from an extended
region, possibly a disc wind, or (ii) a signature of absorption by a disc wind
with a variable covering fraction. Absorption features in the low state may
indicate the presence of an outflow.Comment: 14 pages, accepted for publication in Astronomy & Astrophysic
X-ray Signatures of an Ionized Reprocessor in the Seyfert galaxy Ton S 180
We discuss the hard X-ray properties of the Seyfert galaxy Ton S 180, based
upon the analysis of ASCA data. We find the X-ray flux varied by a factor ~2 on
a time scale of a few thousand seconds. The source showed significantly higher
amplitude of variability in the 0.5-2 keV band than in the 2-10 keV band. The
continuum is adequately parameterized as a Gamma ~ 2.5 power-law across the
0.6--10 keV band . We confirm the recent discovery of an emission line of high
equivalent width, due to Fe K-shell emission from highly-ionized material.
These ASCA data show the Fe line profile to be broad and asymmetric and
tentatively suggest it is stronger during the X-ray flares, consistent with an
origin from the inner parts of an accretion disk. The X-ray spectrum is complex
below 2 keV, possibly due to emission from a blend of soft X-ray lines, which
would support the existence of an ionized reprocessor, most likely due to a
relatively high accretion rate in this source.Comment: 24 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journa
Multi-dimensional modelling of X-ray spectra for AGN accretion-disk outflows III: application to a hydrodynamical simulation
We perform multi-dimensional radiative transfer simulations to compute
spectra for a hydrodynamical simulation of a line-driven accretion disk wind
from an active galactic nucleus. The synthetic spectra confirm expectations
from parameterized models that a disk wind can imprint a wide variety of
spectroscopic signatures including narrow absorption lines, broad emission
lines and a Compton hump. The formation of these features is complex with
contributions originating from many of the different structures present in the
hydrodynamical simulation. In particular, spectral features are shaped both by
gas in a successfully launched outflow and in complex flows where material is
lifted out of the disk plane but ultimately falls back. We also confirm that
the strong Fe Kalpha line can develop a weak, red-skewed line wing as a result
of Compton scattering in the outflow. In addition, we demonstrate that X-ray
radiation scattered and reprocessed in the flow has a pivotal part in both the
spectrum formation and determining the ionization conditions in the wind. We
find that scattered radiation is rather effective in ionizing gas which is
shielded from direct irradiation from the central source. This effect likely
makes the successful launching of a massive disk wind somewhat more challenging
and should be considered in future wind simulations.Comment: 14 pages, 8 figures. Accepted for publication by MNRA
X-ray reverberation in 1H0707-495 revisited
The narrow-line Seyfert 1 galaxy 1H0707-495 has previously been identified as
showing time lags between flux variations in the soft- (0.3-1 keV) and
medium-energy (1-4 keV) X-ray bands that oscillate between positive and
negative values as a function of the frequency of the mode of variation. Here
we measure and analyse the lags also between a harder X-ray band (4-7.5 keV)
and the soft and medium bands, using existing XMM-Newton data, and demonstrate
that the entire spectrum of lags, considering both the full energy range,
0.3-7.5 keV, and the full frequency range, 10^-5 < nu < 10^-2 Hz, are
inconsistent with previous claims of arising as reverberation associated with
the inner accretion disk. Instead we demonstrate that a simple reverberation
model, in which scattering or reflection is present in all X-ray bands,
explains the full set of lags without requiring any ad hoc explanation for the
time lag sign changes. The range of time delays required to explain the
observed lags extends up to about 1800 s in the hard band. The results are
consistent with reverberation caused by scattering of X-rays passing through an
absorbing medium whose opacity decreases with increasing energy and that
partially-covers the source. A high covering factor of absorbing and scattering
circumnuclear material is inferred.Comment: Accepted for publication in MNRA
Integrated assessment of oyster reef ecosystem services: Macrofauna utilization of restored oyster reefs
Within the Harris Creek Oyster Sanctuary in the Maryland portion of Chesapeake Bay, we evaluated relationships between basic oyster reef characteristics and the abundance and biomass of macrofauna. The eight sites selected for these studies included five restored oyster reef sites and three sites suitable for restoration that had not been restored. These sites encompassed a range of oyster biomass density and were spread throughout the sanctuary area. At each site one month prior to each of four sampling periods, divers filled four wire mesh baskets (0.1m2 surface area x 15 cm depth) with material from the site and embedded them so that the surface was flush with the surrounding substratum. In spring, early summer, late summer and fall of 2015, divers collected baskets and returned them to the laboratory where all macrofauna ≥1 mm were collected from each sample and their identity, abundance and biomass were determined. In addition to the abundance and biomass of oysters, we also assessed the amount of surface as the volume of live oysters along with that of any oyster shells whose surface was at least 50% oxic based on coloration (i.e. black shell was presumed to have been buried below the surface in anoxic conditions).
Positive relationships were identified for all three reef characteristics and the three major macrofaunal groups examined. In the majority of seasons, the relationship between both biomass and abundance of the hooked mussel, Ischadium recurvum, as a power function of oyster tissue biomass density, oyster abundance per square meter and surface shell volume. The relationship between oyster reef characteristics and the biomass and abundance of the mud crab, Eurypanopeus depressus, and of the naked goby, Gobiosoma bosc, were always positive but were more variable than that for I. recurvum. These data demonstrate that relationships can be found between oyster reef characteristics and macrofauna abundance and biomass. They further demonstrate that, in many cases, simple measures of reef characteristics such as oyster abundance and shell volume can provide predictions of macrofauna abundance and biomass that are comparable to more labor intensive measures such as oyster tissue biomass
Going places
Journeys. We all make them. Often they take us to exotic places. Sometimes they take us even further. They might take us through time. Or they might take us into a new way of life. There are times too, when we go all over the world and back again only to find that home is, after all, where it’s all happening.
This book contains stories about many different types of journey. We hope you will enjoy travelling into it and finding a world that suits you
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