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