17,794 research outputs found
The origin of blue-shifted absorption features in the X-ray spectrum of PG 1211+143: Outflow or disc?
In some radio-quiet active galaxies (AGN), high-energy absorption features in
the x-ray spectra have been interpreted as Ultrafast Outflows (UFOs) -- highly
ionised material (e.g. Fe XXV and Fe XXVI) ejected at mildly relativistic
velocities. In some cases, these outflows can carry energy in excess of the
binding energy of the host galaxy. Needless to say, these features demand our
attention as they are strong signatures of AGN feedback and will influence
galaxy evolution. For the same reason, alternative models need to be discussed
and refuted or confirmed. Gallo & Fabian proposed that some of these features
could arise from resonance absorption of the reflected spectrum in a layer of
ionised material located above and corotating with the accretion disc.
Therefore, the absorbing medium would be subjected to similar blurring effects
as seen in the disc. A priori, the existence of such plasma above the disc is
as plausible as a fast wind. In this work, we highlight the ambiguity by
demonstrating that the absorption model can describe the ~7.6 keV absorption
feature (and possibly other features) in the quasar PG 1211+143, an AGN that is
often described as a classic example of an UFO. In this model, the 2-10 keV
spectrum would be largely reflection dominated (as opposed to power law
dominated in the wind models) and the resonance absorption would be originating
in a layer between about 6 and 60 gravitational radii. The studies of such
features constitutes a cornerstone for future X-ray observatories like Astro-H
and Athena+. Should our model prove correct, or at least important in some
cases, then absorption will provide another diagnostic tool with which to probe
the inner accretion flow with future missions.Comment: 4 pages. Accepted for publication in MNRAS Letter
Probing the geometry and motion of AGN coronae through accretion disc emissivity profiles
To gain a better understanding of the inner disc region that comprises active
galactic nuclei it is necessary to understand the pattern in which the disc is
illuminated (the emissivity profile) by X-rays emitted from the continuum
source above the black hole (corona). The differences in the emissivity
profiles produced by various corona geometries are explored via general
relativistic ray tracing simulations. Through the analysis of various
parameters of the geometries simulated it is found that emissivity profiles
produced by point source and extended geometries such as cylindrical slabs and
spheroidal coronae placed on the accretion disc are distinguishable. Profiles
produced by point source and conical geometries are not significantly
different, requiring an analysis of reflection fraction to differentiate the
two geometries. Beamed point and beamed conical sources are also simulated in
an effort to model jet-like coronae, though the differences here are most
evident in the reflection fraction. For a point source we determine an
approximation for the measured reflection fraction with the source height and
velocity. Simulating spectra from the emissivity profiles produced by the
various geometries produce distinguishable differences. Overall spectral
differences between the geometries do not exceed 15 per cent in the most
extreme cases. It is found that emissivity profiles can be useful in
distinguishing point source and extended geometries given high quality spectral
data of extreme, bright sources over long exposure times. In combination with
reflection fraction, timing, and spectral analysis we may use emissivity
profiles to discern the geometry of the X-ray source.Comment: 15 pages, 12 figures. Accepted for publication in MNRA
Continuity properties of a factor of Markov chains
Starting from a Markov chain with a finite alphabet, we consider the chain
obtained when all but one symbol are undistinguishable for the practitioner. We
study necessary and sufficient conditions for this chain to have continuous
transition probabilities with respect to the past
A SspI PCR-RFLP detecting a silent allele at the goat CSN2 locus
The comparison between the cDNA sequence obtained
and the published sequences of the goat CSN2 alleles
showed a new single nucleotide polymorphism (SNP)
(transition C-T) at the 180th nucleotide of the ninth exon.
This mutation, which took place at 124 nt from the
polyadenylation site, identifies a silent allele at the CSN2
locus named CSN2 A1.
Since the 9th exon C-T transition creates a SspI
endonuclease restriction site, the SspI digestion
of a PCR product of 360 bp spanning the 9th exon and
flanking regions, would allow carriers for the presence of
thymine to be identified.
The allelic frequency of the CSN2 A1 allele, determined
in 170 goats belonging to an undefined genetic type reared
in the province of Naples (Italy), was 0.23
It has been observed that the sequences
in the 3’ untranslated regions (UTR), proximal to the
polyadenylation site, can affect the mechanism of mRNA
deadenylation and degradation. Therefore, it is reasonable
to hypothesize that the C-T transition might, directly or
indirectly, influence the stability of the mRNA and,
consequently, the amount of protein produced
A longer XMM-Newton look at I Zwicky 1: Distinct modes of X-ray spectral variability
The short-term spectral variability of the narrow-line Seyfert 1 galaxy I
Zwicky 1 (I Zw 1) as observed in an 85 ks XMM-Newton observation is discussed
in detail. I Zw 1 shows distinct modes of variability prior to and after a flux
dip in the broad-band light curve. Before the dip the variability can be
described as arising from changes in shape and normalisation of the spectral
components. Only changes in normalisation are manifested after the dip. The
change in the mode of behaviour occurs on dynamically short timescales in I Zw
1. The data suggest that the accretion-disc corona in I Zw 1 could have two
components that are co-existing. The first, a uniform, physically diffuse
plasma responsible for the "typical'' long-term (e.g. years) behaviour; and a
second compact, centrally located component causing the rapid flux and spectral
changes. This compact component could be the base of a short or aborted jet as
sometimes proposed for radio-quiet active galaxies. Modelling of the average
and time-resolved rms spectra demonstrate that a blurred Compton-reflection
model can describe the spectral variability if we allow for pivoting of the
continuum component prior to the dip.Comment: 8 pages, 9 figures. Accepted by MNRA
Governance mechanisms for effective leadership: The case of Spain.
Most recent work on Boards of Directors has been focused on what can be considered best practices for effective governance in terms of roles, composition, process and style. Furthermore, this literature can be divided among very practically oriented managerial work, and more rigorous, theoretically based work. Most of the empirical research in this area has been centered on Anglo-Saxon countries. Some European countries, like Spain, have a very different governance tradition. Given this context, our present work pursues with three complementary objectives: 1. To ascertain the current status of governance practices in Spain. 2. To obtain evidence about which of these practices may be associated with effective governance. 3. To find out what factors account for the composition of General Managers’ compensation.leadership; effective governance;
Caught in the act: Measuring the changes in the corona that cause the extreme variability of 1H 0707-495
The X-ray spectra of the narrow line Seyfert 1 galaxy, 1H 0707-495, obtained
with XMM-Newton, from time periods of varying X-ray luminosity are analysed in
the context of understanding the changes to the X-ray emitting corona that lead
to the extreme variability seen in the X-ray emission from active galactic
nuclei (AGN). The emissivity profile of the accretion disc, illuminated by the
X-ray emitting corona, along with previous measurements of reverberation time
lags are used to infer the spatial extent of the X-ray source. By fitting a
twice-broken power law emissivity profile to the relativistically-broadened
iron K fluorescence line, it is inferred that the X-ray emitting corona expands
radially, over the plane of the accretion disc, by 25 to 30 per cent as the
luminosity increases, contracting again as the luminosity decreases, while
increases in the measured reverberation lag as the luminosity increases would
require also variation in the vertical extent of the source above the disc. The
spectrum of the X-ray continuum is found to soften as the total X-ray
luminosity increases and we explore the variation in reflected flux as a
function of directly-observed continuum flux. These three observations combined
with simple, first-principles models constructed from ray tracing simulations
of extended coron self-consistently portray an expanding corona whose average
energy density decreases, but with a greater number of scattering particles as
the luminosity of this extreme object increases.Comment: 12 pages, 4 figures. Accepted for publication in MNRA
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