72 research outputs found
An absorption origin for the soft excess in Seyfert 1 active galactic nuclei
(abridged) The soft excess seen in the X-ray spectra of many high mass
accretion rate AGN can be well modelled by reflection from a partially ionised
accretion disc. However, the derived parameters are often extreme, both in
terms of the underlying spacetime and the reflection geometry, and these models
require that the disc is far from hydrostatic equilibrium. An alternative model
uses similarly partially ionised, velocity smeared material but seen in
absorption, though again the derived velocities are extreme, requiring magnetic
driving (in the jet?) rather than a simple line driven disc wind. We find that
while both models give comparably good fits to XMM--Newton data, we favour the
absorption model as, unlike reflection, all the derived spectral indices are
soft. This is as expected by analogy with the correspondingly high mass
accretion rate stellar mass black holes. Furthermore, these X-ray spectra are
consistent with a one--to--one mapping between AGN type and spectral state,
with NLS1's having softer spectra corresponding to the very high state, while
the broad line AGN have Gamma~2 as expected for the high/soft state. We also
use the simultaneous OM data to derive the ratio of disc to total accretion
power which is another tracer of spectral state in X-ray binaries. This does
not always show that the disc in NLS1's contributes less than 80 per cent of
the total power, as expected for a very high state. We suggest that this is an
artifact of the standard disc models used to determine the disc luminosity in
our fits. The disc seen in the very high state of black hole binaries is often
observed to be distorted from the standard shape, and a similar effect in
NLS1's could recover the correspondance between black hole binary spectral
state and AGN type.Comment: 12 pages, 12 figures, submitted to MNRA
Universal spectral shape of AGN with high accretion rate
The spectra of radio quiet and NLS1 galaxies show suprising similarity in
their shape. They seem to scale only with the accretion rate but not with
central black hole mass. We consider two mechanisms modifying the disk
spectrum. First, the outer parts of the disk are irradiated by the flux
emerging from the inner parts. This is due to the scattering of the flux by the
extended hot medium (warm absorber). Second process is connected with the
development of the disk warm Comptonizing skin above the disk and/or coronae.
Our scenario applies only to object with relatively high luminosity to the
Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 4 pages, 5 figures, 1 table, Proc. of the meeting: "The Restless
High-Energy Universe" (Amsterdam, The Netherlands
What can we learn about quasars from alpha_OX measurements in galactic black hole binaries?
We draw a comparison between AGN and Galactic black hole binaries using a
uniform description of spectral energy distribution of these two classes of
accreting X-ray sources. We parametrize spectra of GBHs with an alpha_GBH
parameter which we define as a slope of a nominal power law function between 3
and 20 keV. We show that this parameter can be treated as an equivalent of the
X-ray loudness, alpha_OX, used to describe AGN spectra. We do not find linear
correlation between the alpha_GBH and disc flux (similar to that between
alpha_OX and optical/UV luminosity found in AGN). Instead, we show that
alpha_GBH follows a well defined pattern during a GBH outburst. We find that
alpha_GBH tend to cluster around 1, 1.5 and 2, which correspond to a hard, very
high/intermediate and soft spectral state, respectively. We conclude that
majority of the observed Type 1 radio quiet AGN are in a spectral state
corresponding to a very high/intermediate state of GBHs. The same conclusion is
valid for radio loud AGN. We also study variations of the spectral slopes
(alpha_GBH and the X-ray photon index, Gamma) as a function of disc and
Comptonization fluxes. We discuss these dependencies in the context of
correlations of alpha_OX and Gamma with the optical/UV and X-ray 2 keV fluxes
considered for AGN and quasars.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Compton scattering as the explanation of the peculiar X-ray properties of Cyg X-3
We consider implications of a possible presence of a Thomson-thick,
low-temperature, plasma cloud surrounding the compact object in the binary
system Cyg X-3. The presence of such a cloud was earlier inferred from the
energy-independent orbital modulation of the X-ray flux and the lack of high
frequencies in its power spectra. Here, we study the effect of Compton
scattering by the cloud on the X-ray energy and power spectra, concentrating on
the hard spectral state. The process reduces the energy of the high-energy
break/cut-off in the energy spectra, which allows us to determine the Thomson
optical depth. This, together with the observed cut-off in the power spectrum,
determines the size of the plasma to be 2x10^9 cm. At this size, the cloud will
be in thermal equilibrium in the photon field of the X-ray source, which yields
the cloud temperature of 3 keV, which refines the determination of the Thomson
optical depth to 7. At these parameters, thermal bremsstrahlung emission of the
cloud becomes important as well. The physical origin of the cloud is likely to
be collision of the very strong stellar wind of the companion Wolf-Rayet star
with a small accretion disc formed by the wind accretion. Our model thus
explains the peculiar X-ray energy and power spectra of Cyg X-3.Comment: MNRAS, the version as printed, the title and abstract change
Is the soft excess in active galactic nuclei real?
We systematically analyse all publicly available XMM–Newton spectra of radio-quiet PG quasars. The soft X-ray excess in these objects is well modelled by an additional, cool, Compton scattering region. However, the remarkably constant temperature derived for this component over the whole sample requires a puzzling fine tuning of the parameters. Instead, we propose that the soft excess is an artifact of strong, relativistically smeared, partially ionized absorption. The strong jump in opacity at 0.7 keV from O vii, O viii and iron can lead to an apparent soft excess below this energy, which is trivially constant since it depends on atomic processes. This can have a dramatic effect on the derived spectrum, which has implications for fitting the relativistic smearing of the reflected iron line emission from the disc
Thermal proteome profiling of breast cancer cells reveals proteasomal activation by CDK4/6 inhibitor palbociclib
Palbociclib is a CDK4/6 inhibitor approved for metastatic estrogen receptor-positive breast cancer. In addition to G1 cell cycle arrest, palbociclib treatment results in cell senescence, a phenotype that is not readily explained by CDK4/6 inhibition. In order to identify a molecular mechanism responsible for palbociclib-induced senescence, we performed thermal proteome profiling of MCF7 breast cancer cells. In addition to affecting known CDK4/6 targets, palbociclib induces a thermal stabilization of the 20S proteasome, despite not directly binding to it. We further show that palbociclib treatment increases proteasome activity independently of the ubiquitin pathway. This leads to cellular senescence, which can be counteracted by proteasome inhibitors. Palbociclib-induced proteasome activation and senescence is mediated by reduced proteasomal association of ECM29. Loss of ECM29 activates the proteasome, blocks cell proliferation, and induces a senescence-like phenotype. Finally, we find that ECM29 mRNA levels are predictive of relapse-free survival in breast cancer patients treated with endocrine therapy. In conclusion, thermal proteome profiling identifies the proteasome and ECM29 protein as mediators of palbociclib activity in breast cancer cells
GRS 1915+105: the distance, radiative processes and energy-dependent variability
We present an exhaustive analysis of five broad-band observations of GRS
1915+105 in two variability states, chi and omega, observed simultaneously by
the PCA, HEXTE and OSSE. We find all the spectra well fitted by Comptonization
of disc blackbody photons, with very strong evidence for the presence of a
nonthermal electron component in the Comptonizing plasma. Both the energy and
the power spectra in the chi state are typical to the very high/intermediate
state of black-hole binaries. The spectrum of the omega state is characterized
by a strong blackbody component Comptonized by thermal electrons and a weak
nonthermal tail. We then calculate rms spectra (fractional variability as
functions of energy) for the PCA data. We accurately model the rms spectra by
coherent superposition of variability in the components implied by the spectral
fits, namely a less variable blackbody and more variable Comptonization. The
latter dominates at high energies, resulting in a flattening of the rms at high
energies in most of the data. This is also the case for the spectra of the QPOs
present in the chi state. Then, some of our data require a radial dependence of
the rms of the disc blackbody. We also study the distance to the source, and
find 11 kpc as the most likely value, contrary to a recent claim of a much
lower value.Comment: Accepted to MNRAS, landscape table in a separate fil
Patterns of energy-dependent variability from Comptonization
We study fractional variability as a function of energy from black-hole X-ray
binaries on timescales from milliseconds to hundreds of seconds. We build a
theoretical model of energy-dependent variability in which the X-ray energy
spectrum varies in response to a changing physical parameter. We compare these
models to rms spectra obtained from RXTE PCA observations of black-hole
binaries XTE J1550-564 and XTE J1650-500. We show that two main variability
models are consistent with the data: variable seed photon input in the hard
state and variable power in the Comptonized component in the soft and very high
states. The lack of clear reflection features in the rms spectra implies that
the reflection and the X-ray continuum, when integrated over Fourier
frequencies, are correlated and vary with similar fractional amplitudes. Our
models predict two important features of rms spectra, not possible to be
clearly seen by the PCA due sensitivity limits. At soft X-rays, >~3 keV, we
predict the presence of a break in the rms spectrum at energy directly related
to the seed photon temperature. At higher energies, ~20--30 keV, we predict a
peak in the rms spectrum originating from the variability of the spectrum
produced by a hybrid thermal/non-thermal electron distribution. If these
features are confirmed by broad-band observations, they will impose important
constraints on the origin of the seed photons for Comptonization and the
electron distribution in the hot plasma.Comment: 11 pages, 12 figures, revised version, accepted for publication in
MNRA
Physics of accretion in the millisecond pulsar XTE J1751-305
We have undertaken an extensive study of X-ray data from the accreting
millisecond pulsar XTE J1751-305 observed by RXTE and XMM-Newton during its
2002 outburst. In all aspects this source is similar to a prototypical
millisecond pulsar SAX J1808.4-3658, except for the higher peak luminosity of
13 per cent of Eddington, and the optical depth of the hard X-ray source larger
by factor ~2. Its broad-band X-ray spectrum can be modelled by three
components. We interpret the two soft components as thermal emission from a
colder (kT ~ 0.6 keV) accretion disc and a hotter (~1 keV) spot on the neutron
star surface. We interpret the hard component as thermal Comptonization in
plasma of temperature ~40 keV and optical depth of ~1.5 in a slab geometry. The
plasma is heated by the accretion shock as the material collimated by the
magnetic field impacts on to the surface. The seed photons for Comptonization
are provided by the hotspot, not by the disc. The Compton reflection is weak
and the disc is probably truncated into an optically thin flow above the
magnetospheric radius. Rotation of the emission region with the star creates an
almost sinusoidal pulse profile with rms amplitude of 3.3 per cent. The
energy-dependent soft phase lags can be modelled by two pulsating components
shifted in phase, which is naturally explained by a different character of
emission of the optically thick spot and optically thin shock combined with the
action of the Doppler boosting.Comment: Accepted for publication in MNRAS, 15 pages, 16 figures (references
corrected
How many biological replicates are needed in an RNA-seq experiment and which differential expression tool should you use?
An RNA-seq experiment with 48 biological replicates in each of 2 conditions
was performed to determine the number of biological replicates ()
required, and to identify the most effective statistical analysis tools for
identifying differential gene expression (DGE). When , seven of the nine
tools evaluated give true positive rates (TPR) of only 20 to 40 percent. For
high fold-change genes () the TPR is percent. Two
tools performed poorly; over- or under-predicting the number of differentially
expressed genes. Increasing replication gives a large increase in TPR when
considering all DE genes but only a small increase for high fold-change genes.
Achieving a TPR % across all fold-changes requires . For
future RNA-seq experiments these results suggest , rising to
when identifying DGE irrespective of fold-change is important. For
, superior TPR makes edgeR the leading tool tested. For , minimizing false positives is more important and DESeq outperforms the
other tools.Comment: 21 Pages and 4 Figures in main text. 9 Figures in Supplement attached
to PDF. Revision to correct a minor error in the abstrac
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