159 research outputs found
Disentangling jet and disc emission from the 2005 outburst of XTE J1118+480
The black hole X-ray transient, XTE J1118+480, has now twice been observed in
outburst - 2000 and 2005 - and on both occasions remained in the low/hard X-ray
spectral state. Here we present radio, infrared, optical, soft X-ray and hard
X-ray observations of the more recent outburst. We find that the lightcurves
have very different morphologies compared with the 2000 event and the optical
decay is delayed relative to the X-ray/radio. We attribute this lesser degree
of correlation to contributions of emission from multiple components, in
particular the jet and accretion disc. Whereas the jet seemed to dominate the
broadband spectrum in 2000, in 2005 the accretion disc seems to be more
prominent and we use an analysis of the lightcurves and spectra to distinguish
between the jet and disc emission. There also appears to be an optically thin
component to the radio emission in the 2005 data, possibly associated with
multiple ejection events and decaying as the outburst proceeds. These results
add to the discussion that the term "low/hard state'" covers a wider range of
properties than previously thought, if it is to account for XTE J1118+480
during these two outbursts.Comment: Accepted for publication in MNRA
Revisiting a fundamental test of the disc instability model for X-ray binaries
We revisit a core prediction of the disc instability model (DIM) applied to
X-ray binaries. The model predicts the existence of a critical mass transfer
rate, which depends on disc size, separating transient and persistent systems.
We therefore selected a sample of 52 persistent and transient neutron star and
black hole X-ray binaries and verified if observed persistent (transient)
systems do lie in the appropriate stable (unstable) region of parameter space
predicted by the model. We find that, despite the significant uncertainties
inherent to these kinds of studies, the data are in very good agreement with
the theoretical expectations. We then discuss some individual cases that do not
clearly fit into this main conclusion. Finally, we introduce the transientness
parameter as a measure of the activity of a source and show a clear trend of
the average outburst recurrence time to decrease with transientness in
agreement with the DIM predictions. We therefore conclude that, despite
difficulties in reproducing the complex details of the lightcurves, the DIM
succeeds to explain the global behaviour of X-ray binaries averaged over a long
enough period of time.Comment: 12 pages, 4 figures. Accepted for publication in MNRAS. Version 2:
some typos corrected and references adde
The quasar PG 0844+349 in an X-ray weak state
In March 2009 the well-studied quasar, PG 0844+349, was discovered with Swift
to be in an X-ray weak state. A follow-up XMM-Newton observation several weeks
later generated a good quality spectrum of the source, showing substantial
curvature and spectral hardening. In combination with archival data at two
previous epochs when the source was in a bright state, we examine the long-term
spectral and timing properties of PG 0844+349 spanning nearly ten years and a
factor of ten in brightness. Partial covering and blurred reflection models are
compared to the data at each flux state while attempting to maintain
consistency between the various epochs. In terms of the blurred reflection
model, PG 0844+349 is in a reflection dominated state during the 2009 X-ray
weak observations, which can be understood in terms of light bending. Moreover,
the light bending scenario can also account for the short-term (i.e. ~1000s)
spectral variability in the source. Other models cannot be decisively ruled
out, but we note distinguishing features of the models that can be explored for
in higher signal-to-noise data from current and future observatories.Comment: 11 pages. Accepted for publication in MNRA
Detectability of low energy X-ray spectral components in type 1 AGN
In this paper we examine the percentage of type 1 AGN which require the
inclusion of a soft excess component and/or significant cold absorption in the
modelling of their X-ray spectra obtained by XMM-Newton. We do this by
simulating spectra which mimic typical spectral shapes in order to find the
maximum detectability expected at different count levels. We then apply a
correction to the observed percentages found for the Scott et al. (2011) sample
of 761 sources. We estimate the true percentage of AGN with a soft excess
component to be 75+/-23%, suggesting that soft excesses are ubiquitous in the
X-ray spectra of type 1 AGN. By carrying out joint fits on groups of low count
spectra in narrow z bins in which additional spectral components were not
originally detected, we show that the soft excess feature is recovered with a
mean temperature kT and blackbody to power-law normalisation ratio consistent
with those of components detected in individual high count spectra. Cold
absorption with nH values broadly consistent with those reported in individual
spectra are also recovered. We suggest such intrinsic cold absorption is found
in a minimum of ~5% of type 1 AGN and may be present in up to ~10%.Comment: 10 pages, 6 figures, 4 tables. Accepted for publication in MNRA
Bird-like tracks from the imilchil formation (Middle jurassic, bajocian-bathonian) of the central high atlas, Morocco, in comparison with similar mesozoic tridactylous ichnotaxa
© 2017, Mucchi Editori s.r.l. All rights reserved. Small bird-like tracks have recently been discovered at three outcrops of the Imilchil Formation (Middle Jurassic, Bajocian-Bathonian) in the Central High Atlas of Morocco. The track-bearing strata are part of a marine-continental transitional succession, the studied surfaces being sandy marls and limestones of a brackish depositional environment. The footprints strongly resemble the ichnogenus Trisauropodiscus Ellenberger, 1970, from the Lower Elliot Formation (latest Triassic) of Lesotho, southern Africa and are assigned to Trisauropodiscus isp. These are functionally tridactyl, widely divaricated pes tracks with digit III being longest and a trace of the reverted digit I (hallux) being occasionally imprinted. In contrast to some former studies suggesting Trisauropodiscus as a junior synonym and extramorphological variation of the ornithischian ichnogenus Anomoepus, this ichnotaxon is considered here as a distinctive morphotype among similar theropod tracks found in Jurassic-Cretaceous ichnoassemblages. An amended diagnosis is proposed focusing on the features that are here discussed and considered as key characters of this ichnotaxon. An avian interpretation of the trackmaker is problematical, especially against the background of the stratigraphic range of Trisauropodiscus back to the Late Triassic. Presently, theropods with very bird-like feet are the more likely producers. Future analyses and comparison of Trisauropodiscus with pes skeletons of avian and non-avian theropods might enlighten this
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
Absorption of high-energy gamma rays in Cygnus X-3
The microquasar Cygnus X-3 was detected at high energies by the gamma-ray
space telescopes AGILE and Fermi. The gamma-ray emission is transient,
modulated with the orbital period and seems related to major radio flares, i.e.
to the relativistic jet. The GeV gamma-ray flux can be substantially attenuated
by internal absorption with the ambient X-rays. In this study, we examine
quantitatively the effect of pair production in Cygnus X-3 and put constraints
on the location of the gamma-ray source. Cygnus X-3 exhibits complex temporal
and spectral patterns in X-rays. During gamma-ray flares, the X-ray emission
can be approximated by a bright disk black body component and a non-thermal
tail extending in hard X-rays, possibly related to a corona above the disk. We
calculate numerically the exact optical depth for gamma rays above a standard
accretion disk. Emission and absorption in the corona are also investigated.
GeV gamma rays are significantly absorbed by soft X-rays emitted from the inner
parts of the accretion disk. The absorption pattern is complex and anisotropic.
Isotropization of X-rays due to Thomson scattering in the companion star wind
tends to increase the gamma-ray opacity. Gamma rays from the corona suffer from
strong absorption by photons from the disk and cannot explain the observed
high-energy emission, unless the corona is unrealistically extended. The lack
of absorption feature in the GeV emission indicates that high-energy gamma rays
should be located at a minimum distance ~10^8-10^10 cm from the compact object.
The gamma-ray emission is unlikely to have a coronal origin.Comment: 11 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Spectro-timing analysis of Cygnus X-1 during a fast state transition
We present the analysis of two long, quasi-uninterrupted RXTE observations of
Cygnus X-1 that span several days within a 10 d interval. The spectral
characteristics during this observation cover the region where previous
observations have shown the source to be most dynamic. Despite that the source
behavior on time scales of hours and days is remarkably similar to that on year
time scales. This includes a variety of spectral/temporal correlations that
previously had only been observed over Cyg X-1's long-term evolution.
Furthermore, we observe a full transition from a hard to a soft spectral state
that occurs within less than 2.5 hours - shorter than previously reported for
any other similar Cyg X-1 transition. We describe the spectra with a
phenomenological model dominated by a broken power law, and we fit the X-ray
variability power spectra with a combination of a cutoff power law and
Lorentzian components. The spectral and timing properties are correlated: the
power spectrum Lorentzian components have an energy-dependent amplitude, and
their peak frequencies increase with photon spectral index. Averaged over
3.2-10 Hz, the time lag between the variability in the 4.5-5.7 keV and 9.5-15
keV bands increases with decreasing hardness when the variability is dominated
by the Lorentzian components during the hard state. The lag is small when there
is a large power law noise contribution, shortly after the transition to the
soft state. Interestingly, the soft state not only shows the shortest lags, but
also the longest lags when the spectrum is at its softest and faintest. We
discuss our results in terms of emission models for black hole binaries.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
Theropod courtship: large scale physical evidence of display arenas and avian-like scrape ceremony behaviour by Cretaceous dinosaurs
Relationships between non-avian theropod dinosaurs and extant and fossil birds are a major focus of current paleobiological research. Despite extensive phylogenetic and morphological support, behavioural evidence is mostly ambiguous and does not usually fossilize. Thus, inferences that dinosaurs, especially theropods displayed behaviour analogous to modern birds are intriguing but speculative. Here we present extensive and geographically widespread physical evidence of substrate scraping behavior by large theropods considered as compelling evidence of "display arenas" or leks, and consistent with "nest scrape display" behaviour among many extant ground-nesting birds. Large scrapes, up to 2 m in diameter, occur abundantly at several Cretaceous sites in Colorado. They constitute a previously unknown category of large dinosaurian trace fossil, inferred to fill gaps in our understanding of early phases in the breeding cycle of theropods. The trace makers were probably lekking species that were seasonally active at large display arena sites. Such scrapes indicate stereotypical avian behaviour hitherto unknown among Cretaceous theropods, and most likely associated with terrirorial activity in the breeding season. The scrapes most probably occur near nesting colonies, as yet unknown or no longer preserved in the immediate study areas. Thus, they provide clues to paleoenvironments where such nesting sites occurred
Detection of blueshifted emission and absorption and a relativistic Iron line in the X-ray spectrum of ESO 323-G077
We report on the X-ray observation of the Seyfert 1 ESO323-G077 performed
with XMM-Newton. The spectra show a complex spectrum with conspicuous
absorption and emission features. The continuum emission can be modelled with a
power law with an index of 1.99+/-0.02 in the whole XMM-Newton energy band,
marginally consistent with typical values of Type-I objects. An absorption
component with an uncommonly high equivalent Hydrogen column,
n_H=5.82(+0.12/-0.11)x10^22 cm-2, is affecting the soft part of the spectrum.
Additionally, two warm absorption components are also present. The lower
ionised one has an ionisation parameter of Log(U)=2.14(+0.06/-0.07) and an
outflowing velocity of v=3200(+600/-200) km/s. Two absorption lines located at
~6.7 and ~7.0 keV can be modelled with the highly ionised absorber. The
ionisation parameter and outflowing velocity of the gas measured are
Log(U)=3.26(+0.19/-0.15) and v=1700(+600/-400) km/s, respectively. Four
emission lines were also detected in the soft energy band. The most likely
explanation for these emission lines is that they are associated with an
outflowing gas with a velocity of ~2000 km/s. The data suggest that the same
gas which is causing the absorption could also being responsible of these
emission features. Finally, the spectrum shows the presence of a relativistic
iron emission line likely originated in the accretion disc of a Kerr BH with an
inclination of ~25 deg. We propose a model to explain the observed X-ray
properties which invokes the presence of a two-phase outflow with cone-like
structure and a velocity of the order of 2,000-4,000 km/s. The inner layer of
the cone would be less ionised, or even neutral, than the outer layer. The
inclination angle would be lower than the opening angle of the outflowing cone.Comment: 11 pages, accepted in MNRA
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