824 research outputs found
Bench-to-bedside review: Erythropoietin and its derivatives as therapies in critical care
Author can archive publisher's pdf. Free via Creative Commons: CC-BENCHTOBEDSIDE-2.0. © 2012 BioMed Central Ltd
ASCA and ROSAT observations of NGC5548: discrepant spectral indices
We report on simultaneous ASCA and ROSAT observations of the Seyfert galaxy
NGC5548 made during the ASCA Performance Verification phase. Spectral features
due to a warm absorber and reflection are clearly seen in the X-ray spectra. We
find that the continuum spectral shape differs between the ASCA and ROSAT
datasets. The photon-index obtained from the ROSAT PSPC exceeds that from the
ASCA SIS about 0.4. The discrepancy is clear even in the 0.5-2 keV energy band
over which both detectors are sensitive. The spectra cannot be made consistent
by choosing a more complex model. The problem likely lies in the response curve
(estimated effective area) of one, or both, detectors. There may be important
consequences for a wide range of published results.Comment: 9 pages, accepted for publication in MNRA
NEARBY Platform: Algorithm for Automated Asteroids Detection in Astronomical Images
In the past two decades an increasing interest in discovering Near Earth
Objects has been noted in the astronomical community. Dedicated surveys have
been operated for data acquisition and processing, resulting in the present
discovery of over 18.000 objects that are closer than 30 million miles of
Earth. Nevertheless, recent events have shown that there still are many
undiscovered asteroids that can be on collision course to Earth. This article
presents an original NEO detection algorithm developed in the NEARBY research
object, that has been integrated into an automated MOPS processing pipeline
aimed at identifying moving space objects based on the blink method. Proposed
solution can be considered an approach of Big Data processing and analysis,
implementing visual analytics techniques for rapid human data validation.Comment: IEEE 14th International Conference on Intelligent Computer
Communication and Processing (ICCP), Sep 6-8, 2018, Cluj-Napoca, Romani
First Constraints on Iron Abundance versus Reflection Fraction from the Seyfert~1 Galaxy MCG--6-30-15
We report on a joint ASCA and RXTE observation spanning an 400~ks time
interval of the bright Seyfert~1 galaxy MCG--6-30-15. The data clearly confirm
the presence of a broad skewed iron line ( 266 eV) and
Compton reflection continuum at higher energies reported in our previous paper.
We also investigate whether the gravitational and Doppler effects that affect
the iron line may also be manifest in the reflected continuum. The uniqueness
of this data set is underlined by the extremely good statistics that we obtain
from the approximately four million photons that make up the 2-20 keV RXTE PCA
spectrum alone. This, coupled with the high energy coverage of HEXTE and the
spectral resolution of ASCA in the iron line regime has allowed us to constrain
the relationship between abundance and reflection fraction for the first time
at the 99 per cent confidence level. The reflection fraction is entirely
consistent with a flat disk, i.e. the cold material subtends sr at
the source, to an accuracy of 20 per cent. Monte Carlo simulations show that
the observed strong iron line intensity is explained by an overabundance of
iron by a factor of 2 and an underabundance of the lower-Z elements by a
similar factor. By considering non-standard abundances, a clear and consistent
picture can be made in which both the iron line and reflection continuum come
from the same material such as e.g. an accretion disk.Comment: 8 pages, 8 figures, accepted for publication MNRAS 7/9
Evidence for Rapid Iron K_alpha Line Flux Variability in MCG--6-30-15
This paper employs direct spectral fitting of individual orbital data in
order to measure rapid X-ray iron K_alpha line and continuum spectral slope
variations in Seyfert 1 galaxies with unprecedented temporal resolution.
Application of this technique to a long RXTE observation of MCG--6-30-15
indicates that the line flux does vary on short (~1d) timescales, but that
these variations are not correlated with changes in the continuum flux or
slope. These rapid variations indicate that the line does indeed originate
close to the black hole, confirming predictions based on its very broad
profile. However, the lack of a correlation with the continuum presents
problems for models in which the line variations are driven by those in the
continuum, modified only by light-travel time effects. Instead, it may be that
the line responds according to a physical process with a different time scale,
such as ionization instabilities in the disk, or perhaps that the geometry and
physical picture is more complex than implied by the simplest disk-corona
models.
These data also indicate that the slope of the underlying power-law continuum
(Gamma) shows strong variability and is tightly correlated with the continuum
flux in the sense that the spectrum steepens as the source brightens. All of
these results have been checked with extensive simulations, which also
indicated that a spurious correlation between Gamma and Compton reflection
fraction (R) will result if these quantities are measured from the same
spectra. This casts serious doubts on previous claims of such a Gamma-R
correlation.Comment: Accepted for publication in Ap
X-ray properties of UV-selected star forming galaxies at z~1 in the Hubble Deep Field North
We present an analysis of the X-ray emission from a large sample of
ultraviolet (UV) selected, star forming galaxies with 0.74<z<1.32 in the Hubble
Deep Field North (HDF-N) region. By excluding all sources with significant
detected X-ray emission in the 2 Ms Chandra observation we are able to examine
the properties of galaxies for which the emission in both UV and X-ray is
expected to be predominantly due to star formation. Stacking the X-ray flux
from 216 galaxies in the soft and hard bands produces significant detections.
The derived mean 2-10 keV rest-frame luminosity is 2.97+/-0.26x10^(40) erg/s,
corresponding to an X-ray derived star formation rate (SFR) of 6.0+/-0.6
Msolar/yr. Comparing the X-ray value with the mean UV derived SFR, uncorrected
for attenuation, we find that the average UV attenuation correction factor is
\~3. By binning the galaxy sample according to UV magnitude and colour,
correlations between UV and X-ray emission are also examined. We find a strong
positive correlation between X-ray emission and rest-frame UV emission. A
correlation between the ratio of X-ray-to-UV emission and UV colour is also
seen, such that L(X)/L(UV) increases for redder galaxies. Given that X-ray
emission offers a view of star formation regions that is relatively unaffected
by extinction, results such as these can be used to evaluate the effects of
dust on the UV emission from high-z galaxies. For instance we derive a
relationship for estimating UV attenuation corrections as a function of colour
excess. The observed relation is inconsistent with the Calzetti et al. (2000)
reddening law which over predicts the range in UV attenuation corrections by a
factor of ~100 for the UV selected z~1 galaxies in this sample (abridged).Comment: 10 pages, 7 figures, accepted for publication in MNRA
On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure
We use the method of Press, Rybicki & Hewitt (1992) to search for time lags
and time leads between different energy bands of the RXTE data for MCG-6-30-15.
We tailor our search in order to probe any reverberation signatures of the
fluorescent iron Kalpha line that is thought to arise from the inner regions of
the black hole accretion disk. In essence, an optimal reconstruction algorithm
is applied to the continuum band (2-4keV) light curve which smoothes out noise
and interpolates across the data gaps. The reconstructed continuum band light
curve can then be folded through trial transfer functions in an attempt to find
lags or leads between the continuum band and the iron line band (5-7keV). We
find reduced fractional variability in the line band. The spectral analysis of
Lee et al. (1999) reveals this to be due to a combination of an apparently
constant iron line flux (at least on timescales of few x 10^4s), and flux
correlated changes in the photon index. We also find no evidence for iron line
reverberation and exclude reverberation delays in the range 0.5-50ksec. This
extends the conclusions of Lee et al. and suggests that the iron line flux
remains constant on timescales as short as 0.5ksec. The large black hole mass
(>10^8Msun) naively suggested by the constancy of the iron line flux is
rejected on other grounds. We suggest that the black hole in MCG-6-30-15 has a
mass of M_BH~10^6-10^7Msun and that changes in the ionization state of the disk
may produce the puzzling spectral variability. Finally, it is found that the
8-15keV band lags the 2-4keV band by 50-100s. This result is used to place
constraints on the size and geometry of the Comptonizing medium responsible for
the hard X-ray power-law in this AGN.Comment: 11 pages, 13 postscript figures. Accepted for publication in Ap
AEGIS: The color-magnitude relation for X-ray selected AGN
We discuss the relationship between rest-frame color and optical luminosity
for X-ray sources in the range 0.6<z<1.4 selected from the Chandra survey of
the Extended Groth Strip (EGS). These objects are almost exclusively active
galactic nuclei (AGN). While there are a few luminous QSOs, most are relatively
weak or obscured AGN whose optical colors should be dominated by host galaxy
light. The vast majority of AGN hosts at z~1 are luminous and red, with very
few objects fainter than M_{B}=-20.5 or bluer than U-B=0.6. This places the AGN
in a distinct region of color-magnitude space, on the ``red sequence'' or at
the top of the ``blue cloud'', with many in between these two modes in galaxy
color. A key stage in the evolution of massive galaxies is when star formation
is quenched, resulting in a migration from the blue cloud to the red sequence.
Our results are consistent with scenarios in which AGN either cause or maintain
this quenching. The large numbers of red sequence AGN imply that strong,
ongoing star formation is not a necessary ingredient for AGN activity, as black
hole accretion appears often to persist after star formation has been
terminated.Comment: 5 pages, 2 figures, accepted for publication in AEGIS ApJ Letters
special editio
X-ray vs. Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness
We present optical broadband (B and R) observations of the Seyfert 1 nucleus
NGC 3516, obtained at Wise Observatory from March 1997 to March 2002,
contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data
we increase the temporal baseline of this dataset to 5 years, more than triple
to the coverage we have previously presented for this object. Analysis of the
new data does not confirm the 100-day lag of X-ray behind optical variations,
tentatively reported in our previous work. Indeed, excluding the first year's
data, which drive the previous result, there is no significant correlation at
any lag between the X-ray and optical bands. We also find no correlation at any
lag between optical flux and various X-ray hardness ratios. We conclude that
the close relation observed between the bands during the first year of our
program was either a fluke, or perhaps the result of the exceptionally bright
state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results
of published joint X-ray and UV/optical Seyfert monitoring programs, we
speculate that there are at least two components or mechanisms contributing to
the X-ray continuum emission up to 10 keV: a soft component that is correlated
with UV/optical variations on timescales >1 day, and whose presence can be
detected when the source is observed at low enough energies (about 1 keV), is
unabsorbed, or is in a sufficiently bright phase; and a hard component whose
variations are uncorrelated with the UV/optical.Comment: 9 pages, AJ, in pres
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