2,239 research outputs found
The XMM-Newton Wide Angle Survey (XWAS): the X-ray spectrum of type-1 AGN
We discuss the broad band X-ray properties of one of the largest samples of
X-ray selected type-1 AGN to date (487 objects in total), drawn from the
XMM-Newton Wide Angle Survey. The objects cover 2-10 keV luminosities from
~10^{42}-10^{45} erg s^{-1} and are detected up to redshift ~4. We constrain
the overall properties of the broad band continuum, soft excess and X-ray
absorption, along with their dependence on the X-ray luminosity and redshift
and we discuss the implications for models of AGN emission. We constrained the
mean spectral index of the broad band X-ray continuum to =1.96+-0.02
with intrinsic dispersion sigma=0.27_{-0.02}^{+0.01}. The continuum becomes
harder at faint fluxes and at higher redshifts and luminosities. The dependence
of Gamma with flux is likely due to undetected absorption rather than to
spectral variation. We found a strong dependence of the detection efficiency of
objects on the spectral shape which can have a strong impact on the measured
mean continuum shapes of sources at different redshifts and luminosities. We
detected excess absorption in ~3% of our objects, with column densities ~a few
x10^{22} cm^{-2}. The apparent mismatch between the optical classification and
X-ray properties of these objects is a challenge for the standard AGN
unification model. We found that the fraction of objects with detected soft
excess is ~36%. Using a thermal model, we constrained the soft excess mean
temperature and intrinsic dispersion to ~100 eV and sigma~34 eV. The origin
of the soft excess as thermal emission from the accretion disk or Compton
scattered disk emission is ruled out on the basis of the temperatures detected
and the lack of correlation of the measured temperature with the X-ray
luminosity (abridged).Comment: 13 pages, 24 figures, Accepted for publication in Astronomy and
Astrophysic
X-ray spectra of XMM-Newton serendipitous medium flux sources
We report on the results of a detailed analysis of the X-ray spectral
properties of a large sample of sources detected serendipitously with the
XMM-Newton observatory in 25 selected fields. The survey covers a total solid
angle of ~3.5 deg2 and contains 1137 sources with ~10E-15 < S0.5-10 < 10E-12
erg cm-2 s-1. We find evidence for hardening of the average X-ray spectra of
the sources towards fainter fluxes. We interpret this as indicating a higher
degree of photoelectric absorption amongst the fainter population. Absorption
is detected at 95% confidence in 20% of the sources, but it could certainly be
present in many other sources below our detection capabilities. For Broad Line
AGNs (BLAGNs), we detect absorption in ~10% of the sources with column
densities in the range 10E21 - 10E22 cm-2. The fraction of absorbed Narrow
Emission Line galaxies (NELGs, most with intrinsic X-ray luminosities >10E43
erg s-1, and therefore classified as type 2 AGNs) is significantly higher
(40%), with a hint of moderately higher columns. We do not find evidence for a
redshift evolution of the underlying power law index of BLAGNs, which stays
roughly constant at Gamma ~1.9, with intrinsic dispersion of 0.4. A small
fraction (~7%) of BLAGNs and NELGs require the presence of a soft excess, that
we model as a black body with temperature ranging from 0.1 to 0.3 keV.
Comparing our results on absorption to popular X-ray background synthesis
models, we find absorption in only ~40% of the sources expected. This is due to
a deficiency of heavily absorbed sources (with NH ~10E22 - 10E24 cm-2) in our
sample in comparison with the models. We therefore conclude that the synthesis
models require some revision in their specific parameters.Comment: 20 pages, 30 Postscript figures, A&A in pres
The XMM-SSC survey of hard-spectrum XMM-Newton sources 1: optically bright sources
We present optical and X-ray data for a sample of serendipitous XMM-Newton
sources that are selected to have 0.5-2 keV vs 2-4.5 keV X-ray hardness ratios
which are harder than the X-ray background. The sources have 2-4.5 keV X-ray
flux >= 10^-14 cgs, and in this paper we examine a subsample of 42 optically
bright (r < 21) sources; this subsample is 100 per cent spectroscopically
identified. All but one of the optical counterparts are extragalactic, and we
argue that the single exception, a Galactic M star, is probably a coincidental
association. The X-ray spectra are consistent with heavily absorbed power laws
(21.8 < log NH < 23.4), and all of them appear to be absorbed AGN. The majority
of the sources show only narrow emission lines in their optical spectra,
implying that they are type-2 AGN. Only a small fraction of the sources (7/42)
show broad optical emission lines, and all of these have NH < 10^23 cm^-2. This
implies that ratios of X-ray absorption to optical/UV extinction equivalent to
> 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a
few percent of the population), and may be restricted to broad absorption-line
QSOs. Seven objects appear to have an additional soft X-ray component in
addition to the heavily absorbed power law. We consider the implications of our
results in the light of the AGN unified scheme. We find that the soft
components in narrow-line objects are consistent with the unified scheme
provided that > 4 per cent of broad-line AGN have ionised absorbers that
attenuate their soft X-ray flux by >50 per cent. In at least one of the X-ray
absorbed, broad-line AGN in our sample the X-ray spectrum requires an ionised
absorber, consistent with this picture.Comment: accepted for publication in MNRA
The variable XMM-Newton spectrum of Markarian 766
The narrow-line Seyfert 1 galaxy Markarian 766 was observed for 60 ks with
the XMM-Newton observatory. The source shows a complex X-ray spectrum. The 2-10
keV spectrum can be adequately represented by a power law and broad Fe Ka
emission. Between 0.7 and 2 keV the spectrum is harder and exhibits a flux
deficit with respect to the extrapolated medium energy slope. Below 0.7 keV,
however, there is a strong excess of emission. The RGS spectrum shows an
edge-like feature at 0.7 keV; the energy of this feature is inconsistent with
that expected for an OVII edge from a warm absorber. Markarian 766 varies by a
factor of ~ 2 in overall count rate in the EPIC and RGS instruments on a
timescale of a few thousand seconds, while no significant flux changes are
observed in the ultraviolet with the OM. The X-ray variability is spectrally
dependent with the largest amplitude variability occurring in the 0.4-2 keV
band. The spectral variability can be explained by a change in flux and slope
of the medium energy continuum emission, superimposed on a less variable (or
constant) low energy emission component.Comment: 6 pages, 6 figures, Accepted by A&A for publication in the Special
Issue on 1st science with XMM Newto
The U.K. Deep and Medium Surveys with Rosat: log N-Log S relation
We have carried out a soft X-ray survey of the sky using the ROSAT Position Sensitive Proportional Counter (PSPC) in a region of very low Galactic column density (NH = 6–9 × 1019 cm–2). The data consist of a deep > 70 ks pointing (the Deep Survey) and six pointings at lower sensitivity (13–20 ks; the Medium Survey). We detect a total of 141 sources over the 0.9 deg2 of sky area used. The faintest source detected has a flux of 3.2 × 10–15 erg cm–2 s–1 (0.5–2.0 keV). We present the source number-flux distribution, and we compare it with the distributions constructed from other ROSAT observations and from surveys carried out with X-ray instruments operating at higher energies. We estimate the contribution of the resolved sources to the soft X-ray background by direct comparison of their integrated spectrum with that of the diffuse background: 44 per cent of the extragalactic X-ray background between 0.5 and 2.0 keV is resolved directly into discrete sources whose average spectrum is steeper than that of the background. Integration of our number-flux relation to infinite flux produces a background resolved fraction of 44±565 per cent (0.5–2.0 keV). Limits on the slope of the log N–log S curve at very faint fluxes are set by the measured value of the X-ray background.KOM, ACF and RGM acknowledge the support of the Royal Society.Peer Reviewe
The XMM deep survey in the CDF-S II. a 9-20 keV selection of heavily obscured active galaxies at z>1.7
We present results on a search of heavily obscured active galaxies z>1.7
using the rest-frame 9-20 keV excess for X-ray sources detected in the deep
XMM-CDFS survey. Out of 176 sources selected with the conservative detection
criteria (>8 sigma) in the first source catalogue of Ranalli et al., 46 objects
lie in the redshift range of interest with the median redshift z~2.5. Their
typical rest-frame 10-20 keV luminosity is 1e+44 erg/s, as observed. Among
optically faint objects that lack spectroscopic redshift, four were found to be
strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption
features in their X-ray spectra were used to obtain X-ray spectroscopic
redshifts. Using the X-ray colour-colour diagram based on the rest-frame 3-5
keV, 5-9 keV, and 9-20 keV bands, seven objects were selected for their 9-20
keV excess and were found to be strongly absorbed X-ray sources with column
density of nH > 0.6e+24 cm-2, including two possible Compton thick sources.
While they are emitting at quasar luminosity, ~3/4 of the sample objects are
found to be absorbed by nH > 1e+22 cm-2. A comparison with local AGN at the
matched luminosity suggests an increasing trend of the absorbed source fraction
for high-luminosity AGN towards high redshifts.Comment: 9 pages, 7 figures. Accepted for publication in A&
Averaging the AGN X-ray spectra from deep Chandra fields
The X-ray spectra of Active Galactic Nuclei (AGN) carry the signatures of the
emission from the central region, close to the Super Massive Black Hole (SMBH).
For this reason, the study of deep X-ray spectra is a powerful instrument to
investigate the origin of their emission.
The emission line most often observed in the X-ray spectra of AGN is Fe K. It
is known that it can be broadened and deformed by relativistic effects if
emitted close enough to the central SMBH. In recent statistical studies of the
X-ray spectra of AGN samples, it is found that a narrow Fe line is ubiquitous,
while whether the broad features are as common is still uncertain. We present
here the results of an investigation on the characteristics of the Fe line in
the average X-ray spectra of AGN in deep Chandra fields.
The average spectrum of the AGN is computed using Chandra spectra with more
than 200 net counts from the AEGIS, Chandra Deep Field North and Chandra Deep
Field South surveys. The sample spans a broader range of X-ray luminosities
than other samples studied with stacking methods up to z=3.5. We analyze the
average spectra of this sample using our own averaging method, checking the
results against extensive simulations. Subsamples defined in terms of column
density of the local absorber, luminosity and z are also investigated.
We found a very significant Fe line with a narrow profile in all our samples
and in almost all the subsamples that we constructed. The equivalent width (EW)
of the narrow line estimated in the average spectrum of the full sample is 74
eV. The broad line component is significantly detected in the subsample of AGN
with L<1.43 1E44 cgs and z<0.76, with EW=108 eV.
We concluded that the narrow Fe line is an ubiquitous feature of the X-ray
spectra of the AGN up to z=3.5.The broad line component is significant in the
X-ray spectra of the AGN with low luminosity and low z.Comment: 17 pages, 23 figures, accepted for publication in Astronomy &
Astrophysics. Replacements in the bibliography: Iwasawa K., Mainieri V.,
Brusa M., et al., 2011, arXiv:1111.2447v1 (previously: Iwasawa et al 2011, in
preparation); Mateos S., Carrera F.J., Page M.J., et al., 2010, \aap, 510,
A35 (previously: Mateos S., Warwick R.S., Carrera F.J., et al., 2008, \aap,
492, 51
Measurement of isolated photon production in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV
Isolated photon production is measured in proton-proton and lead-lead
collisions at nucleon-nucleon centre-of-mass energies of 2.76 TeV in the
pseudorapidity range |eta|<1.44 and transverse energies ET between 20 and 80
GeV with the CMS detector at the LHC. The measured ET spectra are found to be
in good agreement with next-to-leading-order perturbative QCD predictions. The
ratio of PbPb to pp isolated photon ET-differential yields, scaled by the
number of incoherent nucleon-nucleon collisions, is consistent with unity for
all PbPb reaction centralities.Comment: Submitted to Physics Letters
Common schizophrenia alleles are enriched in mutation-intolerant genes and maintained by background selection
Schizophrenia is a debilitating psychiatric condition often associated with poor quality of life and decreased life expectancy. Lack of progress in improving treatment outcomes has been attributed to limited knowledge of the underlying biology, although large-scale genomic studies have begun to provide such insight. We report the largest single cohort genome-wide association study of schizophrenia (11,260 cases and 24,542 controls) and through meta-analysis with existing data we identify 50 novel GWAS loci. Using gene-wide association statistics we implicate an additional set of 22 novel associations that map onto a single gene. We show for the first time that the common variant association signal is highly enriched among genes that are intolerant to loss of function mutations and that variants in these genes persist in the population despite the low fecundity associated with the disorder through the process of background selection. Associations point to novel areas of biology (e.g. metabotropic GABA-B signalling and acetyl cholinesterase), reinforce those implicated in earlier GWAS studies (e.g. calcium channel function), converge with earlier rare variants studies (e.g. NRXN1, GABAergic signalling), identify novel overlaps with autism (e.g. RBFOX1, FOXP1, FOXG1), and support early controversial candidate gene hypotheses (e.g. ERBB4 implicating neuregulin signalling). We also demonstrate the involvement of six independent central nervous system functional gene sets in schizophrenia pathophysiology. These findings provide novel insights into the biology and genetic architecture of schizophrenia, highlight the importance of mutation intolerant genes and suggest a mechanism by which common risk variants are maintained in the population
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