89 research outputs found
A multi-wavelength survey of AGN in the XMM-LSS field: I. Quasar selection via the KX technique
AIMS: We present a sample of candidate quasars selected using the
KX-technique. The data cover 0.68 deg^2 of the X-ray Multi-Mirror (XMM)
Large-Scale Structure (LSS) survey area where overlapping multi-wavelength
imaging data permits an investigation of the physical nature of selected
sources. METHODS: The KX method identifies quasars on the basis of their
optical (R and z') to near-infrared (Ks) photometry and point-like morphology.
We combine these data with optical (u*,g'r',i',z') and mid-infrared (3.6-24
micron) wavebands to reconstruct the spectral energy distributions (SEDs) of
candidate quasars. RESULTS: Of 93 sources selected as candidate quasars by the
KX method, 25 are classified as quasars by the subsequent SED analysis.
Spectroscopic observations are available for 12/25 of these sources and confirm
the quasar hypothesis in each case. Even more, 90% of the SED-classified
quasars show X-ray emission, a property not shared by any of the false
candidates in the KX-selected sample. Applying a photometric redshift analysis
to the sources without spectroscopy indicates that the 25 sources classified as
quasars occupy the interval 0.7 < z < 2.5. The remaining 68/93 sources are
classified as stars and unresolved galaxies.Comment: 13 pages, 9 figures, A&A 494, p. 579-589. Replaced with published
version. Fig. 9 in first astro-ph submission has been update
Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey
Our goal is to probe the populations of obscured and unobscured AGN
investigating their optical-IR and X-ray properties as a function of X-ray
flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray
sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength
coverage. The XMDS area is covered with optical photometry from the VVDS and
CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and
X-ray to optical ratio, 132 sources are likely AGN, of which 122 have
unambiguous optical - IR identification. The observed optical and IR spectral
energy distributions of sources are fitted with AGN/galaxy templates in order
to classify them and compute photometric redshifts. 70% of the AGN are fitted
by a type 2 AGN or a star forming galaxy template and are grouped together in a
single class of ``optically obscured'' AGN. They have ``red'' optical colors
and generally show significant X-ray absorption from X-ray spectra or hardness
ratios (N cm). Sources with SEDs typical of type 1 AGN
have ``blue'' optical colors and exhibit X-ray absorption in about 30% of
cases. We performed a stacking analysis for obscured and type 1 AGN. The
stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and
has an average spectral slope of Gamma = 1.6. The subsample of objects fitted
by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 -
1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower
luminosities. X-ray absorption is less common than ``optical'' obscuration and
its incidence is nearly constant with redshift and luminosity. This implies
that X-ray absorption is not necessarily related to optical obscuration.Comment: 33 pages, 21 figures, accepted for publication in A&
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
The XMM-LSS Survey: A well controlled X-ray cluster sample over the D1 CFHTLS area
We present the XMM-LSS cluster catalogue corresponding to the CFHTLS D1 area.
The list contains 13 spectroscopically confirmed, X-ray selected galaxy
clusters over 0.8 deg2 to a redshift of unity and so constitutes the highest
density sample of clusters to date. Cluster X-ray bolometric luminosities range
from 0.03 to 5x10^{44} erg/s. In this study, we describe our catalogue
construction procedure: from the detection of X-ray cluster candidates to the
compilation of a spectroscopically confirmed cluster sample with an explicit
selection function. The procedure further provides basic X-ray products such as
cluster temperature, flux and luminosity. We detected slightly more clusters
with a (0.5-2.0 keV) X-ray fluxes of >2x10^{-14} erg/s/cm^{-2} than we expected
based on expectations from deep ROSAT surveys. We also present the
Luminosity-Temperature relation for our 9 brightest objects possessing a
reliable temperature determination. The slope is in good agreement with the
local relation, yet compatible with a luminosity enhancement for the 0.15 < z<
0.35 objects having 1 < T < 2 keV, a population that the XMM-LSS is identifying
systematically for the first time. The present study permits the compilation of
cluster samples from XMM images whose selection biases are understood. This
allows, in addition to studies of large-scale structure, the systematic
investigation of cluster scaling law evolution, especially for low mass X-ray
groups which constitute the bulk of our observed cluster population. All
cluster ancillary data (images, profiles, spectra) are made available in
electronic form via the XMM-LSS cluster database.Comment: 12 pages 5 figures, MNRAS accepted. The paper with full resolution
cluster images is available at
http://vela.astro.ulg.ac.be/themes/spatial/xmm/LSS/rel_pub_e.htm
Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS Survey
We present the spectral energy distributions (SEDs) of a hard X-ray selected
sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and
132 are AGNs. The sources are detected in a 1 square degree area of the
XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys,
and infrared data from the SWIRE survey are available. Based on a SED fitting
technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of
outliers and identify AGN signatures in 83% of the objects. This fraction is
higher than derived when a spectroscopic classification is available. The
remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The
sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2
(50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by
type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft
X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard
X-ray spectra, consistent with being absorbed. The analysis of the average SEDs
as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent
with a decreasing contribution from the host galaxy at higher luminosities. The
AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by
their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously
found correlation for AGNs between the radio luminosity and the X-ray and the
mid-infrared luminosities. The X-ray-radio correlation can be used to identify
heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN
population responsible for the bulk of the background at E>10 keV are too faint
to be detected even in the deepest current radio surveys.Comment: Accepted for publication in Ap
The XMM-LSS survey: The XMDS/VVDS 4 sigma catalogue
We present a first catalogue of X-ray sources resulting from the central area
of the XMM-LSS (Large Scale Structure survey). We describe the reduction
procedures and the database tools we developed and used to derive a well
defined catalogue of X-ray sources. The present catalogue is limited to a
sub-sample of 286 sources detected at 4 sigma in the 1 deg^2 area covered by
the photometric VVDS (VIRMOS VLT Deep Survey), which allows us to provide
optical and radio identifications. We also discuss the X-ray properties of a
larger X-ray sample of 536 sources detected at > 4 sigma in the full 3 deg^2
area of the XMM Medium Deep Survey (XMDS) independently of the optical
identification. We also derive the logN-logS relationship for a sample of more
than one thousand sources that we discuss in the context of other surveys at
similar fluxes.Comment: 15+6 pages, 12 figures, accepted for publication in Astronomy &
Astrophysics The online catalogue announced in the paper will be accessible
in about 2 weeks due to technical reasons Fig. 2 replaced with a low
resolution on
The XMM-LSS catalogue: X-ray sources and associated optical data. Version I
Following the presentation of the XMM-LSS X-ray source detection package by
Pacaud et al., we provide the source lists for the first 5.5 surveyed square
degrees. The catalogues pertain to the [0.5-2] and [2-10] keV bands and contain
in total 3385 point-like or extended sources above a detection likelihood of 15
in either band. The agreement with deep logN-logS is excellent. The main
parameters considered are position, countrate, source extent with associated
likelihood values. A set of additional quantities such as astrometric
corrections and fluxes are further calculated while errors on the position and
countrate are deduced from simulations. We describe the construction of the
band-merged catalogue allowing rapid sub-sample selection and easy
cross-correlation with external multi-wavelength catalogues. A small optical
CFHTLS multi-band subset of objects is associated wich each source along with
an X-ray/optical overlay. We make the full X-ray images available in FITS
format. The data are available at CDS and, in a more extended form, at the
Milan XMM-LSS database.Comment: 13 pages, 7 figures and 11 tables (fig. 1 and 6 are enclosed in
reduced resolution), MNRAS Latex, accepted by MNRA
Optical identification of XMM sources in the CFHTLS
We present optical spectroscopic identifications of X-ray sources in ~3
square degrees of the XMM-Large Scale Structure survey (XMM-LSS), also covered
by the Canada France Hawaii Telescope Legacy Survey (CFHTLS), obtained with the
AAOmega instrument at the Anglo Australian Telescope. In a flux limited sample
of 829 point like sources in the optical band with g' <~22 mag and the 0.5-2
keV flux > 1x10^{-15}erg/cm^2/s, we observed 695 objects and obtained reliable
spectroscopic identification for 489 sources, ~59% of the overall sample. We
therefore increase the number of identifications in this field by a factor
close to five. Galactic stellar sources represent about 15% of the total
(74/489). About 55% (267/489) are broad-line Active Galactic Nuclei (AGNs)
spanning redshifts between 0.15 and 3.87 with a median value of 1.68. The
optical-to-X-ray spectral index of the broad-line AGNs is 1.47, typical of
optically-selected Type I quasars and is found to correlate with the rest frame
X-ray and optical monochromatic luminosities at 2 keV and 2500 angstroms
respectively. Consistent with previous studies, we find alpha_ox not to be
correlated with z. In addition, 32 and 116 X-ray sources are, respectively
absorption and emission-line galaxies at z<0.76. From a line ratio diagnostic
diagram it is found that in about 50% of these emission line galaxies, the
emission lines are powered significantly by the AGN. Thirty of the XMM sources
are detected at one or more radio frequencies. In addition, 24 sources have
ambiguous identification: in 8 cases, two XMM sources have a single optical
source within 6 arcsecs of each of them, whereas, 2 and 14 XMM sources have,
respectively, 3 and 2 possible optical sources within 6 arcsecs of each of
them.Comment: 15 pages, 14 figures, 5 tables, accepted for publication in MNRA
The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme
Our goal is to characterize AGN populations by comparing their X-ray and
optical classifications. We present a sample of 99 spectroscopically identified
X-ray point sources in the XMM-LSS survey which are significantly detected in
the [2-10] keV band, and with more than 80 counts. We performed an X-ray
spectral analysis for all of these 99 X-ray sources. Introducing the fourfold
point correlation coefficient, we find only a mild correlation between the
X-ray and the optical classifications, as up to 30% of the sources have
differing X-ray and optical classifications: on one hand, 10% of the type 1
sources present broad emission lines in their optical spectra and strong
absorption in the X-rays. These objects are highly luminous AGN lying at high
redshift and thus dilution effects are totally ruled out, their discrepant
nature being an intrinsic property. Their X-ray luminosities and redshifts
distributions are consistent with those of the unabsorbed X-ray sources with
broad emission lines. On the other hand, 25/32 are moderate luminosity AGN,
which are both unabsorbed in the X-rays and only present narrow emission lines
in their optical spectra. The majority of them have an optical spectrum which
is representative of the host galaxy. We finally infer that dilution of the AGN
by the host galaxy seems to account for their nature. 5/25 have been defined as
Seyfert 2. In conclusion, most of these 32 discrepant cases can be accounted
for by the standard AGN unified scheme, as its predictions are not met for only
12% of the 99 X-ray sources. ABRIDGEDComment: 25 pages, 19 figures, Accepted for publication in A&
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