2,967 research outputs found
The 3D soft X-ray cluster-AGN cross-correlation function in the ROSAT NEP survey
X-ray surveys facilitate investigations of the environment of AGNs. Deep
Chandra observations revealed that the AGNs source surface density rises near
clusters of galaxies. The natural extension of these works is the measurement
of spatial clustering of AGNs around clusters and the investigation of relative
biasing between active galactic nuclei and galaxies near clusters.The major
aims of this work are to obtain a measurement of the correlation length of AGNs
around clusters and a measure of the averaged clustering properties of a
complete sample of AGNs in dense environments. We present the first measurement
of the soft X-ray cluster-AGN cross-correlation function in redshift space
using the data of the ROSAT-NEP survey. The survey covers 9x9 deg^2 around the
North Ecliptic Pole where 442 X-ray sources were detected and almost completely
spectroscopically identified. We detected a >3sigma significant clustering
signal on scales s<50 h70^-1 Mpc. We performed a classical maximum-likelihood
power-law fit to the data and obtained a correlation length s_0=8.7+1.2-0.3
h_70-1 Mpc and a slope gamma=1.7$^+0.2_-0.7 (1sigma errors). This is a strong
evidence that AGNs are good tracers of the large scale structure of the
Universe. Our data were compared to the results obtained by cross-correlating
X-ray clusters and galaxies. We observe, with a large uncertainty, that the
bias factor of AGN is similar to that of galaxies.Comment: 4 pages, 2 figure, proceedings of the Conference "At the edge of the
Universe", Sintra Portugal, October 2006. To be published on the Astronomical
Society of the Pacific Conference Series (ASPCS
The ROSAT North Ecliptic Pole Survey: The Optical Identifications
The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All Sky
Survey have been used to construct a contiguous area survey consisting of a
sample of 445 individual X-ray sources above a flux of ~2x10^-14 erg cm^-2 s^-1
in the 0.5-2.0 keV energy band. The NEP survey is centered at RA (2000) = 18h
00m, DEC(2000) = +66deg 33arcmin and covers a region of 80.7 sq. deg at a
moderate Galactic latitude of b = 29.8deg. Hence, the NEP survey is as deep and
covers a comparable solid angle to the ROSAT serendipitous surveys, but is also
contiguous. We have identified 99.6% of the sources and determined redshifts
for the extragalactic objects. In this paper we present the optical
identifications of the NEP catalog of X-ray sources including basic X-ray data
and properties of the sources. We also describe with some detail the optical
identification procedure. The classification of the optical counterparts to the
NEP sources is very similar to that of previous surveys, in particular the
Einstein Extended Medium Sensitivity Survey (EMSS). The main constituents of
the catalog are active galactic nuclei (~49%), either type 1 or type 2
according to the broadness of their permitted emission lines. Stellar
counterparts are the second most common identification class (~34%). Clusters
and groups of galaxies comprise 14%, and BL Lacertae objects 2%. One non-AGN
galaxy, and one planetary nebula have also been found. The NEP catalog of X-ray
sources is a homogeneous sample of astronomical objects featuring complete
optical identification.Comment: Accepted for publication in the ApJS; 33 pages including 12
postscript figures and 3 tables; uses emulateapj.sty. On-line source catalog
at http://www.eso.org/~cmullis/research/nep-catalog.htm
The contribution of faint AGN to the hard X-ray background
Hard X-ray selection is the most efficient way to discriminate between
accretion-powered sources, such as AGN, from sources dominated by starlight.
Hard X-rays are also less affected than other bands by obscuration. We have
then carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the
largely unexplored 5-10 keV band, finding 180 sources in ~50 deg^2 of sky with
flux >5E-14 erg cm-2 s-1. After correction for the non uniform sky coverage
this corresponds to resolving about 30 % of the hard Cosmic X-ray Background
(XRB). Here we report on a first optical spectroscopic identification campaign,
finding 12 AGN out of 14 X-ray error-boxes studied. Seven AGN show evidence for
obscuration in X-ray and optical bands, a fraction higher than in previous
ROSAT or ASCA-ROSAT surveys (at a 95-99 % and 90 % confidence level
respectively), thus supporting the scenario in which a significant fraction of
the XRB is made by obscured AGN.Comment: MNRAS, revised version after minor referee comment
A Hard Medium Survey with ASCA. IV: the Radio-Loud Type 2 QSO AXJ0843+294 2
We discuss the X-ray, optical and radio properties of AX J0843+2942, a high
luminosity Type 2 AGN found in the ASCA Hard Serendipitous Survey. The X-ray
spectrum is best described by an absorbed power-law model with photon index of
Gamma = 1.72 (+0.3 -0.6) and intrinsic absorbing column density of NH = 1.44
(+0.33 -0.52) x 10E23 cm-2. The intrinsic luminosity in the 0.5-10 keV energy
band is ~ 3x10E45 erg s-1, well within the range of quasar luminosities. AX
J0843+2942, positionally coincident with the core of a triple and strong (S_1.4
GHz ~ 1 Jy; P_1.4 GHz ~ 9 x 10E33 erg s-1 Hz-1) radio source, is
spectroscopically identified with a Narrow Line object (intrinsic FWHM of all
the permitted emission lines <= 1200 km s-1) at z=0.398, having line features
and ratios typical of Seyfert-2 like objects. The high X-ray luminosity,
coupled with the high intrinsic absorption, the optical spectral properties and
the radio power, allow us to propose AX J0843+2942 as a Radio-Loud "Type 2
QSO". A discussion of the SED of this object is presented here together with a
comparison with the SED of Ultra Luminous Infrared Galaxies, other "Type 2 QSO"
candidates from the literature, and "normal" Radio-Quiet and Radio-Loud QSOs.Comment: 10 pages, 7 figures, Latex manuscript, Accepted for publication in
Ast ronomy and Astrophysic
Streamlined Variational Inference for Linear Mixed Models with Crossed Random Effects
We derive streamlined mean field variational Bayes algorithms for fitting
linear mixed models with crossed random effects. In the most general situation,
where the dimensions of the crossed groups are arbitrarily large, streamlining
is hindered by lack of sparseness in the underlying least squares system.
Because of this fact we also consider a hierarchy of relaxations of the mean
field product restriction. The least stringent product restriction delivers a
high degree of inferential accuracy. However, this accuracy must be mitigated
against its higher storage and computing demands. Faster sparse storage and
computing alternatives are also provided, but come with the price of diminished
inferential accuracy. This article provides full algorithmic details of three
variational inference strategies, presents detailed empirical results on their
pros and cons and, thus, guides the users on their choice of variational
inference approach depending on the problem size and computing resources
Alcohol drinking and head and neck cancer risk: The joint effect of intensity and duration
BACKGROUND: Alcohol is a well-established risk factor for head and neck cancer (HNC). This study aims to explore the effect of alcohol intensity and duration, as joint continuous exposures, on HNC risk.
METHODS: Data from 26 case-control studies in the INHANCE Consortium were used, including never and current drinkers who drunk ≤10 drinks/day for ≤54 years (24234 controls, 4085 oral cavity, 3359 oropharyngeal, 983 hypopharyngeal and 3340 laryngeal cancers). The dose-response relationship between the risk and the joint exposure to drinking intensity and duration was investigated through bivariate regression spline models, adjusting for potential confounders, including tobacco smoking.
RESULTS: For all subsites, cancer risk steeply increased with increasing drinks/day, with no appreciable threshold effect at lower intensities. For each intensity level, the risk of oral cavity, hypopharyngeal and laryngeal cancers did not vary according to years of drinking, suggesting no effect of duration. For oropharyngeal cancer, the risk increased with durations up to 28 years, flattening thereafter. The risk peaked at the higher levels of intensity and duration for all subsites (odds ratio = 7.95 for oral cavity, 12.86 for oropharynx, 24.96 for hypopharynx and 6.60 for larynx).
CONCLUSIONS: Present results further encourage the reduction of alcohol intensity to mitigate HNC risk
Genetic diversity and relationships among 192 public common bean inbred lines assessed by SSR markers.
Knowledge of germplasm diversity and of relationships among elite breeding materials has a significant impact on the improvement of crop plants and on the development of strategies for genetic resources management and exploration. The present study was conducted to determine the
level of genetic variation and relatedness among some selected common bean varieties by using microsatellite markers. In this investigation, we used 61 SSRs to fingerprint 192 common bean
inbred public lines released over the last 50 years in the U.S.A. All the lines are commercial seed type classes that are grown in the USA and include both dry bean classes and snap beans for the fresh and processing markets.The 344 alleles identified were used as raw data for estimating the amount of diversity and to describe the genetic structure of the commercial bean gene pool. A model-based clustering analysis placed the varieties in six clusters that correspond to major breeding groups plus a set of lines showing evidence of mixed origins. Neighbor-joining tree was constructed to further assess the genetic structure of common bean lines, showing good agreement with the pedigree information and the cluster analysis. A significant fixation index FST, also revealed genetic substructure within the U.S. common bean gene pool with Kidney and Pinto-Great Northern beans being the most different from the other varietal groups.The results of this study - based on a much larger number of SSRs -confirm a previous observation indicating a relatively low
level of genetic variation and a molecular variability that parallels phenotypic characters distinguishing different commercial groups. Our results indicate also a strong subpopulation
structure and provide additional tools for breeding and breeder’s rights implementation
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