114 research outputs found
The X-ray Variability of AGN and its Implications for Observations of Galaxy Clusters
The detection of new clusters of galaxies or the study of known clusters of
galaxies in X-rays can be complicated by the presence of X-ray point sources,
the majority of which will be active galactic nuclei (AGN). This can be
addressed by combining observations from a high angular resolution observatory
(such as Chandra) with deeper data from a more sensitive observatory that may
not be able to resolve the AGN (like XMM). However, this approach is undermined
if the AGN varies in flux between the epochs of the observations. To address
this we measure the characteristic X-ray variability of serendipitously
detected AGN in 70 pairs of Chandra observations, separated by intervals of
between one month and thirteen years. After quality cuts, the full sample
consists of 1511 sources, although the main analysis uses a subset of 416
sources selected on the geometric mean of their flux in the pairs of
observations, which eliminates selection biases. We find a fractional
variability that increases with increasing interval between observations, from
about 0.25 for observations separated by tens of days up to about 0.45 for
observations separated by years. As a rule of thumb, given the
precise X-ray flux of a typical AGN at one epoch, its flux at a second epoch
some years earlier or later can be predicted with a precision of about
due to its variability (ignoring any statistical noise). This is larger than
the characteristic variability of the population by a factor of due
to the uncertainty on the mean flux of the AGN due to a single prior
measurement. The precision can thus be improved with multiple prior flux
measurements (reducing the factor), or by reducing the interval
between observations to reduce the characteristic variability.Comment: 13 pages, 7 figures; accepted for publication in the Open Journal of
Astrophysics; full data table included with source files; comments welcom
Chandra Measurements of a Complete Sample of X-ray Luminous Galaxy Clusters: the Gas Mass Fraction
We present Chandra X-ray measurements of the gas mass fraction out to r500
for a complete sample of the 35 most luminous clusters from the Brightest
Cluster Sample and the Extended Brightest Cluster Sample at redshift
z=0.15-0.30. The sample includes relaxed and unrelaxed clusters, and the data
were analysed independently using two pipelines and two different models for
the gas density and temperature. We measure an average of fgas(r500) = 0.163
+/- 0.032, which is in agreement with the cosmic baryon fraction (Omega_b /
Omega_M = 0.167 +/- 0.006) at the 1-sigma level, after adding the stellar
baryon fraction. Earlier studies reported gas mass fractions significantly
lower than the cosmic baryon fraction at r500, and in some cases higher values
that are consistent with the cosmic baryon fraction towards the virial
radius.In this paper we show that the most X-ray luminous clusters in the
redshift range z=0.15-0.30 have a gas mass fraction that is consistent with the
cosmic value at r500.Comment: MNRAS submitted, comments welcome; 23 pages, 57 figure
A fossil galaxy cluster; an X-ray and optical study of RX J1416.4+2315
We present a detailed X-ray and optical study of a distant fossil system RX
J1416.4+2315 (z=0.13), combining Chandra and XMM-Newton observations, optical
photometry and spectroscopy. X-ray emitting hot gas imaged by both the Chandra
and XMM-Newton shows a globally relaxed spatial distribution, supporting the
idea that fossil groups are old galaxy systems with no recent mergers. However,
the diffuse X-ray emission shows signs of asymmetries in the core of the
system. With a mean gas temperature of ~ 4.0 keV and total gravitational mass
of 3.1 x 10^14 solar mass, within the virial radius, this is better described
as a fossil galaxy cluster rather than a fossil group. The temperature profile
shows no sign of a significant cooler core despite a cooling time dropping to 5
Gyr within the resolved core. We find a mass concentration parameter c_200 ~ 11
which is relatively high for a cluster of this mass, indicative of an early
formation epoch. Using the spectroscopically identified cluster members we
present the galaxy luminosity function for this fossil system. We measure the
velocity dispersion of the galaxies to be ~ 700 km/s based on 18 confirmed
members. The dynamical mass is nearly twice the total gravitational mass
derived from the X-ray analysis. The measured R-band mass-to-light ratio,
within the virial radius, is ~ 440 M/L (solar) which is not unusual for
clusters of galaxies. The central giant elliptical galaxy has discy isophotes
and spectral features typical of elliptical galaxies.Comment: 10 pages, 10 figures, Accepted for publication in MNRA
The XXL Survey:XLII. Scatters and correlations of X-ray proxies in the bright XXL cluster sample
http://irfu.cea.fr/xxl/International audienceContext. Scaling relations between cluster properties embody the formation and evolution of cosmic structure. Intrinsic scatters and correlations between X-ray properties are determined from merger history, baryonic processes, and dynamical state.Aims. We look for an unbiased measurement of the scatter covariance matrix among the three main X-ray observable quantities attainable in large X-ray surveys: temperature, luminosity, and gas mass. This also gives us the cluster property with the lowest conditional intrinsic scatter at fixed mass.Methods. Intrinsic scatters and correlations can be measured under the assumption that the observable properties of the intra-cluster medium hosted in clusters are log-normally distributed around power-law scaling relations. The proposed method is self-consistent, based on minimal assumptions, and requires neither external calibration by weak lensing, or dynamical or hydrostatic masses, nor the knowledge of the mass completeness.Results. We analysed the 100 brightest clusters detected in the XXL Survey and their X-ray properties measured within a fixed radius of 300 kpc. The gas mass is the less scattered proxy (âŒ8%). The temperature (âŒ20%) is intrinsically less scattered than the luminosity (âŒ30%), but it is measured with a larger observational uncertainty. We found some evidence that gas mass, temperature, and luminosity are positively correlated. Time evolutions are in agreement with the self-similar scenario, but the luminosityâtemperature and the gas massâtemperature relations are steeper.Conclusion. Positive correlations between X-ray properties can be determined by the dynamical state and the merger history of the halos. The slopes of the scaling relations are affected by radiative processes
Chandra follow up of the Hectospec Cluster Survey: Comparison of Caustic and Hydrostatic Masses and Constraints on the Hydrostatic Bias
Clusters of galaxies are powerful probes with which to study cosmology and
astrophysics. However, for many applications an accurate measurement of a
cluster's mass is essential. A systematic underestimate of hydrostatic masses
from X-ray observations (the so-called hydrostatic bias) may be responsible for
tension between the results of different cosmological measurements. We compare
X-ray hydrostatic masses with masses estimated using the caustic method (based
on galaxy velocities) in order to explore the systematic uncertainties of both
methods and place new constraints on the level of hydrostatic bias. Hydrostatic
and caustic mass profiles were determined independently for a sample of 44
clusters based on Chandra observations of clusters from the Hectospec Cluster
Survey. This is the largest systematic comparison of its kind. Masses were
compared at a standardised radius () using a model that includes
possible bias and scatter in both mass estimates. The systematics affecting
both mass determination methods were explored in detail. The hydrostatic masses
were found to be systematically higher than caustic masses on average, and we
found evidence that the caustic method increasingly underestimates the mass
when fewer galaxies are used to measure the caustics. We limit our analysis to
the 14 clusters with the best-sampled caustics where this bias is minimised
( galaxies), and find that the average ratio of hydrostatic to caustic
mass at is . We interpret this result
as a constraint on the level of hydrostatic bias, favouring small or zero
levels of hydrostatic bias (less than at the level). However,
we find systematic uncertainties associated with both mass estimation methods
remain at the level, which would permit significantly larger levels
of hydrostatic bias.Comment: 15 pages plus appendices. Updated to match version accepted for
publication in A&A. Updates include additional tests of systematics. Main
results are unchange
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Dietary prophage inducers and antimicrobials: toward landscaping the human gut microbiome.
The approximately 1011 viruses and microbial cells per gram of fecal matter (dry weight) in the large intestine are important to human health. The responses of three common gut bacteria species, and one opportunistic pathogen, to 117 commonly consumed foods, chemical additives, and plant extracts were tested. Many compounds, including Stevia rebaudiana and bee propolis extracts, exhibited species-specific growth inhibition by prophage induction. Overall, these results show that various foods may change the abundances of gut bacteria by modulating temperate phage and suggests a novel path for landscaping the human gut microbiome
CHEX-MATE: pressure profiles of 6 galaxy clusters as seen by SPT and Planck
Pressure profiles are sensitive probes of the thermodynamic conditions and
the internal structure of galaxy clusters. The intra-cluster gas resides in
hydrostatic equilibrium within the Dark Matter gravitational potential.
However, this equilibrium may be perturbed, e.g. as a consequence of thermal
energy losses, feedback and non-thermal pressure supports. Accurate measures of
the gas pressure over the cosmic times are crucial to constrain the cluster
evolution as well as the contribution of astrophysical processes. In this work
we presented a novel algorithm to derive the pressure profiles of galaxy
clusters from the Sunyaev-Zeldovich (SZ) signal measured on a combination of
Planck and South Pole Telescope (SPT) observations. The synergy of the two
instruments made it possible to track the profiles on a wide range of spatial
scales. We exploited the sensitivity to the larger scales of the Planck
High-Frequency Instrument to observe the faint peripheries, and the higher
spatial resolution of SPT to solve the innermost regions. We developed a
two-step pipeline to take advantage of the specifications of each instrument.
We first performed a component separation on the two data-sets separately to
remove the background (CMB) and foreground (galactic emission) contaminants.
Then we jointly fitted a parametric pressure profile model on a combination of
Planck and SPT data. We validated our technique on a sample of 6 CHEX-MATE
clusters detected by SPT. We compare the results of the SZ analysis with
profiles derived from X-ray observations with XMM-Newton. We find an excellent
agreement between these two independent probes of the gas pressure structure.Comment: 19 pages, 13 figures, submitted to A&
The Upper Respiratory Tract as a Microbial Source for Pulmonary Infections in Cystic Fibrosis. Parallels from Island Biogeography
A continuously mixed series of microbial communities inhabits various points of the respiratory tract, with community composition determined by distance from colonization sources, colonization rates, and extinction rates. Ecology and evolution theory developed in the context of biogeography is relevant to clinical microbiology and could reframe the interpretation of recent studies comparing communities from lung explant samples, sputum samples, and oropharyngeal swabs. We propose an island biogeography model of the microbial communities inhabiting different niches in human airways. Island biogeography as applied to communities separated by time and space is a useful parallel for exploring microbial colonization of healthy and diseased lungs, with the potential to inform our understanding of microbial community dynamics and the relevance of microbes detected in different sample types. In this perspective, we focus on the intermixed microbial communities inhabiting different regions of the airways of patients with cystic fibrosis
The XMM Cluster Survey: Forecasting cosmological and cluster scaling-relation parameter constraints
We forecast the constraints on the values of sigma_8, Omega_m, and cluster
scaling relation parameters which we expect to obtain from the XMM Cluster
Survey (XCS). We assume a flat Lambda-CDM Universe and perform a Monte Carlo
Markov Chain analysis of the evolution of the number density of galaxy clusters
that takes into account a detailed simulated selection function. Comparing our
current observed number of clusters shows good agreement with predictions. We
determine the expected degradation of the constraints as a result of
self-calibrating the luminosity-temperature relation (with scatter), including
temperature measurement errors, and relying on photometric methods for the
estimation of galaxy cluster redshifts. We examine the effects of systematic
errors in scaling relation and measurement error assumptions. Using only (T,z)
self-calibration, we expect to measure Omega_m to +-0.03 (and Omega_Lambda to
the same accuracy assuming flatness), and sigma_8 to +-0.05, also constraining
the normalization and slope of the luminosity-temperature relation to +-6 and
+-13 per cent (at 1sigma) respectively in the process. Self-calibration fails
to jointly constrain the scatter and redshift evolution of the
luminosity-temperature relation significantly. Additional archival and/or
follow-up data will improve on this. We do not expect measurement errors or
imperfect knowledge of their distribution to degrade constraints significantly.
Scaling-relation systematics can easily lead to cosmological constraints 2sigma
or more away from the fiducial model. Our treatment is the first exact
treatment to this level of detail, and introduces a new `smoothed ML' estimate
of expected constraints.Comment: 28 pages, 17 figures. Revised version, as accepted for publication in
MNRAS. High-resolution figures available at http://xcs-home.org (under
"Publications"
Comprehensive pharmacogenetic profiling of the epidermal growth factor receptor pathway for biomarkers of response to, and toxicity from, cetuximab
Background
Somatic mutations in the epidermal growth factor receptor (EGFR) intracellular signalling pathways predict non-response to cetuximab in the treatment of advanced colorectal cancer (aCRC). We hypothesized that common germline variants within these pathways may also play similar roles.
Methods
We analysed 54 potentially functional, common, inherited EGFR pathway variants in 815 aCRC patients treated with oxaliplatin-fluoropyrimidine chemotherapy +cetuximab. Primary endpoints were response and skin rash (SR). We had >85% power to detect ORs=1.6 for variants with minor allele frequencies >20%.
Results
We identified five potential biomarkers for response and four for SR, although none remained significant after correction for multiple testing. Our initial data supported a role for Ser313Pro in PIK3R2 in modulating response to cetuximab - in patients with KRAS wild type CRCs, 36.4% of patients with one allele encoding proline responded, as compared to 71.2% of patients homozygous for alleles encoding serine (OR 0.23, 95% CI 0.09-0.56, P=0.0014) and this association was predictive for cetuximab (Pinteraction=0.017); however, independent replication failed to validate this association. No previously proposed predictive biomarkers were validated.
Conclusions
Our study highlights the need to validate potential pharmacogenetic biomarkers. We did not find strong evidence for common germline biomarkers of cetuximab response and toxicity
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