1,468 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
The evolution of the cluster X-ray scaling relations in the WARPS sample at 0.6<z<1.0
The X-ray properties of a sample of 11 high-redshift (0.6<z<1.0) clusters
observed with Chandra and/or XMM are used to investigate the evolution of the
cluster scaling relations. The observed evolution of the L-T and M-L relations
is consistent with simple self-similar predictions, in which the properties of
clusters reflect the properties of the universe at their redshift of
observation. When the systematic effect of assuming isothermality on the
derived masses of the high-redshift clusters is taken into account, the
high-redshift M-T and Mgas-T relations are also consistent with self-similar
evolution. Under the assumption that the model of self-similar evolution is
correct and that the local systems formed via a single spherical collapse, the
high-redshift L-T relation is consistent with the high-z clusters having formed
at a significantly higher redshift than the local systems. The data are also
consistent with the more realistic scenario of clusters forming via the
continuous accretion of material. The slope of the L-T relation at
high-redshift (B=3.29+/-0.38) is consistent with the local relation, and
significantly steeper then the self-similar prediction of B=2. This suggests
that the non-gravitational processes causing the steepening occurred at z>1 or
in the early stages of the clusters' formation, prior to their observation. The
properties of the intra-cluster medium at high-redshift are found to be similar
to those in the local universe. The mean surface-brightness profile slope for
the sample is 0.66+/-0.05, the mean gas mass fractions within R2500 and R200
are 0.073+/-0.010 and 0.12+/-0.02 respectively, and the mean metallicity of the
sample is 0.28+/-0.16 solar.Comment: 23 pages, 17 figures. Accepted for publication in MNRAS. Revised to
match accepted version: reanalysed data with latest calibrations, several
minor changes. Conclusions unchange
The use of the X rays in the diagnosis of aneurisms of the thoracic aorta, dilatations of the aorta and mediastinal new growths
The material on which this thesis is based
has all been got from the Royal Infirmary, Edinburgh. The cases have all been under the charge
of Dr. Bramwell, and a large number of them have
personally come under my own observation.The examination by means of the X rays has
been made in the electrical department of the
above institution by the medical staff in that
department.The aim of this thesis is to show, by the illustration of the cases which have come under observation during the last 5 or 6 years, what results are obtained on X ray examination and what
is its bearing upon the diagnosis of cases.First of all then we will describe briefly
the technique of the operation entailed in making
an examination
Interconnected or disconnected? Promotion of mental health and prevention of mental disorder in the digital age
To date there have been few peer-reviewed studies on the feasibility, acceptability and effectiveness of digital technologies for mental health promotion and disorder prevention. Any evaluation of these evolving technologies is complicated by a lack of understanding about the specific risks and possible benefits of the many forms of internet use on mental health. To adequately meet the mental health needs of today's society, psychiatry must engage in rigorous assessment of the impact of digital technologies
Images, structural properties and metal abundances of galaxy clusters observed with Chandra ACIS-I at 0.1<z<1.3
We have assembled a sample of 115 galaxy clusters at 0.1<z<1.3 with archived
Chandra ACIS-I observations. We present X-ray images of the clusters and make
available region files containing contours of the smoothed X-ray emission. The
structural properties of the clusters were investigated and we found a
significant absence of relaxed clusters (as determined by centroid shift
measurements) at z>0.5. The slope of the surface brightness profiles at large
radii were steeper on average by 15% than the slope obtained by fitting a
simple beta-model to the emission. This slope was also found to be correlated
with cluster temperature, with some indication that the correlation is weaker
for the clusters at z>0.5. We measured the mean metal abundance of the cluster
gas as a function of redshift and found significant evolution, with the
abundances dropping by 50% between z=0.1 and z~1. This evolution was still
present (although less significant) when the cluster cores were excluded from
the abundance measurements, indicating that the evolution is not solely due to
the disappearance of relaxed, cool core clusters (which are known to have
enhanced core metal abundances) from the population at z>0.5.Comment: 23 pages, 12 figures. Accepted for publication in ApJS. Updated to
match published version. Redshifts of two clusters (RXJ1701 and CL0848)
corrected and two observations of MACSJ0744.8 have been combined into one.
Conclusions unchanged. A version with images of all of the clusters is
available at http://hea-www.harvard.edu/~bmaughan/clusters.htm
The WARPS Survey. VIII. Evolution of the Galaxy Cluster X-ray Luminosity Function
We present measurements of the galaxy cluster X-ray Luminosity Function (XLF)
from the Wide Angle ROSAT Pointed Survey (WARPS) and quantify its evolution.
WARPS is a serendipitous survey of the central region of ROSAT pointed
observations and was carried out in two phases (WARPS-I and WARPS-II). The
results here are based on a final sample of 124 clusters, complete above a flux
limit of 6.5 10E-15 erg/s/cm2, with members out to redshift z ~ 1.05, and a sky
coverage of 70.9 deg2. We find significant evidence for negative evolution of
the XLF, which complements the majority of X-ray cluster surveys. To quantify
the suggested evolution, we perform a maximum likelihood analysis and conclude
that the evolution is driven by a decreasing number density of high luminosity
clusters with redshift, while the bulk of the cluster population remains nearly
unchanged out to redshift z ~ 1.1, as expected in a low density Universe. The
results are found to be insensitive to a variety of sources of systematic
uncertainty that affect the measurement of the XLF and determination of the
survey selection function. We perform a Bayesian analysis of the XLF to fully
account for uncertainties in the local XLF on the measured evolution, and find
that the detected evolution remains significant at the 95% level. We observe a
significant excess of clusters in the WARPS at 0.1 < z < 0.3 and LX ~ 2 10E42
erg/s compared with the reference low-redshift XLF, or our Bayesian fit to the
WARPS data. We find that the excess cannot be explained by sample variance, or
Eddington bias, and is unlikely to be due to problems with the survey selection
function.Comment: 13 pages, 12 figures, accepted for publication in MNRA
Chandra Measurements of a Complete Sample of X-ray Luminous Galaxy Clusters: The Luminosity-Mass Relation
We present the results of work involving a statistically complete sample of
34 galaxy clusters, in the redshift range 0.15z0.3 observed with
. We investigate the luminosity-mass () relation for the cluster
sample, with the masses obtained via a full hydrostatic mass analysis. We
utilise a method to fully account for selection biases when modeling the
relation, and find that the relation is significantly different than the
relation modelled when not account for selection effects. We find that the
luminosity of our clusters is 2.20.4 times higher (when accounting for
selection effects) than the average for a given mass, its mass is 30% lower
than the population average for a given luminosity. Equivalently, using the
relation measured from this sample without correcting for selection biases
would lead to the underestimation by 40% of the average mass of a cluster with
a given luminosity. Comparing the hydrostatic masses to mass estimates
determined from the parameter, we find that they are entirely
consistent, irrespective of the dynamical state of the cluster.Comment: 31 pages, 43 figures, accepted for publication in MNRA
An XMM and Chandra view of massive clusters of galaxies to z=1
The X-ray properties of a sample of high redshift (z>0.6), massive clusters
observed with XMM-Newton and Chandra are described, including two exceptional
systems. One, at z=0.89, has an X-ray temperature of T=11.5 (+1.1, -0.9) keV
(the highest temperature of any cluster known at z>0.6), an estimated mass of
(1.4+/-0.2)x10^15 solar masses and appears relaxed. The other, at z=0.83, has
at least three sub-clumps, probably in the process of merging, and may also
show signs of faint filamentary structure at large radii,observed in X-rays. In
general there is a mix of X-ray morphologies, from those clusters which appear
relaxed and containing little substructure to some highly non-virialized and
probably merging systems. The X-ray gas metallicities and gas mass fractions of
the relaxed systems are similar to those of low redshift clusters of the same
temperature, suggesting that the gas was in place, and containing its metals,
by z=0.8. The evolution of the mass-temperature relation may be consistent with
no evolution or with the ``late formation'' assumption. The effect of point
source contamination in the ROSAT survey from which these clusters were
selected is estimated, and the implications for the ROSAT X-ray luminosity
function discussed.Comment: 9 pages, in Carnegie Observatories Astrophysics Series, Vol. 3:
Clusters of Galaxies: Probes of Cosmological Structure and Galaxy Evolution,
ed. J. S. Mulchaey, A. Dressler, and A. Oemler. See
http://www.ociw.edu/ociw/symposia/series/symposium3/proceedings.html for a
full-resolution versio
- âŠ