94 research outputs found
Anisotropy of the galaxy cluster X-ray luminosity-temperature relation
We introduce a new test to study the Cosmological Principle with galaxy
clusters. Galaxy clusters exhibit a tight correlation between the luminosity
and temperature of the X-ray-emitting intracluster medium. While the luminosity
measurement depends on cosmological parameters through the luminosity distance,
the temperature determination is cosmology-independent. We exploit this
property to test the isotropy of the luminosity distance over the full
extragalactic sky, through the normalization of the scaling
relation and the cosmological parameters and . We use two
almost independent galaxy cluster samples: the ASCA Cluster Catalog (ACC) and
the XMM Cluster Survey (XCS-DR1). Interestingly enough, these two samples
appear to have the same pattern for with respect to the Galactic longitude.
We also identify one sky region within (Group A) that
shares very different best-fit values for for both samples. We find the
deviation of Group A to be for ACC and for XCS-DR1.
This tension is not relieved after excluding possible outliers or after a
redshift conversion to the CMB frame is applied. Using also the HIFLUGCS
sample, we show that a possible excess of cool-core clusters in this region,
cannot explain the obtained deviations. Moreover, we tested for a dependence of
the relation on supercluster environment. We indeed find a trend for
supercluster members to be underluminous compared to field clusters. However,
the fraction of supercluster members is similar in the different sky regions.
Constraining and via the redshift evolution of and the
luminosity distance, we obtain approximately the same deviation amplitudes as
for . The observed behavior of for the sky regions that coincide
with the CMB dipole is similar to what was found with other cosmological probes
as well.Comment: 18 pages, 15 figures, accepted for publication in A&
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
Constraining galaxy cluster temperatures and redshifts with eROSITA survey data
The nature of dark energy is imprinted in the large-scale structure of the
Universe and thus in the mass and redshift distribution of galaxy clusters. The
upcoming eROSITA mission will exploit this method of probing dark energy by
detecting roughly 100,000 clusters of galaxies in X-rays. For a precise
cosmological analysis the various galaxy cluster properties need to be measured
with high precision and accuracy. To predict these characteristics of eROSITA
galaxy clusters and to optimise optical follow-up observations, we estimate the
precision and the accuracy with which eROSITA will be able to determine galaxy
cluster temperatures and redshifts from X-ray spectra. Additionally, we present
the total number of clusters for which these two properties will be available
from the eROSITA survey directly. During its four years of all-sky surveys,
eROSITA will determine cluster temperatures with relative uncertainties of
Delta(T)/T<10% at the 68%-confidence level for clusters up to redshifts of
z~0.16 which corresponds to ~1,670 new clusters with precise properties.
Redshift information itself will become available with a precision of
Delta(z)/(1+z)<10% for clusters up to z~0.45. Additionally, we estimate how the
number of clusters with precise properties increases with a deepening of the
exposure. Furthermore, the biases in the best-fit temperatures as well as in
the estimated uncertainties are quantified and shown to be negligible in the
relevant parameter range in general. For the remaining parameter sets, we
provide correction functions and factors. The eROSITA survey will increase the
number of galaxy clusters with precise temperature measurements by a factor of
5-10. Thus the instrument presents itself as a powerful tool for the
determination of tight constraints on the cosmological parameters.Comment: accepted for publication in A&A; 17 pages, 20 figure
Studying the Nature of Dark Energy with Galaxy Clusters
We report on the status of our effort to constrain the nature of dark energy
through the evolution of the cluster mass function. Chandra temperature
profiles for 31 clusters from a local cluster sample are shown. The X-ray
appearance of the proto supermassive binary black hole at the center of the
cluster Abell 400 is described. Preliminary weak lensing results obtained with
Megacam@MMT for a redshift z=0.5 cluster from a distant cluster sample are
given.Comment: 5 pages, to appear in: Aschenbach, B., Burwitz, V., Hasinger, G.,
Leibundgut, B. (eds.), Relativistic Astrophysics and Cosmology - Einstein's
Legacy. ESO Astrophysics Symposia, Springer Verlag, Berlin, German
Discovery of large scale shock fronts correlated with the radio halo and radio relic in the A2163 galaxy cluster
Imprints of galaxy cluster formation processes are visible in the
intracluster medium and can arise in shock fronts, which are detectable via
discontinuities in e.g. the gas temperature and density profiles. In this
study, we investigate the X-ray properties of the intracluster gas and the
radio morphology of the extraordinary cluster A2163. This cluster shows an
irregular morphology in various wavelengths and has one of the most luminous
and extended known radio halos. Additionally, it is one of the hottest clusters
known. We analyze two Suzaku observations of A2163, one in the north-east (NE)
and one in the south-west (SW) direction, and use archival XMM-Newton data to
remove point sources in the field of view. To compare our findings in the X-ray
regime with the radio emission, we obtain radio images of the cluster from an
archival VLA observation at 20cm. We identify three shock fronts in A2163 in
our spectral X-ray study. A clear shock front lies in the NE direction at a
distance of 1.4Mpc from the center, with a Mach number of
, estimated from the temperature discontinuity. This shock
coincides with the position of a known radio relic. We identify two additional
shocks in the SW direction, one with at a distance of
0.7Mpc, which is likely related to a cool core remnant, and a strong shock with
at a distance of 1.3Mpc, which also closely matches the
radio contours. The complex structure of A2163 as well as the different Mach
numbers and shock velocities suggest a merging scenario with two unequal
merging constituents, where two shock fronts emerged in an early stage of the
merger and traveled outwards while an additional shock front developed in front
of the merging cluster cores.Comment: 9 pages, 7 figures, accepted by A&
A New X-ray Selected Sample of Very Extended Galaxy Groups from the ROSAT All-Sky Survey
Some indications for tension have long been identified between cosmological
constraints obtained from galaxy clusters and primary CMB measurements.
Typically, assuming the matter density and fluctuations, as parameterized with
Omega_m and sigma_8, estimated from CMB measurements, many more clusters are
expected than those actually observed. One possible explanation could be that
certain types of galaxy groups or clusters were missed in samples constructed
in previous surveys, resulting in a higher incompleteness than estimated. We
aim to determine if a hypothetical class of very extended, low surface
brightness, galaxy groups or clusters have been missed in previous X-ray
cluster surveys based on the ROSAT All-Sky Survey (RASS). We applied a
dedicated source detection algorithm sensitive also to more unusual group or
cluster surface brightness distributions. We found many known but also a number
of new group candidates, which are not included in any previous X-ray / SZ
cluster catalogs. In this paper, we present a pilot sample of 13 very extended
groups discovered in the RASS at positions where no X-ray source has been
detected previously and with clear optical counterparts. The X-ray fluxes of at
least 5 of these are above the nominal flux-limits of previous RASS cluster
catalogs. They have low mass (; i.e., galaxy
groups), are at low redshift (z<0.08), and exhibit flatter surface brightness
distributions than usual. We demonstrate that galaxy groups were missed in
previous RASS surveys, possibly due to the flat surface brightness
distributions of this potential new population. Analysis of the full sample
will show if this might have a significant effect on previous cosmological
parameter constraints based on RASS cluster surveys. (This is a shortened
version of the abstract - full text in the article)Comment: 18 pages, 7 figures, accepted by A&
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