1,759 research outputs found
2XMM J083026+524133: The most X-ray luminous cluster at redshift 1
In the distant universe X-ray luminous clusters of galaxies are rare objects.
Large area surveys are therefore needed to probe the high luminosity end of the
cluster population at redshifts z >= 1. We correlated extended X-ray sources
from the second XMM-Newton source catalogue (2XMM) with the SDSS in order to
identify new clusters of galaxies. Distant cluster candidates in empty SDSS
fields were imaged in the R and z bands with the Large Binocular Telescope. We
extracted the X-ray spectra of the cluster candidates and fitted thermal plasma
models to the data. We determined the redshift 0.99 +-0.03 for 2XMM
J083026+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 x
10^45 erg/sec this is the most X-ray luminous cluster at redshifts z >= 1. We
measured a gas temperature of 8.2 +- 0.9 keV and and estimate a cluster mass
M(500) = 5.6 x 10^14 M(solar). The optical imaging revealed a rich cluster of
galaxies.Comment: New version, as accepted for publication in Astronomy & Astrophysic
Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster
Galaxy clusters form through a sequence of mergers of smaller galaxy clusters
and groups. Models of diffusive shock acceleration (DSA) suggest that in shocks
that occur during cluster mergers, particles are accelerated to relativistic
energies, similar to supernova remnants. Together with magnetic fields these
particles emit synchrotron radiation and may form so-called radio relics. Here
we report the detection of a radio relic for which we find highly aligned
magnetic fields, a strong spectral index gradient, and a narrow relic width,
giving a measure of the magnetic field in an unexplored site of the universe.
Our observations prove that DSA also operates on scales much larger than in
supernova remnants and that shocks in galaxy clusters are capable of producing
extremely energetic cosmic rays.Comment: Published in Science Express on 23 September 2010, 6 figures,
Supporting Online Material included. This is the author's version of the
work. It is posted here by permission of the AAAS for personal use, not for
redistribution. The definitive version was published in Science, volume 330,
15 October 201
Hyperfine Structure in the Rotational Spectrum of GaBr
The hyperfine structure of the rotational transition J = 2 ← 1 in the vicinity of 9.8 GHz was measured. The analysis yielded the following quadrupole coupling constants: e qv Q (79Br)= 105.78(34) + 0.92(38) (υ +1/2) MHz, e qv Q (81Br) = 88.55 (35) + 0.60 (32) (υ +1/2) MHz, e qv Q (69Ga) = − 86.68 (30) + 0.36 (24) (υ +1/2) MHz, e qv Q (71Ga) = − 54.58 (25) MHz. These results are consistent with systematic trends in the group of (III a/VII a) -compounds and remove some discrepencies in the literature. © 1978, Walter de Gruyter. All rights reserved
Diffuse radio emission in the complex merging galaxy cluster Abell 2069
Galaxy clusters with signs for a recent merger show in many cases extended
diffuse radio features. This emission originates from relativistic electrons
which suffer synchrotron losses due to the intra-cluster magnetic field. The
mechanisms of the particle acceleration and the properties of the magnetic
field are still poorly understood. We search for diffuse radio emission in
galaxy clusters. Here, we study the complex galaxy cluster Abell 2069, for
which X-ray observations indicate a recent merger. We investigate the cluster's
radio continuum emission by deep Westerbork Synthesis Radio Telescope (WSRT)
observations at 346 MHz and a Giant Metrewave Radio Telescope (GMRT)
observation at 322 MHz. We find an extended diffuse radio feature roughly
coinciding with the main component of the cluster. We classify this emission as
a radio halo and estimate its lower limit flux density to 25 +/- 9 mJy.
Moreover, we find a second extended diffuse source located at the cluster's
companion and estimate its flux density to 15 +/- 2 mJy. We speculate that this
is a small halo or a mini-halo. If true, this cluster is the first example of a
double-halo in a single galaxy cluster.Comment: 6 pages, 3 figures, accepted for publication in A&
Radio relics in cosmological simulations
Radio relics have been discovered in many galaxy clusters. They are believed
to trace shock fronts induced by cluster mergers. Cosmological simulations
allow us to study merger shocks in detail since the intra-cluster medium is
heated by shock dissipation. Using high resolution cosmological simulations,
identifying shock fronts and applying a parametric model for the radio emission
allows us to simulate the formation of radio relics. We analyze a simulated
shock front in detail. We find a rather broad Mach number distribution. The
Mach number affects strongly the number density of relativistic electrons in
the downstream area, hence, the radio luminosity varies significantly across
the shock surface. The abundance of radio relics can be modeled with the help
of the radio power probability distribution which aims at predicting radio
relic number counts. Since the actual electron acceleration efficiency is not
known, predictions for the number counts need to be normalized by the observed
number of radio relics. For the characteristics of upcoming low frequency
surveys we find that about thousand relics are awaiting discovery.Comment: 10 pages, 4 figures, Invited talk at the conference "Diffuse
Relativistic Plasmas", Bangalore, 1-4 March 2011; in press in special issue
of Journal of Astrophysics and Astronom
Multiple density discontinuities in the merging galaxy cluster CIZA J2242.8+5301
CIZA J2242.8+5301, a merging galaxy cluster at z=0.19, hosts a double-relic
system and a faint radio halo. Radio observations at frequencies ranging from a
few MHz to several GHz have shown that the radio spectral index at the outer
edge of the N relic corresponds to a shock of Mach number 4.6+/-1.1, under the
assumptions of diffusive shock acceleration of thermal particles in the test
particle regime. Here, we present results from new Chandra observations of the
cluster. The Chandra surface brightness profile across the N relic only hints
to a surface brightness discontinuity (<2-sigma detection). Nevertheless, our
reanalysis of archival Suzaku data indicates a temperature discontinuity across
the relic that is consistent with a Mach number of 2.5+/-0.5, in agreement with
previously published results. This confirms that the Mach number at the shock
traced by the N relic is much weaker than predicted from the radio. Puzzlingly,
in the Chandra data we also identify additional inner small density
discontinuities both on and off the merger axis. Temperature measurements on
both sides of the discontinuities do not allow us to undoubtedly determine
their nature, although a shock front interpretation seems more likely. We
speculate that if the inner density discontinuities are indeed shock fronts,
then they are the consequence of violent relaxation of the dark matter cores of
the clusters involved in the merger.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
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