441 research outputs found
Numerical Procedures for the Calculation of the Stresses in Monocoques II : Diffusion of Tensiles Stringer Loads in Reinforced Flat Panels with Cut-outs
Numerical procedures for the calculation of the stresses in Monocoques I : diffusion of tensile stringer loads in reinforced panels
Prospects of detecting gamma-ray emission from galaxy clusters: cosmic rays and dark matter annihilations
We study the possibility for detecting gamma-ray emission from galaxy
clusters. We consider 1) leptophilic models of dark matter (DM) annihilation
that include a Sommerfeld enhancement (SFE), 2) different representative
benchmark models of supersymmetric DM, and 3) cosmic ray (CR) induced pion
decay. Among all clusters/groups of a flux-limited X-ray sample, we predict
Virgo, Fornax and M49 to be the brightest DM sources and find a particularly
low CR-induced background for Fornax. For a minimum substructure mass given by
the DM free-streaming scale, cluster halos maximize the substructure boost for
which we find a factor above 1000. Since regions around the virial radius
dominate the annihilation flux of substructures, the resulting surface
brightness profiles are almost flat. This makes it very challenging to detect
this flux with imaging atmospheric Cherenkov telescopes. Assuming cold dark
matter with a substructure mass distribution down to an Earth mass and using
extended Fermi upper limits, we rule out the leptophilic models in their
present form in 28 clusters, and limit the boost from SFE in M49 and Fornax to
be < 5. This corresponds to a limit on SFE in the Milky Way of < 3, which is
too small to account for the increasing positron fraction with energy as seen
by PAMELA and challenges the DM interpretation. Alternatively, if SFE is
realized in Nature, this would imply a limiting substructure mass of M_lim >
10^4 M_sol - a problem for structure formation. Using individual cluster
observations, it will be challenging for Fermi to constrain our selection of DM
benchmark models without SFE. The Fermi upper limits are, however, closing in
on our predictions for the CR flux using an analytic model based on
cosmological hydrodynamical cluster simulations. We limit the CR-to-thermal
pressure in nearby bright galaxy clusters of the Fermi sample to < 10% and in
Norma and Coma to < 3%.Comment: 43 pages, 23 figures, 10 tables. Accepted for publication in Phys.
Rev. D: streamlined paper, added a paragraph about detectability to
introduction, few references added, and few typos correcte
AWM 4 - an isothermal cluster observed with XMM-Newton
We present analysis of an XMM observation of the poor cluster AWM 4. The
cluster is relaxed and its X-ray halo is regular with no apparent substructure.
Azimuthally averaged radial spectral profiles suggest that the cluster is
isothermal to a radius of at least 160 kpc, with no evidence of a central
cooling region. Spectral mapping shows some significant temperature and
abundance substructure, but no evidence of strong cooling in the cluster core.
Abundance increases in the core, but not to the extent expected, and we find
some indication of gas mixing. Modeling the three dimensional properties of the
system, we show that ongoing heating by an AGN in the dominant elliptical, NGC
6051, is likely to be responsible for the lack of cooling. We also compare AWM
4 to MKW 4, a cluster of similar mass observed recently with XMM. While the two
systems have similar gravitational mass profiles, MKW 4 has a cool core and
somewhat steeper gas density profile, which leads to a lower core entropy. AWM
4 has a considerably larger gas fraction at 0.1 R200, and we show that these
differences result from the difference in mass between the two dominant
galaxies and the activity cycles of their AGN. We estimate the energy required
to raise the temperature profile of MKW 4 to match that of AWM 4 to be 9x10^58
erg, or 3x10^43 erg/s for 100 Myr, comparable to the likely power output of the
AGN in AWM 4.Comment: Accepted for publication in MNRAS, 18 pages, 1 colour and 11 b&w
postscript figures, corrected author affiliatio
AGN feedback and gas mixing in the core of NGC 4636
Chandra observations of NGC 4636 show disturbances in the galaxy X-ray halo,
including arm-like high surface brightness features (tentatively identified as
AGN driven shocks) and a possible cavity on the west side of the galaxy core.
We present Chandra and XMM spectral maps of NGC 4636 which confirm the presence
of the cavity and show it to be bounded by the arm features. The maps also
reveal a ~15 kpc wide plume of low temperature, high abundance gas extending
25-30 kpc to the southwest of the galaxy. The cavity appears to be embedded in
this plume, and we interpret the structure as being entrained gas drawn out of
the galaxy core during previous episodes of AGN activity. The end of the plume
is marked by a well defined edge, with significant falls in surface brightness,
temperature and abundance, indicating a boundary between galaxy and
group/cluster gas. This may be evidence that as well as preventing gas cooling
through direct heating, AGN outbursts can produce significant gas mixing,
disturbing the temperature structure of the halo and transporting metals out
from the galaxy into the surrounding intra-group medium.Comment: 4 Pages, 2 colour figures, accepted for publication in ApJ Letters.
Version with high quality images at
http://hea-www.harvard.edu/~ejos/files/N4636_hires.pd
Nonthermal Bremsstrahlung and Hard X-ray Emission from Clusters of Galaxies
We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray
(HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these
models, the HXR emission is due to suprathermal electrons with energies of
about 10-200 keV. Under the assumption that the suprathermal electrons form
part of a continuous spectrum of electrons including highly relativistic
particles, we have calculated the inverse Compton (IC) extreme ultraviolet
(EUV), HXR, and radio synchrotron emission by the extensions of the same
populations. For accelerating electron models with power-law momentum spectra
(N[p] propto p^{- mu}) with mu <~ 2.7, which are those expected from strong
shock acceleration, the IC HXR emission exceeds that due to NTB. Thus, these
models are only of interest if the electron population is cut-off at some upper
energy <~1 GeV. Similarly, flat spectrum accelerating electron models produce
more radio synchrotron emission than is observed from clusters if the ICM
magnetic field is B >~ 1 muG. The cooling electron model produces vastly too
much EUV emission as compared to the observations of clusters. We have compared
these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB
models require a nonthermal electron population which contains about 3% of the
number of electrons in the thermal ICM. If the suprathermal electron population
is cut-off at some energy above 100 keV, then the models can easily fit the
observed HXR fluxes and spectral indices in both clusters. For accelerating
electron models without a cutoff, the electron spectrum must be rather steep >~
2.9.Comment: Accepted for publication in the Astrophysical Journal. 10 pages with
5 embedded Postscript figures in emulateapj.sty. An abbreviated abstract
follow
A Chandra Observation of Abell 13: Investigating the Origin of the Radio Relic
We present results from the Chandra X-ray observation of Abell 13, a galaxy
cluster that contains an unusual noncentral radio source, also known as a radio
relic. This is the first pointed X-ray observation of Abell 13, providing a
more sensitive study of the properties of the X-ray gas. The X-ray emission
from Abell 13 is extended to the northwest of the X-ray peak and shows
substructure indicative of a recent merger event. The cluster X-ray emission is
centered on the bright galaxy H of Slee et al. 2001. We find no evidence for a
cooling flow in the cluster. A knot of excess X-ray emission is coincident with
the other bright elliptical galaxy F. This knot of emission has properties
similar to the enhanced emission associated with the large galaxies in the Coma
cluster.
With these Chandra data we are able to compare the properties of the hot
X-ray gas with those of the radio relic from VLA data, to study the interaction
of the X-ray gas with the radio emitting electrons. Our results suggest that
the radio relic is associated with cooler gas in the cluster. We suggest two
explanations for the coincidence of the cooler gas and radio source. First, the
gas may have been uplifted by the radio relic from the cluster core.
Alternatively, the relic and cool gas may have been displaced from the central
galaxy during the cluster merger event.Comment: 11 pages, 9 figures, Accepted for Publication in the Astrophysical
Journal, higher-resolution figures can be found at
http://www.astro.virginia.edu/~amj3r/Abell13
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