166 research outputs found
Measuring the correlation length of intergalactic magnetic fields from observations of gamma-ray induced cascades
Context. The imaging and timing properties of {\gamma}-ray emission from
electromagnetic cascades initiated by very-high-energy (VHE) {\gamma}-rays in
the intergalactic medium depend on the strength B and correlation length
{\lambda}B of intergalactic magnetic fields (IGMF). Aims. We study the
possibility of measuring both B and {\lambda}B via observations of the cascade
emission with {\gamma}-ray telescopes. Methods. For each measurement method, we
find two characteristics of the cascade signal, which are sensitive to the IGMF
B and {\lambda}B values in different combinations. For the case of IGMF
measurement using the observation of extended emission around extragalactic VHE
{\gamma}-ray sources, the two characteristics are the slope of the surface
brightness profile and the overall size of the cascade source. For the case of
IGMF measurement from the time delayed emission, these two characteristics are
the initial slope of the cascade emission light curve and the overall duration
of the cascade signal. Results. We show that measurement of the slope of the
cascade induced extended emission and/or light curve can both potentially
provide measure of the IGMF correlation length, provided it lies within the
range 10 kpc< {\lambda}B <1 Mpc. For correlation lengths outside this range,
gamma-ray observations can provide upper or lower bound on {\lambda}B. The
latter of the two methods holds great promise in the near future for providing
a measurement/constraint using measurements from present/next-generation
{\gamma}-ray-telescopes. Conclusions. Measurement of the IGMF correlation
length will provide an important constraint on its origin. In particular, it
will enable to distinguish between an IGMF of galactic wind origin from an IGMF
of cosmological origin.Comment: 5 pages, 3 figure
An exploration of hadronic interactions in blazars using IceCube
Context: Hadronic models, involving matter (proton or nuclei) acceleration in
blazar jets, imply high energy photon and neutrino emissions due to
interactions of high-energy protons with matter and/or radiation in the source
environment. Aims: This paper shows that the sensitivity of the IceCube
neutrino telescope in its 40-string configuration (IC-40) is already at the
level of constraining the parameter space of purely hadronic scenarios of
activity of blazars. Methods: Assuming that the entire source power originates
from hadronic interactions, and assuming that the models describe the data, we
estimate the expected neutrino flux from blazars based on the observed
gamma-ray flux by Fermi, simultaneously with IC-40 observations. We consider
two cases separately to keep the number of constrainable parameters at an
acceptable level: proton-proton or proton-gamma interactions are dominant.
Comparing the IC-40 sensitivity to the neutrino flux expected from some of the
brightest blazars, we constrain model parameters characterizing the parent
high-energy proton spectrum. Results: We find that when pp interactions
dominate, some constraints on the primary proton spectrum can be imposed. For
instance, for the tightest constrained source 3C 454.3, the very high energy
part of the spectra of blazars is constrained to be harder than E^-2 with
cut-off energies in the range of Ecut >10^18 eV. When interactions of
high-energy protons on soft photon fields dominate, we can find similarly tight
constraints on the proton spectrum parameters. [abridged]Comment: accepted for publication in A&
Neutrino signal from extended Galactic sources in IceCube
We explore the detectability of the neutrino flux from the entire Galactic
Plane or from a part of it with IceCube. We calculate the normalization and the
spectral index of the neutrino power law spectrum from different regions of the
Galactic plane, based on the observed spectral characteristics of the pion
decay gamma-ray diffuse emission observed by the Fermi/LAT telescope in the
energy band above 100 GeV. We compare the neutrino flux calculated in this way
with the sensitivity of IceCube for the detection of extended sources. Assuming
a binned extended source analysis method, we find that the only possible
evidence for neutrino emission for sources located in the Northern hemisphere
is from the Cygnus region after 20 years of exposure. For other parts of the
Galactic Plane even a 20 years exposure with IceCube is not sufficient for the
detection. Taking into account marginal significance of the detectable source
in the Cygnus region, we find a precise position and size of the source region
which optimizes the signal-to-noise ratio for neutrinos. We also calculate the
low-energy threshold above which the neutrino signal could be detected with the
highest signal-to-noise ratio. This calculation of precise source position,
size and energy range, based on the gamma-ray data, could be used to remove the
'trial factor' in the analysis of the real neutrino data of IceCube. We notice
that the diffuse neutrino emission from the inner Galactic Plane in the
Southern Hemisphere is much brighter. A neutrino detector with characteristics
equivalent to IceCube, but placed at the Northern Hemisphere (such as KM3NeT),
would detect several isolated neutrino sources in the Galactic Plane within
just 5 years exposure at 5{\sigma} level. These isolated sources of ~TeV
neutrinos would unambiguously localize sources of cosmic rays which operated
over the last 10 thousand years in the Galaxy.[abridged]Comment: submitted to A&
Reconstructing the projected gravitational potential of Abell 1689 from X-ray measurements
Context. Galaxy clusters can be used as cosmological probes, but to this end,
they need to be thoroughly understood. Combining all cluster observables in a
consistent way will help us to understand their global properties and their
internal structure. Aims. We provide proof of the concept that the projected
gravitational potential of galaxy clusters can directly be reconstructed from
X-ray observations. We also show that this joint analysis can be used to
locally test the validity of the equilibrium assumptions in galaxy clusters.
Methods. We used a newly developed reconstruction method, based on
Richardson-Lucy deprojection, that allows reconstructing projected
gravitational potentials of galaxy clusters directly from X-ray observations.
We applied this algorithm to the well-studied cluster Abell 1689 and compared
the gravitational potential reconstructed from X-ray observables to the
potential obtained from gravitational lensing measurements. [...] Results.
Assuming spherical symmetry and hydrostatic equilibrium, the potentials
recovered from gravitational lensing and from X-ray emission agree very well
beyond 500 kpc. Owing to the fact that the Richardson-Lucy deprojection
algorithm allows deprojecting each line of sight independently, this result may
indicate that non-gravitational effects and/or asphericity are strong in the
central regions of the clusters. Conclusions. We demonstrate the robustness of
the potential reconstruction method based on the Richardson-Lucy deprojection
algorithm and show that gravitational lensing and X-ray emission lead to
consistent gravitational potentials. Our results illustrate the power of
combining galaxy-cluster observables in a single, non-parametric, joint
reconstruction of consistent cluster potentials that can be used to locally
constrain the physical state of the gas.Comment: 8 pages, 4 figures. Accepted in A&
Can early MRI distinguish between Kingella kingae and Gram-positive cocci in osteoarticular infections in young children?
Background: K. kingae is a common causative organism in acute osteoarticular infections (OAIs) in children under 4years of age. Differentiation between K. kingae and Gram-positive cocci (GPC) is of great interest therapeutically. Objective: Our aim was to identify early distinguishing MRI features of OAIs. Materials and methods: Thirty-one children younger than 4years of age with OAI underwent MRI at presentation. Of these, 21 were caused by K. kingae and ten by GPC. Bone and soft tissue reaction, epiphyseal cartilage involvement, bone and subperiosteal abscess formation were compared between the two groups. Interobserver agreement was measured. Results: Bone reaction was less frequent (P = 0.0066) and soft tissue reaction less severe (P = 0.0087) in the K. kingae group. Epiphysis cartilage abscesses were present only in the K. kingae group (P = 0.0118). No difference was found for bone abscess (P = 0.1411), subperiosteal abscess (P = 1) or joint effusion (P = 0.4414). Interobserver agreement was good for all criteria. Conclusion: MRI is useful in differentiating K. kingae from GPC in OAI. Cartilaginous involvement and modest soft tissue and bone reaction suggest K. kinga
Joint cluster reconstructions: Combining free-form lensing and X-rays
Galaxy clusters provide a multitude of observational data across wavelengths
and their structure and morphology are of considerable interest in cosmology as
well as astrophysics. We develop a framework that allows the combination of
lensing and non-lensing observations in a free-form and mesh-free approach to
infer the projected mass distribution of individual galaxy clusters. This
method can be used to test common assumptions on the morphology of clusters in
parametric models. We make use of the lensing reconstruction code SaWLens2 and
expand its capabilities by incorporating an estimate of the projected
gravitational potential based on X-ray data that are deprojected using the
local Richardson-Lucy method and used to infer the Newtonian potential of the
cluster and we discuss how potentially arising numerical artefacts can be
treated. We demonstrate the feasibility of our method on a simplified mock NFW
halo and on a cluster from a realistic hydrodynamical simulation and show how
the combination of X-ray and weak lensing data can affect a free-form
reconstruction, improving the accuracy in the central region in some cases by a
factor of two.Comment: 14 pages, 19 figures, submitted to A&A; revised to match the accepted
versio
Reconstruction of the two-dimensional gravitational potential of galaxy clusters from X-ray and Sunyaev-Zel'dovich measurements
The mass of galaxy clusters is not a direct observable, nonetheless it is
commonly used to probe cosmological models. Based on the combination of all
main cluster observables, that is, the X-ray emission, the thermal
Sunyaev-Zel'dovich (SZ) signal, the velocity dispersion of the cluster
galaxies, and gravitational lensing, the gravitational potential of galaxy
clusters can be jointly reconstructed. We derive the two main ingredients
required for this joint reconstruction: the potentials individually
reconstructed from the observables and their covariance matrices, which act as
a weight in the joint reconstruction. We show here the method to derive these
quantities. The result of the joint reconstruction applied to a real cluster
will be discussed in a forthcoming paper. We apply the Richardson-Lucy
deprojection algorithm to data on a two-dimensional (2D) grid. We first test
the 2D deprojection algorithm on a -profile. Assuming hydrostatic
equilibrium, we further reconstruct the gravitational potential of a simulated
galaxy cluster based on synthetic SZ and X-ray data. We then reconstruct the
projected gravitational potential of the massive and dynamically active cluster
Abell 2142, based on the X-ray observations collected with XMM-Newton and the
SZ observations from the Planck satellite. Finally, we compute the covariance
matrix of the projected reconstructed potential of the cluster Abell 2142 based
on the X-ray measurements collected with XMM-Newton. The gravitational
potentials of the simulated cluster recovered from synthetic X-ray and SZ data
are consistent, even though the potential reconstructed from X-rays shows
larger deviations from the true potential. Regarding Abell 2142, the projected
gravitational cluster potentials recovered from SZ and X-ray data reproduce
well the projected potential inferred from gravitational-lensing observations.
(abridged)Comment: accepted for publication in the journal A&
The XMM Cluster Outskirts Project (X-COP): Physical conditions to the virial radius of Abell 2142
Context. Galaxy clusters are continuously growing through the accretion of
matter in their outskirts. This process induces inhomogeneities in the gas
density distribution (clumping) which need to be taken into account to recover
the physical properties of the intracluster medium (ICM) at large radii. Aims.
We studied the thermodynamic properties in the outskirts (R > R500) of the
massive galaxy cluster Abell 2142 by combining the Sunyaev Zel'dovich (SZ)
effect with the X-ray signal. Methods. We combined the SZ pressure profile
measured by Planck with the XMM-Newton gas density profile to recover radial
profiles of temperature, entropy and hydrostatic mass out to 2R500. We used a
method that is insensitive to clumping to recover the gas density, and we
compared the results with traditional X-ray measurement techniques. Results.
When taking clumping into account, our joint SZ/X-ray entropy profile is
consistent with the predictions from pure gravitational collapse, whereas a
significant entropy flattening is found when the effect of clumping is
neglected. The hydrostatic mass profile recovered using joint X-ray/SZ data
agrees with that obtained from spectroscopic X-ray measurements and with mass
reconstructions obtained through weak lensing and galaxy kinematics.
Conclusions. We found that clumping can explain the entropy flattening observed
by Suzaku in the outskirts of several clusters. When using a method insensitive
to clumping for the reconstruction of the gas density, the thermodynamic
properties of Abell 2142 are compatible with the assumption that the thermal
gas pressure sustains gravity and that the entropy is injected at accretion
shocks, with no need to evoke more exotic physics. Our results highlight the
need for X-ray observations with sufficient spatial resolution, and large
collecting area, to understand the processes at work in cluster outer regions.Comment: 22 pages, 32 figures, accepted in the journal A&
Characterizing galaxy clusters by their gravitational potential: systematics of cluster potential reconstruction
Context. Biases in mass measurements of galaxy clusters are one of the major
limiting systematics in constraining cosmology with clusters. Aims. We aim to
demonstrate that the systematics associated with cluster gravitational
potentials are smaller than the hydrostatic mass bias and that cluster
potentials could therefore be a good alternative to cluster masses in
cosmological studies. Methods. Using cosmological simulations of galaxy
clusters, we compute the biases in the hydrostatic mass (HE mass) and those in
the gravitational potential, reconstructed from measurements at X-ray and
millimeter wavelengths. In particular, we investigate the effects of the
presence of substructures and of non-thermal pressure support on both the HE
mass and the reconstructed potential. Results. We find that the bias in the
reconstructed potential (6%) is less than that of the HE mass (13%), and that
the scatter in the reconstructed potential decreases by about 35% with respect
to that in the HE mass. Conclusions. This study shows that characterizing
galaxy clusters by their gravitational potential is a promising alternative to
using cluster masses in cluster cosmology.Comment: submitted to the journal A&A, 16 pages, 26 figure
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