1,506 research outputs found
Cosmic rays in magnetized intracluster plasma
Recent results are reported on Magnetic Fields in Clusters of Galaxies,
Diffuse Radio Emission, and Radio - X-ray connection in Radio Halos.Comment: 2 pages, 1 figure, Invited talk at the JD15 "Magnetic Fields in
Diffuse Media", IAU XXVII General Assembly, Rio de Janeir
Why are central radio relics so rare?
In this paper we address the question why cluster radio relics that are
connected to shock acceleration, so-called radio gischt, have preferentially
been found in the outskirts of galaxy clusters. By identifying merger shock
waves in cosmological grid simulations, we explore several prescriptions for
relating the energy dissipated in shocks to the energy emitted in the radio
band. None of the investigated models produce detectable radio relics within
100-200 kpc from the cluster centre. All models cause > 50 per cent of the
detectable relic emission at projected distances > 800 kpc. Central radio
relics caused by shocks that propagate along the line-of-sight are rare events
for simple geometrical reasons, and they have a low surface brightness making
them elusive for current instruments. Our simulations show that the radial
distribution of observed relics can be explained by the radial trend of
dissipated kinetic energy in shocks, that increases with distance from the
cluster centre up until half of the virial radius.Comment: 6 pages, 4 figures. MNRAS accepte
A Chandra Snapshot Survey for 3C Radio Galaxies with redshifts between 0.3 and 0.5
This paper contains an analysis of short Chandra observations of 19 3C
sources with redshifts between 0.3 and 0.5 not previously observed in the
X-rays. This sample is part of a project to obtain Chandra data for all of the
extragalactic sources in the 3C catalogue. Nuclear X-ray intensities as well as
any X-ray emission associated with radio jet knots, hotspots or lobes have been
measured in 3 energy bands: soft, medium and hard. Standard X-ray spectral
analysis for the 4 brightest nuclei has been also performed. X-ray emission was
detected for all the nuclei of the radio sources in the current sample with the
exception of 3C 435A. There is one compact steep spectrum (CSS) source while
all the others are FRII radio galaxies. X-ray emission from two galaxy clusters
(3C 19 and 3C 320); from 6 hotspots in 4 radio galaxies (3C 16, 3C 19, 3C
268.2, 3C 313); and extended X-ray emission on kpc scales in 3C 187 and 3C 313,
has been detected.Comment: 33 pages, 22 figures, 7 tables, accepted for publication on the ApJ
Supplement Series. arXiv admin note: text overlap with arXiv:1210.602
The intracluster magnetic field power spectrum in Abell 665
The goal of this work is to investigate the power spectrum of the magnetic
field associated with the giant radio halo in the galaxy cluster A665. For
this, we present new deep Very Large Array total intensity and polarization
observations at 1.4 GHz. We simulated Gaussian random three-dimensional
turbulent magnetic field models to reproduce the observed radio halo emission.
By comparing observed and synthetic radio halo images we constrained the
strength and structure of the intracluster magnetic field. We assumed that the
magnetic field power spectrum is a power law with a Kolmogorov index and we
imposed a local equipartition of energy density between relativistic particles
and field. Under these assumptions, we find that the radio halo emission in
A665 is consistent with a central magnetic field strength of about 1.3 micro-G.
To explain the azimuthally averaged radio brightness profile, the magnetic
field energy density should decrease following the thermal gas density, leading
to an averaged magnetic field strength over the central 1 Mpc^3 of about 0.75
micro-G. From the observed brightness fluctuations of the radio halo, we infer
that the outer scale of the magnetic field power spectrum is ~450 kpc, and the
corresponding magnetic field auto-correlation length is ~100 kpc.Comment: 12 pages, 6 figures, accepted for publication on A&A, language
editing. For a high quality version see
http://erg.ca.astro.it/preprints/a665_halo
New giant radio sources and underluminous radio halos in two galaxy clusters
The aim of this work is to analyse the radio properties of the massive and
dynamical disturbed clusters Abell 1451 and Zwcl 0634.1+4750, especially
focusing on the possible presence of diffuse emission. We present new GMRT 320
MHz and JVLA 1.5 GHz observations of these two clusters. We found that both
Abell 1451 and Zwcl 0634.1+4750 host a radio halo with a typical spectrum
(). Similarly to a few other cases reported in the recent
literature, these radio halos are significantly fainter in radio luminosity
with respect to the current radio power-mass correlations and they are smaller
than classical giant radio halos. These underluminous sources might contribute
to shed light on the complex mechanisms of formation and evolution of radio
halos. Furthermore, we detected a candidate radio relic at large distance from
the cluster center in Abell 1451 and a peculiar head tail radio galaxy in Zwcl
0634.1+4750, which might be interacting with a shock front.Comment: 15 pages, 13 figures, accepted for publication in A&
Abell 1033: birth of a radio phoenix
Extended steep-spectrum radio emission in a galaxy cluster is usually
associated with a recent merger. However, given the complex scenario of galaxy
cluster mergers, many of the discovered sources hardly fit into the strict
boundaries of a precise taxonomy. This is especially true for radio phoenixes
that do not have very well defined observational criteria. Radio phoenixes are
aged radio galaxy lobes whose emission is reactivated by compression or other
mechanisms. Here, we present the detection of a radio phoenix close to the
moment of its formation. The source is located in Abell 1033, a peculiar galaxy
cluster which underwent a recent merger. To support our claim, we present
unpublished Westerbork Synthesis Radio Telescope and Chandra observations
together with archival data from the Very Large Array and the Sloan Digital Sky
Survey. We discover the presence of two sub-clusters displaced along the N-S
direction. The two sub-clusters probably underwent a recent merger which is the
cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum
extended radio source very close to an AGN is proposed to be a newly born radio
phoenix: the AGN lobes have been displaced/compressed by shocks formed during
the merger event. This scenario explains the source location, morphology,
spectral index, and brightness. Finally, we show evidence of a density
discontinuity close to the radio phoenix and discuss the consequences of its
presence.Comment: accepted MNRA
Geometrical and physical properties of the 1982-84 deformation source at Campi Flegrei - Italy
Deformation of the ground surface in volcanic areas is generally recognized as a reliable indicator of unrest, possibly
resulting from the intrusion of fresh magma within the shallow rock layers. The intrusion process is usually
represented by a deformation source such as an ellipsoidal pressurized cavity, embedded within a homogeneous and
elastic half-space. Similar source models allow inferring the depth, the location and the (incremental) volume of the
intrusion, which are very important parameters for volcanic risk implications. However, assuming a homogeneous
and elastic rheology and, assigning a priori the shape and the mechanism of the source (within a very restricted
“library” of available solutions) may bias considerably the inference of source parameters. In complete generality,
any point source deformation, including overpressure sources, may be described in terms of a suitable moment
tensor, while the assumption of an overpressure source strongly restricts the variety of allowable moment tensors.
In particular, by assuming a pressurized cavity, we rule out the possibility that either shear failure may precede
magma emplacement (seismically induced intrusion) or may accompany it (mixed tensile and shear mode fracture).
Another possibility is that a pre-existent weakness plane may be chosen by the ascending magma (fracture
toughness heterogeneity). We perform joint inversion of levelling and EDM data (part of latter are unpublished),
collected during the 1982-84 unrest at Campi Flegrei caldera: a 43% misfit reduction is obtained for a general
moment source if the elastic heterogeneities computed from seismic tomography are accouted for. The inferred
source is at 5.2 km depth but cannot be interpreted as a simple pressurized cavity. Moreover, if mass conservation
is accounted for, magma emplaced within a shallow source must come from a (generally deeper) reservoir, which
is usually assumed to be deep enough to be simply neglected. At Campi Flegrei, seismic tomography indicates
that the “deep” magma source is rather shallow (at 7-8 km depth), so that its presence should be included in any
thorough attempt to source modeling. Taking into account a deflating source at 7.5 km depth (represented either as
a horizontal sill or as an isotropic cavity) and an inflating moment source, the fit of both levelling and EDM data
improves further (misfit reduction 80%), but still the best fitting moment source (at 5.5 km depth) falls outside the
range of pressurized ellipsoidal cavities. The shallow moment source may be decomposed in a tensile and a shear
dislocation. No clue is obtained that the shear and the tensile mechanisms should be located in different positions.
Our favourite interpretation is in terms of a crack opening in mixed tensile and shear mode, as would be provided
by fluid magma unwelding pre-stressed solid rock. Although this decomposition of the source is not unique, the
proposed solution is physically motivated by the minimum overpressure requirement. An important implication of
this new interpretation is that the magma emplaced in the shallow moment source during the 1982-84 unrest was
not added to already resident magma at the same position
A new interpretation of the 1982-84 unrest episode at Campi Flegrei caldera (Italy) by numerical inversion
The 1982-84 unrest episode at Campi Flegrei was characterized by huge deformation (about 1.8 m uplift) located inside the caldera and significant gravity variations correlated with the elevation changes (about -213 Gal/m). Due to the bell shape of the uplift, the source is usually interpreted to have a fixed spherical shape.
In the present study, we combine simple point source mechanisms (dipoles and double couples) to represent arbitrary sources such as sphere, ellipsoid or sill.
The models are realized by Finite Element and the medium may be characterized by elastic heterogeneities.
We study the deformation detected by leveling and EDM techniques by coupling the FE forward models with an inversion procedure.
The potential point sources are contained in a volume of 888 km located beneath Pozzuoli, the site of maximum displacement.
We calculate the displacement field at each data point for each basic mechanism and we compare the result with the observed value.
From the inversion of geodetic data we retrieve the best-fitting source parameters, without fixing the shape a priori.
The best-fitting source is located beneath Pozzuoli at about 4.8 km b.s.l. and undergoes to horizontal compression and vertical dilatation
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