407 research outputs found
Occurence and Luminosity Functions of Giant Radio Halos from Magneto-Turbulent Model
We calculate the probability to form giant radio halos (~ 1 Mpc size) as a
function of the mass of the host clusters by using a Statistical
Magneto-Turbulent Model (Cassano & Brunetti, these proceedings). We show that
the expectations of this model are in good agreement with the observations for
viable values of the parameters. In particular, the abrupt increase of the
probability to find radio halos in the more massive galaxy clusters (M >
2x10^{15} solar masses) can be well reproduced. We calculate the evolution with
redshift of such a probability and find that giant radio halos can be powered
by particle acceleration due to MHD turbulence up to z~0.5 in a LCDM cosmology.
Finally, we calculate the expected Luminosity Functions of radio halos (RHLFs).
At variance with previous studies, the shape of our RHLFs is characterized by
the presence of a cut-off at low synchrotron powers which reflects the
inefficiency of particle acceleration in the case of less massive galaxy
clusters.Comment: 4 pages, to appear in a dedicated issue of the Journal of the Korean
Astronomical Society (JKAS). Proceedings of the "International conference on
Cosmic Rays and Magnetic Fields in Large Scale Structure", Busan, Korea, 200
Rise and Fall of Radio Halos in Simulated Merging Galaxy Clusters
We present the first high resolution MHD simulation of cosmic-ray electron
reacceleration by turbulence in cluster mergers. We use an idealised model for
cluster mergers, combined with a numerical model for the injection, cooling and
reacceleration of cosmic-ray electrons, to investigate the evolution of cluster
scale radio emission in these objects. In line with theoretical expectations,
we for the first time, show in a simulation that reacceleration of CRe has the
potential to reproduce key observables of radio halos. In particular, we show
that clusters evolve being radio loud or radio quiet, depending on their
evolutionary stage during the merger. We thus recover the observed transient
nature of radio halos. In the simulation the diffuse emission traces the
complex interplay between spatial distribution of turbulence injected by the
halo infall and the spatial distribution of the seed electrons to reaccelerate.
During the formation and evolution of the halo the synchrotron emission spectra
show the observed variety: from power-laws with spectral index of 1 to 1.3 to
curved and ultra-steep spectra with index
Radio-continuum surveys with SKA and LOFAR: a first look at the perspectives for radio mini-halos
Diffuse synchrotron radio emission has been observed in a number of cool-core
clusters on scales comparable to that of the cooling region. These radio
sources are called `mini-halos'. In order to understand their origin, which is
still unclear, joint radio and X-ray statistical studies of large cluster
samples are necessary to investigate the radio mini-halo properties and their
connection with the cluster thermodynamics. We here extend our previous
explorative study and investigate the perspectives offered by surveys in the
radio continuum with LOFAR and SKA, in particular examining the effect of the
intra-cluster magnetic field in the mini-halo region for the first time. By
considering the minimum flux detectable in radio surveys and exploiting the
correlation observed for known mini-halos, we estimate the
detection limits achievable by future radio observational follow-up of X-ray
cluster samples, such as HIFLUGCS and eROSITA. This allows us to estimate the
maximum number of radio mini-halos that can potentially be discovered in future
surveys as a function of redshift and magnetic field strength. We show that
future radio surveys with LOFAR and SKA1 (at 140 MHz and 1.4 GHz) have the
potential to discover ~1,000-10,000 radio mini-halo candidates up to redshift
z=1. We further note that future SKA1 radio surveys at redshift z>0.6 will
allow us to distinguish between different magnetic fields in the mini-halo
region, because higher magnetic fields are expected to produce more powerful
mini-halos, thus implying a larger number of mini-halo detected at high
redshift. For example, the non-detection with SKA1 of mini-halos at z>0.6 will
suggest a low magnetic field (B < few G). The synergy of these radio
surveys with future X-ray observations and theoretical studies is essential in
establishing the radio mini-halo physical nature. [abridged]Comment: Accepted for publication in A&A; 9 pages, 9 figures. Revised to match
the corrected version after language editin
Occurrence of radio minihalos in a mass-limited sample of galaxy clusters
We investigate the occurrence of radio minihalos --- diffuse radio sources of
unknown origin observed in the cores of some galaxy clusters --- in a
statistical sample of 58 clusters drawn from the Planck Sunyaev-Zel'dovich
cluster catalog using a mass cut (). We
supplement our statistical sample with a similarly-sized non-statistical sample
mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray
and radio data, which includes lower-mass clusters. Where necessary (for 9
clusters), we reanalyzed the Very Large Array archival radio data to determine
if a mihinalo is present. Our total sample includes all 28 currently known and
recently discovered radio minihalos, including 6 candidates. We classify
clusters as cool-core or non-cool core according to the value of the specific
entropy floor in the cluster center, rederived or newly derived from the
Chandra X-ray density and temperature profiles where necessary (for 27
clusters). Contrary to the common wisdom that minihalos are rare, we find that
almost all cool cores - at least 12 out of 15 (80%) - in our complete sample of
massive clusters exhibit minihalos. The supplementary sample shows that the
occurrence of minihalos may be lower in lower-mass cool-core clusters. No
minihalos are found in non-cool-cores or "warm cores". These findings will help
test theories of the origin of minihalos and provide information on the
physical processes and energetics of the cluster cores.Comment: 34 pages, accepted for publication in ApJ. Added a section
"Definition of a minihalo" and an appendix "Radio size and average surface
brigthtness of minihalos and halos
Low frequency follow up of radio halos and relics in the GMRT Radio Halo Cluster Survey
We performed GMRT low frequency observations of the radio halos, relics and
new candidates belonging to the GMRT Radio Halo Cluster Sample first observed
at 610 MHz. High sensitivity imaging was performed using the GMRT at 325 MHz
and 240 MHz. The properties of the diffuse emission in each cluster were
compared to our 610 MHz images and/or literature information available at other
frequencies, in order to derive the integrated spectra over a wide frequency
range.Beyond the classical radio halos, whose spectral index is in the
range (S), we found sources with
. This result supports the idea that the spectra of the
radiating particles in radio halos is not universal, and that inefficient
mechanisms of particle acceleration are responsible for their origin. We also
found a variety of brightness distributions, i.e. centrally peaked as well as
clumpy halos. Even though the thermal and relativistic plasma tend to occupy
the same cluster volume, in some cases a positional shift between the radio and
X-ray peaks of emission is evident. Our observations also revealed the
existence of diffuse cluster sources which cannot be easily classified either
as halos or relics. New candidate relics were found in A1300 and in A1682, and
in some clusters "bridges" of radio emission have been detected, connecting the
relic and radio halo emission. Combining our new data with literature
information, we derived the LogL-LogP correlation for
radio halos, and investigated the possible trend of the spectral index of radio
halos with the temperature of the intracluster medium.Comment: 19 pages, 10 figures, 5 tables, accepted for publication on A&
Intrinsic and Extrinsic Stability of Ovonic-Switching Devices
The time evolution of current and voltage in Ovonicswitching devices is affected, on one side, by parasitic
elements due to contacts and connectors and, on the other one, by the internal-relaxation mechanisms of the material itself. The two aspects, respectively termed here \u201cintrinsic\u201d and \u201cextrinsic\u201d dynamics, are investigated in this paper on the basis of the time-dependent, trap-limited conduction model proposed by the authors for investigating this type of devices
Transport scaling limits of ovonic Devices: a simulative approach
The transport scaling limits of Ovonic devices are studied by means of a numerical solution of a time- and space-dependent transport models based on a set of equations that provide a good physical grasp of the microscopic process at hand. The predictivity of the approach has been confirmed through the comparison with recent experimental results where the parasitic effects have been reduced by the use of top-technology measuring equipments. The present analysis is performed for the AgInSbTe chalcogenide, since this material exibits a steep threshold-switching dynamics which makes it promising for high-speed non-volatile memory applications
Vinca il migliore. L’agonismo nel mondo naturale dal nucleo atomico alla scala cosmica
Il progetto 2016 del ciclo La Curiosità fa lo Scienziato ha proposto una riflessione sui temi della competizione e collaborazione nel mondo naturale, coinvolgendo gli ambiti disciplinari di fisica, chimica e scienze naturali.
In tutto il mondo naturale osservabile equilibri e dinamiche sono riconducibili ad entità in competizione, secondo regole che si vanno scoprendo man mano che aumenta la capacità di interrogare la natura. In fisica entrano nell’arena poche forze fondamentali, dalla micro scala atomica fino alla macro scala cosmologica. In chimica diversi composti competono nel determinare l’evoluzione e gli effetti delle reazioni. Nel mondo biologico il rapporto che si stabilisce in un determinato ambiente tra le varie specie animali e vegetali, legate tra loro da relazioni antagonistiche, ma anche mutualistiche, è condizione indispensabile per la sopravvivenza.
Gli incontri previsti all’interno del Progetto, dedicati a platee di età diverse, hanno sempre incluso conversazioni a tema arricchite da esperimenti includendo in talune circostanze anche esecuzioni di brani musicali. Le attività hanno complessivamente raggiunto un pubblico di circa 1200 persone.
Nel seguito vengono elencate e brevemente descritte le varie attività svolte. Il materiale documentale prodotto durante il Progetto è messo a disposizione a libero accesso sul sito del Dipartimento di Scienze Fisiche, Informatiche e Matematiche dell’Università di Modena e Reggio Emilia al link:
http://www.outreach.fim.unimore.it/site/home/divulgazione/la-curiosita-fa-lo-scienziat
Discovery of a radio relic in the low mass, merging galaxy cluster PLCK G200.9-28.2
Radio relics at the peripheries of galaxy clusters are tracers of the elusive
cluster merger shocks. We report the discovery of a single radio relic in the
galaxy cluster PLCK G200.9-28.2 (, ) using the Giant Metrewave Radio Telescope at 235 and 610 MHz and
the Karl G. Jansky Very Large Array at 1500 MHz. The relic has a size of Mpc, an arc-like morphology and is located at 0.9 Mpc from the
X-ray brightness peak in the cluster. The integrated spectral index of the
relic is . The spectral index map between 235 and 610 MHz shows
steepening from the outer to the inner edge of the relic in line with the
expectation from a cluster merger shock. Under the assumption of diffusive
shock acceleration, the radio spectral index implies a Mach number of
for the shock. The analysis of archival XMM Newton data shows that
PLCK G200.9-28.2 consists of a northern brighter sub-cluster, and a southern
sub-cluster in a state of merger. This cluster has the lowest mass among the
clusters hosting single radio relics. The position of the Planck Sunyaev
Ze'ldovich effect in this cluster is offset by 700 kpc from the X-ray peak in
the direction of the radio relic, suggests a physical origin for the offset.
Such large offsets in low mass clusters can be a useful tool to select
disturbed clusters and to study the state of merger.Comment: 10 pages, 7 figures, 4 tables. Accepted for publication in MNRA
Intrinsic Electric Oscillations of Ovonic Devices towards the TeraHerz limit
The time-dependent response of Ovonic devices to an electric potential ramp signal
is analysed by means of an enhanced version of a previously published time-dependent charge-
transport model proposed by the authors. Depending on the inevitable parasitics of the system,
either stable or oscillating solutions are found according to the position of the load line. The
model also allows for speculations on the potential of Ovonic materials in the design of high-
frequency oscillating circuits close to the terahertz range
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