3,285 research outputs found
Clan structure analysis and new physics signals in pp collisions at LHC
The study of possible new physics signals in global event properties in pp
collisions in full phase space and in rapidity intervals accessible at LHC is
presented. The main characteristic is the presence of an elbow structure in
final charged particle MD's in addition to the shoulder observed at lower c.m.
energies.Comment: 9 pages, talk given at Focus on Multiplicity (Bari, Italy, June 2004
Scenarios for multiplicity distributions in pp collisions in the TeV energy region
Possible scenarios based on available experimental data and phenomenological
knowledge of the GeV energy region are extended to the TeV energy region in the
framework of the weighted superposition mechanism of soft and semi-hard events.
KNO scaling violations, forward-backward multiplicity correlations, Hq vs. q
oscillations and shoulder structures are discussed.Comment: 10 pages, 10 figures, talk given at "Focus on Multiplicity" (Bari,
Italy, June 2004
Forward-backward multiplicity correlations and leakage parameter behaviour in asymmetric high energy collisions
Continuing previous work, forward-backward multiplicity correlations are
studied in asymmetric collisions in the framework of the weighted superposition
mechanism of different classes of events. New parameters for the asymmetric
clan distribution and for the particle leakage from clans in one hemisphere to
the opposite one are introduced to effectively classify different classes of
collisions. This tool should be used to explore forward-backward multiplicity
correlations in AB and pA collisions in present and future experiments at RHIC
and LHC.Comment: 14 pages, 2 figures, latex 2e with amsmat
The physics of the radio emission in the quiet side of the AGN population with the SKA
Despite targets of many multiwavelength campaigns, the main physical
processes at work in AGN are still under debate. In particular the origin of
the radio emission and the mechanisms involved are among the open questions in
astrophysics. In the radio-loud AGN population the radio emission is linked to
the presence of bipolar outflows of relativistic jets. However, the large
majority of the AGN population do not form powerful highly-relativistic jets on
kpc scales and are characterized by radio luminosity up to 10^23 W/Hz at 1.4
GHz, challenging our knowledge on the physical processes at the basis of the
radio emission in radio-quiet objects. The main mechanisms proposed so far are
synchrotron radiation from mildly relativistic mini-jets, thermal
cyclo-synchrotron emission by low-efficiency accretion flow (like ADAF or
ADIOS), or thermal free-free emission from the X-ray heated corona or wind. The
difficulty in understanding the main mechanism involved is related to the
weakness of these objects, which precludes the study of non-local radio-quiet
AGN. Multifrequency, high-sensitivity radio observations are crucial to
constrain the nature of the power engine, and they may help in distinguishing
between the contribution from star formation and AGN activity. The advent of
the SKA, with its sub-arcsecond resolution and unprecedented sensitivity will
allow us to investigate these processes in radio-quiet AGN, even at high
redshift for the first time. Both the broad-band radio spectrum and the
polarization information will help us in disentangling between non-thermal and
thermal origin of the radio emission. The jump in sensitivity of a few order of
magnitudes at the (sub-)uJy level will enable us to detect radio emission from
a large number of radio-quiet AGN at high redshift, providing a fundamental
step in our understanding of their cosmological evolution. (Abridged)Comment: 7 pages, to appear as part of 'Continuum Science' in Proceedings of
'Advancing Astrophysics with the SKA (AASKA14)
A double radio halo in the close pair of galaxy clusters Abell 399 and Abell 401
Radio halos are faint radio sources usually located at the center of merging
clusters of galaxies. These diffuse radio sources are rare, having so far been
found only in about 30 clusters of galaxies, suggesting that particular
conditions are needed to form and maintain them. It is interesting to
investigate the presence of radio halos in close pairs of interacting clusters
in order to possibly clarify their origin in relation to the evolutionary state
of the merger. In this work, we study the case of the close pair of galaxy
clusters A399 and A401. A401 is already known to contain a faint radio halo,
while a hint of diffuse emission in A399 has been suggested based on the NVSS.
To confirm this possibility, we analyzed deeper Very Large Array observations
at 1.4 GHz of this cluster. We find that the central region of A399 is
permeated by a diffuse low-surface brightness radio emission that we classify
as a radio halo with a linear size of about 570 kpc and a central brightness of
0.3 micro-Jy/arcsec^2. Indeed, given their comparatively small projected
distance of about 3 Mpc, the pair of galaxy clusters A401 and A399 can be
considered as the first example of double radio halo system. The discovery of
this double halo is extraordinary given the rarity of these radio sources in
general and given that current X-ray data seem to suggest that the two clusters
are still in a pre-merger state. Therefore, the origin of the double radio halo
is likely to be attributed to the individual merging histories of each cluster
separately, rather than to the result of a close encounter between the two
systems.Comment: 6 pages, 4 figures, accepted for publication in A&
The giant radio halo in Abell 2163
New radio data is presented for the rich cluster Abell 2163. The cluster
radio emission is characterized by the presence of a radio halo, which is one
of the most powerful and extended halos known so far. In the NE peripheral
cluster region, we also detect diffuse elongated emission, which we classify as
a cluster relic. The cluster A2163 is very hot and luminous in X-ray. Its
central region is probably in a highly non relaxed state, suggesting that this
cluster is likely to be a recent merger. The existence of a radio halo in this
cluster confirms that halos are associated with hot massive clusters, and
confirms the connection between radio halos and cluster merger processes. The
comparison between the radio emission of the halo and the cluster X-ray
emission shows a close structural similarity. A power law correlation is found
between the radio and X-ray brightness, with index = 0.64. We also report the
upper limit to the hard X-ray emission, obtained from a BeppoSAX observation.
We discuss the implications of our results.Comment: 7 pages, 8 figures (5 in ps and 3 in gif), Accepted for publication
in Astron. Astrop
Clan Properties in Parton Showers
By considering clans as genuine elementary subprocesses, i.e., intermediate
parton sources in the Simplified Parton Shower model, a generalized version of
this model is defined. It predicts analytically clan properties at parton level
in agreement with the general trends observed experimentally at hadronic level
and in Monte Carlo simulations both at partonic and hadronic level. In
particular the model shows a linear rising in rapidity of the average number of
clans at fixed energy of the initial parton and its subsequent bending for
rapidity intervals at the border of phase space, and approximate energy
independence of the average number of clans in fixed rapidity intervals. The
energy independence becomes stricter by properly normalizing the average number
of clans.Comment: (27 pages in Plain TeX plus 10 Postscript Figures, all compressed via
uufiles) DFTT 7/9
Diffuse radio emission in a REFLEX cluster
Deep Very Large Array radio observations are presented for the REFLEX
clusters RXCJ0437.1+0043 and RXCJ1314.4-2515. They are at similar distance and
show similar X-ray luminosity, but they are quite different in X-ray structure.
Indeed RXCJ0437.1+0043 is regular and relaxed, whereas RXCJ1314.4-2515 is
characterized by substructure and possible merging processes. The radio images
reveal no diffuse emission in RXCJ0437.1+0043, and a complex diffuse structure
in RXCJ1314.4-2515. The diffuse source in the latter cluster consists of a
central radio halo which extends to the West toward the cluster periphery and
bends to the North to form a possible relic. Another extended source is
detected in the eastern cluster peripheral region. Although there could be
plausible optical identifications for this source, it might also be a relic
candidate owing to its very steep spectrum. The present results confirm the
tight link between diffuse cluster radio sources and cluster merger processes.Comment: Accepted for publication in A&A. Figures here have been degraded to
reduce their size. A version with full resolution figures is available at
http://www.ira.cnr.it/~lferetti/OUTGOING/papREFLEX.ps.g
Young radio sources: the duty-cycle of the radio emission and prospects for gamma-ray emission
The evolutionary stage of a powerful radio source originated by an AGN is
related to its linear size. In this context, compact symmetric objects (CSOs),
which are powerful and intrinsically small objects, should represent the young
stage in the individual radio source life. However, the fraction of young radio
sources in flux density-limited samples is much higher than what expected from
the number counts of large radio sources.This indicates that a significant
fraction of young radio sources does not develop to the classical
Fanaroff-Riley radio galaxies,suggesting an intermittent jet activity. As the
radio jets are expanding within the dense and inhomogeneous interstellar
medium,the ambient may play a role in the jet growth, for example slowing down
or even disrupting its expansion when a jet-cloud interaction takes place.
Moreover, this environment may provide the thermal seed photons that scattered
by the lobes' electrons may be responsible for high energy emission, detectable
by Fermi-LAT.Comment: 4 pages, 5 figures; 2011 Fermi Symposium proceedings - eConf C11050
A giant radio halo in the low luminosity X-ray cluster Abell 523
Radio halos are extended and diffuse non-thermal radio sources present at the
cluster center, not obviously associated with any individual galaxy. A strong
correlation has been found between the cluster X-ray luminosity and the halo
radio power. We observe and analyze the diffuse radio emission present in the
complex merging structure Abell 523, classified as a low luminosity X-ray
cluster, to discuss its properties in the context of the halo total radio power
versus X-ray luminosity correlation. We reduced VLA archive observations at 1.4
GHz to derive a deep radio image of the diffuse emission, and compared radio,
optical, and X-ray data. Low-resolution VLA images detect a giant radio halo
associated with a complex merging region. The properties of this new halo agree
with those of radio halos in general discussed in the literature, but its radio
power is about a factor of ten higher than expected on the basis of the cluster
X-ray luminosity. Our study of this giant radio source demonstrates that radio
halos can also be present in clusters with a low X-ray luminosity. Only a few
similar cases have so far been found . This result suggests that this source
represent a new class of objects, that cannot be explained by classical radio
halo models. We suggest that the particle reacceleration related to merging
processes is very efficient and/or the X-ray luminosity is not a good indicator
of the past merging activity of a cluster.Comment: 5 pages, 6 figures, Astronomy and Astrophysics Letter in pres
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