9,746 research outputs found
A network-based rating system and its resistance to bribery
We study a rating system in which a set of individuals (e.g., the customers of a restaurant) evaluate a given service (e.g, the restaurant), with their aggregated opinion determining the probability of all individuals to use the service and thus its generated revenue. We explicitly model the influence relation by a social network, with individuals being influenced by the evaluation of their trusted peers. On top of that we allow a malicious service provider (e.g., the restaurant owner) to bribe some individuals, i.e., to invest a part of his or her expected income to modify their opinion, therefore influencing his or her final gain. We analyse the effect of bribing strategies under various constraints, and we show under what conditions the system is bribery-proof, i.e., no bribing strategy yields a strictly positive expected gain to the service provider
X-rays from Radio-Galaxies: BeppoSAX Observations
We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their
X-ray spectra are quite varied, and perhaps surprisingly, any similarity
between radio-loud AGN and Seyfert galaxies is the exception rather than the
rule. When detected, reprocessing features (iron line and reflection) are
generally weak, suggesting two possible scenarios: either: (1) non-thermal
(jet?) radiation dilutes the X-ray emission from the disk in radio-loud
objects, or (2) the solid angle subtended by the X-ray reprocessing material is
smaller in radio-loud than in radio-quiet AGN due to different characteristics
of the accretion disk itself.Comment: 6 pages, to appear in `Life Cycles of Radio Galaxies', ed. J. Biretta
et al., New Astronomy Review
The Chandra view of the 3C/FRI sample of low luminosity radio-galaxies
We present results from Chandra observations of the 3C/FRI sample of low
luminosity radio-galaxies. We detected a power-law nuclear component in 12
objects out of the 18 with available data. In 4 galaxies we detected nuclear
X-ray absorption at a level of about N_H= (0.2-6)e22 cm-2. X-ray absorbed
sources are associated with the presence of highly inclined dusty disks (or
dust filaments projected onto the nuclei) seen in the HST images. This suggests
the existence of a flattened X-ray absorber, but of much lower optical depth
than in classical obscuring tori. We thus have an un-obstructed view toward
most FR~I nuclei while absorption plays only a marginal role in the remaining
objects. Three pieces of evidence support an interpretation for a jet origin
for the X-ray cores: i) the presence of strong correlations between the nuclear
luminosities in the radio, optical and X-ray bands, extending over 4 orders of
magnitude and with a much smaller dispersion (about 0.3 dex) when compared to
similar trends found for other classes of AGNs, pointing to a common origin for
the emission in the three bands; ii) the close similarity of the broad-band
spectral indices with the sub-class of BL Lac objects sharing the same range of
extended radio-luminosity, in accord with the FRI/BL Lacs unified model; iii)
the presence of a common luminosity evolution of spectral indices in both FRI
and BL Lacs. The low luminosities of the X-ray nuclei, regardless of their
origin, strengthens the interpretation of low efficiency accretion in low
luminosity radio-galaxies.Comment: Accepted in A&
SMAUG: a new technique for the deprojection of galaxy clusters
This paper presents a new technique for reconstructing the spatial
distributions of hydrogen, temperature and metal abundance of a galaxy cluster.
These quantities are worked out from the X-ray spectrum, modeled starting from
few analytical functions describing their spatial distributions. These
functions depend upon some parameters, determined by fitting the model to the
observed spectrum. We have implemented this technique as a new model in the
XSPEC software analysis package. We describe the details of the method, and
apply it to work out the structure of the cluster A1795. We combine the
observation of three satellites, exploiting the high spatial resolution of
Chandra for the cluster core, the wide collecting area of XMM-Newton for the
intermediate regions and the large field of view of Beppo-SAX for the outer
regions. We also test the validity and precision of our method by i) comparing
its results with those from a geometrical deprojection, ii) examining the
spectral residuals at different radii of the cluster and iii) reprojecting the
unfolded profiles and comparing them directly to the measured quantities. Our
analytical method yields the parameters defining the spatial functions directly
from the spectra. Their explicit knowledge allows a straightforward derivation
of other indirect physical quantities like the gravitating mass, as well as a
fast and easy estimate of the profiles uncertainties.Comment: 24 pages, 11 figures, 3 tables; emulateapj; accepted for publication
in the Astrophysical Journa
XMM-Newton unveils the type 2 nature of the BLRG 3C 445
We present an observation of XMM-Newton that unambiguously reveals the
``Seyfert 2'' nature of the Broad Line Radio Galaxy 3C 445. For the first time
the soft excess of this source has been resolved. It consists of unobscured
scattered continuum flux and emission lines, likely produced in a warm
photoionized gas near the pole of an obscuring torus. The presence of
circumnuclear (likely stratified) matter is supported by the complex
obscuration of the nuclear region. Seventy percent of the nuclear radiation
(first component) is indeed obscured by a column density ~4*10^{23} cm^{-2},
and 30 % (second component) is filtered by ~7* 10^{22} cm^{-2}. The first
component is nuclear radiation directly observed by transmission through the
thicker regions. The second one is of more uncertain nature. If the observer
has a deep view into the nucleus but near the edge of the torus, it could be
light scattered by the inner wall of the torus and/or by photoionized gas
within the Broad Line Region observed through the thinner rim of the
circumnuclear matter.Comment: MNRAS Letters, in pres
High-energy neutrinos from FR0 radio-galaxies?
The sources responsible for the emission of high-energy ( 100 TeV)
neutrinos detected by IceCube are still unknown. Among the possible candidates,
active galactic nuclei with relativistic jets are often examined, since the
outflowing plasma seems to offer the ideal environment to accelerate the
required parent high-energy cosmic rays. The non-detection of single point
sources or -- almost equivalently -- the absence, in the IceCube events, of
multiplets originating from the same sky position, constrains the cosmic
density and the neutrino output of these sources, pointing to a numerous
population of faint sources. Here we explore the possibility that FR0
radiogalaxies, the population of compact sources recently identified in large
radio and optical surveys and representing the bulk of radio-loud AGN
population, can represent suitable candidates for neutrino emission. Modeling
the spectral energy distribution of a FR0 radiogalaxy recently associated to a
-ray source detected by the Large Area Telescope onboard Fermi, we
derive the physical parameters of its jet, in particular the power carried by
it. We consider the possible mechanisms of neutrino production, concluding that
reactions in the jet between protons and ambient radiation is too
inefficient to sustain the required output. We propose an alternative scenario,
in which protons, accelerated in the jet, escape from it and diffuse in the
host galaxy, producing neutrinos as a result of scattering with the
interstellar gas, in strict analogy with the processes taking place in
star-forming galaxies.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Jets and outflows in Radio Galaxies: implications for AGN feedback
One of the main debated astrophysical problems is the role of the AGN
feedback in galaxy formation. It is known that massive black holes have a
profound effect on the formation and evolution of galaxies, but how black holes
and galaxies communicate is still an unsolved problem. For Radio Galaxies,
feedback studies have mainly focused on jet/cavity systems in the most massive
and X-ray luminous galaxy clusters. The recent high-resolution detection of
warm absorbers in some Broad Line Radio Galaxies allow us to investigate the
interplay between the nuclear engine and the surrounding medium from a
different perspective. We report on the detection of warm absorbers in two
Broad Line Radio Galaxies, 3C 382 and 3C 390.3, and discuss the physical and
energetic properties of the absorbing gas. Finally, we attempt a comparison
between radio-loud and radio-quiet outflows.Comment: To be published in the proceedings of High Energy Phenomena in
Relativistic Outflows III (HEPRO III, IJMPCS). 4 pages, 2 figure
A Chandra archival study of the temperature and metal abundance profiles in hot Galaxy Clusters at 0.1 < z < 0.3
We present the analysis of the temperature and metallicity profiles of 12
galaxy clusters in the redshift range 0.1--0.3 selected from the Chandra
archive with at least ~20,000 net ACIS counts and kT>6 keV. We divide the
sample between 7 Cooling-Core (CC) and 5 Non-Cooling-Core (NCC) clusters
according to their central cooling time. We find that single power-laws can
describe properly both the temperature and metallicity profiles at radii larger
than 0.1 r_180 in both CC and NCC systems, showing the NCC objects steeper
profiles outwards. A significant deviation is only present in the inner 0.1
r_180. We perform a comparison of our sample with the De Grandi & Molendi
BeppoSAX sample of local CC and NCC clusters, finding a complete agreement in
the CC cluster profile and a marginally higher value (at ~1sigma) in the inner
regions of the NCC clusters. The slope of the power-law describing kT(r) within
0.1 r_180 correlates strongly with the ratio between the cooling time and the
age of the Universe at the cluster redshift, being the slope >0 and
tau_c/tau_age<=0.6 in CC systems.Comment: 12 pages, 6 figures, Accepted for publication by the Astrophysical
Journa
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