2,283 research outputs found
Inverse Compton X-rays from the radio galaxy 3C 219
We report the results from a Chandra observation of the powerful nearby
(z=0.1744) radio galaxy 3C 219. We find evidence for non-thermal X-ray emission
from the radio lobes which fits fairly well with a combination of inverse
Compton scattering of Cosmic Microwave Background radiation and of nuclear
photons with the relativistic electrons in the lobes. The comparison between
radio synchrotron and IC emission yields a magnetic field strength
significantly lower (about a factor 3) than that calculated under minimum
energy conditions; the source energetics is then dominated by the relativistic
particles.Comment: 5 pages, 2 color figures, Accepted for publication in MNRAS pink
page
GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample
We present the results of the GMRT cluster radio halo survey. The main
purposes of our observational project are to measure which fraction of massive
galaxy clusters in the redshift range z=0.2--0.4 hosts a radio halo, and to
constrain the expectations of the particle re--acceleration model for the
origin of the non--thermal radio emission. We selected a complete sample of 50
clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues.
In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at
610 MHz for all clusters still lacking high sensitivity radio information, i.e.
16 eBCS and 7 REFLEX clusters, thus completing the radio information for the
whole sample. The typical sensitivity in our images is in the range 1Jy b. We found a radio halo in A697, a diffuse
peripheral source of unclear nature in A781, a core--halo source in Z7160, a
candidate radio halo in A1682 and ``suspect'' central emission in Z2661.
Including the literature information, a total of 10 clusters in the sample host
a radio halo. A very important result of our work is that 25 out of the 34
clusters observed with the GMRT do not host extended central emission at the
sensitivity level of our observations, and for 20 of them firm upper limits to
the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey
shows that radio halos are not common, and our findings on the fraction of
giant radio halos in massive clusters are consistent with the statistical
expectations based on the re--acceleration model. Our results favour primary to
secondary electron models.Comment: A&A in press, 17 pages, 12 figures, 4 tables Version with high
quality figures available on web at
http://www.ira.inaf.it/~tventuri/pap/Venturi_web.pd
Concrete and abstract concepts in school age children
The aim of this study is to highlight what kind of information distinguishes abstract and concrete conceptual knowledge in different aged children. A familiarity-rating task has shown that 8-year-olds judged concrete concepts as very familiar while abstract concepts were judged as much less familiar with ratings increasing substantially from age 10 to age 12, according to literature showing that abstract terms are not mastered until adolescence (Schwanenflugel, 1991). The types of relation elicited by abstract and concrete concepts during development were investigated in an association production task. At all considered age levels, concrete concepts mainly activated attributive and thematic relations as well as, to a much lesser extent, taxonomic relations and stereotypes. Abstract concepts, instead, elicited mainly thematic relations and, to a much lesser extent, examples and taxonomic relations. The patterns of relations elicited were already differentiated by age 8, becoming more specific in abstract concepts with age
Obscuring Active Galactic Nuclei with Nuclear Starburst Disks
We assess the potential of nuclear starburst disks to obscure the
Seyfert-like AGN that dominate the hard X-ray background at z~1. Over 1200
starburst disk models, based on the theory developed by Thompson et al., are
calculated for five input parameters: the black hole mass, the radial size of
the starburst disk, the dust-to-gas ratio, the efficiency of angular momentum
transport in the disk, and the gas fraction at the outer disk radius. We find
that a large dust-to-gas ratio, a relatively small starburst disk, a
significant gas mass fraction, and efficient angular momentum transport are all
important to produce a starburst disk that can potentially obscure an AGN. The
typical maximum star-formation rate in the disks is ~10 solar masses per year.
Assuming no mass-loss due to outflows, the starburst disks feed gas onto the
black hole at rates sufficient to produce hard X-ray luminosities of
10^{43}-10^{44} erg s^{-1}. The starburst disks themselves should be detectable
at mid-infrared and radio wavelengths; at z=0.8, the predicted fluxes are ~1
mJy at 24microns and ~10-30 microJy at 1.4GHz. Thus, we predict a large
fraction of radio/X-ray matches in future deep radio surveys. The starburst
disks should be easily distinguished from AGN in future 100microns surveys by
Herschel with expected fluxes of ~5 mJy. Any AGN-obscuring starbursts will be
associated with hot dust, independent of AGN heating, resulting in observable
signatures for separating galactic and nuclear star-formation. Finally, because
of the competition between gas and star-formation, nuclear starbursts will be
associated with lower-luminosity AGN. Thus, this phenomenon is a natural
explanation for the observed decrease in the fraction of obscured AGN with
luminosity.Comment: 13 pages, 12 figures, 3 in color; accepted by Ap
The resolved fraction of the Cosmic X-ray Background
We present the X-ray source number counts in two energy bands (0.5-2 and 2-10
keV) from a very large source sample: we combine data of six different surveys,
both shallow wide field and deep pencil beam, performed with three different
satellites (ROSAT, Chandra and XMM-Newton). The sample covers with good
statistics the largest possible flux range so far: [2.4*10^-17 - 10^-11] cgs in
the soft band and [2.1*10^-16 - 8*10^{-12}]cgs in the hard band. Integrating
the flux distributions over this range and taking into account the (small)
contribution of the brightest sources we derive the flux density generated by
discrete sources in both bands. After a critical review of the literature
values of the total Cosmic X--Ray Background (CXB) we conclude that, with the
present data, the 94.3%, and 88.8% of the soft and hard CXB can be ascribed to
discrete source emission. If we extrapolate the analytical form of the Log
N--Log S distribution beyond the flux limit of our catalog in the soft band we
find that the flux from discrete sources at ~3*10^-18 cgs is consistent with
the entire CXB, whereas in the hard band it accounts for only 93% of the total
CXB at most, hinting for a faint and obscured population to arise at even
fainter fluxes.Comment: Accepted for publication in Ap
Cosmic rays and Radio Halos in galaxy clusters : new constraints from radio observations
Clusters of galaxies are sites of acceleration of charged particles and
sources of non-thermal radiation. We report on new constraints on the
population of cosmic rays in the Intra Cluster Medium (ICM) obtained via radio
observations of a fairly large sample of massive, X-ray luminous, galaxy
clusters in the redshift interval 0.2--0.4. The bulk of the observed galaxy
clusters does not show any hint of Mpc scale synchrotron radio emission at the
cluster center (Radio Halo). We obtained solid upper limits to the diffuse
radio emission and discuss their implications for the models for the origin of
Radio Halos. Our measurements allow us to derive also a limit to the content of
cosmic ray protons in the ICM. Assuming spectral indices of these protons delta
=2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these
limits are one order of magnitude deeper than present EGRET upper limits, while
they are less stringent for steeper spectra and lower magnetic fields.Comment: 14 pages, 5 figures, ApJ Letter, accepte
The deepest X-ray look at the Universe
The origin of the X-ray background, in particular at hard (2-10 keV)
energies, has been a debated issue for more than 30 years. The Chandra deep
fields provide the deepest look at the X-ray sky and are the best dataset to
study the X-ray background. We searched the Chandra Deep Field South for X-ray
sources with the aid of a dedicated wavelet-based algorithm. We are able to
reconstruct the Log N-Log S source distribution in the soft (0.5-2 keV) and
hard (2-10 keV) bands down to limiting fluxes of 2x10^{-17} erg s^{-1} cm^{-2}
and 2x10^{-16} erg s^{-1} cm^{-2}, respectively. These are a factor ~5 deeper
than previous investigations. We find that the soft relation continues along
the extrapolation from higher fluxes, almost completely accounting for the soft
X-ray background. On the contrary, the hard distribution shows a flattening
below ~2x10^{-14} erg s^{-1} cm^{-2}. Nevertheless, we can account for >68% of
the hard X-ray background, with the main uncertainty being the sky flux itself.Comment: Accepted for publication on ApJL. Two figures, requires emulateapj5
(included
ISO investigates the nature of extremely-red hard X-ray sources responsible for the X-ray background
We analyse very deep X-ray and mid-IR surveys in common areas of the Lockman
Hole and the HDF North to study the sources of the X-ray background (XRB) and
to test the standard obscured accretion paradigm. We detect with ISO a rich
population of X-ray luminous sources with red optical colours, including a
fraction identified with Extremely Red Objects (R-K > 5) and galaxies with SEDs
typical of normal massive ellipticals or spirals at z ~ 1. The high 0.5-10 keV
X-ray luminosities of these objects (1E43-1E45 erg/s) indicate that the
ultimate energy source is gravitational accretion, while the X-ray to IR flux
ratios and the X-ray spectral hardness show evidence of photoelectric
absorption at low X-ray energies. An important hint on the physics comes from
the mid-IR data at 6.7 and 15 um, well reproduced by model spectra of
completely obscured quasars under standard assumptions and l.o.s. optical
depths tau ~ 30-40. Other predictions of the standard XRB picture, like the
distributions of intrinsic bolometric luminosities and the relative fractions
of type-I and -II objects (1:3), are also consistent with our results. Obscured
gravitational accretion is then confirmed as being responsible for the bulk of
the X-ray background, since we detect in the IR the down-graded energy
photoelectrically absorbed in X-rays: 63% of the faint 5-10 keV XMM sources are
detected in the mid-IR by Fadda et al. (2001). However, although as much as 90%
of the X-ray energy production could be converted to IR photons, no more than
20% of the Cosmic IR Background can be attributed to X-ray loud AGNs.Comment: 7 pages, 5 postscript figures, ApJ submitte
The contribution of Quasars to the Far Infrared Background
Recent observational results obtained with SCUBA, COBE and ISO have greatly
improved our knowledge of the infrared and sub-mm background radiation. These
limits become constraining given the realization that most AGNs are heavily
obscured and must reradiate strongly in the IR/sub-mm. Here we predict the
contribution of AGNs to the IR/sub-mm background, starting from measurements of
the hard X-ray background. We show that an application of what we know of AGN
Spectral Energy Distributions (SEDs) and the IR background requires that a
significant fraction of the 10-150 micron background comes from AGNs. This
conclusion can only be avoided if obscured AGNs are intrinsically brighter in
the X-rays (with respect to the optical-UV) than unobscured AGNs, contrary to
``unified schemes'' for AGNs, or have a dust to gas ratio much lower (< 0.1)
than Galactic. We show that these results are rather robust and not strongly
dependent on the details of the modeling.Comment: 13 pages, 1 figure, Astrophysical Journal, in pres
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