2,246 research outputs found
The REX survey: a search for Radio Emitting X-ray sources
We present the scientific goals, the strategy and the first results of the
REX project, an effort aimed at creating a sizable and statistically complete
sample of Radio Emitting X-ray sources (REX) using the available data from a
VLA survey and the ROSAT PSPC archive. Through a positional cross-correlation
of the two data sets we have derived a sample of about 1600 REX. Among the 393
REX identified so far a high fraction is represented by AGNs, typically radio
loud QSOs and BL Lacs. The remaining sources are galaxies, typically radio
galaxies isolated or in cluster. Thanks to the low flux limits in the radio and
in the X-ray band and the large area of sky covered by the survey, we intend to
derive a new complete and unbiased sample of BL Lacs which will contain both
``RBL'' and ``XBL'' type objects. In this way, the apparent dichotomy resulting
from the current samples of BL Lacs will be directly analyzed in a unique
sample. Moreover, the high number of BL Lacs expected in the REX sample (about
200) will allow an accurate estimate of their statistical properties. To date,
we have discovered 15 new BL Lacs and 11 BL Lac candidates with optical
properties intermediate between those of a typical elliptical galaxy and those
of a typical BL Lac object. These objects could harbour weak sources of
non-thermal continuum in their nuclei and, if confirmed, they could represent
the faint tail of the BL Lac population. The existence of such ``weak'' BL Lacs
is matter of discussion in recent literature and could lead to a re-assessment
of the defining criteria of a BL Lac and, consequently, to a revision of their
cosmological and statistical properties.Comment: 30 pages, 13 figures, accepted for publication to Ap
Emission Line AGNs from the REX survey: Results from optical spectroscopy
We present 71 Emission Line objects selected from the REX survey. Except for
3 of them, for which the presence of an active nucleus is dubious, all these
sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line
radiogalaxies). In addition, we present the spectra of other 19 AGNs included
in a preliminary version of the REX catalog but not in the final one. The
majority (80) of the 90 sources presented in this paper is newly discovered.
Finally, we present the general properties in the radio and in the X-ray band
of all the AGNs discovered so far in the REX survey.Comment: 27 pages. To be published in Astronomy and Astrophysics, Supplement
Series. Better quality figures can be asked to the autho
An X-ray review of MS1054-0321: hot or not?
XMM-Newton observations are presented for the z=0.83 cluster of galaxies
MS1054-0321, the highest redshift cluster in the Einstein Extended Medium
Sensitivity Survey (EMSS). The temperature inferred by the XMM-Newton data,
T=7.2 (+0.7, -0.6) keV, is much lower than the temperature previously reported
from ASCA data, T=12.3 (+3.1, -2.2) keV (Donahue et al. 1998), and a little
lower than the Chandra temperature, T=10.4(+1.7, -1.5) keV, determined by
Jeltema et al. 2001. The discrepancy between the newly derived temperature and
the previously derived temperatures is discussed in detail. If one allows the
column density to be a free parameter, then the best fit temperature becomes
T=8.6 (+1.2, -1.1) keV, and the best fit column density becomes N_(H)=1.33
(+0.15 -0.14) x 10^20 atoms/cm^2. The iron line is well detected in the
XMM-Newton spectrum with a value for the abundance of Z=0.33 (+0.19 -0.18)
Zsol, in very good agreement with previous determinations. The derived XMM
X-ray luminosity for the overall cluster in the 2-10 keV energy band is
L_X=(3.81 +/- 0.19) x 10^44 h^-2 erg s^-1 while the bolometric luminosity is
L_BOL=(8.05+/-0.40) x 10^44 h^-2 erg s^-1. The XMM-Newton data confirm the
substructure in the cluster X-ray morphology already seen by ROSAT and in much
more detail by Chandra. The central weak lensing clump is coincident with the
main cluster component and has a temperature T=8.1 (+1.3, -1.2) keV. The
western weak lensing clump coincides with the western X-ray component which is
much cooler with a temperature T=5.6 (+0.8, -0.6)$ keV. Given the newly
determined temperature, MS1054-0321 is no longer amongst the hottest clusters
known.Comment: To appear in the A&A main Journal, 13 pages including 3 postscript
figures and 4 tables. Figs. 1, 4, 5 and 7 are too large and are not given
here. The whole paper as pdf file is posted at
http://www.ira.cnr.it/~gioia/PUB/publications.htm
Rainfall and land use empirically coupled to forecast landslides in the Esino river basin, central Italy
Abstract. A coupled empirical approach to highlight relationships between rainfall, vegetation segmentation, and landslide occurrence is discussed. To reveal such links, two important rainfall events, which occurred over the Esino river basin in central Italy in November 2013 and May 2014, were analysed. The correlation between rainfall and landslides was evaluated by applying an intensity–duration (ID) threshold method, whereas the correlation between vegetation segmentation and landslides was investigated using morphological spatial pattern analysis (MSPA). This coupled approach represents an attempt to find both timing and location of landslide occurrence through an empirical (black box) analysis. Results showed: (i) the ID minimum threshold proposed in a previous study (Gioia et al., 2015) to be verified as an effective equation to assess the rainfall conditions likely to trigger landslides in the study area ("when"), and (ii) the core areas and the fragmented vegetation structures defined by the MSPA to be the most affected by slope failures ("where"). These encouraging findings prompt additional testing and the application of such a coupled empirical approach so that it is possible to achieve an integrated basis for landslide forecasting
Marangoni shocks in unobstructed soap-film flows
It is widely thought that in steady, gravity-driven, unobstructed soap-film
flows, the velocity increases monotonically downstream. Here we show
experimentally that the velocity increases, peaks, drops abruptly, then lessens
gradually downstream. We argue theoretically and verify experimentally that the
abrupt drop in velocity corresponds to a Marangoni shock, a type of shock
related to the elasticity of the film. Marangoni shocks induce locally intense
turbulent fluctuations and may help elucidate the mechanisms that produce
two-dimensional turbulence away from boundaries.Comment: 4 pages, 5 figures, published in PR
A Hard Medium Survey with ASCA. IV: the Radio-Loud Type 2 QSO AXJ0843+294 2
We discuss the X-ray, optical and radio properties of AX J0843+2942, a high
luminosity Type 2 AGN found in the ASCA Hard Serendipitous Survey. The X-ray
spectrum is best described by an absorbed power-law model with photon index of
Gamma = 1.72 (+0.3 -0.6) and intrinsic absorbing column density of NH = 1.44
(+0.33 -0.52) x 10E23 cm-2. The intrinsic luminosity in the 0.5-10 keV energy
band is ~ 3x10E45 erg s-1, well within the range of quasar luminosities. AX
J0843+2942, positionally coincident with the core of a triple and strong (S_1.4
GHz ~ 1 Jy; P_1.4 GHz ~ 9 x 10E33 erg s-1 Hz-1) radio source, is
spectroscopically identified with a Narrow Line object (intrinsic FWHM of all
the permitted emission lines <= 1200 km s-1) at z=0.398, having line features
and ratios typical of Seyfert-2 like objects. The high X-ray luminosity,
coupled with the high intrinsic absorption, the optical spectral properties and
the radio power, allow us to propose AX J0843+2942 as a Radio-Loud "Type 2
QSO". A discussion of the SED of this object is presented here together with a
comparison with the SED of Ultra Luminous Infrared Galaxies, other "Type 2 QSO"
candidates from the literature, and "normal" Radio-Quiet and Radio-Loud QSOs.Comment: 10 pages, 7 figures, Latex manuscript, Accepted for publication in
Ast ronomy and Astrophysic
The turbulent flow in a slug: a re-examination
The transition to turbulence in pipe flow proceeds through several distinct stages, eventually producing aggressively expanding regions of fluctuations, ‘slugs’, surrounded by laminar flow. By examining mean-velocity profiles, fluctuating-velocity profiles and Reynolds stress profiles, the seminal study of Wygnanski & Champagne (J. Fluid Mech., vol. 59 (2), 1973, 281–335) concluded that the flow inside slugs is ‘identical’ to fully turbulent flow. Although this conclusion is widely accepted, upon closer examination of their analysis, we find that their data cannot be used to substantiate this conclusion. We resolve this conflict via new experiments and simulations wherein we pair slugs and fully turbulent flow at the same value of Reynolds number (Re). We conclude that the flow inside a slug is indeed indistinguishable from a fully turbulent flow but only when the two flows share the same value of Re. Our work highlights the rich Re-dependence of transitional pipe flows
Small-scale universality in the spectral structure of transitional pipe flows
Turbulent flows are not only everywhere, but every turbulent flow is the same at small scales. The extraordinary simplification engendered by this "small-scale universality" is a hallmark of turbulence theory. However, on the basis of the restrictive assumptions invoked by A. N. Kolmogorov to demonstrate this universality, it is widely thought that only idealized turbulent flows conform to this framework. Using experiments and simulations that span a wide range of Reynolds number, we show that small-scale universality governs the spectral structure of a class of flows with no apparent ties to the idealized flows: transitional pipe flows. Our results not only extend the universality of Kolmogorov\u27s framework beyond expectation but also establish an unexpected link between transitional pipe flows and Kolmogorovian turbulence
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