121 research outputs found
How cooperation between EU states shapes counterterrorism policies
Cooperation between EU states has become increasingly important in the fight against terrorism. But what types of cooperation are most beneficial? Drawing on a new study, Mario Gilli and Paolo Tedeschi find that intelligence cooperation is associated with more efficient defensive policies than unanimous political cooperation
Political regimes and government's reaction to terrorism. A simple model
Why do autocratic regimes have sometimes a weak reaction to terrorism, and conversely why do democratic regimes sometimes react harshly? More generally, what are the determinants of governments' reaction to terrorism? And, what are the determinants of terrorism and of its dynamic? In the last years, there have been many empirical studies conducted on these questions. We believe, however, that a general theoretical model is missing, which would significantly help empirical research and its interpretation. This paper is a first attempt towards the construction of such a general model. In this model, individual human capital, government responsiveness and economic development are seen as potential factors influencing individual choice to use terrorism as a political strategy. In this paper, we elaborate a strategic model of terrorism with a specific focus on domestic terrorism, in which the individual choice of joining a terrorist organization is a way of pursing specific political aims. This decision is in turn influenced by the interaction between terrorist' activity, political engagement and government policies. We are able to prove that three different political regimes form the equilibrium outcomes, and that there is not a simple monotonic relationship between a regime accountability and terrorism' repression. It is then also perfectly possible for a democratic regime to harshly repress terrorism and for an autocratic polity to be tolerant on terrorism
X-ray variability with WFXT: AGNs, transients and more
The Wide Field X-ray Telescope (WFXT) is a proposed mission with a high
survey speed, due to the combination of large field of view (FOV) and effective
area, i.e. grasp, and sharp PSF across the whole FOV. These characteristics
make it suitable to detect a large number of variable and transient X-ray
sources during its operating lifetime. Here we present estimates of the WFXT
capabilities in the time domain, allowing to study the variability of thousand
of AGNs with significant detail, as well as to constrain the rates and
properties of hundreds of distant, faint and/or rare objects such as X-ray
Flashes/faint GRBs, Tidal Disruption Events, ULXs, Type-I bursts etc. The
planned WFXT extragalactic surveys will thus allow to trace variable and
transient X-ray populations over large cosmological volumes.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009 (arXiv:1010.5889). To appear in Memorie della
Societ\`a Astronomica Italiana 2010 - Minor corrections to text
AGN in overdense environments at high- with AXIS
Overdense regions at high redshift () are perfect laboratories
to study the relations between environment and SMBH growth, and the AGN
feedback processes on the surrounding galaxies and diffuse gas. In this white
paper, we discuss how AXIS will 1) constrain the AGN incidence in
protoclusters, as a function of parameters such as redshift, overdensity, mass
of the structure; 2) search for low-luminosity and obscured AGN in the
satellite galaxies of luminous QSOs at , exploiting the large galaxy
density around such biased objects; 3) probe the AGN feedback on the proto-ICM
via the measurement of the AGN contribution to the gas ionization and
excitation, and the detection of extended X-ray emission from the ionized gas
and from radio jets; 4) discover new large-scale structures in the wide and
deep AXIS surveys as spikes in the redshift distribution of X-ray sources.
These goals can be achieved only with an X-ray mission with the capabilities of
AXIS, ensuring a strong synergy with current and future state-of-the-art
facilities in other wavelengths. This White Paper is part of a series
commissioned for the AXIS Probe Concept Mission; additional AXIS White Papers
can be found at http://axis.astro.umd.edu/ with a mission overview at
https://arxiv.org/abs/2311.00780.Comment: 8 pages, 3 figures. This White Paper is part of a series commissioned
for the AXIS Probe Concept Mission; additional AXIS White Papers can be found
at the AXIS website (http://axis.astro.umd.edu/) with a mission overview at
arXiv:2311.0078
Piercing Through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields: I. X-ray Spectral and Long-term Variability Analyses
We present a detailed X-ray spectral analysis of 1152 AGNs selected in the
Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (). By fitting spectra with physical models, 436 (38%)
sources with are confirmed to be highly
obscured, including 102 Compton-thick (CT) candidates. We propose a new
hardness-ratio measure of the obscuration level which can be used to select
highly obscured AGN candidates. The completeness and accuracy of applying this
method to our AGNs are 88% and 80%, respectively. The observed logN-logS
relation favors cosmic X-ray background models that predict moderate (i.e.,
between optimistic and pessimistic) CT number counts. 19% (6/31) of our highly
obscured AGNs that have optical classifications are labeled as broad-line AGNs,
suggesting that, at least for part of the AGN population, the heavy X-ray
obscuration is largely a line-of-sight effect, i.e., some high-column-density
clouds on various scales (but not necessarily a dust-enshrouded torus) along
our sightline may obscure the compact X-ray emitter. After correcting for
several observational biases, we obtain the intrinsic NH distribution and its
evolution. The CT-to-highly-obscured fraction is roughly 52% and is consistent
with no evident redshift evolution. We also perform long-term (~17 years in the
observed frame) variability analyses for 31 sources with the largest number of
counts available. Among them, 17 sources show flux variabilities: 31% (5/17)
are caused by the change of NH, 53% (9/17) are caused by the intrinsic
luminosity variability, 6% (1/17) are driven by both effects, and 2 are not
classified due to large spectral fitting errors.Comment: 32 pages, 21 figures, 9 tables, accepted for publication in Ap
A Classic Type 2 QSO
In the Chandra Deep Field South 1Msec exposure we have found, at redshift
3.700 +- 0.005, the most distant Type 2 AGN ever detected. It is the source
with the hardest X-ray spectrum with redshift z>3. The optical spectrum has no
detected continuum emission to a 3sigma detection limit of ~3 10^{-19}
ergs/s/cm^2/AA and shows narrow lines of Ly_alpha, CIV, NV, HeII, OVI, [OIII],
and CIII]. Their FWHM line widths have a range of ~700-2300 km/s with an
average of approximately ~1500 km/s. The emitting gas is metal rich (Z ~2.5-3
Z_solar). In the X-ray spectrum of 130 counts in the 0.5-7 keV band there is
evidence for intrinsic absorption with N_H > 10^{24} cm^{-2}. An iron K_alpha
line with rest frame energy and equivalent width of ~6.4 keV and ~1 keV,
respectively, in agreement with the obscuration scenario, is detected at a
2sigma level. If confirmed by our forthcoming XMM observations this would be
the highest redshift detection of FeK_alpha. Depending on the assumed cosmology
and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for
absorption is ~10^{45 +- 0.5} ergs/s, which makes our source a classic example
of the long sought Type 2 QSOs. From standard population synthesis models,
these sources are expected to account for a relevant fraction of the
black-hole-powered QSO distribution at high redshift.Comment: 24 LaTeX pages including 6 postscript figures. Revised version,
accepted by Ap
The Chandra Deep Field South: the 1 Million Second
We present the main results from our 940 ksec observation of the Chandra Deep
Field South (CDFS), using the source catalog described in an accompanying paper
(Giacconi et al. 2001). We extend the measurement of source number counts to
5.5e-17 erg/cm^2/s in the soft 0.5-2 keV band and 4.5e-16 erg/cm^2/s in the
hard 2-10 keV band. The hard band LogN-LogS shows a significant flattening
(slope~=0.6) below ~1e-14 erg/cm^2/s, leaving at most 10-15% of the X-ray
background (XRB) to be resolved, the main uncertainty lying in the measurement
of the total flux of the XRB. On the other hand, the analysis in the very hard
5-10 keV band reveals a relatively steep LogN-LogS (slope ~=1.3) down to 1e-15
erg/cm^2/s. Together with the evidence of a progressive flattening of the
average X-ray spectrum near the flux limit, this indicates that there is still
a non negligible population of faint hard sources to be discovered at energies
not well probed by Chandra, which possibly contribute to the 30 keV bump in the
spectrum of the XRB. We use optical redshifts and identifications, obtained
with the VLT, for one quarter of the sample to characterize the combined
optical and X-ray properties of the CDFS sample. Different source types are
well separated in a parameter space which includes X-ray luminosity, hardness
ratio and R-K color. Type II objects, while redder on average than the field
population, have colors which are consistent with being hosted by a range of
galaxy types. Type II AGN are mostly found at z<~1, in contrast with
predictions based on AGN population synthesis models, thus suggesting a
revision of their evolutionary parameters.Comment: Accepted by The Astrophysical Journal, 24 pages, 8 figures, 1 color
jpg plate (fig.1
Systematic search for lensed X-ray sources in the CLASH fields
We search for unresolved X-ray emission from lensed sources in the FOV of 11
CLASH clusters with Chandra data. We consider the solid angle in the lens plane
corresponding to a magnification , that amounts to a total of ~100
arcmin. Our main goal is to assess the efficiency of massive clusters as
cosmic telescopes to explore the faint end of X-ray extragalactic source
population. We search for X-ray emission from strongly lensed sources
identified in the optical, and perform an untargeted detection of lensed X-ray
sources. We detect X-ray emission only in 9 out of 849 lensed/background
optical sources. The stacked emission of the sources without detection does not
reveal any signal in any band. Based on the untargeted detection, we find 66
additional X-ray sources that are consistent with being lensed sources. After
accounting for completeness and sky coverage, we measure for the first time the
soft- and hard-band number counts of lensed X-ray sources. The results are
consistent with current modelization of the AGN population distribution. The
distribution of de-lensed fluxes of the sources identified in moderately deep
CLASH fields reaches a flux limit of ~ and ~ erg/s/cm
in the soft and hard bands, respectively. We conclude that, in order to match
the depth of the CDFS exploiting massive clusters as cosmic telescopes, the
required number of cluster fields is about two orders of magnitude larger than
that offered by the 20 years Chandra archive. A significant step forward will
be made when future X-ray facilities, with ~1' angular resolution and large
effective area, will allow the serendipitous discovery of rare, strongly lensed
high- X-ray sources, enabling the study of faint AGN activity in early
Universe and the measurement of gravitational time delays in the X-ray
variability of multiply imaged AGN.Comment: Accepted for publication in A&
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