201 research outputs found
Fiscal shocks, public debt, and long-term interest rate dynamics
Public finances worldwide have been severely hit by the 2008-2009 Great Recession, stimulating the debate on the consequences of growing fiscal imbalances. Building on Paesani et al. (2006), this paper focuses on the USA, Germany and Italy over the 1983-2009 period and studies the effects of fiscal shocks and government debt accumulation on long-term interest rates, both nationally and across borders. Based on a a theoretical framework, the empirical analysis disentangles permanent and transitory components of interest rates dynamics .nding that sustained debt accumulation leads, at least temporarily, to higher long-term interest rates. The is particularly true for the Italian case. There is also evidence of signi.cant cross-country linkages, mainly between Italy and the USA.
Detection and period measurements of GX1+4 at hard x ray energies with the SIGMA telescope
The galactic Low Mass X ray Binary GX1+4 was detected by the coded aperture hard X ray gamma ray SIGMA telescope during the Feb. to April 1991 observations of the galactic center regions. The source, whose emission varied during the survey of a factor greater than 40 pct., reached a maximum luminosity in the 40 to 140 energy range of 1.03 x 10(exp 37) erg/s (D = 8.5 kpc), thus approaching the emission level of the 1970 to 1980 high state. Two minute flux pulsations were detected on Mar. 22 and on Mar. 31 and Apr. 1. Comparison with the last period measurements shows that the current spin-down phase of GX1+4 is ending. Concerning the proposed association of this source with the galactic center 511 keV annihilation emission, upper limits were derived
Speeding up Simplification of Polygonal Curves using Nested Approximations
We develop a multiresolution approach to the problem of polygonal curve
approximation. We show theoretically and experimentally that, if the
simplification algorithm A used between any two successive levels of resolution
satisfies some conditions, the multiresolution algorithm MR will have a
complexity lower than the complexity of A. In particular, we show that if A has
a O(N2/K) complexity (the complexity of a reduced search dynamic solution
approach), where N and K are respectively the initial and the final number of
segments, the complexity of MR is in O(N).We experimentally compare the
outcomes of MR with those of the optimal "full search" dynamic programming
solution and of classical merge and split approaches. The experimental
evaluations confirm the theoretical derivations and show that the proposed
approach evaluated on 2D coastal maps either shows a lower complexity or
provides polygonal approximations closer to the initial curves.Comment: 12 pages + figure
The Agile Alert System For Gamma-Ray Transients
In recent years, a new generation of space missions offered great
opportunities of discovery in high-energy astrophysics. In this article we
focus on the scientific operations of the Gamma-Ray Imaging Detector (GRID)
onboard the AGILE space mission. The AGILE-GRID, sensitive in the energy range
of 30 MeV-30 GeV, has detected many gamma-ray transients of galactic and
extragalactic origins. This work presents the AGILE innovative approach to fast
gamma-ray transient detection, which is a challenging task and a crucial part
of the AGILE scientific program. The goals are to describe: (1) the AGILE
Gamma-Ray Alert System, (2) a new algorithm for blind search identification of
transients within a short processing time, (3) the AGILE procedure for
gamma-ray transient alert management, and (4) the likelihood of ratio tests
that are necessary to evaluate the post-trial statistical significance of the
results. Special algorithms and an optimized sequence of tasks are necessary to
reach our goal. Data are automatically analyzed at every orbital downlink by an
alert pipeline operating on different timescales. As proper flux thresholds are
exceeded, alerts are automatically generated and sent as SMS messages to
cellular telephones, e-mails, and push notifications of an application for
smartphones and tablets. These alerts are crosschecked with the results of two
pipelines, and a manual analysis is performed. Being a small scientific-class
mission, AGILE is characterized by optimization of both scientific analysis and
ground-segment resources. The system is capable of generating alerts within two
to three hours of a data downlink, an unprecedented reaction time in gamma-ray
astrophysics.Comment: 34 pages, 9 figures, 5 table
Multiwavelength observations of 3C 454.3. I. The AGILE 2007 November campaign on the "Crazy Diamond"
[Abridged] We report on a multiwavelength observation of the blazar 3C 454.3
(which we dubbed "crazy diamond") carried out on November 2007 by means of the
astrophysical satellites AGILE, INTEGRAL, Swift, the WEBT Consortium, and the
optical-NIR telescope REM. 3C 454.3 is detected at a level
during the 3-week observing period, with an average flux above 100 MeV of
\phcmsec. The gamma-ray
spectrum can be fit with a single power-law with photon index between 100 MeV and 1 GeV. We detect significant
day-by-day variability of the gamma-ray emission during our observations, and
we can exclude that the fluxes are constant at the 99.6% ()
level. The source was detected typically around 40 degrees off-axis, and it was
substantially off--axis in the field of view of the AGILE hard X-ray imager.
However, a 5-day long ToO observation by INTEGRAL detected 3C 454.3 at an
average flux of about \phcmsec with
an average photon index of between 20--200
keV. Swift also detected 3C 454.3 with a flux in the 0.3--10 keV energy band in
the range \phcmsec{} and a photon index in the
range . In the optical band, both WEBT and REM
show an extremely variable behavior in the band. A correlation analysis
based on the entire data set is consistent with no time-lags between the
gamma-ray and the optical flux variations. Our simultaneous multifrequency
observations strongly indicate that the dominant emission mechanism between 30
MeV and 30 GeV is dominated by inverse Compton scattering of relativistic
electrons in the jet on the external photons from the broad line region.Comment: Accepted for publication in ApJ. Abridged Abstract. 37 pages, 14
Figures, 3 Table
Direct Evidence for Hadronic Cosmic-Ray Acceleration in the Supernova Renmant IC 443
The Supernova Remnant (SNR) IC 443 is an intermediate-age remnant well known
for its radio, optical, X-ray and gamma-ray energy emissions. In this Letter we
study the gamma-ray emission above 100 MeV from IC 443 as obtained by the AGILE
satellite. A distinct pattern of diffuse emission in the energy range 100 MeV-3
GeV is detected across the SNR with its prominent maximum (source "A")
localized in the Northeastern shell with a flux F = (47 \pm 10) 10^{-8} photons
cm^{-2} s^{-1} above 100 MeV. This location is the site of the strongest shock
interaction between the SNR blast wave and the dense circumstellar medium.
Source "A" is not coincident with the TeV source located 0.4 degree away and
associated with a dense molecular cloud complex in the SNR central region. From
our observations, and from the lack of detectable diffuse TeV emission from its
Northeastern rim, we demonstrate that electrons cannot be the main emitters of
gamma-rays in the range 0.1-10 GeV at the site of the strongest SNR shock. The
intensity, spectral characteristics, and location of the most prominent
gamma-ray emission together with the absence of co-spatial detectable TeV
emission are consistent only with a hadronic model of cosmic-ray acceleration
in the SNR. A high-density molecular cloud (cloud "E") provides a remarkable
"target" for nucleonic interactions of accelerated hadrons: our results show
enhanced gamma-ray production near the molecular cloud/shocked shell
interaction site. IC 443 provides the first unambiguous evidence of cosmic-ray
acceleration by SNRs.Comment: 5 pages, 2 figures; accepted by ApJLetters on Jan 21, 201
The flaring blazars of the first 1.5 years of the AGILE mission
We report the AGILE gamma-ray observations and the results of the
multiwavelength campaigns on seven flaring blazars detected by the mission:
During two multiwavelength campaigns, we observed gamma-ray activity from two
Flat Spectrum Radio Quasars of the Virgo region, e.g. 3C 279 and 3C 273 (the
latter being the first extragalactic source simultaneously observed with the
gamma-ray telescope and the hard X ray imager of the mission). Due to the large
FOV of the AGILE/GRID instrument, we achieved an almost continuous coverage of
the FSRQ 3C 454.3. The source showed flux above 10E-6 photons/cm2/s (E > 100
MeV) and showed day by day variability during all the AGILE observing periods.
In the EGRET era, the source was found in high gamma-ray activity only once. An
other blazar, PKS 1510-089 was frequently found in high gamma-ray activity. S5
0716+71, an intermediate BL Lac object, exhibited a very high gamma-ray
activity and fast gamma-ray variability during a period of intense optical
activity. We observed high gamma-ray activity from W Comae, a BL Lac object,
and Mrk 421, an high energy peaked BL Lac object. For this source, a
multiwavelength campaign from optical to TeV has been performed
Episodic Transient Gamma-Ray Emission from the Microquasar Cygnus X-1
Cygnus X-1 is the archetypal black hole (BH) binary system in our Galaxy. We
report the main results of an extensive search for transient gamma-ray emission
from Cygnus X-1 carried out in the energy range 100 MeV - 3 GeV by the AGILE
satellite, during the period 2007 July - 2009 October. The total exposure time
is about 300 days, during which the source was in the "hard" X-ray spectral
state. We divided the observing intervals in 2 or 4 week periods, and searched
for transient and persistent emission. We report an episode of significant
transient gamma-ray emission detected on 2009, October 16 in a position
compatible with Cygnus X-1 optical position. This episode, occurred during a
hard spectral state of Cygnus X-1, shows that a 1-2 day time variable emission
above 100 MeV can be produced during hard spectral states, having important
theoretical implications for current Comptonization models for Cygnus X-1 and
other microquasars. Except for this one short timescale episode, no significant
gamma-ray emission was detected by AGILE. By integrating all available data we
obtain a 2 upper limit for the total integrated flux of
in the energy range
100 MeV - 3 GeV. We then clearly establish the existence of a spectral cutoff
in the energy range 1-100 MeV that applies to the typical hard state outside
the flaring period and that confirms the historically known spectral cutoff
above 1 MeV.Comment: Accepted for publication by ApJ on the 9th of Feb 2010, 5 pages, 3
figure
Gamma-Ray Localization of Terrestrial Gamma-Ray Flashes
Terrestrial Gamma-Ray Flashes (TGFs) are very short bursts of high energy
photons and electrons originating in Earth's atmosphere. We present here a
localization study of TGFs carried out at gamma-ray energies above 20 MeV based
on an innovative event selection method. We use the AGILE satellite Silicon
Tracker data that for the first time have been correlated with TGFs detected by
the AGILE Mini-Calorimeter. We detect 8 TGFs with gamma-ray photons of energies
above 20 MeV localized by the AGILE gamma-ray imager with an accuracy of 5-10
degrees at 50 MeV. Remarkably, all TGF-associated gamma rays are compatible
with a terrestrial production site closer to the sub-satellite point than 400
km. Considering that our gamma rays reach the AGILE satellite at 540 km
altitude with limited scattering or attenuation, our measurements provide the
first precise direct localization of TGFs from space.Comment: 5 pages, 4 figures, 1 table, available at
http://prl.aps.org/abstract/PRL/v105/i12/e12850
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