68 research outputs found
Fermi-LAT Observation of Quiet Solar Emission
The Large Area Telescope (LAT) on board Fermi has detected high-energy gamma
rays from the quiet Sun produced by interactions of cosmic-ray nucleons with
the solar surface and cosmic-ray electrons with solar photons in the
heliosphere. Such observations provide a probe of the extreme conditions near
the solar atmosphere and photosphere and permit the study of the modulation of
cosmic rays over the inner heliosphere. For the first year of Fermi
observations the solar modulation was at its minimum corresponding to a maximum
cosmic-ray flux and, hence, maximum gamma-ray emission from the Sun. We discuss
the study of the quiescent solar emission, including spectral analysis of its
two components, disk and inverse Compton, using the first-year data of the
mission and models using the electron spectrum measured by Fermi.Comment: 2009 Fermi Symposium; eConf Proceedings C09112
Solar gamma rays and modulation of cosmic rays in the inner heliosphere
The first evidence of the gamma-ray emission from the quiescent Sun was found
in the archival EGRET data that was later confirmed by Fermi-LAT observations
with high significance. This emission is produced by Galactic cosmic rays (CRs)
penetrating the inner heliosphere and inter- acting with the solar atmosphere
and optical photons. The solar emission is characterized by two spatially and
spectrally distinct components: (i) disk emission due to the CR cascades in the
solar atmosphere, and (ii) spatially extended inverse Compton (IC) emission due
to the CR electrons scattering off of solar photons. The intensity of both
components associated with Galactic CRs anti-correlate with the level of the
solar activity being the brightest during solar minimum. In this paper we
discuss updates of the models of the IC component of the emission based on CR
measurements made at different levels of solar activity, and we make
predictions for e- ASTROGAM and AMEGO, proposed low-energy gamma-ray missions.Comment: 7 pages, 3 figures, Proceedings of the 35th International Cosmic Ray
Conference, ICRC201
Multi-Party Media Partisanship Attention Score. Estimating Partisan Attention of News Media Sources Using Twitter Data in the Lead-up to 2018 Italian Election
The ongoing radical transformations in communication ecosystems have brought up concerns about the risks of partisan selective exposure and ideological polarization. Tra- ditionally, partisan selective exposure is measured by cross-tabulating survey responses to questions on vote intentions and media consumption. This process is expensive, limits the number of news outlets taken into account and is prone to the typical biases of self-reported data. Building upon previous works and with a specific focus on the online media environment, we introduce a new method to measure partisan media attention in a multi-party political system using Twitter data from 2018 Italian general election. Our first research question addresses the effectiveness of this method by measuring the extent to which our estimates correlate with partisan newspaper consumption measured by the latest Italian National Election Studies (ITANES) survey. Once established the reliability of our method, we employ these scores and measures to analyze the Italian digital media ecosystem in the lead-up to March 2018 election. The traditionally high level of political parallelism that characterizes both the Italian press and TV sectors is only partially reflected in a digital media ecosystem where partisan news sources seem to coexist with cross-partisan outlets. Results also point out that certain online partisan communities tend to rely more on exclusive news media sources
It takes a village to manipulate the media: coordinated link sharing behavior during 2018 and 2019 Italian elections.
Over the last few years, a proliferation of attempts to define, understand and fight the spread of problematic information in contemporary media ecosystems emerged. Most of these attempts focus on false content and/or bad actors detection. In this paper, we argue for a wider ecological focus. Using the frame of media manipulation and a revised version of the âcoordinated inauthentic behaviorâ original definition, the paper presents a study based on an unprecedented combination of Facebook data, accessed through the CrowdTangle API, and two datasets of Italian political news stories published in the run-up to the 2018 Italian general election and 2019 European election. By focusing on actorsâ collective behavior, we identified several networks of pages, groups, and verified public profiles (âentitiesâ), that shared the same political news articles on Facebook within a very short period of time. Some entities in our networks were openly political, while others, despite sharing political content too, deceptively presented themselves as entertainment venues. The proportion of inauthentic entities in a network affects the wideness of the range of news media sources they shared, thus pointing to different strategies and possible motivations. The paper has both theoretical and empirical implications: it frames the concept of âcoordinated inauthentic behaviorâ in existing literature, introduces a method to detect coordinated link sharing behavior and points out different strategies and methods employed by networks of actors willing to manipulate the media and public opinion
Gleam: the GLAST Large Area Telescope Simulation Framework
This paper presents the simulation of the GLAST high energy gamma-ray
telescope. The simulation package, written in C++, is based on the Geant4
toolkit, and it is integrated into a general framework used to process events.
A detailed simulation of the electronic signals inside Silicon detectors has
been provided and it is used for the particle tracking, which is handled by a
dedicated software. A unique repository for the geometrical description of the
detector has been realized using the XML language and a C++ library to access
this information has been designed and implemented.Comment: 10 pages, Late
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
GLAST Large Area Telescope simulation tools
This paper presents the simulation of the GLAST high energy gamma-ray telescope. The simulation package, written in C++, is based on the Geant4 toolkit, and it is integrated into a general framework used to process events. A detailed simulation of the electronic signals inside silicon detectors has been provided and it is used for the particle tracking, which is handled by a dedicated software. A unique repository for the geometrical description of the detector has been realized using the XML language and a C++ library to access this information has been designed and implemented. A new event display based on the HepRep protocol is being implemented. The GLAST satellite parameters derived from the simulation are used in a fast simulator to obtain a "snapshot" of the gamma-ray sky. This paper outlines the contribution developed by the Italian GLAST software group
Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi
Gamma-Ray Pulsar Bonanza
Most of the pulsars we know about were detected through their radio emission; a few are known to pulse gamma rays but were first detected at other wavelengths (see the Perspective by
Halpern
). Using the Fermi Gamma-Ray Space Telescope,
Abdo
et al.
(p.
840
, published online 2 July; see the cover) report the detection of 16 previously unknown pulsars based on their gamma-ray emission alone. Thirteen of these coincide with previously unidentified gamma-ray sources, solving the 30-year-old mystery of their identities. Pulsars are fast-rotating neutron stars. With time they slow down and cease to radiate; however, if they are in a binary system, they can have their spin rates increased by mass transfer from their companion stars, starting a new life as millisecond pulsars. In another study,
Abdo
et al.
(p.
845
) report the detection of gamma-ray emission from the globular cluster 47 Tucanae, which is coming from an ensemble of millisecond pulsars in the cluster's core. The data imply that there are up to 60 millisecond pulsars in 47 Tucanae, twice as many as predicted by radio observations. In a further companion study,
Abdo
et al.
(p.
848
, published online 2 July) searched Fermi Large Area Telescope data for pulsations from all known millisecond pulsars outside of stellar clusters, finding gamma-ray pulsations for eight of them. Their properties resemble those of other gamma-ray pulsars, suggesting that they share the same basic emission mechanism. Indeed, both sets of pulsars favor emission models in which the gamma rays are produced in the outer magnetosphere of the neutron star
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