arXiv subjects

Francesco Longo

Publications and source records attributed to Francesco Longo.

At least 73 records · Page 4Linked to original sources

arXiv2015

Prospects for Gamma-Ray Bursts detection by the Cherenkov Telescope Array

The first Gamma-Ray Burst (GRB) catalog presented by the Fermi-Large Area Telescope (LAT) collaboration includes 28 GRBs, detected above 100 MeV over the first three years since the launch of the Fermi mission. However, more than 100 GRBs are expected to be found over a period of six years of data collection thanks to a new detection algorithm and to the development of a new LAT event reconstruction, the so-called "Pass 8." Our aim is to provide revised prospects for GRB alerts in the CTA era in light of these new LAT discoveries. We focus initially on the possibility of GRB detection with the Large Size Telescopes (LSTs). Moreover, we investigate the contribution of the Middle Size Telescopes (MSTs), which are crucial for the search of larger areas on short post trigger timescales. The study of different spectral components in the prompt and afterglow phase, and the limits on the Extragalactic background light are highlighted. Different strategies to repoint part of - or the entire array - are studied in detail.

astro-ph.HE↗

arXiv2015

Gamma-Ray Burst observations with Fermi

After seven years of science operation, the Fermi mission has brought great advances in the study of Gamma-ray Bursts (GRBs). Over 1600 GRBs have been detected by the Gamma-ray Burst Monitor, and more than 100 of these are also detected by the Large Area Telescope above 30 MeV. We will give an overview of these observations, presenting the common properties in the GRB temporal and spectral behavior at high energies. We will also highlight the unique characteristics of some individual bursts. The main physical implications of these results will be discussed, along with open questions regarding GRB modeling in their prompt and temporally-extended emission phases.

astro-ph.HE↗

arXiv2015

Temporal properties of bright BGO GRBs detected by Fermi

We present results of an analysis of a sample of bright Gamma-Ray Bursts (GRBs) detected by Fermi-GBM up to more than 1 MeV, which were collected during six years of Fermi operations. In particular, we focus on the GRB durations over several energy bands of the prompt emission of a subsample of bright GRBs detected up to 10 MeV by GBM and, when possible, up to 1 GeV by Fermi-LAT, thus expanding the Duration-Energy relationship in GRB light curves to high energies for the first time. We find that the relationship for these energetic GRBs is flatter than reported for other samples, suggesting that the high- and low-energy emission mechanisms are closely related.

astro-ph.HE↗

arXiv2014

GAMMA-LIGHT: High-Energy Astrophysics above 10 MeV

High-energy phenomena in the cosmos, and in particular processes leading to the emission of gamma- rays in the energy range 10 MeV - 100 GeV, play a very special role in the understanding of our Universe. This energy range is indeed associated with non-thermal phenomena and challenging particle acceleration processes. The technology involved in detecting gamma-rays is challenging and drives our ability to develop improved instruments for a large variety of applications. GAMMA-LIGHT is a Small Mission which aims at an unprecedented advance of our knowledge in many sectors of astrophysical and Earth studies research. The Mission will open a new observational window in the low-energy gamma-ray range 10-50 MeV, and is configured to make substantial advances compared with the previous and current gamma-ray experiments (AGILE and Fermi). The improvement is based on an exquisite angular resolution achieved by GAMMA-LIGHT using state-of-the-art Silicon technology with innovative data acquisition. GAMMA-LIGHT will address all astrophysics issues left open by the current generation of instruments. In particular, the breakthrough angular resolution in the energy range 100 MeV - 1 GeV is crucial to resolve patchy and complex features of diffuse sources in the Galaxy as well as increasing the point source sensitivity. This proposal addresses scientific topics of great interest to the community, with particular emphasis on multifrequency correlation studies involving radio, optical, IR, X-ray, soft gamma-ray and TeV emission. At the end of this decade several new observatories will be operational including LOFAR, SKA, ALMA, HAWK, CTA. GAMMA-LIGHT will "fill the vacuum" in the 10 MeV-10 GeV band, and will provide invaluable data for the understanding of cosmic and terrestrial high-energy sources.

astro-ph.IM↗

arXiv2014

DIRAC framework evaluation for the $\boldsymbol{Fermi}$-LAT and CTA experiments

DIRAC (Distributed Infrastructure with Remote Agent Control) is a general framework for the management of tasks over distributed heterogeneous computing environments. It has been originally developed to support the production activities of the LHCb (Large Hadron Collider Beauty) experiment and today is extensively used by several particle physics and biology communities. Current ($Fermi$ Large Area Telescope -- LAT) and planned (Cherenkov Telescope Array -- CTA) new generation astrophysical/cosmological experiments, with very large processing and storage needs, are currently investigating the usability of DIRAC in this context. Each of these use cases has some peculiarities: $Fermi$-LAT will interface DIRAC to its own workflow system to allow the access to the grid resources, while CTA is using DIRAC as workflow management system for Monte Carlo production and analysis on the grid. We describe the prototype effort that we lead toward deploying a DIRAC solution for some aspects of $Fermi$-LAT and CTA needs.

astro-ph.IM↗

arXiv2013

A new concept of y-ray telescope. LArGO: Liquid Argon Gamma-ray Observatory

LArGO (Liquid Argon Gamma-ray Observatory) consists of a new design for a $γ$-ray telescope, which exploits the idea of using a Liquid Argon Time Projection Chamber (LAr-TPC) as tracker-converter. Particle tracking in LAr-TPC can efficiently starts since the primary photon vertex. Indeed, while in the present space telescopes the incident photon converts in a tungsten foil, which is a passive material, in a LAr-TPC this conversion happens in LAr itself, which is fully active. In this proceeding is described a plausible design for the tracker-converter detector which fulfills the constraints on conversion efficiency, angular resolution, and wide field of view. It is demonstrated how this design can provide an unprecedented angular resolution for a $γ$-ray telescope, leading to a significant improvement in sensitivity and most important disclosing the possibility to detect the polarization of $γ$-ray emission.

astro-ph.IM↗

arXiv2013

Constraints on Lorentz Invariance Violation with Fermi-LAT Observations of Gamma-Ray Bursts

Some Quantum Gravity (QG) theories allow for a violation of Lorentz invariance (LIV), manifesting as a dependence of the velocity of light in vacuum on its energy. If such a dependence exists, then photons of different energies emitted together by a distant source will arrive at the Earth at different times. High-energy (GeV) transient emissions from distant astrophysical sources such as Gamma-ray Bursts (GRBs) and Active Galaxy Nuclei can be used to search for and constrain LIV. The Fermi collaboration has previously analyzed two GRBs in order to put constraints on the dispersion parameter in vacuum, and on the energy scale at which QG effects causing LIV may arise. We used three different methods on four bright GRBs observed by the Fermi-LAT to get more stringent and robust constraints. No delays have been detected and strong limits on the QG energy scale are derived: for linear dispersion we set tight constraints placing the QG energy scale above the Planck mass; a quadratic leading LIV effect is also constrained.

astro-ph.HE↗

arXiv2011

Multi-Wavelength Observations of the HBL Object 1ES 1011+496 in Spring 2008

In the spring of 2008 MAGIC organised multi-wavelength (MWL) observations of the blazar 1ES 1011+496. 1ES 1011+496 is a high-frequency peaked BL Lac object discovered at VHE gamma-rays by MAGIC in spring 2007 during an optical outburst reported by the Tuorla Blazar Monitoring Programme. MAGIC re-observed the source during the 2008 MWL campaign which also included the Metsähovi, KVA, Swift and AGILE telescopes. This was the first MWL campaign on this source that also included VHE coverage. MAGIC observed 1ES 1011+496 from March 4th to May 24th 2008 for a total of 27.9 hours, of which 20 h remained after quality cuts. The observations resulted in a detection of the source a ~7 sigma significance level with a mean flux and spectral index similar to those during the discovery. Here we will present the results of the MAGIC observations of the source in combination with contemporaneous observations at other wavelengths (radio, optical, X-rays, high energy gamma-rays) and discuss their implications on the modelling of the spectral energy distribution.

astro-ph.HE↗

arXiv2011

Observational evidence of dissipative photospheres in gamma-ray bursts

The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B: The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, on the magnetic field strength, and on the relation between the energy densities of thermal photons and of the electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease of the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power law slope, alpha, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.

astro-ph.HE↗

arXiv2010

A General Simulation Framework for Supply Chain Modeling: State of the Art and Case Study

Nowadays there is a large availability of discrete event simulation software that can be easily used in different domains: from industry to supply chain, from healthcare to business management, from training to complex systems design. Simulation engines of commercial discrete event simulation software use specific rules and logics for simulation time and events management. Difficulties and limitations come up when commercial discrete event simulation software are used for modeling complex real world-systems (i.e. supply chains, industrial plants). The objective of this paper is twofold: first a state of the art on commercial discrete event simulation software and an overview on discrete event simulation models development by using general purpose programming languages are presented; then a Supply Chain Order Performance Simulator (SCOPS, developed in C++) for investigating the inventory management problem along the supply chain under different supply chain scenarios is proposed to readers.

cs.OH↗

arXiv2009

MAGIC observation of GRB080430

Gamma-ray bursts are cosmological sources emitting radiation from the gamma-rays to the radio band. Substantial observational efforts have been devoted to the study of GRBs during the prompt phase, i.e. the initial burst of high-energy radiation, and during the longer-lasting afterglows. In spite of many successes in interpreting these phenomena there are still several open key questions about the fundamental emission processes, their energetics and the environment. Moreover, independently of their modeling, GRB spectra are remarkably simple, being satisfactorily fitted with power-laws, and therefore offer a very valuable tool to probe the extragalactic background light distribution affecting all high-energy observations of cosmological sources. Observations carried out with Cherenkov telescopes, as MAGIC, can be fundamental for all these scientific topics. GRB080430, being at a rather moderate redshift, z~0.76, and well-studied in the optical, although observed only a few hours after the high-energy event, is a good test case to evaluate the perspective for late-afterglow observations with ground based GeV/TeV observatories.

astro-ph.HE↗

arXiv2009

MAGIC observation of GRB090102

On January 2, the MAGIC-I Telescope observed GRB090102 (z=1.55) under particularly good observation conditions. Using the recently upgraded MAGIC-1 sum trigger system, upper limits down to below 50 GeV have been obtained. This is the first time that the new trigger system was fully exploited for a Gamma-ray Burst (GRB) observation and shows the capabilities of the MAGIC observatory for future follow-up observations.

astro-ph.HE↗

arXiv2009

GRB Observations with the MAGIC Telescopes

MAGIC is currently the most suitable instrument to perform observations of the prompt an early afterglow emission from Gamma Ray Bursts (GRBs) at energies above 25 GeV. The instrument is designed to have the lowest possible energy threshold and fastest reaction time to GRB Coordinates Network (GCN) alerts. The MAGIC-I telescope started to follow-up GCN alerts in the beginning of 2005. Since then, more than 50 GRB candidates were observed. Just now MAGIC-II, the second telescope has started to operate. Both telescopes together will have a factor of three better sensitivity.

astro-ph.HE↗

arXiv2008

GEANT4 and CREME96 comparison using only proton fluxes

CREME96 and GEANT4 are two well known particle transport codes through matter in space science. We present a comparison between the proton fluxes outgoing from an aluminium target, obtained by using both tools. The primary proton flux is obtained by CREME96 only, covering an energy range from MeV to hundreds GeV with the same result in both cases. We studied different thickness targets and two different GEANT4 physics lists in order to show how the spectra of the outgoing proton fluxes are modified. Our findings show consistent agreement of simulation data by each tool, with regards both GEANT4 physics lists and every thickness target analysed.

physics.comp-ph↗

arXiv2007

Simulation of prompt emission from GRBs with a photospheric component and its detectability by GLAST

The prompt emission from gamma-ray bursts (GRBs) still requires a physical explanation. Studies of time-resolved GRB spectra, observed in the keV-MeV range, show that a hybrid model consisting of two components, a photospheric and a non-thermal component, in many cases fits bright, single-pulsed bursts as well as, and in some instances even better than, the Band function. With an energy coverage from 8 keV up to 300 GeV, GLAST will give us an unprecedented opportunity to further investigate the nature of the prompt emission. In particular, it will give us the possibility to determine whether a photospheric component is the determining feature of the spectrum or not. Here we present a short study of the ability of GLAST to detect such a photospheric component in the sub-MeV range for typical bursts, using simulation tools developed within the GLAST science collaboration.

astro-ph↗

arXiv2007

The connection between Gamma-ray bursts and Supernovae Ib/c

It has been established that Gamma-Ray Bursts (GRBs) are connected to Supernovae (SNe) explosions of Type Ib/c. We intend to test whether the hypothesis of Type Ib/c SNe from different massive progenitors can reproduce the local GRB rate as well as the GRB rate as a function of redshift. We aim to predict the GRB rate at very high redshift under different assumptions about galaxy formation and star formation histories in galaxies. We assume different star formation histories in galaxies of different morphological type: ellipticals, spirals and irregulars. We explore different hypotheses concerning the progenitors of Type Ib/c SNe. We find an excellent agreement between the observed GRB local rate and the predicted Type Ib/c SN rate in irregular galaxies, when a range for single Wolf-Rayet stars of 40-100 M_sun is adopted. We also predict the cosmic Type Ib/c SN rate by taking into account all the galaxy types in an unitary volume of the Universe and we compare it with the observed cosmic GRB rate as a function of redshift. By assuming the formation of spheroids at high redshift, we predict a cosmic Type Ib/c SN rate, which is always higher than the GRB rate, suggesting that only a small fraction (0.1-1 %) of Type Ib/c SNe become GRBs. In particular, we find a ratio between the cosmic GRB rate and the cosmic Type Ib/c rate in the range 0.001-0.01, in agreement with previous estimates. Finally, due to the high star formation in spheroids at high redshift, which is our preferred scenario for galaxy formation, we predict more GRBs at high redshift than in the hierarchical scenario for galaxy formation, a prediction which awaits to be proven by future observations.

astro-ph↗

arXiv2003

Are we observing black hole spins in jetted Gamma-Ray Bursts?

We note that if the GRB phenomenon follows from the collapse of a massive object forming a black hole and a torus accreting into it, the resulting ejecta can be only related to the mass and angular momentum that characterize the black hole. The opening angle of the fireball then corresponds to a black hole spin. If indeed the fireballs opening angles are as small as recent estimates reported in the literature suggest, then GRB would be produced by almost maximally rotating BH.

astro-ph↗

arXiv2002

Simulation of the AGILE gamma-ray imaging detector performance: part I

We present in this paper the results of a comprehensive GEANT simulation of the Gamma-Ray Imaging Detector (GRID) being developed for the AGILE space astrophysics mission. The GRID is designed to be sensitive in the 30 MeV - 50 GeV energy range, with excellent imaging and timing capabilities, and with a very large field-of-view (~3 sr). In this paper (Paper I) we present the GRID baseline geometry, a model for the charged particle and albedo-photon backgrounds for an equatorial orbit of 550-600 km altitude, and the main results of the first level (Level-1 Trigger) on-board data processing. Our simulations show that the GRID Level-1 data processing is expected to be capable of decreasing by a factor of ~ 20 the charged particle background (from \~2 kHz to below 100 Hz), and by a factor of ~ 30 the albedo-photon background. The gamma-ray photon detection efficiency by the imaging GRID is simulated to be particularly efficient, varying between 39% and 17% depending on photon energies and incident directions.

astro-ph↗