Sant'Ambrogio di Torino, Italy
Sant'Ambrogio di Torino, Italy

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Striani E.,University of Rome Tor Vergata | Tavani M.,University of Rome Tor Vergata | Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Piano G.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 49 more authors.
Astrophysical Journal Letters | Year: 2011

We report on the extremely intense and fast gamma-ray flare above 100MeV detected by AGILE from the Crab Nebula in mid-April 2011. This event is the fourth of a sequence of reported major gamma-ray flares produced by the Crab Nebula in the period 2007/mid-2011. These events are attributed to strong radiative and plasma instabilities in the inner Crab Nebula, and their properties are crucial for theoretical studies of fast and efficient particle acceleration up to 1015 eV. Here we study the very rapid flux and spectral evolution of the event that on 2011 April 16 reached the record-high peak flux of F = (26 5) × 10-6 photons cm-2 s -1 with a rise-time timescale that we determine to be in the range 6-10hr. The peak flaring gamma-ray spectrum reaches a distinct maximum near 500MeV with no substantial emission above 1 GeV. The very rapid rise time and overall evolution of the Crab Nebula flare strongly constrain the acceleration mechanisms and challenge MHD models. We briefly discuss the theoretical implications of our observations. © 2011. The American Astronomical Society. All rights reserved.

Striani E.,University of Rome Tor Vergata | Vercellone S.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,University of Rome Tor Vergata | Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 63 more authors.
Astrophysical Journal | Year: 2010

We present the gamma-ray data of the extraordinary flaring activity above 100 MeV from the flat spectrum radio quasar 3C 454.3 detected by AGILE during the month of 2009 December. 3C 454.3, which has been among the most active blazars of the FSRQ type since 2007, has been detected in the gamma-ray range with a progressively rising flux since 2009 November 10. The gamma-ray flux reached a value comparable with that of the Vela pulsar on 2009 December 2. Remarkably, between 2009 December 2 and 3, the source more than doubled its gamma-ray emission and became the brightest gamma-ray source in the sky with a peak flux of Fγ,p = (2000 ± 400) × 10-8ph cm-2 s-1 for a 1 day integration above 100 MeV. The gamma-ray intensity decreased in the following days with the source flux remaining at large values near Fγ ≃ (1000 ± 200) × 10-8 ph cm-2 s-1 for more than a week. This exceptional gamma-ray flare dissipated among the largest ever detected intrinsic radiated power in gamma-rays above 100 MeV (Lγ,source,peak ≃ 3 × 1046 erg s-1, for a relativistic Doppler factor of δ ≃ 30). The total isotropic irradiated energy of the month-long episode in the range 100 MeV-3 GeV is Eγ,iso ≃ 1056 erg. We report the intensity and spectral evolution of the gamma-ray emission across the flaring episode. We briefly discuss the important theoretical implications of our detection. © 2010. The American Astronomical Society. All rights reserved.

Sabatini S.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Sabatini S.,University of Rome Tor Vergata | Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,University of Rome Tor Vergata | And 64 more authors.
Astrophysical Journal Letters | Year: 2010

Cygnus X-1 (Cyg X-1) is the archetypal black hole 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-3GeV 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 2-4 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 Cyg X-1 optical position. This episode, which occurred during a hard spectral state of Cyg 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 Cyg 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 2σ upper limit for the total integrated flux of F γ,U.L. = 3 × 10-8 ph cm-2 s-1 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. © 2010. The American Astronomical Society. All rights reserved..

Giuliani A.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Fuschino F.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Vianello G.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Marisaldi M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 64 more authors.
Astrophysical Journal Letters | Year: 2010

Short gamma-ray bursts (GRBs), typically lasting less than 2 s, are a special class of GRBs of great interest. We report the detection by the AGILE satellite of the short GRB 090510 which shows two clearly distinct emission phases: a prompt phase lasting ∼200ms and a second phase lasting tens of seconds. The prompt phase is relatively intense in the 0.3-10 MeV range with a spectrum characterized by a large peak/cutoff energy near 3 MeV; in this phase, no significant high-energy gamma-ray emission is detected. At the end of the prompt phase, intense gamma-ray emission above 30 MeV is detected showing a power-law time decay of the flux of the type t -1.3 and a broadband spectrum remarkably different from that of the prompt phase. It extends from sub-MeV to hundreds of MeV energies with a photon index α ≃ 1.5. GRB 090510 provides the first case of a short GRB with delayed gamma-ray emission. We present the timing and spectral data of GRB 090510 and briefly discuss its remarkable properties within the current models of gamma-ray emission of short GRBs. © 2010. The American Astronomical Society. All rights reserved..

Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,University of Rome Tor Vergata | Giuliani A.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Chen A.W.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 63 more authors.
Astrophysical Journal Letters | Year: 2010

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 100MeV from IC 443 as obtained by the AGILE satellite. A distinct pattern of diffuse emission in the energy range 100MeV-3GeV is detected across the SNR with its prominent maximum (source "A") localized in the northeastern shell with a flux F = (47±10) × 10?8 photons cm?2 s?1 above 100MeV. 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° 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 0.1-10GeV 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 cospatial 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. © 2010. The American Astronomical Society. All rights reserved.

Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,University of Rome Tor Vergata | Marisaldi M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Labanti C.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 70 more authors.
Physical Review Letters | Year: 2011

Strong electric discharges associated with thunderstorms can produce terrestrial gamma-ray flashes (TGFs), i.e., intense bursts of x-rays and γ-rays lasting a few milliseconds or less. We present in this Letter new TGF timing and spectral data based on the observations of the Italian Space Agency AGILE satellite. We determine that the TGF emission above 10 MeV has a significant power-law spectral component reaching energies up to 100 MeV. These results challenge TGF theoretical models based on runaway electron acceleration. The TGF discharge electric field accelerates particles over the large distances for which maximal voltages of hundreds of megavolts can be established. The combination of huge potentials and large electric fields in TGFs can efficiently accelerate particles in large numbers, and we reconsider here the photon spectrum and the neutron production by photonuclear reactions in the atmosphere. © 2010 The American Physical Society.

Striani E.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Striani E.,University of Rome Tor Vergata | Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,University of Rome Tor Vergata | And 25 more authors.
Astrophysical Journal | Year: 2013

Gamma-ray emission from the Crab Nebula has been recently shown to be unsteady. In this paper, we study the flux and spectral variability of the Crab above 100 MeV on different timescales ranging from days to weeks. In addition to the four main intense and day-long flares detected by AGILE and Fermi-LAT between 2007 September and 2012 September, we find evidence for week-long and less intense episodes of enhanced gamma-ray emission that we call "waves." Statistically significant "waves" show timescales of 1-2 weeks, and can occur by themselves or in association with shorter flares. We present a refined flux and spectral analysis of the 2007 September-October gamma-ray enhancement episode detected by AGILE that shows both "wave" and flaring behavior. We extend our analysis to the publicly available Fermi-LAT data set and show that several additional "wave" episodes can be identified. We discuss the spectral properties of the 2007 September "wave"/flare event and show that the physical properties of the "waves" are intermediate between steady and flaring states. Plasma instabilities inducing "waves" appear to involve spatial distances l ∼ 1016 cm and enhanced magnetic fields B ∼ (0.5-1) mG. Day-long flares are characterized by smaller distances and larger local magnetic fields. Typically, the deduced total energy associated with the "wave" phenomenon (Ew ∼ 1042 erg, where Ew is the kinetic energy of the emitting particles) is comparable with that associated to the flares, and can reach a few percent of the total available pulsar spin-down energy. Most likely, flares and waves are the product of the same class of plasma instabilities that we show acting on different timescales and radiation intensities. © 2013. The American Astronomical Society. All rights reserved.

Coppi B.,MIUR | Airoldi A.,University Degli Studi | Albanese R.,University Degli Studi | Ambrosino G.,University Degli Studi | And 25 more authors.
Nuclear Fusion | Year: 2013

The scientific goal of the Ignitor experiment is to approach, for the first time, the ignition conditions of a magnetically confined D-T plasma. The IGNIR collaboration between Italy and Russia is centred on the construction of the core of the Ignitor machine in Italy and its installation and operation within the Triniti site (Troitsk). A parallel initiative has developed that integrates this programme, involving the study of plasmas in which high-energy populations are present, with ongoing research in high-energy astrophysics, with a theory effort involving the National Institute for High Mathematics, and with INFN and the University of Pisa for the development of relevant nuclear and optical diagnostics. The construction of the main components of the machine core has been fully funded by the Italian Government. Therefore, considerable attention has been devoted towards identifying the industrial groups having the facilities necessary to build these components. An important step for the Ignitor programme is the adoption of the superconducting MgB2 material for the largest poloidal field coils (P14) that is compatible with the He-gas cooling system designed for the entire machine. The progress made in the construction of these coils is described. An important advance has been made in the reconfiguration of the cooling channels of the toroidal magnet that can double the machine duty cycle. A facility has been constructed to test the most important components of the ICRH system at full scale, and the main results of the tests carried out are presented. The main physics issues that the Ignitor experiment is expected to face are analysed considering the most recent developments in both experimental observations and theory for weakly collisional plasma regimes. Of special interest is the I-regime that has been investigated in depth only recently and combines advanced confinement properties with a high degree of plasma purity. This is a promising alternative to the high-density L-regime that had been observed by the Alcator experiment and whose features motivated the Ignitor project. The provisions that are incorporated in the machine design, and in that of the plasma chamber in particular, in order to withstand or prevent the development of macroscopic instabilities with deleterious amplitudes are presented together with relevant analyses. © 2013 IAEA, Vienna.

Cardillo M.,National institute for astrophysics | Cardillo M.,University of Rome Tor Vergata | Tavani M.,National institute for astrophysics | Tavani M.,University of Rome Tor Vergata | And 8 more authors.
Astronomy and Astrophysics | Year: 2014

The middle-aged supernova remnant (SNR) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays. For the first time for a SNR, the gamma-ray missions AGILE and Fermi have established the spectral continuum below 200 MeV, which can be attributed to a neutral pion emission. Confirming the hadronic origin of the gamma-ray emission near 100 MeV is then of the greatest importance. Our paper is focused on a global re-assessment of all available data and models of particle acceleration in W44 with the goal of determining the hadronic and leptonic contributions to the overall spectrum on a firm ground. We also present new gamma-ray and CO NANTEN2 data on W44 and compare them to recently published AGILE and Fermi data. Our analysis strengthens previous studies and observations of the W44 complex environment and provides new information for more detailed modeling. In particular, we determine that the average gas density of the regions emitting 100 MeV-10 GeV gamma-rays is relatively high (n ~ 250-300 cm-3). The hadronic interpretation of the gamma-ray spectrum of W44 is viable and supported by strong evidence. It implies a relatively large value for the average magnetic field (B ≥ 102 μG) in the SNR surroundings, which is a sign of field amplification by shock-driven turbulence. Our new analysis establishes that the spectral index of the proton energy distribution function is p1 = 2.2 ± 0.1 at low energies and p2 = 3.2 ± 0.1 at high energies. We critically discuss hadronic versus leptonic-only models of emission taking radio and gamma-ray data into account simultaneously. We find that the leptonic models are disfavored by the combination of radio and gamma-ray data. Having determined the hadronic nature of the gamma-ray emission on firm ground, a number of theoretical challenges remain to be addressed. © ESO 2014.

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