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Sant'Ambrogio di Torino, Italy

Bartoli B.,University of Naples Federico II | Bartoli B.,Complesso Universitario Of Monte Santangelo | Bernardini P.,University of Salento | Bernardini P.,National Institute of Nuclear Physics, Italy | And 95 more authors.
Astrophysical Journal | Year: 2015

The ARGO-YBJ air shower detector monitored the Crab Nebula gamma-ray emission from 2007 November to 2013 February. The integrated signal, consisting of 3.3 × 105 events, reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in the energy range 0.3-20 TeV can be described by a power law function dN/dE = I 0 (E/2 TeV)-α, with a flux normalization I 0 = (5.2 ± 0.2) × 10-12 photons cm-2 s-1 TeV-1 and α = 2.63 ± 0.05, corresponding to an integrated flux above 1 TeV of 1.97 × 10-11 photons cm-2 s-1. The systematic error is estimated to be less than 30% for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cutoff dN/dE = I 0 (E/2 TeV)-α exp (-E/E cut), the lower limit of the cutoff energy E cut is 12 TeV, at 90% confidence level. Our extended data set allows the study of the TeV emission over long timescales. Over five years, the light curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 × 10-2. A correlated analysis with Fermi-LAT data over 4.5 yr using the light curves of the two experiments gives a Pearson correlation coefficient r = 0.56 ± 0.22. Concerning flux variations on timescales of days, a "blind" search for flares with a duration of 1-15 days gives no excess with a significance higher than four standard deviations. The average rate measured by ARGO-YBJ during the three most powerful flares detected by Fermi-LAT is 205 ± 91 photons day-1, consistent with the average value of 137 ± 10 day-1. © 2015. The American Astronomical Society. All rights reserved.

Verrecchia F.,Earth Observation Directorate | Verrecchia F.,Consorzio Interuniversitario Fisica Spaziale | Pittori C.,Earth Observation Directorate | Bulgarelli A.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 3 more authors.
Advances in Space Research | Year: 2013

AGILE pointed observations performed from July 9, 2007 to October 30, 2009 cover a very large time interval, with a γ-ray data archive useful to perform monitoring studies of medium to high brightness γ-ray sources in the 30 MeV-50 GeV energy range. The first AGILE Gamma-Ray Imaging Detector (GRID) catalog (Pittori et al.; 2009) included a significance-limited (4σ) sample of 47 sources (1AGL), detected with a conservative analysis over the first-year of operations. We present a variability study of the 1AGL sources over the complete AGILE pointed Observation Blocks (OBs) dataset. In the analysis here reported we used data obtained with an improved full Field of View (FOV) event filter, on a much larger (about 27.5 months) dataset, integrating data on the OB timescales, mostly ranging between 4 and 30 days. The data processing resulted in an improved source list as compared to the 1AGL one. We present here our results on the variability of some of these sources. © 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.

Giuliani A.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Cardillo M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Cardillo M.,University of Rome Tor Vergata | Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 43 more authors.
Astrophysical Journal Letters | Year: 2011

We present the AGILE gamma-ray observations in the energy range 50MeV-10GeV of the supernova remnant (SNR) W44, one of the most interesting systems for studying cosmic-ray production. W44 is an intermediate-age SNR (∼20, 000 years) and its ejecta expand in a dense medium as shown by a prominent radio shell, nearby molecular clouds, and bright [SII] emitting regions. We extend our gamma-ray analysis to energies substantially lower than previous measurements which could not conclusively establish the nature of the radiation. We find that gamma-ray emission matches remarkably well both the position and shape of the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay. Here we demonstrate that (1) hadron-dominated models are consistent with all W44 multiwavelength constraints derived from radio, optical, X-ray, and gamma-ray observations; (2) ad hoc lepton-dominated models fail to explain simultaneously the well-constrained gamma-ray and radio spectra, and require a circumstellar density much larger than the value derived from observations; and (3) the hadron energy spectrum is well described by a power law (with index s = 3.0 ± 0.1) and a low-energy cut-off at E c = 6 ± 1GeV. Direct evidence for pion emission is then established in an SNR for the first time. © 2011. The American Astronomical Society. All rights reserved..

Vittorini V.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Tavani M.,University of Rome Tor Vergata | Pucella G.,ENEA | And 20 more authors.
Astrophysical Journal Letters | Year: 2011

Strong gamma-ray flares from the Crab Nebula have been recently discovered by AGILE and confirmed by Fermi-LAT. We study here the spectral evolution in the gamma-ray energy range above 50MeV of the 2010 September flare that was simultaneously detected by AGILE and Fermi-LAT. We revisit the AGILE spectral data and present an emission model based on rapid (within 1 day) acceleration followed by synchrotron cooling. We show that this model successfully explains both the published AGILE and Fermi-LAT spectral data showing a rapid rise and a decay within 2 and 3days. Our analysis constrains the acceleration timescale and mechanism, the properties of the particle distribution function, and the local magnetic field. The combination of very rapid acceleration, emission well above 100MeV, and the spectral evolution consistent with synchrotron cooling contradicts the idealized scenario predicting an exponential cutoff at photon energies above 100MeV. We also consider a variation of our model based on even shorter acceleration and decay timescales, which can be consistent with the published averaged properties. © 2011. The American Astronomical Society. All rights reserved.

Mignone A.,University of Turin | Striani E.,National institute for astrophysics | Striani E.,University of Rome Tor Vergata | Tavani M.,National institute for astrophysics | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate the dynamical propagation of the South-East jet from the Crab pulsar interacting with supernova ejecta by means of three-dimensional relativistic magnetohydrodynamic (MHD) numerical simulations with the PLUTO code. The initial jet structure is set up from the inner regions of the Crab nebula. We study the evolution of hot, relativistic hollow outflows initially carrying a purely azimuthal magnetic field. Our jet models are characterized by different choices of the outflow magnetization (σ parameter) and the bulk Lorentz factor (γ j). We show that the jet is heavily affected by the growth of current-driven kink instabilities causing considerable deflection throughout its propagation length. This behaviour is partially stabilized by the combined action of larger flow velocities and/or reduced magnetic field strengths. We find that our best jet models are characterized by relatively large values of σ (≳1) and small values of γ j ≃ 2. Our results are in good agreement with the recent X-ray (Chandra) data of the Crab nebula South-East jet indicating that the jet changes direction of propagation on a time-scale of the order of few years. The 3D models presented here may have important implications in the investigation of particle acceleration in relativistic outflows. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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