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

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Aleksic J.,IFAE | Alvarez E.A.,Complutense University of Madrid | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 169 more authors.
Astrophysical Journal | Year: 2012

We present the results of five years (2005-2009) of MAGIC observations of the BL Lac object PG 1553+113 at very high energies (VHEs; E > 100GeV). Power-law fits of the individual years are compatible with a steady mean photon index Γ = 4.27 ± 0.14. In the last three years of data, the flux level above 150GeV shows a clear variability (probability of constant flux < 0.001%). The flux variations are modest, lying in the range from 4% to 11% of the Crab Nebula flux. Simultaneous optical data also show only modest variability that seems to be correlated with VHE gamma-ray variability. We also performed a temporal analysis of (all available) simultaneous Fermi/Large Area Telescope data of PG 1553+113 above 1GeV, which reveals hints of variability in the 2008-2009 sample. Finally, we present a combination of the mean spectrum measured at VHEs with archival data available for other wavelengths. The mean spectral energy distribution can be modeled with a one-zone synchrotron self-Compton model, which gives the main physical parameters governing the VHE emission in the blazar jet. © 2012 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.

Verrecchia F.,Science Data Center | Verrecchia F.,National institute for astrophysics | Pittori C.,Science Data Center | Pittori C.,National institute for astrophysics | And 98 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. We present a variability study of a sample of bright ?-ray (30 Mev?50 Gev) sources. This sample is an extension of the first AGILE catalogue of ?-ray sources (1AGL), obtained using the complete set of AGILE observations in pointing mode performed during a 2.3 year period from July 9, 2007 until October 30, 2009. Methods. The dataset of AGILE pointed observations covers a long time interval and its ?-ray data archive is useful for monitoring studies of medium-to-high brightness ?-ray sources. In the analysis reported here, we used data obtained with an improved event filter that covers a wider field of view, on a much larger (about 27.5 months) dataset, integrating data on observation block time scales, which mostly range from a few days to thirty days. Results. The data processing resulted in a better characterized source list than 1AGL was, and includes 54 sources, 7 of which are new high galactic latitude (|BII| ? 5) sources, 8 are new sources on the galactic plane, and 20 sources from the previous catalogue with revised positions. Eight 1AGL sources (2 high-latitude and 6 on the galactic plane) were not detected in the final processing either because of low OB exposure and/or due to their position in complex galactic regions. We report the results in a catalogue of all the detections obtained in each single OB, including the variability results for each of these sources. In particular, we found that 12 sources out of 42 or 11 out of 53 are variable, depending on the variability index used, where 42 and 53 are the number of sources for which these indices could be calculated. Seven of the 11 variable sources are blazars, the others are Crab pulsar+nebula, LS I +61?303, Cyg X-3, and 1AGLR J2021+4030. © ESO 2013.

Feroci M.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Costa E.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Del Monte E.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Donnarumma I.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 92 more authors.
Astronomy and Astrophysics | Year: 2010

Context. SuperAGILE is the hard X-ray monitor of the AGILE gamma ray mission, in orbit since 23 April 2007. It is an imaging experiment based on a set of four independent silicon strip detectors, equipped with one-dimensional coded masks, operating in the nominal energy range 18-60 keV. Aims. The main goal of SuperAGILE is the observation of cosmic sources simultaneously with the main gamma-ray AGILE experiment, the Gamma Ray Imaging Detector (GRID). Given its ~steradian-wide field of view and its ~15 mCrab day-sensitivity, SuperAGILE is also well suited to the long-term monitoring of Galactic compact objects and the detection of bright transients. Methods. The SuperAGILE detector properties and design allow for a 6 arcmin angular resolution in each of the two independent orthogonal projections of the celestial coordinates. Photon by photon data are continuously available by means of experiment telemetry, and are used to derive images and fluxes of individual sources, with integration × depending on the source intensity and position in the field of view. Results. We report on the main scientific results achieved by SuperAGILE over its first two years in orbit, until April 2009. The scientific observations started in mid-July 2007, with the science verification phase, continuing during the complete AGILE Cycle 1 and the first half of Cycle 2. Despite the largely non-uniform sky coverage, due to the pointing strategy of the AGILE mission, a few tens of Galactic sources were monitored, some× for unprecedently long continuous periods, leading to the detection also of several bursts and outbursts. Approximately one gamma ray burst per month was detected and localized, allowing for prompt multiwavelength observations. A few extragalactic sources in bright states were occasionally detected as well. The light curves of sources measured by SuperAGILE are made publicly available on the web in almost real-time. To enable a proper scientific use of these, we provide the reader with the relevant scientific and technical background. © ESO, 2010.

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..

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