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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 115 more authors.
Chinese Physics C | Year: 2014

The energy spectrum of cosmic Hydrogen and Helium nuclei has been measured below the so-called "knee" by using a hybrid experiment with a wide field-of-view Cherenkov telescope and the Resistive Plate Chamber (RPC) array of the ARGO-YBJ experiment at 4300 m above sea level. The Hydrogen and Helium nuclei have been well separated from other cosmic ray components by using a multi-parameter technique. A highly uniform energy resolution of about 25% is achieved throughout the whole energy range (100-700 TeV). The observed energy spectrum is compatible with a single power law with index γ=-2. 63±0.06. © 2014 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.


Bernlohr K.,Max Planck Institute for Nuclear Physics | Bernlohr K.,Humboldt University of Berlin | Barnacka A.,Polish Academy of Sciences | Becherini Y.,Ecole Polytechnique - Palaiseau | And 45 more authors.
Astroparticle Physics | Year: 2013

The Cherenkov Telescopes Array (CTA) is planned as the future instrument for very-high-energy (VHE) gamma-ray astronomy with a wide energy range of four orders of magnitude and an improvement in sensitivity compared to current instruments of about an order of magnitude. Monte Carlo simulations are a crucial tool in the design of CTA. The ultimate goal of these simulations is tofind the most cost-effective solution for given physics goals and thus sensitivity goals or to find, for a given cost, the solution best suited for different types of targets with CTA. Apart from uncertain component cost estimates, the main problem in this procedure is the dependence on a huge number of configuration parameters, both in specifications of individual telescope types and in the array layout. This is addressed by simulation of a huge array intended as a superset of many different realistic array layouts, and also by simulation of array subsets for different telescope parameters. Different analysis methods-inuse with current installations andextended (or developed specifically) for CTA -are applied to the simulated data sets for deriving the expected sensitivity of CTA. In this paper we describe the current status of this iterative approach to optimize the CTA design and layout. © 2012 Elsevier B.V. All rights reserved.


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 119 more authors.
Astrophysical Journal | Year: 2013

We report the observation of a very high energy γ-ray source whose position is coincident with HESS J1841?055. This source has been observed for 4.5 years by the ARGO-YBJ experiment from 2007 November to 2012 July. Its emission is detected with a statistical significance of 5.3 standard deviations. Parameterizing the source shape with a two-dimensional Gaussian function, we estimate an extension δ= (0.40+0.32?0.22)°, which is consistent with the HESS measurement. The observed energy spectrum is dN/dE = (9.0±1.6) ?10?13(E/5 TeV)?2.32± 0.23 photons cm?2 s?1 TeV?1, in the energy range 0.950 TeV. The integral ? -ray flux above 1 TeV is 1.3±0.4 Crab, which is 3.2±1.0 times the flux derived by HESS. The differences in the flux determination between HESS and ARGO-YBJ and possible counterparts at other wavelengths are discussed. © 2013. The American Astronomical Society. All rights reserved.


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 82 more authors.
Astrophysical Journal | Year: 2015

This paper reports on the measurement of the large-scale anisotropy in the distribution of cosmic-ray arrival directions using the data collected by the air shower detector ARGO-YBJ from 2008 January to 2009 December, during the minimum of solar activity between cycles 23 and 24. In this period, more than 2 × 1011 showers were recorded with energies between ∼1 and 30 TeV. The observed two-dimensional distribution of cosmic rays is characterized by two wide regions of excess and deficit, respectively, both of relative intensity ∼10-3 with respect to a uniform flux, superimposed on smaller size structures. The harmonic analysis shows that the large-scale cosmic-ray relative intensity as a function of R.A. can be described by the first and second terms of a Fouries series. The high event statistics allow the study of the energy dependence of the anistropy, showing that the amplitude increases with energy, with a maximum intensity at ∼10 TeV, and then decreases while the phase slowly shifts toward lower values of R.A. with increasing energy. The ARGO-YBJ data provide accurate observations over more than a decade of energy around this feature of the anisotropy spectrum. © 2015. The American Astronomical Society. All rights reserved..


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 94 more authors.
Astrophysical Journal | Year: 2014

The search for gamma-ray burst (GRB) emission in the energy range of 1-100 GeV in coincidence with the satellite detection has been carried out using the Astrophysical Radiation with Ground-based Observatory at YangBaJing (ARGO-YBJ) experiment. The high-altitude location (4300 m a.s.l.), the large active surface (6700 m2of Resistive Plate Chambers), the wide field of view (2 sr, limited only by the atmospheric absorption), and the high duty cycle (>86%) make the ARGO-YBJ experiment particularly suitable to detect short and unexpected events like GRBs. With the scaler mode technique, i.e., counting all the particles hitting the detector with no measurement of the primary energy and arrival direction, the minimum threshold of 1 GeV can be reached, overlapping the direct measurements carried out by satellites. During the experiment lifetime from 2004 December 17 to 2013 February 7, a total of 206 GRBs occurring within the ARGO-YBJ field of view (zenith angle θ ≤ 45°) have been analyzed. This is the largest sample of GRBs investigated with a ground-based detector. Two light curve models have been assumed and since in both cases no significant excess has been found, the corresponding fluence upper limits in the 1-100 GeV energy region have been derived, with values as low as 10-5erg cm-2. The analysis of a subset of 24 GRBs with known redshift has been used to constrain the fluence extrapolation to the GeV region together with possible cutoffs under different assumptions on the spectrum. © 2014. The American Astronomical Society. All rights reserved..


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 94 more authors.
Astrophysical Journal | Year: 2014

The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by Fermi-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from 2007 November to 2013 January, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of 10° × 10°, finding a source extension σext= 1.°8 ± 0.°5. The observed differential energy spectrum is dN/dE = (2.5 ± 0.4) × 10-11(E/1 TeV)-2.6 ± 0.3 photons cm-2 s-1 TeV-1, in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by Fermi-LAT and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with supernova remnants (SNRs) indicates that the particle acceleration inside a superbubble is similar to that in an SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution. © 2014. The American Astronomical Society. All rights reserved.


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 93 more authors.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2015

The ARGO-YBJ experiment is a full coverage array of Resistive Plate Chambers (RPCs) with an active area of 5800 m2. In order to eliminate the response difference of the charge readout from the RPCs, a calibration procedure is carried out with the iso-gradient method. This method also allows the extension of the absolute calibration with the muon telescope including scintillation detectors to all the RPCs in the array. The overall systematic uncertainty in measurements of the number of particles by the RPCs is 10.7%. In general, the method gives results consistent with those from a totally different approach also used in the experiment. © 2015 Elsevier B.V. All rights reserved.


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 81 more authors.
Astrophysical Journal, Supplement Series | Year: 2016

We report on the extensive multi-wavelength observations of the blazar Markarian 421 (Mrk 421) covering radio to γ-rays, during the 4.5 year period of ARGO-YBJ and Fermi common operation time, from 2008 August to 2013 February. These long-term observations, extending over an energy range of 18 orders of magnitude, provide a unique chance to study the variable emission of Mrk 421. In particular, due to the ARGO-YBJ and Fermi data, the entire energy range from 100 MeV to 10 TeV is covered without any gap. In the observation period, Mrk 421 showed both low- and high-activity states at all wavebands. The correlations among flux variations in different wavebands were analyzed. The X-ray flux is clearly correlated with the TeV γ-ray flux, while the GeV γ-rays only show a partial correlation with the TeV γ-rays. Radio and UV fluxes seem to be weakly or not correlated with the X-ray and γ-ray fluxes. Seven large flares, including five X-ray flares and two GeV γ-ray flares with variable durations (3-58 days), and one X-ray outburst phase were identified and used to investigate the variation of the spectral energy distribution with respect to a relative quiescent phase. During the outburst phase and the seven flaring episodes, the peak energy in X-rays is observed to increase from sub-keV to a few keV. The TeV γ-ray flux increases up to 0.9-7.2 times the flux of the Crab Nebula. The behavior of GeV γ-rays is found to vary depending on the flare, a feature that leads us to classify flares into three groups according to the GeV flux variation. Finally, the one-zone synchrotron self-Compton model was adopted to describe the emission spectra. Two out of three groups can be satisfactorily described using injected electrons with a power-law spectral index around 2.2, as expected from relativistic diffuse shock acceleration, whereas the remaining group requires a harder injected spectrum. The underlying physical mechanisms responsible for different groups may be related to the acceleration process or to the environment properties. © 2016. The American Astronomical Society. All rights reserved.


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.

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