The development of a new camera based on the use of Silicon Photomultipliers (SiPM) proposed for the Cherenkov Telescope Array (CTA), which represents a new generation of ground based very high ...energy gamma ray observatory, is one of the main items of the Italian Institute of Nuclear Physics (INFN). In the R&D framework a single channel electronic charge preamplifier has been developed to improve the performance of photon cameras equipped with High Density NUV – HD SiPM produced by Fondazione Bruno Kessler (FBK) with a micro cell of 30 μm x 30 μm and 6 mm x 6 mm total area. The single channel preamplifier will be used as basic component for a 16-channel electronic board prototype to test the 8 x 8 NUV – HD SiPM modules proposed to equip a pSCT (Schwarzschild-Couder Telescope prototype) camera. In this work the results of tests on the single channel preamplifier prototype to optimize the SiPM performances will be presented.
ABSTRACT We present the third Fermi Large Area Telescope (LAT) source catalog (3FGL) of sources in the 100 MeV-300 GeV range. Based on the first 4 yr of science data from the Fermi Gamma-ray Space ...Telescope mission, it is the deepest yet in this energy range. Relative to the Second Fermi LAT catalog, the 3FGL catalog incorporates twice as much data, as well as a number of analysis improvements, including improved calibrations at the event reconstruction level, an updated model for Galactic diffuse γ-ray emission, a refined procedure for source detection, and improved methods for associating LAT sources with potential counterparts at other wavelengths. The 3FGL catalog includes 3033 sources above significance, with source location regions, spectral properties, and monthly light curves for each. Of these, 78 are flagged as potentially being due to imperfections in the model for Galactic diffuse emission. Twenty-five sources are modeled explicitly as spatially extended, and overall 238 sources are considered as identified based on angular extent or correlated variability (periodic or otherwise) observed at other wavelengths. For 1010 sources we have not found plausible counterparts at other wavelengths. More than 1100 of the identified or associated sources are active galaxies of the blazar class; several other classes of non-blazar active galaxies are also represented in the 3FGL. Pulsars represent the largest Galactic source class. From source counts of Galactic sources we estimate that the contribution of unresolved sources to the Galactic diffuse emission is ∼3% at 1 GeV.
ABSTRACT The third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT (3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of sources detected between 100 MeV and ...300 GeV with a Test Statistic greater than 25, between 2008 August 4 and 2012 July 31. The 3LAC includes 1591 AGNs located at high Galactic latitudes ( ), a 71% increase over the second catalog based on 2 years of data. There are 28 duplicate associations, thus 1563 of the 2192 high-latitude gamma-ray sources of the 3FGL catalog are AGNs. Most of them (98%) are blazars. About half of the newly detected blazars are of unknown type, i.e., they lack spectroscopic information of sufficient quality to determine the strength of their emission lines. Based on their gamma-ray spectral properties, these sources are evenly split between flat-spectrum radio quasars (FSRQs) and BL Lacs. The most abundant detected BL Lacs are of the high-synchrotron-peaked (HSP) type. About 50% of the BL Lacs have no measured redshifts. A few new rare outliers (HSP-FSRQs and high-luminosity HSP BL Lacs) are reported. The general properties of the 3LAC sample confirm previous findings from earlier catalogs. The fraction of 3LAC blazars in the total population of blazars listed in BZCAT remains non-negligible even at the faint ends of the BZCAT-blazar radio, optical, and X-ray flux distributions, which hints that even the faintest known blazars could eventually shine in gamma-rays at LAT-detection levels. The energy-flux distributions of the different blazar populations are in good agreement with extrapolation from earlier catalogs.
ABSTRACT Most of the celestial γ rays detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays ...interact with interstellar nucleons and photons. Conventional point-source and extended-source studies rely on the modeling of this diffuse emission for accurate characterization. Here, we describe the development of the Galactic Interstellar Emission Model (GIEM), which is the standard adopted by the LAT Collaboration and is publicly available. This model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse-Compton emission produced in the Galaxy. In the GIEM, we also include large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20° and we observe an enhanced emission toward their base extending in the north and south Galactic directions and located within ∼4° of the Galactic Center.
Fermi-LAT highlights on Supernova Remnants Di Venere, L.; de Palma, F.
Nuclear and particle physics proceedings,
April-June 2018, 2018-04-00, Letnik:
297-299
Journal Article
Recenzirano
Supernova Remnants (SNRs) are a very well studied class of objects in our Galaxy and are closely related to the origin of Cosmic Rays (CRs), being candidates to host the acceleration process of ...Galactic CRs. Accelerated particles in SNRs can produce γ-rays through interactions with gas (nucleon-nucleon interactions or e± bremsstrahlung) or low-energy photons (inverse Compton scattering by electrons). In more than eight years of data taking, the Large Area Telescope (LAT) onboard the Fermi satellite, has detected more than 30 SNRs in the γ-ray energy band. The energy range in which Fermi-LAT is sensitive, namely from less than 100 MeV up to a few hundred GeV, is crucial to provide information on the physical processes occurring at the source and disentangle between lepton-based and hadron-based interpretation models. The Fermi-LAT collaboration has recently performed a systematic study of the known SNRs, producing the first Supernova Remnant Catalog in the GeV energy range. The spatial and spectral information obtained allow a systematic study of the SNR characteristics, which, together with multi-wavelength information, provide more general constraints on the CR population of our Galaxy. We present here the latest results from the observations of Galactic SNRs by Fermi-LAT, with a particular focus on the recent results obtained in the first SNR catalog.
The Fermi bubbles are two large structures in the gamma-ray sky extending to 55degrees above and below the Galactic center. We analyze 50 months of Fermi Large Area Telescope data between 100 MeV and ...500 GeV above 10degrees in Galactic latitude to derive the spectrum and morphology of the Fermi bubbles. We thoroughly explore the systematic uncertainties that arise when modeling the Galactic diffuse emission through two separate approaches. The gamma-ray spectrum is well described by either a log parabola or a power law with an exponential cutoff. We exclude a simple power law with more than 7sigma significance. The power law with an exponential cutoff has an index of 1.9 + or - 0.2 and a cutoff energy of 110 + or - 50 GeV. We find that the gamma-ray luminosity of the bubbles is (ProQuest: Formulae and/or non-USASCII text omitted)erg s super(-1). We confirm a significant enhancement of gamma-ray emission in the southeastern part of the bubbles, but we do not find significant evidence for a jet. No significant variation of the spectrum across the bubbles is detected. The width of the boundary of the bubbles is estimated to be (ProQuest: Formulae and/or non-USASCII text omitted) deg. Both inverse Compton (IC) models and hadronic models including IC emission from secondary leptons fit the gamma-ray data well. In the IC scenario, synchrotron emission from the same population of electrons can also explain the WMAP and Planck microwave haze with a magnetic field between 5 and 20 mu G.
The Fermi Large Area Telescope (LAT) has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using ...2 yr of observations with the Fermi LAT, Local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range 0.1 - 100 GeV , presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of 2.0 × 10 − 12 erg cm − 2 s − 1 on the 0.1 - 100 GeV energy flux (95% confidence level). This revised upper limit remains consistent with the observed correlation between gamma-ray luminosity and star formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10 . Its spectrum is consistent with a power law with photon index Γ = 2.4 0.1 stat + syst and a 0.1 - 100 GeV energy flux of ( 5.6 0.6 stat + syst ) × 10 − 12 erg cm − 2 s − 1 . M31 is detected to be extended with a 4 significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0 4 and no offset from the center of the galaxy, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic center excess found in Fermi-LAT observations of the MW.
ABSTRACT
We report a characterization of the multiband flux variability and correlations of the nearby (z = 0.031) blazar Markarian 421 (Mrk 421) using data from Metsähovi, Swift, Fermi-LAT, MAGIC, ...FACT, and other collaborations and instruments from 2014 November till 2016 June. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F>1 TeV < 1.7 × 10−12 ph cm−2 s−1) and in the 2–10 keV (X-ray) band (F$_{2-10\, \mathrm{keV}}\lt 3.6\times 10^{-11}$ erg cm−2 s−1), during which the Swift-BAT data suggest an additional spectral component beyond the regular synchrotron emission. The highest flux variability occurs in X-rays and very high-energy (E > 0.1 TeV) γ-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HRkeV and HRTeV show the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 d centred at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 d centred at a time lag of $43^{+9}_{-6}$ d. This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a lognormal function in most of the energy bands probed, indicating that the variability in Mrk 421 is likely produced by a multiplicative process.
ABSTRACT
M 87 is one of the closest (z = 0.004 36) extragalactic sources emitting at very high energies (VHE, E > 100 GeV). The aim of this work is to locate the region of the VHE gamma-ray emission ...and to describe the observed broad-band spectral energy distribution (SED) during the low VHE gamma-ray state. The data from M 87 collected between 2012 and 2015 as part of a MAGIC monitoring programme are analysed and combined with multiwavelength data from Fermi-LAT, Chandra, HST, EVN, VLBA, and the Liverpool Telescope. The averaged VHE gamma-ray spectrum can be fitted from ∼100 GeV to ∼10 TeV with a simple power law with a photon index of (−2.41 ± 0.07), while the integral flux above 300 GeV is $(1.44\pm 0.13)\times 10^{-12}\, \mathrm{cm}^{-2}\, \mathrm{s}^{-1}$. During the campaign between 2012 and 2015, M 87 is generally found in a low-emission state at all observed wavelengths. The VHE gamma-ray flux from the present 2012–2015M 87 campaign is consistent with a constant flux with some hint of variability ($\sim 3\, \sigma$) on a daily time-scale in 2013. The low-state gamma-ray emission likely originates from the same region as the flare-state emission. Given the broad-band SED, both a leptonic synchrotron self-Compton and a hybrid photohadronic model reproduce the available data well, even if the latter is preferred. We note, however, that the energy stored in the magnetic field in the leptonic scenario is very low, suggesting a matter-dominated emission region.
A High Efficiency fast-Response GAmma detector (HERGA) for the identification of radionuclides and for gamma-ray imaging is presented. The detector is compact, ideal for real-time analysis, with low ...power consumption, suitable for industrial process control, environmental monitoring and localization of nuclear wastes. A prototype consisting of 16 CsI(Tl) scintillators coupled to photomultiplier tubes (PMTs) with a digital readout was built. The scintillators are arranged in a 4 × 4 matrix; a 7 × 7 mask composed by plastic and tungsten tiles has been used to validate the imaging figures of merit according to coded mask principle. A 2D Kolmogorov–Smirnov pattern recognition statistical method has been developed, allowing to achieve a point spread function (PSF) of a few mrad for pointlike sources. The Minimum Detectable Activity (MDA) was also determined in the case of pointlike radioactive sources.
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