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Serguei Vorobiov, C. L. Naumann, 2012, published scientific conference contribution

Found in: ključnih besedah
Keywords: gamma-ray astronomy, front-end electronics, integrated ASIC, trigger, readout
Published: 25.04.2014; Views: 2160; Downloads: 12
URL Fulltext (0,00 KB)

The NECTAr project
Serguei Vorobiov, J-F. Glicenstein, 2011, published scientific conference contribution

Found in: ključnih besedah
Keywords: gamma ray astronomy, Cherenkov Telescope Arrays, NECTAr front-end electronics
Published: 05.08.2014; Views: 2612; Downloads: 17
URL Fulltext (0,00 KB)

Radioactive isotopes in ground waters of Slovenia
Katarina Kovačič, 2015, doctoral dissertation

Abstract: Within the duration of three projects (J7-0363, L1-0437, L1-4280), in the period from 2008 to 2013, measurements of tritium (3H), 40K and γ-ray emitters of uranium (238U, 226Ra and 210Pb) and thorium (228Ra and 228Th) decay series in groundwaters of Slovenia were carried out. Tritium was also measured in precipitation at selected locations. Groundwater sampling sites were selected in order to cover the most representative lithological units that are found in Slovenia and were at the same time evenly distributed throughout the country. In total, 281 samples were measured from 120 different locations. Measurements in the period 2009-2013 have shown that tritium concentrations in precipitation in Ljubljana are for about 30% lower compared to those in Vienna, however they are in line with seasonal fluctuations observed in Vienna. This was an important information in the process of the reconstruction of past tritium concentrations in precipitation in Ljubljana (tritium curve) for the missing period between 1953 and 1981. Tritium curve represents the input data when determining the average age of water. In determining the influence of various parameters on the final interpretation of the groundwater age it has been found that the greatest uncertainty contribute the measurements of the tritium curve and the measurements of tritium in groundwater samples. Also, the interaction of other factors (continental effect, rainfall regime, infiltration) and the lack of knowledge of them may have a major impact on the misinterpretation of groundwater age. Based on tritium concentration, groundwaters were divided into 4 major categories, into groundwaters that are older than 100 years (tritium concentration was below the detection limit), groundwaters, where the older component prevails (concentration of tritium was between 0 and 2.5 TU), groundwaters with the age between 30 and 60 years (the concentration of tritium was on average 8 TU) and recent groundwaters with age up to 15 years (tritium concentration of about 6 TU). The natural background of γ-ray emitters was determined for each lithological unit. The most represented radionuclide was 40K with concentrations always above the detection limit. A dependency of 40K concentrations on nitrate was observed, especially in the recharge areas, where limestone prevailed. Other γ-ray emitters revealed some dependence on nitrate concentration, especially both radium isotopes (226Ra and 228Ra). The concentration of 210Pb in groundwaters is mainly due to the connection with the surface and consequently with atmospheric 210Pb. The highest concentrations of 238U and 228Ra were measured in groundwaters from recharge areas in clastic rocks with a clay component. In contrast to the expected, concentrations for all γ-ray emitters (except 210Pb) were the lowest or even not detectable, in groundwaters from recharge areas in igneous and metamorphic rocks. Among γ-ray emitters and categorization based on tritium concentrations, elevated concentrations of 238U were observed in groundwaters with prevailing older component. Elevated concentrations of 40K were also observed in them. The largest scattering of 210Pb and 40K concentrations was observed in recent waters, which indicates a connection with the surface and application of fertilizers.
Found in: ključnih besedah
Keywords: Groundwaters, Slovenia, tritium input curve, natural gamma-ray emitters background.
Published: 05.10.2015; Views: 3453; Downloads: 118
.pdf Fulltext (8,62 MB)

A search for Galactic transients disguised as gamma-ray bursts
Valentina La Parola, Cristiano Guidorzi, Aurora Clerici, 2015, original scientific article

Abstract: A significant fraction of cosmological gamma-ray bursts (GRBs) are characterised by a fast rise and exponential decay (FRED) temporal structure. This is not a distinctive feature of this class, since it is observed in many Galactic transients and is likely descriptive of a sudden release of energy followed by a diffusion process. Possible evidence has recently been reported by Tello et al. (2012) for a Galactic contamination in the sample of FRED GRBs discovered with Swift. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue up to September 2014. We selected 181 FRED GRBs (2/3 with unknown redshift) and considered different subsamples. We tested the degree of isotropy through the dipole and the quadrupole moment distributions, both with reference to the Galaxy and in a coordinate-system-independent way, as well as with the two-point angular autocovariance function. In addition, we searched for possible indicators of a Galactic origin among the spectral and temporal properties of individual GRBs. We found marginal (~3 sigma) evidence for an excess of FREDs with unknown redshift towards the Galactic plane compared with what is expected for an isotropic distribution corrected for the non-uniform sky exposure. However, when we account for the observational bias against optical follow-up observations of low-Galactic latitude GRBs, the evidence for anisotropy decreases to ~2 sigma. In addition, we found no statistical evidence for different spectral or temporal properties from the bulk of cosmological GRBs. We found marginal evidence for the presence of a disguised Galactic population among Swift GRBs with unknown redshift. The estimated fraction is f=(19 +- 11)%, with an upper limit of 34% (90% confidence).
Found in: ključnih besedah
Keywords: gamma-ray burst, statistical, Galactic transient
Published: 03.02.2016; Views: 2128; Downloads: 0
.pdf Fulltext (173,64 KB)

Gabrijela Zaharijas, Collaboration Fermi LAT, M. Ackermann, 2016, original scientific article

Abstract: We present a catalog of sources detected above 50 GeV by the Fermi-Large Area Telescope (LAT) in 80 months of data. The newly delivered Pass8 event-level analysis allows the detection and characterization of sources in the 50 GeV–2 TeV energy range. In this energy band, Fermi-LAT has detected 360 sources, which constitute the second catalog of hard Fermi-LAT sources (2FHL). The improved angular resolution enables the precise localization of point sources (∼1 7 radius at 68% C.L.) and the detection and characterization of spatially extended sources. We find that 86% of the sources can be associated with counterparts at other wavelengths, of which the majority (75%) are active galactic nuclei and the rest (11%) are Galactic sources. Only 25% of the 2FHL sources have been previously detected by Cherenkov telescopes, implying that the 2FHL provides a reservoir of candidates to be followed up at very high energies. This work closes the energy gap between the observations performed at GeV energies by Fermi-LATon orbit and the observations performed at higher energies by Cherenkov telescopes from the ground.
Found in: ključnih besedah
Keywords: catalogs – gamma-rays
Published: 25.02.2016; Views: 2514; Downloads: 0
.pdf Fulltext (2,26 MB)

Prospects for Indirect Dark Matter Searches with the Cherenkov Telescope Array (CTA)
Gabrijela Zaharijas, John Carr, 2015, published scientific conference contribution

Abstract: The Cherenkov Telescope Array (CTA) will have a unique chance of discovery for a large range of masses in Weakly Interacting Massive Particles models of dark matter. The principal target for dark matter searches with CTA is the centre of the Galactic Halo. The best strategy is to perform CTA observations within a few degrees of the Galactic Centre, with the Galactic Centre itself and the most intense diffuse emission regions removed from the analysis. Assuming a cuspy dark matter density profile for the Milky Way, 500 hours of observations in this region provide sensitivities to and below the thermal cross-section of dark matter annihilations, for masses between a few hundred GeV and a few tens of TeV; therefore CTA will have a significant chance of discovery in some models. Since the dark matter density in the Milky Way is far from certain in the inner kpc region, other targets are also proposed for observation, like ultra-faint dwarf galaxies such as Segue 1 with 100 hours per year proposed. Beyond these two observational targets, further alternatives, such as Galactic dark clumps, will be considered closer to the actual date of CTA operations. Sensitivity predictions for dark matter searches are given on the various targets taking into account the latest instrument response functions expected for CTA together with a discussion on the systematic uncertainties from the backgrounds.
Found in: ključnih besedah
Keywords: dark matter, gamma-ray astronomy
Published: 01.03.2016; Views: 2098; Downloads: 155
.pdf Fulltext (724,07 KB)

Gabrijela Zaharijas, B. L. Winer, 2016, original scientific article

Abstract: The Fermi Large Area Telescope (LAT) has provided the most detailed view to date of the emission toward the Galactic center (GC) in high-energy γ-rays. This paper describes the analysis of data taken during the first 62 months of the mission in the energy range 1–100 GeV from a 15° × 15° region about the direction of the GC. Specialized interstellar emission models (IEMs) are constructed to enable the separation of the γ-ray emissions produced by cosmic ray particles interacting with the interstellar gas and radiation fields in the Milky Way into that from the inner ∼1 kpc surrounding the GC, and that from the rest of the Galaxy. A catalog of point sources for the 15° × 15° region is self-consistently constructed using these IEMs: the First Fermi-LAT Inner Galaxy Point Source Catalog (1FIG). The spatial locations, fluxes, and spectral properties of the 1FIG sources are presented, and compared with γ-ray point sources over the same region taken from existing catalogs. After subtracting the interstellar emission and point-source contributions a residual is found. If templates that peak toward the GC are used to model the positive residual the agreement with the data improves, but none of the additional templates tried account for all of its spatial structure. The spectrum of the positive residual modeled with these templates has a strong dependence on the choice of IEM.
Found in: ključnih besedah
Keywords: cosmic rays – Galaxy: center – gamma-rays: general – gamma-rays: ISM – radiation mechanisms: non-thermal
Published: 02.03.2016; Views: 2364; Downloads: 176
.pdf Fulltext (6,36 MB)

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