1,721,515 research outputs found
Very-high-energy gamma rays and neutrinos: The search for PeVatrons
Full text linkSince its discovery more than one hundred years ago, the origin of the cosmic-ray flux measured on Earth is still unknown: in order to explain the energy region below the knee, supernova remnants are usually addressed, even though no clear indication of PeV energies has been observed so far in such a kind of sources. However, recently, the Galactic Center region has been detected as a multi-TeV gamma-ray emitter: in the case of hadronic origin of the radiation, this would imply the existence of PeV primary protons. Hence, this detection triggers the search for a PeVatron at the center of our Galaxy. In order to identify the origin of the emission, a multi-messenger strategy appears suitable: in fact, in the hadronic scenario, neutrinos would constitute a natural counterpart of the electromagnetic emission. The fundamental role of neutrinos in disentangling the origin of the observed gamma rays is here discussed
Search for high-energy neutrinos from GRB130427A with the ANTARES neutrino telescope
Full text linkANTARES is the first deep under-sea high-energy astrophysical neutrino telescope, in operation since 2008, in the Northern Hemisphere. In the light of a multi-messenger approach, one of the most ever intense (photon fluence Fγ ≃10-3 erg/cm2) and close (redshift z = 0.34) transient γ-source, GRB130427A, is considered in the ANTARES physics program for a co-incident search for photons and high-energy neutrinos. The first time-dependent analysis on GRBs neutrino emissions has been performed for this source: Konus-Wind parameters of the γ time-dependent spectrum are used to predict the expected neutrino flux from each peak of the burst, through the numerical calculation code NeuCosmA. An extended maximum likelihood ratio search is performed in order to maximize the discovery probability of prompt neutrinos from the burst: at the end, ANTARES sensitivity to this source is evaluated to be E2Φv ∼ 1 -10 GeV/cm2 in the energy range from 2 x 105 GeV to 2 x 107 GeV
Search for PeVatrons in VHE gamma rays and neutrinos
Full text linkSince its discovery more than one hundred years ago, the origin of the cosmic-ray (CR) flux measured on Earth is still unknown: in order to explain the region below the knee, supernova remnants (SNRs) are usually addressed as PeV cosmic accelerators. In particular, young SNRs are potential candidates since they might act as PeVatrons at least during the initial stage of their evolution. However, no clear indication of PeV energies has been observed so far in such a kind of sources, including the brightest TeV SNR, RX J1713-3946.7. Recently, the Galactic Center region has been detected as a very-high-energy (VHE) gamma-ray emitter. Two emission regions have been resolved by H.E.S.S.: a point source, spatially associated to the known radio source SgrA, and a diffuse flux, characterised by a simple power law gamma-ray spectrum with no visible cut-off up to gamma-ray energies of about 35 TeV. Such a detection triggers the search for PeVatron at the center of our Galaxy. A clear evidence of the hadronic nature of the emission would be the detection of a neutrino counterpart. I will here discuss the potentials of the next generation neutrino telescopes
Search for muon neutrinos from GRBs with the ANTARES neutrino telescope
Full text linkANTARES is the largest operational neutrino telescope in the Northern hemisphere, located in the deep water of the Mediterranean Sea, offshore Toulon. One of its main scientific goals concerns the identification of hadronic astrophysical accelerators through the detection of high-energy neutrinos. Among these sources, Gamma-Ray Bursts (GRBs) constitutes promising candidates because they are the most bright sources in the Universe. Their transient nature allows to drastically reduce the expected background when both a temporal and spatial correlation with the observed gamma-ray prompt emission is required. Cosmic neutrinos could be produced in the interaction between accelerated protons and intense radiation fields in the jet. Two different approaches have been adopted in the search: a stacked analysis with a sample of GRBs observable using the full ANTARES data set (from 2008 to 2016) and an individual search from some of the brightest GRBs (with gamma-ray fluence greater than 10-4 erg/cm2) occurred in the same time period. The methods and the results of these searches for muon neutrinos are here presented. The stacking analysis allows to constrain the contribution to the diffuse flux of neutrinos from this population of sources. In the bright GRB analysis, instead, the internal shock and the photospheric scenarios have been investigated and limits in the parameter space of the fireball model are derived individually. Since no events have been detected in spatial and temporal coincidence with GRBs in any of the searches, upper limits on neutrino fluence are derived both for individual bright sources and for the GRB population sample
Cosmic ray electrons released by supernova remnants
No full textThe process that allows cosmic rays (CRs) to escape from their sources and be released into the Galaxy is still largely unknown. The comparison between CR electron and proton spectra measured at Earth suggests that electrons are released with a spectrum steeper than protons by Δsep 0.3 for energies above 10 GeV and by Δsep 1.2 above 1 TeV. Assuming that both species are accelerated at supernova remnant shocks, we here explore two possible scenarios that can in principle justify steeper electron spectra: (i) energy losses due to synchrotron radiation in an amplified magnetic field and (ii) time-dependent acceleration efficiency. We account for magnetic field amplification produced by either CR-induced instabilities or by magnetohydrodynamics instabilities my means of a parametric description. We show that both mechanisms are required to explain the electron spectrum. In particular, synchrotron losses can only produce a significant electron steepening above 1 TeV, while a time-dependent acceleration can explain the spectrum at lower energies if the electron injection into diffusive shock acceleration is inversely proportional to the shock speed. We discuss observational and theoretical evidences supporting such a behaviour. Furtheore, we predict two additional spectral features: a spectral break below few GeV (as required by existing observations) due to the acceleration efficiency drop during the adiabatic phase, and a spectral hardening above 20 TeV (where no data are available yet) resulting from electrons escaping from the shock precursor
From the Document to Knowledge: the Constructive History of the Wooden Ring of the Dome of Santa Maria del Fiore|Dal documento alla conoscenza: la storia costruttiva della cerchiatura lignea della cupola di Santa Maria del Fiore
No full textThe paper presents the results of the research recently carried out on the chesnut chain of Santa Maria del Fiore dome, in Florence: probably the less investigated element of Brunelleschi's masterpiece. Thanks to the recent laser-scanner survey of the chain - carried out by the Opera del Duomo in Florence (studio Scaletti, Florence) - it has been possible to reconstruct the history of a really peculiar artifact (for materials and configuration) which always accompanied the debates on the stability of the dome, without ever becoming the protagonist. By comparing the results to historical treatises, the attempt was to date the different constructive elements of the wooden ring, thus enhancing the global knowledge on the dome. The analysis of the detailed notes drawn up by the workers led to localize the interventions on the chain. This allowed to distinguish the original technologies from the further interventions, thus mapping the technical advances that have taken place in the meantime during the centuries. In particular, the investigation carried out on a specific joint (E) aims to demonstrate the validity of a research method, which finds in the monument itself the first document of its complex history
A time-dependent search for high-energy neutrinos from bright GRBs with ANTARES
Full text linkAstrophysical point-like neutrino sources, like Gamma-Ray Bursts (GRBs), are one of the main targets for neutrino telescopes, since they are among the best candidates for Ultra-High-Energy Cosmic Ray (UHECR) acceleration. From the interaction between the accelerated protons and the intense radiation fields of the source jet, charged mesons are produced, which then decay into neutrinos. The methods and the results of a search for high-energy neutrinos in spatial and temporal correlation with the detected gamma-ray emission are presented for four bright GRBs observed between 2008 and 2013: a time-dependent analysis, optimised for each flare of the selected bursts, is performed to predict detailed neutrino spectra. The internal shock scenario of the fireball model is investigated, relying on the neutrino spectra computed through the numerical code NeuCosmA. The analysis is optimized on a per burst basis, through the maximization of the signal discovery probability. Since no events in ANTARES data passed the optimised cuts, 90% C.L. upper limits are derived on the expected neutrino fluences
BEST BEFORE? ADVANCED GEOMATICS FOR PRESERVATION AND PRESERVATION OF ADVANCED GEOMATICS
Full text linkThis discussion paper deals with the task of preserving past geometrical surveys and keeping them in use. The continuous advancement in technology caused a quick ageing of both survey techniques and their outcomes. This may result into a predetermined expiration date after which they risk losing their quality and usefulness (hence the title «Best Before»). Facing this risk, this paper focuses on the unceasing relevance of past surveys both as a legacy in the history of technique and as a fundamental source of information on heritage buildings, and thus a support to the preservation activity. The present paper offers an overview of the most recent attempts of preserving and keeping in use the paper surveys of the past
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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