325,690 research outputs found
Very-high-energy gamma rays and neutrinos: The search for PeVatrons
No full textSince 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 PeVatrons in VHE gamma rays and neutrinos
No full textSince 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
No full textANTARES 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
A time-dependent search for high-energy neutrinos from bright GRBs with ANTARES
No full textAstrophysical 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
In memory of Giorgio Celli (1935-2011)
No full textThe text is an obituary in memory of Giorgio Celli, Emeritus Professor of Entomology at the Alma Mater Studiorum University of Bologna. A list of his publications is give
Search for high-energy neutrinos from GRB130427A with the ANTARES neutrino telescope
No full textANTARES 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
KM3NeT: status and perspectives of neutrino astronomy
No full textKM3NeT is a multi-purpose neutrino observatory, being currently deployed at the bottom of the Mediterranean Sea. It consists of two detectors: ORCA and ARCA (for Oscillation and Astroparticle Research with Cosmics in the Abyss, respectively). ARCA will instrument 1 Gton of seawater, with the primary goal of detecting cosmic neutrinos with energies between several tens of GeV and PeV. Due to its position in the Northern Hemisphere, ARCA will provide an optimal view of the Southern sky, including the Galactic Center. In April 2021, a major step has be taken in the construction of ARCA, bringing the number of detection lines from one to six. ORCA, also currently running in a six-string configuration, is a smaller (∼ few Mtons) and denser array, optimized for the detection of atmospheric neutrinos in the 1 − 100 GeV. It can also perform low-energy neutrino astronomy studies, e.g. searching for MeV-scale neutrinos expected at core-collapse supernovae. In this contribution, some of the key scientific cases in the field of neutrino astronomy are reviewed and perspectives for their investigation with KM3NeT are presented
Particle escape from supernova remnant shocks: gamma-ray and cosmic-ray signatures
No full textIn the context of the supernova remnant (SNR) paradigm for the origin of Galactic cosmic rays (CRs), the escape process of accelerated particles represents a fundamental piece of information to interpret both the observed CR spectrum and the gamma-ray spectral signatures emerging from these sources. Under the assumption that in the spatial region immediately outside of the remnant the diffusion coefficient is suppressed with respect to the average Galactic one, we found that a significant fraction of particles can still be located inside the SNR long time after their nominal release from the acceleration region. This fact results into a gamma-ray spectrum arising from hadronic collisions that resembles a broken power law, similar to those observed in several middle-aged SNRs. Above the break, the spectral steepening is determined by the diffusion coefficient outside of the SNR and by the time dependence of maximum energy. Consequently, the comparison between SNR data and model predictions will possibly allow to determine these two quantities. Additionally, by further assuming that protons and electrons are accelerated at SNR shocks with the same slope, CR spectral measurements on Earth can then be reproduced if electrons are injected with a spectrum steeper than protons for energies above 10 GeV. A possible scenario that can in principle justify the observed steeper electron spectrum relies on the combination of energy losses, due to synchrotron radiation in an amplified magnetic field, and time dependent acceleration efficiency
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
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