ABSTRACT
The 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. Furthermore, 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.
ABSTRACT
The escape process of particles accelerated at supernova remnant (SNR) shocks is one of the poorly understood aspects of the shock acceleration theory. Here we adopt a phenomenological ...approach to study the particle escape and its impact on the gamma-ray spectrum resulting from hadronic collisions both inside and outside of a middle-aged SNR. 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 show that a significant fraction of particles are still located inside the SNR long time after their nominal release from the acceleration region. This fact results into a gamma-ray spectrum 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 the model prediction and actual data will contribute to determining these two quantities, the former being particularly relevant within the predictions of the gamma-ray emission from the halo of escaping particles around SNRs, which could be detected with future Cherenkov telescope facilities. We also calculate the spectrum of runaway particles injected into the Galaxy by an individual remnant. Assuming that the acceleration stops before the SNR enters the snowplow phase, we show that the released spectrum can be a featureless power law only if the accelerated spectrum is ∝ p−α with α > 4.
ABSTRACT
Observations from the radio to the gamma-ray wavelengths indicate that supernova remnant (SNR) shocks are sites of effective particle acceleration. It has been proposed that the presence of ...dense clumps in the environment where supernovae explode might have a strong impact on the shape of the hadronic gamma-ray spectrum. Here we present a detailed numerical study of the penetration of relativistic protons into clumps that are engulfed by a SNR shock, taking into account the magneto-hydrodynamical properties of the background plasma. We show that the spectrum of protons inside clumps is much harder than that in the diffuse inter-clump medium, and we discuss the implications for the formation of the spectrum of hadronic gamma-rays, which no longer reflects the acceleration spectrum of protons, resulting substantially modified inside the clumps owing to propagation effects. For the Galactic SNR RX J1713.7 – 3946, we show that a hadronic scenario including dense clumps inside the remnant shell is able to reproduce the broadband gamma-ray spectrum from GeV to TeV energies. Moreover, we argue that small clumps crossed by the shock could provide a natural explanation for the non-thermal X-ray variability observed in some hotspots of RX J1713.7 – 3946. Finally, we discuss the detectability of gamma-ray emission from clumps with the upcoming Cherenkov Telescope Array, and the possible detection of the clumps themselves through molecular lines.
Context. Star clusters constitute a significant part of the stellar population in our Galaxy. The feedback processes they exert on the interstellar medium impact multiple physical processes from the ...chemical to the dynamical evolution of the Galaxy. In addition, young and massive stellar clusters might act as efficient particle accelerators and contribute to the production of cosmic rays. Aims. We aim at evaluating the wind luminosity driven by the young (< 30 Myr) Galactic open stellar clusters observed by the Gaia space mission. This is crucial for determining the energy channeled into accelerated particles. Methods. To do this, we developed a method relying on the number, magnitude, and line-of-sight extinction of the stars observed per cluster. Assuming that the stellar mass function follows a Kroupa mass distribution and accounting for the maximum stellar mass allowed by the age and mass of the parent cluster, we conservatively estimated the mass and wind luminosity of 387 local clusters within the second data release of Gaia . Results. We compared the results of our computation with recent estimates of young cluster masses. With respect to these, our sample is three times more abundant, particularly above a few thousand solar masses. This is of the utmost relevance for predicting the gamma-ray emission resulting from the interaction of accelerated particles. The cluster wind luminosity distribution we obtained extends up to 3 × 10 38 erg s −1 . This is a promising feature in terms of potential particle acceleration scenarios.
We discuss the discovery potential of extended Very-High-Energy (VHE) neutrino sources by the future KM3 Neutrino Telescope (KM3NeT) in the context of the constraining power of the Cherenkov ...Telescope Array (CTA), designed for deep surveys of the sky in VHE gamma rays. The study is based on a comparative analysis of sensitivities of KM3NeT and CTA. We show that a minimum gamma-ray energy flux of E2ϕγ(10TeV)>1×10−12 TeV cm−2 s−1 is required to identify a possible neutrino counterpart with a 3σ significance and 10 years of KM3NeT observations with upgoing muons, if the source has an angular size of Rsrc=0.1∘ and emits gamma rays with an E−2 energy spectrum through a full hadronic mechanism. This minimum gamma-ray flux is increased to the level of E2ϕγ(10TeV)>2×10−11 TeV cm−2 s−1 in case of sources with radial extension of Rsrc=2.0∘. The analysis methods are applied to the supernova remnant RX J1713.7-3946 and the Galactic Center Ridge, as well as to the recent HAWC catalog of multi-TeV gamma-ray sources.
Gamma-Ray Bursts (GRBs) are considered very interesting astrophysical sources to be studied in the context of neutrino astronomy. Indeed, their jet composition continues to be an open issue. Within ...the framework of the fireball model, mesons can be produced during photo-hadronic interactions occurring in the internal shocks between shells emitted by the central engine. From their decays, high-energy gamma rays and neutrinos are expected to be generated. Neutrino telescopes are particularly interested in GRBs: being the most powerful explosions observable in the Universe, they are potentially able to achieve the energetics required to contribute to the diffuse astrophysical neutrino flux measured few years ago by IceCube. The analysis here presented relies on the search of time and space coincidences between neutrinos and the GRB emission. The undersea neutrino telescope ANTARES, operational in the Northern hemisphere since 2008 in its full configuration, plays an important role in the cosmic neutrino searches. In this contribution, the results of a stacked search for astrophysical muon neutrinos are presented for 784 GRBs in the period 2007-2017 using the ANTARES data. Given the absence of coincidences between a neutrino and a GRB in the considered sample, this analysis has allowed to constrain the contribution of the detected GRB population to the neutrino diffuse flux to be less than 10% around 100 TeV, within the context of the internal shock model. In addition, for the first time in this kind of analysis, the uncertainties on the not well characterised GRB parameters, needed to compute the neutrino flux expectations, are taken into account for each individual burst and then propagated to the diffuse flux from GRBs and to the estimated stacked upper limit.
This 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.
ABSTRACT
Supernova remnants (SNRs) represent a powerful laboratory to study the cosmic ray acceleration processes at shocks, and their relation to the properties of the circumstellar medium. With the ...aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0–24.8 GHz with the Medicina and Sardinia Radio Telescopes, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum well fitted by a power-law function (S ∝ ν−α), with spectral index α = 0.45 ± 0.05 for NGC 6992 and α = 0.49 ± 0.01 for the southern shell and without any indication of a spectral break. The spectra are significantly flatter than the whole Cygnus Loop spectrum (α = 0.54 ± 0.01), suggesting a departure from the plain shock acceleration mechanisms, which for NGC 6992 could be related to the ongoing transition towards a radiative shock. We model the integrated spectrum of the whole SNR considering the evolution of the maximum energy and magnetic field amplification. Through the radio spectral parameters, we infer a magnetic field at the shock of 10 μG. This value is compatible with purely adiabatic compression of the interstellar magnetic field, suggesting that the amplification process is currently inefficient.
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