Abstracts of Interest
Selected by:
Bruce Dawson
Abstract: 1812.03070
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Title:Present status and prospects of the Tunka Radio Extension
(Submitted on 7 Dec 2018)
Abstract: The Tunka Radio Extension (Tunka-Rex) is a digital radio array operating in the frequency band of 30-80 MHz and detecting radio emission from air-showers produced by cosmic rays with energies above 100 PeV. The experiment is installed at the site of the TAIGA (Tunka Advanced Instrument for cosmic rays and Gamma Astronomy) observatory and performs joint measurements with the co-located particle and air-Cherenkov detectors in passive mode receiving a trigger from the latter. Tunka-Rex collects data since 2012, and during the last five years went through several upgrades. As a result the density of the antenna field was increased by three times since its commission. In this contribution we present the latest results of Tunka-Rex experiment, particularly an updated analysis and efficiency study, which have been applied to the measurement of the mean shower maximum as a function of energy for cosmic rays of energies up to EeV. The future plans are also discussed: investigations towards an energy spectrum of cosmic rays with Tunka-Rex and their mass composition using a combination of Tunka-Rex data with muon measurements by the particle detector Tunka-Grande.
Abstract: 1812.02960
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Full Text: [ PostScript, PDF]
Title:Proton acceleration in colliding stellar wind binaries
(Submitted on 7 Dec 2018)
Abstract: The interaction between the strong winds in stellar colliding-wind binary (CWB) systems produces two shock fronts, delimiting the wind collision region (WCR). There, particles are expected to be accelerated mainly via diffusive shock acceleration (DSA). We investigate the injection and the acceleration of protons in typical CWB systems by means of Monte Carlo simulations, with both a test-particle approach and a non-linear method modelling a shock locally modified by the backreaction of the accelerated protons. We use magnetohydrodynamic simulations to determine the background plasma in the WCR and its vicinity. This allows us to consider particle acceleration at both shocks, on either side of the WCR, with a realistic large-scale magnetic field. We highlight the possible effects of particle acceleration on the local shock profiles at the WCR. We include the effect of magnetic field amplification due to resonant streaming instability (RSI), and compare results without and with the backreaction of the accelerated protons. In the latter case we find a lower flux of the non-thermal proton population, and a considerable magnetic field amplification. This would significantly increase the synchrotron losses of relativistic electrons accelerated in CWB systems, lowering the maximal energy they can reach and strongly reducing the inverse Compton fluxes. As a result, $γ$-rays from CWBs would be predominantly due to the decay of neutral pions produced in nucleon-nucleon collisions. This might provide a way to explain why, in the vast majority of cases, CWB systems have not been identified as $γ$-ray sources, while they emit synchrotron radiation.
Abstract: 1812.02950
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Full Text: [ PostScript, PDF]
Title:Origin of the First Cosmic Rays
(Submitted on 7 Dec 2018)
Abstract: Nonthermal phenomena are ubiquitous in the Universe, and cosmic rays (CRs) play various roles in different environments. When, where, and how CRs are first generated since the Big Bang? We argue that blast waves from the first cosmic explosions at z~20 lead to Weibel mediated nonrelativistic shocks and CRs can be generated by the diffusive shock acceleration mechanism. We show that protons are accelerated at least up to sub-GeV energies, and the fast velocity component of supernova ejecta is likely to allow CRs to achieve a few GeV in energy. We discuss other possible accelerators of the first CRs, including accretion shocks due to the cosmological structure formation. These CRs can play various roles such as the magnetic field amplification in the early universe.
Abstract: 1710.02883
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Title:Particle-level model for radar based detection of high-energy neutrino cascades
(Submitted on 8 Oct 2017 (v1), last revised 6 Dec 2018 (this version, v4))
Abstract: We present a particle-level model for calculating the radio scatter of incident RF radiation from the plasma formed in the wake of a particle shower. We incorporate this model into a software module ("RadioScatter"), which calculates the collective scattered signal using the individual particle equations of motion, accounting for plasma effects, transmitter and receiver geometries, refraction at boundaries, and antenna gain patterns. We find appreciable collective scattering amplitudes with coherent phase for a range of geometries, with high geometric and volumetric acceptance. Details of the calculation are discussed, as well as the implementation of RadioScatter into GEANT4. A laboratory test of our model, currently scheduled at SLAC in 2018, with the goal of measuring the time-dependent characteristics of the reflecting plasma, is also described. Prospects for a future in-ice, high-energy neutrino detector, along with comparison to current detection strategies, are presented.
Abstract: 1812.02678
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Title:Exploring the legacy of big stargazing events
(Submitted on 4 Dec 2018)
Abstract: We assess the impact of annual large-scale stargazing events, following the success of Stargazing Live and based on our experiences running events in Portsmouth, UK
Abstract: 1812.02662
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Full Text: [ PostScript, PDF]
Title:Carpet-2 search for PeV gamma rays associated with IceCube high-energy neutrino events
(Submitted on 6 Dec 2018)
Abstract: Carpet-2 is an air-shower array at Baksan Valley, Russia, equipped with a large-area (175 m^2) muon detector, which makes it possible to separate primary photons from hadrons. We report the first results of the search for primary photons with energies E_γ>1 PeV, directionally associated with IceCube high-energy neutrino events, in the data obtained in 3080 days of Carpet-2 live time.
Abstract: 1812.02125
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Title:The behaviour of galactic cosmic ray intensity during solar activity cycle 24
(Submitted on 5 Dec 2018)
Abstract: We have studied long-term variations of galactic cosmic ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time-lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for solar activity cycles 20-23 to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of cycle 24. Cross-plots of SSN vs GCR show a clear difference between the odd-numbered and even-numbered cycles. Linear and elliptical models have been fit to the data with the linear fit and elliptical model proving the more suitable model for even-numbered and odd-numbered solar activity cycles respectively, in agreement with previous literature. Through the application of these methods for the 24th solar activity cycle, it has been shown that cycle 24 experienced a lag of 2-4 months and follows the trend of the preceding activity cycles albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for cycle 24, as the ellipse model doesn't show a significant improvement, which is also in agreement with previous even-numbered cycles.
Abstract: 1812.01615
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Full Text: [ PostScript, PDF]
Title:Star Clusters Across Cosmic Time
(Submitted on 4 Dec 2018)
Abstract: Star clusters stand at the intersection of much of modern astrophysics: the interstellar medium, gravitational dynamics, stellar evolution, and cosmology. Here we review observations and theoretical models for the formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle with several phases: (1) Clusters form in hierarchically-structured, accreting molecular clouds that convert gas into stars at a low rate per dynamical time until feedback disperses the gas. (2) The densest parts of the hierarchy resist gas removal long enough to reach high star formation efficiency, becoming dynamically-relaxed and well-mixed. These remain bound after gas removal. (3) In the first $\sim 100$ Myr after gas removal, clusters disperse moderately fast, through a combination of mass loss and tidal shocks by dense molecular structures in the star-forming environment. (4) After $\sim 100$ Myr, clusters lose mass via two-body relaxation and shocks by giant molecular clouds, processes that preferentially affect low-mass clusters and cause a turnover in the cluster mass function to appear on $\sim 1-10$ Gyr timescales. (5) Even after dispersal, some clusters remain coherent and thus detectable in chemical or action space for multiple galactic orbits. In the next decade a new generation of space- and AO-assisted ground-based telescopes will enable us to test and refine this picture.
Abstract: 1812.01031
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Full Text: [ PostScript, PDF]
Title:A $γ$-ray determination of the Universe's star-formation history
(Submitted on 3 Dec 2018)
Abstract: The light emitted by all galaxies over the history of the Universe produces the extragalactic background light (EBL) at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for $γ$ rays via photon-photon interactions, leaving an imprint in the spectra of distant $γ$-ray sources. We measure this attenuation using {739} active galaxies and one gamma-ray burst detected by the {\it Fermi} Large Area Telescope. This allows us to reconstruct the evolution of the EBL and determine the star-formation history of the Universe over 90\% of cosmic time. Our star-formation history is consistent with independent measurements from galaxy surveys, peaking at redshift $z\sim2$. Upper limits of the EBL at the epoch of re-ionization suggest a turnover in the abundance of faint galaxies at $z\sim 6$.
Abstract: 1812.01036
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Full Text: [ PostScript, PDF]
Title:Tau Neutrinos in IceCube, KM3NeT and the Pierre Auger Observatory
(Submitted on 3 Dec 2018)
Abstract: In 2018, the IceCube collaboration reported evidence for the identification of a blazar as an astrophysical neutrino source. That evidence is briefly summarised here before focusing on the prospects of tau neutrino physics in IceCube, both at high energies (astrophysical neutrinos) and at lower energies (atmospheric neutrino oscillations). In addition, future neutrino detectors such as KM3NeT and the IceCube Upgrade and their tau neutrino physics potential are discussed. Finally, the detection mechanism for high-energy (tau) neutrinos in the Pierre Auger Observatory and the resulting flux upper limits are presented.
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