Abstracts of Interest
Selected by:
Peter Marinos
Abstract: 1910.08602
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Full Text: [ PostScript, PDF]
Title:Wolf-Rayet stars in young massive star clusters as potential sources of Galactic cosmic rays
(Submitted on 18 Oct 2019)
Abstract: For most elements, the isotopic ratios seen in cosmic rays (CRs) are similar to those in the solar wind. The most important exception to this is $^{22}$Ne/$^{20}$Ne where the CR value is $\sim 5$ times that of the solar wind. According to most recent models of nucleosynthesis, a large amount of $^{22}\mathrm{Ne}$ is generated in Wolf-Rayet (WR) stars. In the winds of carbon sequence WR stars, i.e., WC stars, the isotopic ratio $^{22}\mathrm{Ne}$/$^{20}\mathrm{Ne}$ can be much larger than in the solar wind. Here, we consider CRs produced by $^{22}\mathrm{Ne}$-enriched WR winds in compact clusters of young massive stars like Westerlund~1. We assume that efficient CR acceleration in clusters occurs over the WR lifetime in an ensemble of shock waves from multiple massive star winds. We estimate the fraction of all Galactic CRs such sources may produce for a given set of parameters.
Abstract: 1910.09435
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Full Text: [ PostScript, PDF]
Title:Background Rejection in Atmospheric Cherenkov Telescopes using Recurrent Convolutional Neural Networks
(Submitted on 21 Oct 2019)
Abstract: In this work, we present a new, high performance algorithm for background rejection in imaging atmospheric Cherenkov telescopes. We build on the already popular machine-learning techniques used in gamma-ray astronomy by the application of the latest techniques in machine learning, namely recurrent and convolutional neural networks, to the background rejection problem. Use of these machine-learning techniques addresses some of the key challenges encountered in the currently implemented algorithms and helps to significantly increase the background rejection performance at all energies.
We apply these machine learning techniques to the H.E.S.S. telescope array, first testing their performance on simulated data and then applying the analysis to two well known gamma-ray sources. With real observational data we find significantly improved performance over the current standard methods, with a 20-25\% reduction in the background rate when applying the recurrent neural network analysis. Importantly, we also find that the convolutional neural network results are strongly dependent on the sky brightness in the source region which has important implications for the future implementation of this method in Cherenkov telescope analysis.
Abstract: 1910.09693
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Full Text: [ PostScript, PDF]
Title:Toward the Detection of Relativistic Image Doubling in Imaging Atmospheric Cerenkov Telescopes
(Submitted on 21 Oct 2019)
Abstract: Cosmic gamma-ray photons incident on the upper atmosphere create air showers that move to the Earth's surface with superluminal speed, relative to the air. Even though many of these air showers remain superluminal all along their trajectories, the shower's velocity component toward a single Imaging Atmospheric Cherenkov Telescope (IACT) may drop from superluminal to subluminal. When this happens, an IACT that is able to resolve the air shower both in time and angle should be able to document an unusual optical effect known as relativistic image doubling (RID). The logic of RID is that the shower appears to precede its own Cherenkov radiation when its speed component toward the IACT is superluminal, but appears to trail its own Cherenkov radiation when its speed component toward the IACT is subluminal. The result is that the IACT will see the shower start not at the top of the atmosphere but in the middle -- at the point along the shower's path where its radial velocity component drops to subluminal. Images of the shower would then be seen by the IACT to go both up and down simultaneously. A simple simulation demonstrating this effect is presented. Clear identification of RID would confirm in the atmosphere a novel optical imaging effect caused not by lenses but solely by relativistic kinematics, and may aid in the accuracy of path and speed reconstructions of the relativistic air shower.
Abstract: 1910.10168
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Full Text: [ PostScript, PDF]
Title:Realistic modeling of wind and supernovae shocks in star clusters: addressing ${\rm ^{22}Ne/^{20}Ne}$ and other problems in Galactic cosmic rays
(Submitted on 22 Oct 2019)
Abstract: Cosmic ray (CR) sources leave signatures in the isotopic abundances of CRs. Current models of Galactic CRs that consider supernovae (SNe) shocks as the main sites of particle acceleration cannot satisfactorily explain the higher ${\rm ^{22}Ne/^{20}Ne}$ ratio in CRs compared to the interstellar medium. Although stellar winds from massive stars have been invoked, their contribution relative to SNe ejecta has been taken as a free parameter. Here we present a theoretical calculation of the relative contributions of wind termination shocks (WTSs) and SNe shocks in superbubbles, based on the hydrodynamics of winds in clusters, the standard stellar mass function, and stellar evolution theory. We find that the contribution of WTSs towards the total CR production is at least $25\%$, which rises to $\gtrsim 50\%$ for young ($\lesssim 10$ Myr) clusters, and explains the observed $^{22}{\rm Ne}/^{20} {\rm Ne}$ ratio. We argue that since the progenitors of apparently isolated supernovae remnants (SNRs) are born in massive star clusters, both WTS and SNe shocks can be integrated into a combined scenario of CRs being accelerated in massive clusters. This scenario is consistent with the observed ratio of SNRs to $\gamma$-ray bright ($L_\gamma \gtrsim 10^{35}$ erg s$^{-1}$) star clusters, as predicted by star cluster mass function. Moreover, WTSs can accelerate CRs to PeV energies, and solve other longstanding problems of the standard supernova paradigm of CR acceleration.
Abstract: 1910.10193
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Full Text: [ PostScript, PDF]
Title:First M87 Event Horizon Telescope Results and the Role of ALMA
(Submitted on 22 Oct 2019)
Abstract: In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super-massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein's theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/ submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents - and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.
Abstract: 1910.10439
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Full Text: [ PostScript, PDF]
Title:Neutrino production from proton-proton interactions in binary-driven hypernovae
(Submitted on 23 Oct 2019)
Abstract: We estimate the neutrino emission from the decay chain of the $\pi$-meson and $\mu$-lepton, produced by proton-proton inelastic scattering in energetic ($E_{\rm iso}\gtrsim 10^{52}$ erg) long gamma-ray bursts (GRBs), within the type I binary-driven hypernova (BdHN) model. The BdHN I progenitor is binary system composed of a carbon-oxygen star (CO$_{\rm core}$) and a neutron star (NS) companion. The CO$_{\rm core}$ explosion as supernova (SN) triggers a massive accretion process onto the NS. For short orbital periods of few minutes, the NS reaches the critical mass, hence forming a black hole (BH). Recent numerical simulations of the above scenario show that the SN ejecta becomes highly asymmetric, creating a \textit{cavity} around the newborn BH site, due to the NS accretion and gravitational collapse. Therefore, the electron-positron ($e^{\pm}$) plasma created in the BH formation, during its isotropic and expanding self-acceleration, engulfs different amounts of ejecta baryons along different directions, leading to a direction-dependent Lorentz factor. The protons engulfed inside the high-density ($\sim10^{23}$~particle/cm$^3$) ejecta reach energies in the range $1.24\lesssim E_p\lesssim 6.14$ GeV and interact with the unshocked protons in the ejecta. The protons engulfed from the low density region around the BH reach energies $\sim 1$ TeV and interact with the low-density ($\sim1$~particle/cm$^3$) protons of the interstellar medium (ISM). The above interactions give rise, respectively, to neutrino energies $E_{\nu}\leq 2$ GeV and $10\leq E_{\nu}\leq 10^3$ GeV, and for both cases we calculate the spectra and luminosity.
Abstract: 1910.10856
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Full Text: [ PostScript, PDF]
Title:Empirical relations for gravitational-wave asteroseismology of binary neutron star mergers
(Submitted on 24 Oct 2019)
Abstract: We construct new, multivariate empirical relations for measuring neutron star radii and tidal deformabilities from the dominant gravitational wave frequency in the post-merger phase of binary neutron star mergers. The relations determine neutron star radii and tidal deformabilities for specific neutron star masses with consistent accuracy and depend only on two observables: the post-merger peak frequency $f_{\rm peak}$ and the chirp mass $M_{\rm chirp}$. The former could be measured with good accuracy from gravitational waves emitted in the post-merger phase using next-generation detectors, whereas the latter is already obtained with good accuracy from the inspiral phase with present-day detectors. Our main data set consists of a gravitational wave catalogue obtained with CFC/SPH simulations. We also extract the $f_{\rm peak}$ frequency from the publicly available CoRe data set, obtained through grid-based GRHD simulations and find good agreement between the extracted frequencies of the two data sets. As a result, we can construct empirical relations for the combined data sets. Furthermore, we investigate empirical relations for two secondary peaks, $f_{2-0}$ and $f_{\rm spiral}$, and show that these relations are distinct in the whole parameter space, in agreement with a previously introduced spectral classification scheme. Finally, we show that the spectral classification scheme can be reproduced using machine-learning techniques.
Abstract: 1910.11246
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Full Text: [ PostScript, PDF]
Title:Implications of the search for optical counterparts during the first six months of the Advanced LIGO's and Advanced Virgo's third observing run: possible limits on the ejecta mass and binary properties
(Submitted on 24 Oct 2019)
Abstract: The multi-messenger observation of GW170817 has opened a new window into the Universe and proved that neutron star mergers not only emit gravitational waves, but in addition, can give rise to a variety of electromagnetic signatures in multiple wavelengths. Within the first half of the third observing run of the Advanced LIGO and Virgo detectors, there have been a number of gravitational wave candidates of compact binary systems for which at least one component is potentially a neutron star. In this article, we look at the candidates S190425z, S190426c, S190510g, S190901ap, and S190910h, predicted to have potentially a non-zero remnant mass, in more detail. All these triggers have been followed up with extensive campaigns by the astronomical community doing electromagnetic searches for their optical counterparts, such as kilonovae. Furthermore, the events have not been retracted by the LIGO and Virgo collaboration, however, according to the released classification, there is a high probability that some of these events might not be of extraterrestrial origin. Assuming that the triggers are caused by a compact binary coalescence, not due to noise, and that the individual source locations have been covered during the EM follow-up campaigns, we employ three different kilonova models and apply them to derive constraints on the matter ejection consistent with the existing gravitational-wave trigger information but the non-existing kilonova. These upper bounds on the ejecta mass can be related to limits on the maximum mass of the binary neutron star candidate S190425z and to constraints on the mass-ratio, spin, and NS compactness for the potential black hole-neutron star candidate S190426c. Our results show that deeper observations for future gravitational wave events near the horizon limit of the advanced detectors are essential.
Abstract: 1910.11265
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Full Text: [ PostScript, PDF]
Title:Simulation of collisionless shocks in plasmas with high metallicity
(Submitted on 24 Oct 2019)
Abstract: The results of hybrid simulation of low-beta supercritical quasi-parallel shocks in high metallicity plasma are presented. The structure of upstream and downstream turbulence is addressed and velocities of the corresponding scattering centers are derived. It is shown that independently of their chemical composition the shocks experience self-reformation process. However, the period of self-reformation as well as the wave spectrum is greatly affected by the presence of substantial admixture of weakly charged heavy ions. Also the downstream magnetic field amplification is stronger for the high metallicity case.
Abstract: 1910.11283
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Full Text: [ PostScript, PDF]
Title:The 3D Kinematics of Gas in the Small Magellanic Cloud
(Submitted on 24 Oct 2019)
Abstract: We investigate the kinematics of neutral gas in the Small Magellanic Cloud (SMC) and test the hypothesis that it is rotating in a disk. To trace the 3D motions of the neutral gas distribution, we identify a sample of young, massive stars embedded within it. These are stars with radial velocity measurements from spectroscopic surveys and proper motion measurements from Gaia, whose radial velocities match with dominant HI components. We compare the observed radial and tangential velocities of these stars with predictions from the state-of-the-art rotating disk model based on high-resolution 21 cm observations of the SMC from the Australian Square Kilometer Array Pathfinder telescope. We find that the observed kinematics of gas-tracing stars are inconsistent with disk rotation. We conclude that the kinematics of gas in the SMC are more complex than can be inferred from the integrated radial velocity field. As a result of violent tidal interactions with the LMC, non-rotational motions are prevalent throughout the SMC, and it is likely composed of distinct sub-structures overlapping along the line of sight.
Abstract: 1910.11288
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Full Text: [ PostScript, PDF]
Title:Rapid bound-state formation of Dark Matter in the Early Universe
(Submitted on 24 Oct 2019)
Abstract: The thermal decoupling description of dark matter (DM) and co-annihilating partners is reconsidered. If DM is realized at around the TeV-mass region or above, even the heaviest electroweak force carriers could act as long-range forces, leading to the existence of meta-stable DM bound states. The formation and subsequent decay of the latter further deplete the relic density during the freeze-out process on top of the Sommerfeld enhancement, allowing for larger DM masses. While so far the bound-state formation was described via the emission of an on-shell mediator ($W^{\pm}$, $Z$, $H$, $g$, photon or exotic), we point out that this particular process does not have to be the dominant scattering-bound state conversion channel in general. If the mediator is coupled in a direct way to any relativistic species present in the Early Universe, the bound-state formation can efficiently occur through particle scattering, where a mediator is exchanged virtually. To demonstrate that such a virtually stimulated conversion process can dominate the on-shell emission even for all temperatures, we analyze a simplified model where DM is coupled to only one relativistic species in the primordial plasma through an electroweak-scale mediator. We find that the bound-state formation cross section via particle scattering can exceed the on-shell emission by up to several orders of magnitude.
Abstract: 1910.11387
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Full Text: [ PostScript, PDF]
Title:Proton acceleration in pulsar magnetospheres
(Submitted on 24 Oct 2019)
Abstract: Pulsars have been identified as good candidates for the acceleration of cosmic rays, up to ultra-high energies. However, a precise description of the acceleration processes at play is still to be established. Using 2D particle-in-cell simulations, we study proton acceleration in axisymmetric pulsar magnetospheres. Protons and electrons are extracted from the neutron star surface by the strong electric field induced by the rotation of the star, and electrons and positrons are produced in the magnetosphere through pair production process. As pair production has a crucial impact on electromagnetic fields, on gaps and thus on particle acceleration, we study its influence on the maximum energy and luminosity of protons escaping the magnetosphere. Protons are accelerated and escape in all our simulations. However, the acceleration sites are different for the protons and the pairs. As shown in previous studies, pairs are accelerated to their highest energies at the Y-point and in the equatorial current sheet, where magnetic reconnection plays and important role. In contrast, protons gain most of their kinetic energy below the light-cylinder radius within the separatrix current layers, but they are not confined within the equatorial current sheet. They can reach $15\%$ to $75\%$ of the maximum Lorentz factor obtained by the acceleration through the full vacuum potential drop from pole to equator, respectively for a high and a low yield of pair production. Their luminosity can reach $0.5\%$ to $2\%$ of the theoretical spin down luminosity of an aligned pulsar, respectively for a low and a high yield of pair production. These estimates support that millisecond pulsars could accelerate cosmic rays up to PeV energies and that new born millisecond pulsars could accelerate cosmic rays up to ultra-high energies.
Abstract: 1910.11464
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Full Text: [ PostScript, PDF]
Title:A Neutral Beam Model for High-Energy Neutrino Emission from the Blazar TXS 0506+056
(Submitted on 25 Oct 2019)
Abstract: The IceCube collaboration reported a $\sim 3.5\sigma$ excess of $13\pm5$ neutrino events in the direction of the blazar TXS 0506+56 during a $\sim$6 month period in 2014-2015, as well as the ($\sim3\sigma$) detection of a high-energy muon neutrino during an electromagnetic flare in 2017. We explore the possibility that the 2014-2015 neutrino excess and the 2017 multi-messenger flare are both explained in a common physical framework that relies on the emergence of a relativistic neutral beam in the blazar jet due to interactions of accelerated cosmic rays (CRs) with photons. We demonstrate that the neutral beam model provides an explanation for the 2014-2015 neutrino excess without violating X-ray and $\gamma$-ray constraints, and also yields results consistent with the detection of one high-energy neutrino during the 2017 flare. If both neutrino associations with TXS 05065+056 are real, our model requires that (i) the composition of accelerated CRs is light, with a ratio of helium nuclei to protons $\gtrsim5$, (ii) a luminous external photon field ($\sim 10^{46}$ erg s$^{-1}$) variable (on year-long timescales) is present, and (iii) the CR injection luminosity as well as the properties of the dissipation region (i.e., Lorentz factor, magnetic field, and size) vary on year-long timescales.
Abstract: 1910.11578
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Full Text: [ PostScript, PDF]
Title:Propagation, cocoon formation, and resultant destabilization of relativistic jets
(Submitted on 25 Oct 2019)
Abstract: A cocoon is a by-product of a propagating jet that results from shock heating at the jet head. Herein, considering simultaneous cocoon formation, we study the stability of relativistic jets propagating through the uniform ambient medium. Using a simple analytic argument, we demonstrate that independent from the jet launching condition, the effective inertia of the jet is larger than that of the cocoon when the fully relativistic jet oscillates radially owing to the pressure mismatch between jet and cocoon. In such situations, it is expected that the onset condition for the oscillation-induced Rayleigh-Taylor instability is satisfied at the jet interface, resulting in the destabilization of the relativistic jet during its propagation. We have quantitatively verified and confirmed our prior expectation by performing relativistic hydrodynamic simulations in three dimensions. The possible occurrences of the Richtmyer-Meshkov instability, oscillation-induced centrifugal instability, and Kelvin-Helmholtz instability are also discussed.
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