Abstracts of Interest
Selected by:
Peter Marinos
Abstract: 1909.03734
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Title:The Crab Nebula Spectrum at ~100 TeV Measured with MAGIC under Very Large Zenith Angles
(Submitted on 9 Sep 2019)
Abstract: The Crab Nebula was discovered as the first very-high-energy gamma-ray source by the Whipple Observatory in 1989. Thirty years after its discovery it is still the reference source and the standard candle for Imaging Atmospheric Cherenkov Telescopes (IACTs). Its spectrum has been measured from the cm radio band to energies up to tens of TeV. Some studies reported a possible but still debated cut-off in its spectrum at few tens of TeV. The MAGIC collaboration is currently investigating the spectrum of the Crab Nebula by using the Very Large Zenith Angle observation technique. The latter provides a significantly increased collection area for energies above 10 TeV. The details of these MAGIC observations will be presented.
Abstract: 1909.04140
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Title:The highest-speed local dark matter particles come from the Large Magellanic Cloud
(Submitted on 9 Sep 2019)
Abstract: Using N-body simulations of the Large Magellanic Cloud (LMC's) passage through the Milky Way (MW), tailored to reproduce observed kinematic properties of both galaxies, we show that the high-speed tail of the Solar Neighborhood dark matter distribution is overwhelmingly of LMC origin. Two populations contribute at high speeds: 1) Particles that were once bound to the LMC, and 2) MW halo particles that have been accelerated owing to the response of the halo to the recent passage of the LMC. These particles reach speeds of 700-900 km/s with respect to the Earth, above the local escape speed of the MW. The high-speed particles follow trajectories similar to the Solar reflex motion, with peak velocities reached in June. For low-mass dark matter, these high-speed particles can dominate the signal in direct-detection experiments, extending the reach of the experiments to lower mass and elastic scattering cross sections even with existing data sets. Our study shows that even non-disrupted MW satellite galaxies can leave a significant dark-matter footprint in the Solar Neighborhood.
Abstract: 1909.04537
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Title:Prediction of the strength and timing of sunspot cycle 25 reveal decadal-scale space environmental conditions
(Submitted on 10 Sep 2019)
Abstract: The Sun's activity cycle governs the radiation, particle and magnetic flux in the heliosphere creating hazardous space weather. Decadal-scale variations define space climate and force the Earth's atmosphere. However, predicting the solar cycle is challenging. Current understanding indicates a short window for prediction best achieved at previous cycle minima. Utilizing magnetic field evolution models for the Sun's surface and interior we perform the first century-scale, data-driven simulations of solar activity and present a scheme for extending the prediction window to a decade. Our ensemble forecast indicates cycle 25 would be similar or slightly stronger than the current cycle and peak around 2024. Sunspot cycle 25 may thus reverse the substantial weakening trend in solar activity which has led to speculation of an imminent Maunder-like grand minimum and cooling global climate. Our simulations demonstrate fluctuation in the tilt angle distribution of sunspots is the dominant mechanism responsible for solar cycle variability.
Abstract: 1909.05032
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Title:Photometric light curves classification with machine learning
(Submitted on 10 Sep 2019)
Abstract: The Large Synoptic Survey Telescope will complete its survey in 2022 and produce terabytes of imaging data each night. To work with this massive onset of data, automated algorithms to classify astronomical light curves are crucial. Here, we present a method for automated classification of photometric light curves for a range of astronomical objects. Our approach is based on the gradient boosting of decision trees, feature extraction and selection, and augmentation. The solution was developed in the context of The Photometric LSST Astronomical Time Series Classification Challenge (PLAsTiCC) and achieved one of the top results in the challenge.
Abstract: 1909.05779
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Title:A fast radio burst in the direction of the Virgo cluster
(Submitted on 12 Sep 2019)
Abstract: The rate of fast radio bursts (FRBs) in the direction of nearby galaxy clusters is expected to be higher than the mean cosmological rate if intrinsically faint FRBs are numerous. In this paper, we describe a targeted search for faint FRBs near the core of the Virgo cluster using the Australian Square Kilometer Array Pathfinder telescope. During 300 hr of observations, we discovered one burst, FRB 180417, with dispersion measure DM $=474.8$ cm$^{-3}$pc. The FRB was promptly followed up by several radio telescopes for 27 h, but no repeat bursts were detected. An optical follow-up of FRB 180417 using the PROMPT5 telescope revealed no new sources down to an $R$-band magnitude of 20.1. We argue that FRB 180417 is likely behind the Virgo cluster as the Galactic and intracluster DM contribution are small compared to the DM of the FRB, and there are no galaxies in the line of sight. The non-detection of FRBs from Virgo constrains the faint-end slope, $\alpha<1.52$ (at 68\% confidence limit), and the minimum luminosity, $L_{\rm min}\gtrsim 2\times 10^{40}$ erg s$^{-1}$ (at 68\% confidence limit), of the FRB luminosity function assuming cosmic FRB rate of $10^4$ FRBs sky$^{-1}$ day$^{-1}$ with flux above 1 Jy located out to redshift of 1. Further FRB surveys of galaxy clusters with high-sensitivity instruments will tighten the constraints on the faint end of the luminosity function and, thus, are strongly encouraged.
Abstract: 1909.05867
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Title:Jet Propagation in Neutron Star Mergers and GW170817
(Submitted on 12 Sep 2019)
Abstract: The gravitational wave event from the binary neutron star (BNS) merger GW170817 and the following multi-messenger observations present strong evidence for i) merger ejecta expanding with substantial velocities and ii) a relativistic jet which had to propagate through the merger ejecta. The ejecta's expansion velocity is not negligible for the jet head motion, which is a fundamental difference from the other systems like collapsars and active galactic nuclei. Here we present an analytic model of the jet propagation in an expanding medium. In particular, we notice a new term in the expression of the breakout time and velocity. In parallel, we perform a series of over a hundred 2D numerical simulations of jet propagation. The BNS merger ejecta is prepared based on numerical relativity simulations of a BNS merger with the highest-resolution to date. We show that our analytic results agree with numerical simulations over a wide parameter space. Then we apply our analytic model to GW170817, and obtain two solid constraints on: i) the central engine luminosity as $L_{iso,0} \sim 3\times10^{49}-2.5\times10^{52}$ erg s$^{-1}$, and on ii) the delay time between the merger and engine activation $t_0-t_m < 1.3$ s. The engine power implies that the apparently-faint \textit{short} gamma-ray burst (\textit{s}GRB) \textit{s}GRB 170817A is similar to typical \textit{s}GRBs if observed on-axis.
Abstract: 1909.05874
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Title:Radio Emission from Supernovae in the Very Early Phase: Implications for the Dynamical Mass Loss of Massive Stars
(Submitted on 12 Sep 2019)
Abstract: Recent high-cadence transient surveys and rapid follow-up observations indicate that some massive stars may dynamically lose their own mass within decades before supernovae (SNe). Such a mass-loss forms `confined' circumstellar medium (CSM); a high density material distributed only within a small radius ($\lesssim 10^{15}$ cm with the mass-loss rate of 0.01 $\sim 10^{-4} M_\odot$ yr$^{-1}$). While the SN shock should trigger particle acceleration and magnetic field amplification in the `confined' CSM, synchrotron emission may be masked in centimeter wavelengths due to free-free absorption; the millimeter range can however be a potential new window. We investigate the time evolution of synchrotron radiation from the system of a red super giant surrounded by the `confined' CSM, relevant to typical type II-P SNe. We show that synchrotron millimeter emission is generally detectable, and the signal can be used as a sensitive tracer of the nature of the `confined' CSM; it traces different CSM density parameter space than in the optical. Furthermore, our simulations show that the `confined' CSM efficiently produces secondary electrons and positrons through proton inelastic collisions, which can become main contributors to the synchrotron emission in several ten days since the SN. We predict that the synchrotron emission is detectable by ALMA, and suggest that it will provide a robust evidence of the existence of the `confined' CSM.
Abstract: 1909.05913
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Title:First Constraint on Coherent Elastic Neutrino-Nucleus Scattering in Argon
(Submitted on 12 Sep 2019)
Abstract: Coherent elastic neutrino-nucleus scattering (CEvNS) is the dominant neutrino scattering channel for neutrinos of energy $E_\nu < 100$ MeV. We report a limit for this process using data collected in an engineering run of the 29 kg CENNS-10 liquid argon detector located 27.5 m from the Oak Ridge National Laboratory Spallation Neutron Source (SNS) Hg target with $4.2\times 10^{22}$ protons on target. The dataset yielded $< 7.4$ observed CEvNS events implying a cross section for the process, averaged over the SNS pion decay-at-rest flux, of $<3.4 \times 10^{-39}$ cm$^{2}$, a limit within twice the Standard Model prediction. This is the first limit on CEvNS from an argon nucleus and confirms the earlier CsI non-standard neutrino interaction constraints from the collaboration. This run demonstrated the feasibility of the ongoing experimental effort to detect CEvNS with liquid argon.
Abstract: 1909.05984
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Title:High-energy emission from GRBs: 10 years with Fermi-LAT
(Submitted on 12 Sep 2019)
Abstract: In 2018, the Fermi mission celebrated its first decade of operation. In this time, the Large Area Telescope (LAT) has been very successful in detecting the high-energy emission (>100 MeV) from Gamma-Ray Bursts (GRBs). The analysis of particularly remarkable events - such as GRB 080916C, GRB 090510 and GRB 130427A - has been presented in dedicated publications. Here we present the results of a new systematic search for high-energy emission from the full sample of GRBs detected in 10 years by the Fermi Gamma-Ray Burst Monitor, as well as Swift, AGILE, Integral and IPN bursts, featuring a detection efficiency more than 50\%\ better than previous works, and returning 186 detections during 10 years of LAT observations. This milestone marks a vast improvement from the 35 events contained in the first LAT GRB catalog (covering the first 3 years of Fermi operations). We assess the characteristics of the GRB population at high energy with unprecedented sensitivity, covering aspects such as temporal properties, energetics and spectral index of the high-energy emission. Finally, we show how the LAT observations can be used to inform theory, in particular the prospects for very high-energy emission.
Abstract: 1909.06230
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Title:Electron Acceleration in the Crab Nebula
(Submitted on 13 Sep 2019)
Abstract: We study electron and positron acceleration at the termination shock of a striped pulsar wind. Drift motion along the shock surface keeps either electrons or positrons -but not both, close to the equatorial plane of the pulsar, where they are accelerated by the first-order Fermi process. Their energy spectrum is a power law, and both the X-ray flux and photon index of the Crab Nebula, as measured by NuSTAR, can be reproduced for sufficiently large downstream turbulence levels. The implication that one sign of charge is preferentially accelerated in pulsar wind nebulae is potentially important for the interpretation of the positron fraction in cosmic-rays.
Abstract: 1909.06269
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Title:The role of the H$_2$ adiabatic index in the formation of the first stars
(Submitted on 13 Sep 2019)
Abstract: The adiabatic index of H$_2\,$ ($\gamma_{\mathrm{H_2}}$) is non-constant at temperatures between $100-10^4\,\mathrm{K}$ due to the large energy spacing between its rotational and vibrational modes. For the formation of the first stars at redshifts 20 and above, this variation can be significant because primordial molecular clouds are in this temperature range due to the absence of efficient cooling by dust and metals. We study the possible importance of variations in $\gamma_{\mathrm{H_2}}$ for the primordial initial mass function by carrying out 80 3D gravito-hydrodynamic simulations of collapsing clouds with different random turbulent velocity fields, half using fixed $\gamma_{\rm H_2} = 7/5$ in the limit of classical diatomic gas (used in earlier works) and half using an accurate quantum mechanical treatment of $\gamma_{\mathrm{H_2}}$. We use the adaptive mesh refinement code FLASH with the primordial chemistry network from KROME for this study. The simulation suite produces almost 400 stars, with masses from $0.02 - 50$ M$_\odot$ (mean mass $\sim 10.5\,\mathrm{M_{\odot}}$ and mean multiplicity fraction $\sim 0.4$). While the results of individual simulations do differ when we change our treatment of $\gamma_{\mathrm{H_2}}$, we find no statistically significant differences in the overall mass or multiplicity distributions of the stars formed in the two sets of runs. We conclude that, at least prior to the onset of radiation feedback, approximating H$_2$ as a classical diatomic gas with $\gamma_{\rm H_2} = 7/5$ does not induce significant errors in simulations of the fragmentation of primordial gas. Nonetheless, we recommend using the accurate formulation of the H$_2\,$ adiabatic index in primordial star formation studies since it is not computationally more expensive and provides a better treatment of the thermodynamics.
Abstract: 1909.06288
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Title:The Issue with Diffusive Shock Acceleration
(Submitted on 13 Sep 2019)
Abstract: We discuss the recent developments in the theory of diffusive shock acceleration (DSA) by using both first-principle kinetic plasma simulations and analytical theory based on the solution of the convection/diffusion equation. In particular, we show how simulations reveal that the spectra of accelerated particles are significantly steeper than the $E^{-2}$ predicted by the standard theory of DSA for strong shocks, in agreement with several observational pieces of evidence. We single out which standard assumptions of test-particle and non-linear DSA are violated in the presence of strong (self-generated) magnetic turbulence and put forward a novel theory in better agreement with the particle spectra inferred with multi-wavelength observations of young SN remnants, radio-supernovae, and Galactic cosmic rays.
Abstract: 1909.06320
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Title:Resonance production of keV sterile neutrinos in core-collapse supernovae and lepton number diffusion
(Submitted on 13 Sep 2019)
Abstract: We investigate how hypothetical particles - sterile neutrinos - can be produced in the interior of exploding supernovae via the resonant conversion of $\bar\nu_\mu$ and $\bar \nu_\tau$. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account back reaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, exponentially sensitive to the temperature in the inner supernovae regions, making robust predictions of challenging. In order to understand whether this production affects supernova evolution and can therefore be constrained, detailed simulations including the effects of sterile neutrinos are needed.
Abstract: 1909.04642
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Full Text: [ PostScript, PDF]
Title:Water Vapor on the Habitable-Zone Exoplanet K2-18b
(Submitted on 10 Sep 2019)
Abstract: Ever since the discovery of the first exoplanet, astronomers have made steady progress towards finding and probing planets in the habitable zone of their host stars, where the conditions could be right for liquid water to form and life to sprawl. Results from the Kepler mission indicate that the occurrence rate of habitable-zone Earths and super-Earths may be as high as 5-20%. Despite this abundance, probing the conditions and atmospheric properties on any of these habitable-zone planets is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid water clouds in the atmosphere of the 8.6 M$_\oplus$ habitable-zone planet K2-18b. With a 33 day orbit around a cool M3 dwarf, K2-18b receives virtually the same amount of total radiation from its host star ($1441\pm80$ W/m$^2$) as the Earth receives from the Sun (1370 W/m$^2$), making it a good candidate to host liquid water clouds. In this study we observed eight transits using HST/WFC3 in order to achieve the necessary sensitivity to detect water vapor. While the thick gaseous envelope of K2-18b means that it is not a true Earth analogue, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with present-day telescopes.
Abstract: 1909.05218
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Title:Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b
(Submitted on 11 Sep 2019)
Abstract: In the past decade, observations from space and ground have found H$_2$O to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous, extrasolar planets. Being the main molecular carrier of oxygen, H$_2$O is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of H$_2$O is of great significance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected. Super-Earths -- planets lighter than ten M$_\oplus$ -- around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here we report the detection of an H$_2$O spectroscopic signature in the atmosphere of \planet\ -- an eight M$_\oplus$ planet in the habitable-zone of an M-dwarf -- with high statistical confidence (ADI = 5.0, $\sim$3.6$\sigma$). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/WFC3 camera, and analysed with our dedicated, publicly available, algorithms. While the suitability of M-dwarfs to host habitable worlds is still under discussion, \planet\ offers an unprecedented opportunity to get insight into the composition and climate of habitable-zone planets.
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