Abstracts of Interest
Selected by:
Fabien Voisin
Abstract: 1911.12977
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Title:Positron Excess from Cosmic Ray Interactions in Galactic Molecular Clouds
(Submitted on 29 Nov 2019)
Abstract: The recent data on cosmic ray positron flux measured near the Earth by the \textit{Alpha Magnetic Spectrometer} (AMS-02) experiment extends to TeV energy. The positron flux measured in GeV$^2$ m$^{-2}$ sec$^{-1}$ sr$^{-1}$ rises with energy and shows a peak near a few hundred GeV. This rising positron flux cannot be explained by interactions of cosmic rays with interstellar hydrogen gas. Due to the progress in multi-wavelength astronomy, many new Galactic Molecular Clouds (GMCs) have been discovered in our Galaxy recently. We use the updated list of GMCs, which are distributed in the Galactic plane, to find the secondary positrons produced in them in interactions of cosmic rays with molecular hydrogen. Moreover, by analysing the Fermi LAT data, new GMCs have been discovered away from the Galactic plane. We also include some of these GMCs closest to the Earth where cosmic ray interactions are producing secondaries. After including these GMCs we show that the positron excess can be mostly well explained, with a small contribution from unknown extra component near the peak.
Abstract: 1911.12828
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Title:Discovery of a Powerful >10^61 erg AGN Outburst in Distant Galaxy Cluster SPT-CLJ0528-5300
(Submitted on 28 Nov 2019)
Abstract: We present ~103 ks of Chandra observations of the galaxy cluster SPT-CLJ0528-5300 (SPT0528, z=0.768). This cluster harbors the most radio-loud (L_1.4GHz = 1.01 x 10^33 erg/s/Hz) central AGN of any cluster in the South Pole Telescope (SPT) SZ survey with available X-ray data. We find evidence of AGN-inflated cavities in the X-ray emission, which are consistent with the orientation of the jet direction revealed by ATCA radio data. The combined probability that two such depressions -- each at ~1.4-1.8sigma significance, oriented ~180 degrees apart and aligned with the jet axis -- would occur by chance is 0.1%. At >10^61 erg, the outburst in SPT0528 is among the most energetic known in the universe, and certainly the most powerful known at z>0.25. This work demonstrates that such powerful outbursts can be detected even in shallow X-ray exposures out to relatively high redshifts (z~0.8), providing an avenue for studying the evolution of extreme AGN feedback. The ratio of the cavity power (P_cav = 9.4+/-5.8 x 10^45 erg/s) to the cooling luminosity (L_cool = 1.5+/-0.5 x 10^44 erg/s) for SPT0528 is among the highest measured to date. If, in the future, additional systems are discovered at similar redshifts with equally high P_cav/L_cool ratios, it would imply that the feedback/cooling cycle was not as gentle at high redshifts as in the low-redshift universe.
Abstract: 1911.11570
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Title:Magnetic energy dissipation and origin of non-thermal spectra in radiatively efficient relativistic sources
(Submitted on 26 Nov 2019)
Abstract: The dissipation of turbulent magnetic fields is an appealing scenario to explain the origin of non-thermal particles in high-energy astrophysical sources. However, it has been suggested that the particle distribution may effectively thermalise when the radiative (synchrotron and/or Inverse Compton) losses are severe. Inspired by recent PIC simulations of relativistic turbulence, which show that electrons are impulsively heated in intermittent current sheets by a strong electric field aligned with the local magnetic field, we instead argue that in plasmas where the particle number density is dominated by the pairs (electron-positron and electron-positron-ion plasmas): (i) as an effect of fast cooling and of different injection times, the electron energy distribution is ${\rm d}n_e/{\rm d}\gamma\propto\gamma^{-2}$ for $\gamma\lesssim\gamma_{\rm heat}$ (the Lorentz factor $\gamma_{\rm heat}$ being close to the equipartition value), while the distribution steepens at higher energies; (ii) since the time scales for the turbulent fields to decay and for the photons to escape are of the same order, the magnetic and the radiation energy densities in the dissipation region are comparable; (iii) if the mass energy of the plasma is dominated by the ion component, the pairs with a Lorentz factor smaller than a critical one (of the order of the proton-to-electron mass ratio) become isotropic, while the pitch angle remains small otherwise. The outlined scenario is consistent with the typical conditions required to reproduce the Spectral Energy Distribution of blazars, and allows one to estimate the magnetisation of the emission site. Finally, we show that turbulence within the Crab Nebula may power the observed gamma-ray flares if the pulsar wind is nearly charge-separated at high latitudes.
Abstract: 1911.12077
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Title:Cherenkov Telescope Array (CTA) Contributions to the 36th International Cosmic Ray Conference (ICRC2019)
(Submitted on 27 Nov 2019)
Abstract: List of contributions from the Cherenkov Telescope Array (CTA) Consortium presented at the 36th International Cosmic Ray Conference, July 24 - August 1 2019, Madison, WI, EUA.
Abstract: 1911.11829
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Title:Deciphering the Nature of the Pulsar Wind Nebula CTB 87 with XMM-Newton
(Submitted on 26 Nov 2019)
Abstract: CTB 87 (G74.9+1.2) is an evolved supernova remnant (SNR) which hosts a peculiar pulsar wind nebula (PWN). The X-ray peak is offset from that observed in radio and lies towards the edge of the radio nebula. The putative pulsar, CXOU~J201609.2+371110, was first resolved with \textit{Chandra} and is surrounded by a compact and a more extended X-ray nebula. Here we use a deep {\textit{XMM-Newton}} observation to examine the morphology and evolutionary stage of the PWN and to search for thermal emission expected from a supernova shell or reverse shock interaction with supernova ejecta. We do not find evidence of thermal X-ray emission from the SNR and place an upper limit on the electron density of 0.05~cm$^{-3}$ for a plasma temperature $kT\sim 0.8$ keV. The morphology and spectral properties are consistent with a $\sim$20~kyr-old relic PWN expanding into a stellar wind-blown bubble. We also present the first X-ray spectral index map from the PWN and show that we can reproduce its morphology by means of 2D axisymmetric relativistic hydrodynamical simulations.
Abstract: 1911.11809
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Full Text: [ PostScript, PDF]
Title:Constraints on Neutrino Emission from Nearby Galaxies Using the 2MASS Redshift Survey and IceCube
(Submitted on 26 Nov 2019)
Abstract: The distribution of galaxies within the local universe is characterized by anisotropic features. Observatories searching for the production sites of astrophysical neutrinos can take advantage of these features to establish directional correlations between a neutrino dataset and overdensities in the galaxy distribution in the sky. The results of two correlation searches between a seven-year time-integrated neutrino dataset from the IceCube Neutrino Observatory, and the 2MASS Redshift Survey (2MRS) catalog are presented here. The first analysis searches for neutrinos produced via interactions between diffuse intergalactic Ultra-High Energy Cosmic Rays (UHECRs) and the matter contained within galaxies. The second analysis searches for low-luminosity sources within the local universe, which would produce subthreshold multiplets in the IceCube dataset that directionally correlate with galaxy distribution. No significant correlations were observed in either analyses. Constraints are presented on the flux of neutrinos originating within the local universe through diffuse intergalactic UHECR interactions, as well as on the density of standard candle sources of neutrinos at low luminosities.
Abstract: 1911.12833
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Full Text: [ PostScript, PDF]
Title:Emission from a Pulsar Wind Nebula: Application to the Persistent Radio Counterpart of FRB 121102
(Submitted on 28 Nov 2019)
Abstract: The properties of fast radio bursts (FRBs) indicate that the physical origin of this type of astrophysical phenomenon is related to neutron stars. The first detected repeating source, FRB 121102, is associated with a persistent radio counterpart. In this paper, we propose that this radio counterpart could arise from a pulsar wind nebula powered by a magnetar without surrounding supernova ejecta. Its medium is a stratified structure produced by a progenitor wind. The model parameters are constrained by the spectrum of the counterpart emission, the size of the nebula, and the large but decreasing rotation measure (RM) of the repeating bursts. In addition, the observed dispersion measure is consistent with the assumption that all of the RM comes from the shocked medium.
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