Abstracts of Interest
Selected by:
Sabrina Einecke
Abstract: 1807.11069
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Title: Variability and Optical Polarization Can Probe the Neutrino and Electromagnetic Emission Mechanisms of TXS~0506+056
(Submitted on 29 Jul 2018 (v1), last revised 2 Aug 2018 (this version, v2))
Abstract: The association of the high-energy neutrino event IceCube-170922A with the flaring blazar TXS~0506+056 indicates that hadronic processes may operate in a blazar jet. We perform semi-analytical spectral fitting of the multi-wavelength emission to obtain estimates of the jet physical parameters, and find that the multi-wavelength emission can be explained by either a proton synchrotron scenario or an electron inverse Compton scattering scenario. In the proton synchrotron scenario, a strong magnetic field of $10-100$~G is required, implying that the particle acceleration is likely driven by magnetic energy dissipation such as magnetic reconnection events. The inverse Compton scenario implies a magnetic field of $0.1-1$~G. Thus the particle acceleration is likely driven by the kinetic energy dissipation such as shocks. We also discuss the neutrino production in the context of single-zone and multi-zone models based on the above two scenarios. We demonstrate that the variability and optical polarization signatures can be used to distinguish the two scenarios due to their drastically different magnetic field. Specifically, the proton synchrotron scenario may show orphan fast variability in the low-energy spectral component on top of the active state, with an optical polarization degree $\lesssim 10\%$ throughout the active state. The inverse Compton scattering scenario instead predicts co-variability of the low- and high-energy components on both short and long time scales, as well as a strongly variable optical polarization degree that can reach $\gtrsim 20\%$. Our results suggest that optical polarization measurements and well-resolved multi-wavelength light curves can be used to understand the electromagnetic and high-energy neutrino emissions by TXS~0506+056 and similar events in the future.
Abstract: 1807.11239
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Title: Synchrotron radiation from secondary electrons in SNR shocks
(Submitted on 30 Jul 2018)
Abstract: The secondary electrons and positrons (SEPs) generated by the hadronic interactions in supernova remnants (SNRs) will give rise to synchrotron radiation in the magnetic field of the SNR shock, which may be a powerful way to distinguish whether the gamma-ray emission from SNRs is leptonic or hadronic origin, and whether cosmic rays are accelerated in SNR shocks. Here we provide a method that directly use the observed gamma-ray flux from SNRs, created by pion decays, to derive the SEP distribution and hence the synchrotron spectrum. We apply the method to SNRs RX J1713.7-3946, IC 443 and W44 and find: (1) If the gamma-rays from the young SNR RX J1713.7-3946 are produced by hadronic interactions, the SEP synchrotron radiation may show a spectral bump at $10^{-5}-10^{-2}$eV dominating the primary electrons' synchrotron component, if the magnetic field in the SNR shock is $\gtrsim 0.5$mG; (2) In the leptonic model for the gamma-rays from RX J1713.7-3946, the SEPs from subdominant hadronic processes may still produce a flat synchrotron spectrum and a flux in the level of a few $10^{-13} \rm erg~cm^{-2}s^{-1}$ at $\gtrsim100$ keV; (3) If the gamma-rays from middle-aged SNRs IC443 and W44 are produced by hadronic interactions, the SEP synchrotron radiation can well account for the observed radio flux and spectral slopes, supporting the hadronic origin of the gamma-rays in these two SNRs; (4) The requirement that the SEP radiation derived by the observed gamma-ray spectra, assumed to be hadronic origin, cannot exceed the observed radio flux constrain the magnetic fields in the SNR shocks to be $\lesssim 2$mG, $\lesssim 350 \mu$G, and $\lesssim400 \mu$G for RX J1713.7-3946, IC 443 and W44, respectively. Future microwave to far-infrared and hard ($>100$~keV) X-ray observations are encouraged to detect the SEP radiation, and constrain the SNR ability of cosmic ray acceleration.
Abstract: 1807.11263
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Title: Estimation of the height of the first interaction in gamma-ray showers observed by Cherenkov telescopes
(Submitted on 30 Jul 2018)
Abstract: Very high energy gamma rays entering the atmosphere initiate Extensive Air Showers (EAS). The Cherenkov light induced by an EAS can be observed by ground-based telescopes to study the primary gamma rays. An important parameter of an EAS, determining its evolution, is the height of the first interaction of the primary particle. However, this variable cannot be directly measured by Cherenkov telescopes. We study two simple, independent methods for the estimation of the first interaction height. We test the methods using the Monte Carlo simulations for the 4 Large Size Telescopes (LST) that are part of the currently constructed Cherenkov Telescope Array (CTA) Observatory. We find that using such an estimated parameter in the gamma/hadron separation can bring a mild improvement (~10-20%) in the sensitivity in the energy range ~30-200 GeV.
Abstract: 1807.11497
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Title: On estimating the atomic hydrogen column density from the H I 21 cm emission spectra
(Submitted on 30 Jul 2018)
Abstract: The 21 cm hyperfine transition of the atomic hydrogen (HI) in ground state is a powerful probe of the neutral gas content of the universe. This radio frequency transition has been used routinely for decades to observe, both in emission and absorption, HI in the Galactic interstellar medium as well as in extragalactic sources. In general, however, it is not trivial to derive the physically relevant parameters like temperature, density or column density from these observations. Here, we have considered the issue of column density estimation from the HI 21 cm emission spectrum for sightlines with a non-negligible optical depth and a mix of gas at different temperatures. The complicated radiative transfer and a lack of knowledge about the relative position of gas clouds along the sightline often make it impossible to uniquely separate the components, and hinders reliable estimation of column densities in such cases. Based on the observed correlation between the 21 cm brightness temperature and optical depth, we propose a method to get an unbiased estimate of the HI column density using only the 21 cm emission spectrum. This formalism is further used for a large sample to study the spin temperature of the neutral interstellar medium.
Abstract: 1807.11815
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Title: The cosmic ray shadow of the Moon observed with the ANTARES neutrino telescope
(Submitted on 31 Jul 2018)
Abstract: One of the main objectives of the ANTARES telescope is the search for point-like neutrino sources. Both the pointing accuracy and the angular resolution of the detector are important in this context and a reliable way to evaluate this performance is needed. In order to measure the pointing accuracy of the detector, one possibility is to study the shadow of the Moon, i.e. the deficit of the atmospheric muon flux from the direction of the Moon induced by the absorption of cosmic rays. Analysing the data taken between 2007 and 2016, the Moon shadow is observed with $3.5\sigma$ statistical significance. The detector angular resolution for downward-going muons is 0.73$^{\circ}\pm0.14^{\circ}.$ The resulting pointing performance is consistent with the expectations. An independent check of the telescope pointing accuracy is realised with the data collected by a shower array detector onboard of a ship temporarily moving around the ANTARES location.
Abstract: 1808.00041
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Title: Are 10 EeV cosmic rays extragalactic? Theory of cosmic ray diffusion at high energy
(Submitted on 26 Jul 2018)
Abstract: Auger Collaboration has reported a large-scale anisotropy in the arrival directions of cosmic rays above 8 EeV. The dipole direction, at 125$^\circ$ from the Galactic center, is taken as an indication of an extragalactic origin of these cosmic rays. We show, both theoretically and by direct numerical simulations, that this is not necessarily true. Intermediate mass nuclei originating in the Galaxy and diffusing in the Galactic magnetic field can have a dipole anisotropy pointing away from the Galactic center. Our theory predicts a quadrupole anisotropy of the same order of magnitude as the dipole.
Abstract: 1808.00559
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Title: Pointing optimization for IACTs on indirect dark matter searches
(Submitted on 1 Aug 2018)
Abstract: We present a procedure to optimize the offset angle (usually also known as the wobble distance) and the signal integration region for the observations and analysis of extended sources by Imaging Atmospheric Cherenkov Telescopes (IACTs) such as MAGIC, HESS, VERITAS or (in the near future), CTA. Our method takes into account the off-axis instrument performance and the emission profile of the gamma-ray source. We take as case of study indirect dark matter searches (where an a priori knowledge on the expected signal morphology can be assumed) and provide optimal pointing strategies to perform searches of dark matter on a set of dwarf spheroidal galaxies with current and future IACTs.
Abstract: 1808.00683
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Title: Near-Infrared Spectroscopy of SN 2017eaw in 2017: Carbon Monoxide and Dust Formation in a Type II-P Supernova
(Submitted on 2 Aug 2018)
Abstract: The origin of dust in the early Universe has been the subject of considerable debate. Core-collapse supernovae (ccSNe), which occur several million years after their massive progenitors form, could be a major source of that dust, as in the local universe several ccSNe have been observed to be copious dust producers. Here we report nine near-infrared (0.8 - 2.5 micron) spectra of the Type II-P SN 2017eaw in NGC 6946, spanning the time interval 22 - 205 days after discovery. The specta show the onset of CO formation and continuum emission at wavelengths greater than 2.1 micron from newly-formed hot dust, in addition to numerous lines of hydrogen and metals, which reveal the change in ionization as the density of much of the ejecta decreases. The observed CO masses estimated from an LTE model are typically 10^{-4} Msun during days 124 - 205, but could be an order of magnitude larger if non-LTE conditions are present in the emitting region. The timing of the appearance of CO is remarkably consistent with chemically controlled dust models of Sarangi & Cherchneff.
Abstract: 1808.00977
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Title: Relativistic supernova 2009bb exploded close to an atomic gas cloud
(Submitted on 2 Aug 2018)
Abstract: The potential similarity of the powering mechanisms of relativistic SNe and GRBs allowed us to make a prediction that relativistic SNe are born in environments similar to those of GRBs, that is, ones which are rich in atomic gas. Here we embark on testing this hypothesis by analysing the properties of the host galaxy NGC 3278 of the relativistic SN 2009bb. This is the first time the atomic gas properties of a relativistic SN host are provided and the first time resolved 21 cm-hydrogen-line (HI) information is provided for a host of an SN of any type in the context of the SN position. We obtained radio observations with ATCA covering the HI line, and optical integral field unit spectroscopy observations with MUSE. The atomic gas distribution of NGC 3278 is not centred on the optical galaxy centre, but instead around a third of atomic gas resides in the region close to the SN position. This galaxy has a few times lower atomic and molecular gas masses than predicted from its SFR. SN 2009bb exploded close to the region with the highest SFR density and the lowest age (~5.5 Myr, corresponding to the initial mass of the progenitor star ~36 Mo). As for GRB hosts, the gas properties of NGC 3278 are consistent with a recent inflow of gas from the intergalactic medium, which explains the concentration of atomic gas close to the SN position and the enhanced SFR. Super-solar metallicity at the position of the SN (unlike for most GRBs) may mean that relativistic explosions signal a recent inflow of gas (and subsequent star formation), and their type (GRBs or SNe) is determined either i) by the metallicity of the inflowing gas, so that metal-poor gas results in a GRB explosion and metal-rich gas results in a relativistic SN explosion without an accompanying GRB, or ii) by the efficiency of gas mixing, or iii) by the type of the galaxy.
Abstract: 1808.01176
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Full Text: [ PostScript, PDF]
Title: A MST catalogue of gamma-ray source candidates above 10 GeV and at Galactic latitudes higher than 20 degrees
(Submitted on 3 Aug 2018)
Abstract: We describe a catalogue of gamma-ray source candidates, selected using the Minimum Spanning Tree (MST) algorithm on the 9-years Fermi-LAT sky (Pass 8) at energies higher than 10 GeV. The extragalactic sky at absolute Galactic latitudes above 20 degrees has been investigated using rather restrictive selection criteria, resulting in a total sample of 1342 sources. Of these, 249 are new detections, not previously associated with gamma-ray catalogues. A large fraction of them have interesting counterparts, most likely blazars. In this paper the main results on the catalogue selection and search of counterparts are reported.
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