Abstracts of Interest
Selected by:
Gavin Rowell
Abstract: 1712.05739
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Title: Cosmic Neutrinos
(Submitted on 15 Dec 2017)
Abstract: Neutrinos are key astronomical messengers, because they are undeflected by magnetic field and unattenuated by electromagnetic interaction. After the first detection of extraterrestrial neutrinos in the TeV-PeV region by Neutrino Telescopes we are entering a new epoch where neutrino astronomy becomes possible. In this paper I briefly review the main issues concerning cosmological neutrinos and their experimental observation.
Abstract: 1712.05609
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Full Text: [ PostScript, PDF]
Title: Gamma-ray emission from the black hole's vicinity in AGN
(Submitted on 15 Dec 2017)
Abstract: Non-thermal magnetospheric processes in the vicinity of supermassive black holes have attracted particular attention in recent times. Gap-type particle acceleration accompanied by curvature and Inverse Compton radiation could in principle lead to variable gamma-ray emission that may be detectable with current instruments. We shortly comment on the occurrence of magnetospheric gaps and the realisation of different potentials. The detection of rapid variability becomes most instructive by imposing a constraint on possible gap sizes, thereby limiting extractable gap powers and allowing to assess the plausibility of a magnetospheric origin. The relevance of this is discussed for the radio galaxies Cen A, M87 and IC310. The detection of magnetospheric gamma-ray emission generally allows for a sensitive probe of the near-black-hole region and is thus of prime interest for advancing our understanding of the (astro)physics of extreme environments
Abstract: 1712.05407
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Full Text: [ PostScript, PDF]
Title: Atomic data and spectral modeling constraints from high-resolution X-ray observations of the Perseus cluster with Hitomi
(Submitted on 14 Dec 2017)
Abstract: The Hitomi SXS spectrum of the Perseus cluster, with $\sim$5 eV resolution in the 2-9 keV band, offers an unprecedented benchmark of the atomic modeling and database for hot collisional plasmas. It reveals both successes and challenges of the current atomic codes. The latest versions of AtomDB/APEC (3.0.8), SPEX (3.03.00), and CHIANTI (8.0) all provide reasonable fits to the broad-band spectrum, and are in close agreement on best-fit temperature, emission measure, and abundances of a few elements such as Ni. For the Fe abundance, the APEC and SPEX measurements differ by 16%, which is 17 times higher than the statistical uncertainty. This is mostly attributed to the differences in adopted collisional excitation and dielectronic recombination rates of the strongest emission lines. We further investigate and compare the sensitivity of the derived physical parameters to the astrophysical source modeling and instrumental effects. The Hitomi results show that an accurate atomic code is as important as the astrophysical modeling and instrumental calibration aspects. Substantial updates of atomic databases and targeted laboratory measurements are needed to get the current codes ready for the data from the next Hitomi-level mission.
Abstract: 1712.05395
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Full Text: [ PostScript, PDF]
Title: The Einstein@Home Gamma-Ray Pulsar Survey II. Source Selection, Spectral Analysis and Multi-wavelength Follow-up
(Submitted on 14 Dec 2017)
Abstract: We report on the analysis of 13 gamma-ray pulsars discovered in the Einstein@Home blind search survey using Fermi Large Area Telescope (LAT) Pass 8 data. The 13 new gamma-ray pulsars were discovered by searching 118 unassociated LAT sources from the third LAT source catalog (3FGL), selected using the Gaussian Mixture Model (GMM) machine learning algorithm on the basis of their gamma-ray emission properties being suggestive of pulsar magnetospheric emission. The new gamma-ray pulsars have pulse profiles and spectral properties similar to those of previously-detected young gamma-ray pulsars. Follow-up radio observations have revealed faint radio pulsations from two of the newly-discovered pulsars, and enabled us to derive upper limits on the radio emission from the others, demonstrating that they are likely radio-quiet gamma-ray pulsars. We also present results from modeling the gamma-ray pulse profiles and radio profiles, if available, using different geometric emission models of pulsars. The high discovery rate of this survey, despite the increasing difficulty of blind pulsar searches in gamma rays, suggests that new systematic surveys such as presented in this article should be continued when new LAT source catalogs become available.
Abstract: 1712.05392
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Full Text: [ PostScript, PDF]
Title: Coincident detection significance in multimessenger astronomy
(Submitted on 14 Dec 2017)
Abstract: We derive a Bayesian criterion for assessing whether signals observed in two separate data sets originate from a common source. The Bayes factor for a common vs. unrelated origin of signals includes an overlap integral of the posterior distributions over the common source parameters. Focusing on multimessenger gravitational-wave astronomy, we apply the method to the spatial and temporal association of independent gravitational-wave and electromagnetic (or neutrino) observations. As an example, we consider the coincidence between the recently discovered gravitational-wave signal GW170817 from a binary neutron star merger and the gamma-ray burst GRB 170817A: we find that the common source model is enormously favored over a model describing them as unrelated signals.
Abstract: 1712.05331
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Full Text: [ PostScript, PDF]
Title: Galactic and Extragalactic Samples of Supernova Remnants: How They Are Identified and What They Tell Us
(Submitted on 14 Dec 2017)
Abstract: Supernova remnants (SNRs) arise from the interaction between the ejecta of a supernova (SN) explosion and the surrounding circumstellar and interstellar medium. Some SNRs, mostly nearby SNRs, can be studied in great detail. However, to understand SNRs as a whole, large samples of SNRs must be assembled and studied. Here, we describe the radio, optical, and X-ray techniques which have been used to identify and characterize almost 300 Galactic SNRs and more than 1200 extragalactic SNRs. We then discuss which types of SNRs are being found and which are not. We examine the degree to which the luminosity functions, surface-brightness distributions and multi-wavelength comparisons of the samples can be interpreted to determine the class properties of SNRs and describe efforts to establish the type of SN explosion associated with a SNR. We conclude that in order to better understand the class properties of SNRs, it is more important to study (and obtain additional data on) the SNRs in galaxies with extant samples at multiple wavelength bands than it is to obtain samples of SNRs in other galaxies
Abstract: 1712.05070
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Full Text: [ PostScript, PDF]
Title: A Molecular Line Survey toward the Nearby Galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45-m Radio Telescope: Impact of an AGN on 1 kpc Scale Molecular Abundances
(Submitted on 14 Dec 2017)
Abstract: It is important to investigate the relationships between the power sources and the chemical compositions of galaxies for understanding the scenario of galaxy evolution. We carried out an unbiased molecular line survey towards AGN host galaxy NGC1068, and prototypical starburst galaxies, NGC 253 and IC 342, with the Nobeyama 45-m telescope in the 3-mm band. The advantage of this line survey is that the obtained spectra have the highest angular resolution ever obtained with single-dish telescopes. In particular, the beam size of this telescope is ~15"--19", which is able to spatially separate the nuclear molecular emission from that of the starburst ring (d~30") in NGC 1068. We successfully detected approximately 23 molecular species in each galaxy, and calculated rotation temperatures and column densities. We estimate the molecular fractional abundances with respect to 13CO and CS molecules and compare them among three galaxies in order to investigate the chemical signatures of an AGN environment. As a result, we found clear trends on the abundances of molecules surrounding the AGN on 1 kpc scale. HCN, H13CN, CN, 13CN, and HC3N are more abundant, and CH3CCH is deficient in NGC 1068 compared with the starburst galaxies. High abundances of HCN, H13CN, and HC3N suggest that the circumnuclear disk in NGC 1068 is in a high-temperature environment. The reason for the non-detection of CH3CCH is likely to be dissociation by high energy radiation or less sublimation of a precursor of CH3CCH from grains.
Abstract: 1712.05044
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Full Text: [ PostScript, PDF]
Title: Identifying Exoplanets with Deep Learning: A Five Planet Resonant Chain around Kepler-80 and an Eighth Planet around Kepler-90
(Submitted on 13 Dec 2017)
Abstract: NASA's Kepler Space Telescope was designed to determine the frequency of Earth-sized planets orbiting Sun-like stars, but these planets are on the very edge of the mission's detection sensitivity. Accurately determining the occurrence rate of these planets will require automatically and accurately assessing the likelihood that individual candidates are indeed planets, even at low signal-to-noise ratios. We present a method for classifying potential planet signals using deep learning, a class of machine learning algorithms that have recently become state-of-the-art in a wide variety of tasks. We train a deep convolutional neural network to predict whether a given signal is a transiting exoplanet or a false positive caused by astrophysical or instrumental phenomena. Our model is highly effective at ranking individual candidates by the likelihood that they are indeed planets: 98.8% of the time it ranks plausible planet signals higher than false positive signals in our test set. We apply our model to a new set of candidate signals that we identified in a search of known Kepler multi-planet systems. We statistically validate two new planets that are identified with high confidence by our model. One of these planets is part of a five-planet resonant chain around Kepler-80, with an orbital period closely matching the prediction by three-body Laplace relations. The other planet orbits Kepler-90, a star which was previously known to host seven transiting planets. Our discovery of an eighth planet brings Kepler-90 into a tie with our Sun as the star known to host the most planets.
Abstract: 1712.04947
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Full Text: [ PostScript, PDF]
Title: The dependence of cosmic ray driven galactic winds on halo mass
(Submitted on 13 Dec 2017)
Abstract: Galactic winds regulate star formation in disk galaxies and help to enrich the circum-galactic medium. They are therefore crucial for galaxy formation, but their driving mechanism is still poorly understood. Recent studies have demonstrated that cosmic rays (CRs) can drive outflows if active CR transport is taken into account. Using hydrodynamical simulations of isolated galaxies with virial masses between $10^{10}$ and $10^{13}\mathrm{~M_\odot}$, we study how the properties of CR-driven winds depend on halo mass. CRs are treated in a two-fluid approximation and their transport is modelled through isotropic or anisotropic diffusion. We find that CRs are only able to drive mass-loaded winds beyond the virial radius in haloes with masses below $10^{12}\mathrm{~M_\odot}$. For our lowest examined halo mass, the wind is roughly spherical and has velocities of $\sim20\mathrm{~km\;s^{-1}}$. With increasing halo mass, the wind becomes biconical and can reach ten times higher velocities. The mass loading factor drops rapidly with virial mass, a dependence that approximately follows a power-law with a slope between $-1$ and $-2$. This scaling is slightly steeper than observational inferences, and also steeper than commonly used prescriptions for wind feedback in cosmological simulations. The slope is quite robust to variations of the CR injection efficiency or the CR diffusion coefficient. In contrast to the mass loading, the energy loading shows no significant dependence on halo mass. While these scalings are close to successful heuristic models of wind feedback, the CR-driven winds in our present models are not yet powerful enough to fully account for the required feedback strength.
Abstract: 1712.04515
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Full Text: [ PostScript, PDF]
Title: New Distances to Four Supernova Remnants
(Submitted on 12 Dec 2017)
Abstract: Distances are found for four supernova remnants SNRs without previous distance measurements. HI spectra and HI channel maps are used to determine the maximum velocity of HI absorption for four supernova remnants (SNRs). We examined $^{13}$CO emission spectra and channel maps to look for possible molecular gas associated with each SNR, but did not find any. The resulting distances for the SNRs are $3.5 \pm 0.2$ kpc (G$24.7+0.6$), $4.7 \pm 0.3$ kpc (G$29.6+0.1$) , $ 4.1 \pm 0.5$ kpc (G$41.5+0.4$) and $4.5\pm 0.4 - 9.0 \pm 0.4$ kpc (G$57.2+0.8$).
Abstract: 1712.04316
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Full Text: [ PostScript, PDF]
Title: On the radio and GeV-TeV gamma-ray emission connection in Fermi blazars
(Submitted on 12 Dec 2017)
Abstract: The Fermi-LAT revealed that the census of the gamma-ray sky is dominated by blazars. Looking for a possible connection between radio and gamma-ray emission is a central issue for understanding the blazar physics, and various works were dedicated to this topic. However, while a strong and significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range, the connection between radio and very high energy (VHE, E>0.1 TeV) emission is still elusive. The main reason is the lack of a homogeneous VHE sky coverage, due to the operational mode of the imaging atmospheric Cherenkov telescopes. With the present work we aim to quantify and assess the significance of the possible connection between high-resolution radio emission, on milliarcsecond scale, and GeV-TeV gamma-ray emission in blazars. For achieving our goal we extract two large and unbiased blazar samples from the 1FHL and 2FHL Fermi catalogs, above 10 GeV and 50 GeV, respectively. To investigate how the correlation evolves as the gamma-ray energy increases, we perform the same analysis by using the 0.1-300 GeV 3FGL gamma-ray energy fluxes. When we consider the 0.1-300 GeV gamma-ray energy range, we find a strong and significant correlation for all of the blazar sub-classes. Conversely, when we consider the gamma-ray emission above 10 GeV the correlation with the radio emission vanishes, with the exception of the blazar sub-class of high synchrotron peaked objects.
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