Abstracts of Interest
Selected by:
Roger Clay
Abstract: 1902.01657
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Title:Probing restarting activity in hard X-ray selected giant radio galaxies
(Submitted on 5 Feb 2019)
Abstract: With their sizes larger than 0.7 Mpc, Giant Radio Galaxies (GRGs) are the largest individual objects in the Universe. To date, the reason why they reach such enormous extensions is still unclear. One of the proposed scenarios suggests that they are the result of multiple episodes of jet activity. Cross-correlating the INTEGRAL+Swift AGN population with radio catalogues (NVSS, FIRST, SUMSS), we found that 22% of the sources are GRG (a factor four higher than those selected from radio catalogues). Remarkably, 80% of the sample shows signs of restarting radio activity. The X-ray properties are consistent with this scenario, the sources being in a high-accretion, high-luminosity state with respect to the previous activity responsible for the radio lobes.
Abstract: 1902.01675
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Full Text: [ PostScript, PDF]
Title:The mass, location and heating of the dust in the Cassiopeia A supernova remnant
(Submitted on 5 Feb 2019)
Abstract: We model the thermal dust emission from dust grains heated by synchrotron radiation and by particle collisions, under conditions appropriate for four different shocked and unshocked gas components of the Cassiopeia A (Cas A) supernova remnant (SNR). By fitting the resulting spectral energy distributions (SEDs) to the observed SNR dust fluxes, we determine the required mass of dust in each component. We find the observed SED can be reproduced by $\sim 0.6 \, {\rm M_\odot}$ of silicate grains, the majority of which is in the unshocked ejecta and heated by the synchrotron radiation field. Warmer dust, located in the X-ray emitting reverse shock and blastwave regions, contribute to the shorter wavelength infrared emission but make only a small fraction of the total dust mass. Carbon grains can at most make up $\sim 25 \%$ of the total dust mass. Combined with estimates for the gas masses, we obtain dust-to-gas mass ratios for each component, which suggest that the condensation efficiency in the ejecta is high, and that dust in the shocked ejecta clumps is well protected from destruction by sputtering in the reverse shock.
Abstract: 1902.01695
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Full Text: [ PostScript, PDF]
Title:Mapping the stellar age of the Milky Way bulge with the VVV. I. The method
(Submitted on 5 Feb 2019)
Abstract: Recent observational programmes are providing a global view of the Milky Way bulge that serves as template for detailed comparison with models and extragalactic bulges. A number of surveys (i.e. VVV, GIBS, GES, ARGOS, BRAVA, APOGEE) are producing comprehensive and detailed extinction, metallicity, kinematics and stellar density maps of the Galactic bulge with unprecedented accuracy. However, the still missing key ingredient is the distribution of stellar ages across the bulge. To overcome this limitation, we aim to age-date the stellar population in several bulge fields with the ultimate goal of deriving an age map of the Bulge. This paper presents the methodology and the first results obtained for a field along the Bulge minor axis, at $b=-6^\circ$. We use a new PSF-fitting photometry of the VISTA Variables in the Vía Láctea (VVV) survey data to construct deep color-magnitude diagrams of the bulge stellar population down to $\sim$ 2 mag below the Main Sequence turnoff. We find the bulk of the bulge stellar population in the observed field along the minor axis to be at least older than $\sim$ 7.5 Gyr. In particular, when the metallicity distribution function spectroscopically derived by GIBS is used, the best fit to the data is obtained with a combination of synthetic populations with ages in between $\sim$ 7.5 Gyr and 11 Gyr. However, the fraction of stars younger than $\sim$ 10 Gyr strongly depends upon the number of Blue Straggler Stars present in the bulge. Simulations show that the observed color-magnitude diagram of the bulge in the field along the minor axis is incompatible with the presence of a conspicuous population of intermediate-age/young (i.e. $\lesssim 5$ Gyr) stars.
Abstract: 1902.01776
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Full Text: [ PostScript, PDF]
Title:Radio detection in the multi-messenger context
(Submitted on 5 Feb 2019)
Abstract: The present work discusses the development of the radio technique for detection of ultra-high energy air-showers induced by cosmic radiation, and the prospects of its application in the future multi-messenger activities, particularly for detection of ultra-high energy cosmic rays, gamma rays and neutrinos. It gives an overview of the results achieved by the modern digital radio arrays, as well as discuss present challenges and future prospects.}
Abstract: 1902.01783
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Full Text: [ PostScript, PDF]
Title:Fundamental Physics and Cosmology in the Extremely Large Telescopes Era
(Submitted on 5 Feb 2019)
Abstract: The observational evidence for the recent acceleration of the universe demonstrates that canonical theories of cosmology and particle physics are incomplete (or possibly incorrect) and that new physics is out there, waiting to be discovered. A crucial task for the next generation of astrophysical facilities is to search for, identify and ultimately characterise this new physics. I outline the theoretical arguments pointing to this new physics, and then discuss some unique contributions of the ELTs towards this endeavour, including tests of the stability of nature's fundamental couplings, tests of the behaviour of gravity in the strong-field regime, and mapping the expansion history of the universe in the deep matter era, by both direct and indirect means. I also briefly highlight how the ELTs can optimally complement other planned ground and space facilities, and stress the requirements that these searches impose on the instruments currently being built or developed.
Abstract: 1902.02426
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Full Text: [ PostScript, PDF]
Title:Machine Vision and Deep Learning for Classification of Radio SETI Signals
(Submitted on 6 Feb 2019)
Abstract: We apply classical machine vision and machine deep learning methods to prototype signal classifiers for the search for extraterrestrial intelligence. Our novel approach uses two-dimensional spectrograms of measured and simulated radio signals bearing the imprint of a technological origin. The studies are performed using archived narrow-band signal data captured from real-time SETI observations with the Allen Telescope Array and a set of digitally simulated signals designed to mimic real observed signals. By treating the 2D spectrogram as an image, we show that high quality parametric and non-parametric classifiers based on automated visual analysis can achieve high levels of discrimination and accuracy, as well as low false-positive rates. The (real) archived data were subjected to numerous feature-extraction algorithms based on the vertical and horizontal image moments and Huff transforms to simulate feature rotation. The most successful algorithm used a two-step process where the image was first filtered with a rotation, scale and shift-invariant affine transform followed by a simple correlation with a previously defined set of labeled prototype examples. The real data often contained multiple signals and signal ghosts, so we performed our non-parametric evaluation using a simpler and more controlled dataset produced by simulation of complex-valued voltage data with properties similar to the observed prototypes. The most successful non-parametric classifier employed a wide residual (convolutional) neural network based on pre-existing classifiers in current use for object detection in ordinary photographs. These results are relevant to a wide variety of research domains that already employ spectrogram analysis from time-domain astronomy to observations of earthquakes to animal vocalization analysis.
Abstract: 1902.02535
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Full Text: [ PostScript, PDF]
Title:Magnetic field and ISM in the local Galactic disc
(Submitted on 7 Feb 2019)
Abstract: Correlation analysis is obtained among Faraday rotation measure, HI column density, thermal and synchrotron radio brightness using archival all-sky maps of the Galaxy. A method is presented to calculate the magnetic strength and its line-of-sight (LOS) component, volume gas densities, effective LOS depth, effective scale height of the disk) from these data in a hybrid way. Applying the method to archival data, all-sky maps of the local magnetic field strength and its parallel component are obtained, which reveal details of local field orientation.
Abstract: 1902.02557
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Full Text: [ PostScript, PDF]
Title:The Role of Magnetic Fields in Setting the Star Formation Rate and the Initial Mass Function
(Submitted on 7 Feb 2019)
Abstract: Star-forming gas clouds are strongly magnetized, and their ionization fractions are high enough to place them close to the regime of ideal magnetohydrodyamics on all but the smallest size scales. In this review we discuss the effects of magnetic fields on the star formation rate (SFR) in these clouds, and on the mass spectrum of the fragments that are the outcome of the star formation process, the stellar initial mass function (IMF). Current numerical results suggest that magnetic fields by themselves are minor players in setting either the SFR or the IMF, changing star formation rates and median stellar masses only by factors of $\sim 2-3$ compared to non-magnetized flows. However, the indirect effects of magnetic fields, via their interaction with star formation feedback in the form of jets, photoionization, radiative heating, and supernovae, could have significantly larger effects. We explore evidence for this possibility in current simulations, and suggest avenues for future exploration, both in simulations and observations.
Abstract: 1902.02561
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Full Text: [ PostScript, PDF]
Title:Angular and Abundance Distribution of High-energy Gamma Rays and Neutrons Simulated by GEANT4 Code for Solar Flares
(Submitted on 7 Feb 2019)
Abstract: In the solar flare observed on June 3, 2012, high energy gamma-rays and neutrons were observed. The event includes a remarkable feature of a high neutron/gamma-ratio in the secondary particles. We have examined whether this high n/$\gamma$-ratio can be explained by simulation. As a result of simulations using the GEANT4 program, the high n/$\gamma$-ratio may be reproduced for the case that helium and other heavy ions were dominantly accelerated in the flare.
Abstract: 1902.02562
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Full Text: [ PostScript, PDF]
Title:Plasmas in Gamma-Ray Bursts: particle acceleration, magnetic fields, radiative Processes and environments
(Submitted on 7 Feb 2019)
Abstract: Being the most extreme explosions in the universe, gamma-ray bursts (GRBs) provide a unique laboratory to study various plasma physics phenomena. The complex lightcurve and broad-band, non-thermal spectra indicate a very complicated system on the one hand, but on the other hand provide a wealth of information to study it. In this chapter I focus on recent progress in some of the key unsolved physical problems. These include: (1) Particle acceleration and magnetic field generation in shock waves; (2) Possible role of strong magnetic fields in accelerating the plasmas, and accelerating particles via magnetic reconnection process; (3) Various radiative processes that shape the observed lightcurve and spectra, both during the prompt and the afterglow phases, and finally (4) GRB environments and their possible observational signature.
Abstract: 1902.02720
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Full Text: [ PostScript, PDF]
Title:Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
(Submitted on 7 Feb 2019)
Abstract: Planets form in protoplanetary disks and inherit their chemical compositions. It is thus crucial to map the distribution and investigate the formation of simple organics, such as formaldehyde and methanol, in protoplanetary disks. We analyze ALMA observations of the nearby disk-jet system around the T Tauri star DG Tau in the o-H$_2$CO $3_{1,2}-2_{1,1}$ and CH$_3$OH $3_{-2,2}-4_{-1,4}$ E, $5_{0,5}-4_{0,4}$ A transitions at an unprecedented resolution of $\sim0.15"$, i.e., $\sim18$ au at a distance of 121 pc. The H$_2$CO emission originates from a rotating ring extending from $\sim40$ au with a peak at $\sim62$ au, i.e., at the edge of the 1.3mm dust continuum. CH$_3$OH emission is not detected down to an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para ratio of 1.8-2.8 the ring- and disk-height-averaged H$_2$CO column density is $\sim0.3-4\times10^{14}$ cm$^{-2}$, while that of CH$_3$OH is $<0.04-0.7\times10^{14}$ cm$^{-2}$. In the inner $40$ au no o-H$_2$CO emission is detected with an upper limit on its beam-averaged column density of $\sim0.5-6\times10^{13}$ cm$^{-2}$. The H$_2$CO ring in the disk of DG Tau is located beyond the CO iceline (R$_{\rm CO}\sim30$ au). This suggests that the H$_2$CO abundance is enhanced in the outer disk due to formation on grain surfaces by the hydrogenation of CO ice. The emission peak at the edge of the mm dust continuum may be due to enhanced desorption of H$_2$CO in the gas phase caused by increased UV penetration and/or temperature inversion. The CH$_3$OH/H$_2$CO abundance ratio is $<1$, in agreement with disk chemistry models. The inner edge of the H$_2$CO ring coincides with the radius where the polarization of the dust continuum changes orientation, hinting at a tight link between the H$_2$CO chemistry and the dust properties in the outer disk and at the possible presence of substructures in the dust distribution.
Abstract: 1902.02737
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Full Text: [ PostScript, PDF]
Title:Coherent radio emission from the electron beam sudden appearance
(Submitted on 7 Feb 2019)
Abstract: We report on the measurement of coherent radio emission from the electron beam sudden appearance at the Telescope Array Electron Light Source facility. This emission was detected by four independent radio detector setups sensitive to frequencies ranging from 50 MHz up to 12.5 GHz. We show that this phenomenon can be understood as a special case of coherent transition radiation by comparing the observed results with simulations. The in-nature application of this signal is given by the emission of cosmic ray or neutrino induced particle cascades traversing different media such as air, rock and ice.
Abstract: 1902.02822
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Full Text: [ PostScript, PDF]
Title:CORSIKA 8 - Towards a modern framework for the simulation of extensive air showers
(Submitted on 7 Feb 2019)
Abstract: Current and future challenges in astroparticle physics require novel simulation tools to achieve higher precision and more flexibility. For three decades the FORTRAN version of CORSIKA served the community in an excellent way. However, the effort to maintain and further develop this complex package is getting increasingly difficult. To overcome existing limitations, and designed as a very open platform for all particle cascade simulations in astroparticle physics, we are developing CORSIKA 8 based on modern C++ and Python concepts. Here, we give a brief status report of the project.
Abstract: 1902.02856
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Full Text: [ PostScript, PDF]
Title:The ALMA Development Roadmap
(Submitted on 7 Feb 2019)
Abstract: The present document outlines a roadmap for future developments that will significantly expand ALMA's capabilities and enable it to produce even more exciting science in the coming decades. The proposed developments are motivated by the groundbreaking results achieved by ALMA during its first five years of operation. The roadmap described here is based on input on new scientific directions and technical feasibility of future developments from the ALMA Science Advisory Committee (ASAC), the community, and technical documents.
The Working Group recommends that the top development priority, based on scientific merit and technical feasibility, is to broaden the receiver IF bandwidth by at least a factor two, and to upgrade the associated electronics and correlator. These developments will advance a wide range of scientific studies by significantly reducing the time required for blind redshift surveys, chemical spectral scans, and deep continuum surveys. In order of scientific priority, receiver upgrades are recommended for intermediate (200-425 GHz), low (< 200 GHz), and high (> 425 GHz) frequencies.
The Working Group recommends that the receiver and throughput developments proceed as soon as fiscally and technically feasible. As a first step, a technical and scientific group should be formed to formalize the top-level requirements. A team of systems engineers should then be charged with flowing these requirements down to the subsystems to form a consistent new set of minimum requirements, which future development projects would have to meet. Given that upgrading the throughput will impact many ALMA subsystems, the Working Group recommends that a team within ALMA be charged with coordinating and monitoring these developments. (Abbreviated)
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