Abstracts of Interest
Selected by:
Violet Harvey
Abstract: 1902.07813
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Title:Revealing a new region of gamma-ray emission in the vicinity of HESS J1825-137
(Submitted on 20 Feb 2019)
Abstract: HESS J1825-137 is a bright very-high-energy (VHE) gamma-ray source that has been firmly established as a pulsar wind nebula (PWN), and one of the most extended gamma-ray objects within this category. The progenitor supernova remnant (SNR) for this PWN has not been firmly established. We carried out an analysis of gamma-ray observations in the region of HESS J1825-137 with the Fermi-LAT which reveal emission in the direction away from the Galactic plane. The region lies beyond the PWN and reaches a distance from the pulsar compatible with the supposed location of the SNR shock front. The spectrum of the gamma-rays is hard with a photon index of $\sim 1.9$ in the 10-250 GeV range. Several scenarios for the origin of the emission are discussed, including the SNR as a source of high-energy particles and the "leakage"' of leptons from the PWN.
Abstract: 1902.08124
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Title:Report on Tests and Measurements of Hadronic Interaction Properties with Air Showers
(Submitted on 21 Feb 2019)
Abstract: We present a summary of recent tests and measurements of hadronic interaction properties with air showers. This report has a special focus on muon density measurements. Several experiments reported deviations between simulated and recorded muon densities in extensive air showers, while others reported no discrepancies. We combine data from eight leading air shower experiments to cover shower energies from PeV to tens of EeV. Data are combined using the z-scale, a unified reference scale based on simulated air showers. Energy-scales of experiments are cross-calibrated. Above 10 PeV, we find a muon deficit in simulated air showers for each of the six considered hadronic interaction models. The deficit is increasing with shower energy. For the models EPOS-LHC and QGSJet-II.04, the slope is found significant at 8 sigma.
Abstract: 1902.08429
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Title:Science Case for a Wide Field-of-View Very-High-Energy Gamma-Ray Observatory in the Southern Hemisphere
(Submitted on 22 Feb 2019)
Abstract: We outline the science motivation for SGSO, the Southern Gamma-Ray Survey Observatory. SGSO will be a next-generation wide field-of-view gamma-ray survey instrument, sensitive to gamma-rays in the energy range from 100 GeV to hundreds of TeV. Its science topics include unveiling galactic and extragalactic particle accelerators, monitoring the transient sky at very high energies, probing particle physics beyond the Standard Model, and the characterization of the cosmic ray flux. SGSO will consist of an air shower detector array, located in South America. Due to its location and large field of view, SGSO will be complementary to other current and planned gamma-ray observatories such as HAWC, LHAASO, and CTA.
Abstract: 1902.06173
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Title:Understanding the cosmic ray positron flux
(Submitted on 16 Feb 2019)
Abstract: Recent precision measurements of the flux of cosmic ray positrons by the Alpha Magnetic Spectrometer show that the spectrum has a marked softening feature for energies close to one TeV. A possible interpretation of this result is that the observed feature measures the maximum energy of a new hard source of positrons perhaps associated to dark matter self--annihilation or decay, or to positron accelerators. A gradual hardening of the positron flux centered at $E \simeq 25$~GeV can also be understood as the signature of the transition where the new source overtakes the conventional component due to secondary production. This interpretation is simple and attractive, but it is not unique. The alternative possibility, that the positron flux is entirely of secondary origin, remains viable. In such a scenario the spectral softening observed by AMS for positrons is generated by energy loss effects, and a feature of similar, but not identical structure should be also visible in the $e^-$ spectrum. Spectral features similar to both the hardening and softening of the positron flux are in fact observed for electrons and call for a consistent explanation. Precision measurements of the $e^+$ and $e^-$ spectra in the TeV and multi--TeV energy range are crucial to clarify the problem.
Abstract: 1902.06271
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Title:The Efficiency of Coherent Radiation from Relativistic Shocks
(Submitted on 17 Feb 2019)
Abstract: We discuss a mechanism for intense electromagnetic wave emission at an astrophysical relativistic shock in a magnetized collisionless plasma. At the magnetized shock, the particle reflection by a compressed magnetic field of the shock produces a ring-like distribution in momentum, which gives rise to plasma instabilities. Intense and coherent high-frequency electromagnetic waves will be emitted if the synchrotron maser instability (SMI) is excited, whereas non-propagating magnetic fluctuations will be generated when the Weibel instability (WI) is the dominant mode. The problem is of great astrophysical interest because if intense radiation is emitted, the interaction with the upstream medium induces a large-amplitude electrostatic field (or Wakefield), which may play a role for the acceleration of ultra-high-energy cosmic rays. We review our recent effort to measure the efficiency of the electromagnetic wave emission using fully self-consistent, two-dimensional (2D) particle-in-cell (PIC) simulations for pair plasmas. We found that the emission efficiency in 2D was systematically lower than one dimensional (1D) PIC simulation results. However, the power remains finite even when the WI is active to generate large-amplitude magnetic fluctuations. Astrophysical implications of the present results are briefly discussed.
Abstract: 1902.06064
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Title:Spectral and morphological study of the gamma radiation of the middle-aged supernova remnant HB 21
(Submitted on 16 Feb 2019)
Abstract: We investigate the nature of the accelerated particles responsible for the production of the gamma-ray emission observed from the middle-aged supernova remnant (SNR) HB 21. The analysis of more than nine years of Fermi LAT data leads to the observation of an extended emission positionally in agreement with the SNR HB 21. The bulk of this gamma-ray emission is detected from the remnant; photons up to $\sim$10\,GeV show clear evidence of curvature at the lower energies. The remnant is characterized by an extension of $0^{\circ}.83$, that is, 30\% smaller than claimed in previous studies. The increased statistics allows us also to resolve a point-like source at the edge of the remnant, in proximity to a molecular cloud of the Cyg OB7 complex. In the southern part of the remnant, a hint of an additional gamma-ray excess in correspondence to shocked molecular clouds is observed. The spectral energy distribution of the SNR shows evidence of a break around 400 MeV, which can be properly fitted within both the hadronic and leptonic scenario. The pion-decay mechanism reproduces well the gamma rays, postulating a proton spectrum with a slope $\sim 2.5$ and with a steepening around tens of GeV, which could be explained by the energy-dependent escape of particles from the remnant. In the leptonic scenario the electron spectrum within the SNR matches closely the locally measured spectrum. This remarkable and novel result shows that SNR HB 21 could be a direct contributor to the population of Galactic electrons. In the leptonic scenario, we find that the local electron spectrum with a break around 2 GeV, closely evokes the best-fitting parental spectrum within this SNR. If such a scenario is confirmed, this would indicate that the SNR might be a source of Galactic background electrons.
Abstract: 1902.08442
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Full Text: [ PostScript, PDF]
Title:Narrow-band search for gravitational waves from known pulsars using the second LIGO observing run
(Submitted on 22 Feb 2019 (v1), last revised 25 Feb 2019 (this version, v2))
Abstract: Isolated spinning neutron stars, asymmetric with respect to their rotation axis, are expected to be sources of continuous gravitational waves. The most sensitive searches for these sources are based on accurate matched filtering techniques, that assume the continuous wave to be phase-locked with the pulsar beamed emission. While matched filtering maximizes the search sensitivity, a significant signal-to-noise ratio loss will happen in case of a mismatch between the assumed and the true signal phase evolution. Narrow-band algorithms allow for a small mismatch in the frequency and spin-down values of the pulsar while integrating coherently the entire data set. In this paper we describe a narrow-band search using LIGO O2 data for the continuous wave emission of 33 pulsars. No evidence for a continuous wave signal has been found and upper-limits on the gravitational wave amplitude, over the analyzed frequency and spin-down volume, have been computed for each of the targets. In this search we have surpassed the spin-down limit for some of the pulsars already present in the O1 LIGO narrow-band search, such as J1400\textminus6325 J1813\textminus1246, J1833\textminus1034, J1952+3252, and for new targets such as J0940\textminus5428 and J1747\textminus2809. For J1400\textminus6325, J1833\textminus1034 and J1747\textminus2809 this is the first time the spin-down limit is surpassed.
Abstract: 1902.08507
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Title:Searches for Gravitational Waves from Known Pulsars at Two Harmonics in 2015-2017 LIGO Data
(Submitted on 22 Feb 2019 (v1), last revised 25 Feb 2019 (this version, v2))
Abstract: We present a search for gravitational waves from 222 pulsars with rotation frequencies $\gtrsim 10$ Hz. We use advanced LIGO data from its first and second observing runs spanning 2015-2017, which provides the highest sensitivity gravitational-wave data so far obtained. In this search we target emission from both the $l = m = 2$ mass quadrupole mode, with a frequency at twice that of the pulsar's rotation, and from the $l = 2$, $m = 1$ mode, with a frequency at the pulsar rotation frequency. The search finds no evidence for gravitational-wave emission from any pulsar at either frequency. For the $l = m = 2$ mode search, we provide updated upper limits on the gravitational-wave amplitude, mass quadrupole moment, and fiducial ellipticity for 167 pulsars, and the first such limits for a further 55. For 20 young pulsars these results give limits that surpass those inferred from the pulsars' spin-down. For the Crab and Vela pulsars our results constrain gravitational-wave emission to account for less than 0.017% and 0.18% of the spin-down luminosity, respectively. For the recycled millisecond pulsar J0711-6830 our limits are only a factor of 1.3 above the spin-down limit, assuming the canonical value of $10^{38}$ kg m$^2$ for the star's moment of inertia, and imply a gravitational-wave derived upper limit on the star's ellipticity of $1.2\!\times\!10^{-8}$. We also place new limits on the emission amplitude at the rotation frequency of the pulsars.
Abstract: 1902.06009
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Title:A new fast radio burst in the datasets containing the Lorimer burst
(Submitted on 15 Feb 2019)
Abstract: We report the discovery of a new fast radio burst (FRB), FRB~010312, in archival data from a 1.4\,GHz survey of the Magellanic Clouds using the multibeam receiver on the Parkes 64\,m-diameter radio telescope. These data sets include the Lorimer burst (FRB~010724), which it pre-dates and which we also re-detect. The new burst has a much higher dispersion measure of 1187\,cm$^{-3}$pc. The burst is one of the broadest found to date, the second earliest FRB known, and the ninth FRB discovered with a dispersion measure larger than 1000\,cm$^{-3}\,$pc. Our discovery indicates that there are likely to be more burst events still to be found in the existing Parkes data archive.
Abstract: 1902.06586
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Title:Probing the space time around a black hole with X-ray variability
(Submitted on 18 Feb 2019)
Abstract: In the past decades, the phenomenology of fast time variations of high-energy flux from black-hole binaries has increased, thanks to the availability of more and more sophisticated space observatories, and a complex picture has emerged. Recently, models have been developed to interpret the observed signals in terms of fundamental frequencies connected to General Relativity, which has opened a promising way to measure the prediction of GR in the strong-field regime. I review the current standpoint both from the observational and theoretical side and show that these systems are the most promising laboratories for testing GR and the observations available today suggest that the next observational facilities can lead to a breakthrough in the field.
Abstract: 1902.06944
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Title:Background for a gamma-ray satellite on a low-Earth orbit
(Submitted on 19 Feb 2019)
Abstract: The different background components in a low-Earth orbit have been modeled in the 10 keV to 100 GeV energy range. The model is based on data from previous instruments and it considers both primary and secondary particles, charged particles, neutrons and photons. The necessary corrections to consider the geomagnetic cutoff are applied to calculate the flux at different inclinations and altitudes for a mean solar activity. Activation simulations from such a background have been carried out using the model of a possible future gamma-ray mission (e-ASTROGAM). The event rates and spectra from these simulations were then compared to those from the isotopes created by the particles present in the South Atlantic Anomaly (SAA). The primary protons are found to be the main contributor of the activation, while the contributions of the neutrons, and that of the secondary protons can be considered negligible. The long-term activation from the passage through the SAA becomes the main source of background at high inclination (i$\gtrsim10^\circ$). The used models have been collected in a Python class openly available on github.
Abstract: 1902.08515
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Full Text: [ PostScript, PDF]
Title:Longwave radiative analysis of cloudy scattering atmospheres using a Net Exchange Formulation
(Submitted on 22 Feb 2019)
Abstract: The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer equation. It was proposed in 1967 by Green [1] for atmospheric sciences and by Hottel [2] for engineering sciences. Until now, the NEF has been used only in a very few cases for atmospheric studies. Recently we have developped a longwave radiative code based on this formulation for a GCM of the Mars planet. Here, we will present results for the Earth atmosphere, obtained with a Monte Carlo Method based on the NEF. In this method, fluxes are not addressed any more. The basic variables are the net exchange rates (NER) between each pair of atmospheric layer (i, j), i.e. the radiative power emitted by i and absorbed by j minus the radiative power emitted by j and absorbed by i. The graphical representation of the NER matrix highlights the radiative exchanges that dominate the radiative budget of the atmosphere and allows one to have a very good insight of the radiative exchanges. Results will be presented for clear sky atmospheres with Mid-Latitude Summer and Sub-Arctic Winter temperature profiles, and for the same atmospheres with three different types of clouds. The effect of scattering on longwave radiative exchanges will also be analysed.
Abstract: 1902.07977
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Title:Towards the forecast of atmospheric parameters and optical turbulence above an astronomical site on 24h time scale
(Submitted on 21 Feb 2019)
Abstract: Forecast of the atmospheric parameters and optical turbulence applied to the ground-based astronomy is very crucial mainly for the queue scheduling. So far, most efforts have been addressed by our group in developing algorithms for the optical turbulence (CN2) and annexed integrated astroclimatic parameters and quantifying the performances of the Astro-Meso-Nh package in reconstructing such parameters. Besides, intensive analyses on the Meso-Nh performances= in reconstructing atmospheric parameters relevant for the ground-based astronomy has been carried out. Our studies referred always to the night time regime. To extend the applications of our studies to the day time regime, we present, in this contribution, preliminary results obtained by comparing model outputs and measurements of classical atmospheric parameter relevant for the ground-based astronomy in night and day time. We chose as a test case, the Roque de los Muchachos Observatory (Canary Islands), that offers a very extended set of measurements provided by different sensors belonging to different telescopes on the same summit/Observatory. The convective regime close to the ground typical of the day time is pretty different from the stable regime characterising the night time. This study aims therefore to enlarge the domain of validity of the Astro-Meso-Nh code to new turbulence regimes and it permits to cover the total 24 hours of a day. Such an approach will permit not only an application to solar telescopes (e.g. EST) but also applications to a much extended set of scientific fields, not only in astronomical context such as satellite communications.
Abstract: 1902.07575
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Title:Using Nagios to monitor the Telescope Manager (TM) of the Square Kilometre Array (SKA)
(Submitted on 20 Feb 2019)
Abstract: SKA (Square Kilometer Array), currently under design, will be a huge radio-astronomical facility, whose management will be performed by a suite of software applications called Telescope Manager (SKA TM) via the TANGO framework. In order to ensure the proper and uninterrupted operation of TM, a local monitoring and control system (TM.LMC) is being developed, with the goal to perform monitoring, lifecycle control and fault management of TM. For the monitoring activity, central in TM.LMC, Nagios (automated by the lifecycle management tool Chef) has been proposed as main toolkit to check resources, services and status of every TM application both at generic and performance level: for this latter purpose, a custom agent has been developed. This led to an integrated fault management module, based on Nagios-Chef integration, which can efficiently handle any abnormal situation
Abstract: 1902.07092
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Full Text: [ PostScript, PDF]
Title:pizza: An open-source pseudo-spectral code for spherical quasi-geostrophic convection
(Submitted on 19 Feb 2019 (v1), last revised 25 Feb 2019 (this version, v2))
Abstract: We present a new pseudo-spectral open-source code nicknamed pizza. It is dedicated to the study of rapidly-rotating Boussinesq convection under the 2-D spherical quasi-geostrophic approximation, a physical hypothesis that is appropriate to model the turbulent convection that develops in planetary interiors. The code uses a Fourier decomposition in the azimuthal direction and supports both a Chebyshev collocation method and a sparse Chebyshev integration formulation in the cylindrically-radial direction. It supports several temporal discretisation schemes encompassing multi-step time steppers as well as diagonally-implicit Runge-Kutta schemes. The code has been tested and validated by comparing weakly-nonlinear convection with the eigenmodes from a linear solver. The comparison of the two radial discretisation schemes has revealed the superiority of the Chebyshev integration method over the classical collocation approach both in terms of memory requirements and operation counts. The good parallelisation efficiency enables the computation of large problem sizes with $\mathcal{O}(10^4\times 10^4)$ grid points using several thousands of ranks. This allows the computation of numerical models in the turbulent regime of quasi-geostrophic convection characterised by large Reynolds $Re$ and yet small Rossby numbers $Ro$. A preliminary result obtained for a strongly supercritical numerical model with a small Ekman number of $10^{-9}$ and a Prandtl number of unity yields $Re\simeq 10^5$ and $Ro \simeq 10^{-4}$. pizza is hence an efficient tool to study spherical quasi-geostrophic convection in a parameter regime inaccessible to current global 3-D spherical shell models.
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