Abstracts of Interest
Selected by:
Roger Clay
Abstract: 1908.10326
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Title:Temporal and Spatial Evolutions of a Large Sunspot Group and Great Auroral Storms around the Carrington Event in 1859
(Submitted on 27 Aug 2019)
Abstract: The Carrington event is considered to be one of the most extreme space weather events in observational history within a series of magnetic storms caused by extreme interplanetary coronal mass ejections (ICMEs) from a large and complex active region (AR) emerged on the solar disk. In this article, we study the temporal and spatial evolutions of the source sunspot active region and visual aurorae, and compare this storm with other extreme space weather events on the basis of their spatial evolution. Sunspot drawings by Schwabe, Secchi, and Carrington describe the position and morphology of the source AR at that time. Visual auroral reports from the Russian Empire, Iberia, Ireland, Oceania, and Japan fill the spatial gap of auroral visibility and revise the time series of auroral visibility in mid to low magnetic latitudes (MLATs). The reconstructed time series is compared with magnetic measurements and shows the correspondence between low to mid latitude aurorae and the phase of magnetic storms. The spatial evolution of the auroral oval is compared with those of other extreme space weather events in 1872, 1909, 1921, and 1989 as well as their storm intensity, and contextualizes the Carrington event, as one of the most extreme space weather events, but likely not unique.
Abstract: 1908.11743
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Full Text: [ PostScript, PDF]
Title:Multiple Regions of Shock-accelerated Particles during a Solar Coronal Mass Ejection
(Submitted on 30 Aug 2019)
Abstract: The Sun is an active star that can launch large eruptions of magnetised plasma into the heliosphere, called coronal mass ejections (CMEs). These ejections can drive shocks that accelerate particles to high energies, often resulting in radio emission at low frequencies (<200 MHz). To date, the relationship between the expansion of CMEs, shocks and particle acceleration is not well understood, partly due to the lack of radio imaging at low frequencies during the onset of shock-producing CMEs. Here, we report multi-instrument radio, white-light and ultraviolet imaging of the second largest flare in Solar Cycle 24 (2008-present) and its associated fast CME (3038+/-288 km/s). We identify the location of a multitude of radio shock signatures, called herringbones, and find evidence for shock accelerated electron beams at multiple locations along the expanding CME. These observations support theories of non-uniform, rippled shock fronts driven by an expanding CME in the solar corona.
Abstract: 1908.10873
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Full Text: [ PostScript, PDF]
Title:An empirical infrared transit spectrum of Earth: opacity windows and biosignatures
(Submitted on 28 Aug 2019)
Abstract: The Atmospheric Chemistry Experiment's Fourier Transform Spectrometer on the SCISAT satellite has been measuring infrared transmission spectra of Earth during Solar occultations since 2004. We use these data to build an infrared transit spectrum of Earth. Regions of low atmospheric opacity, known as windows, are of particular interest, as they permit observations of the planet's lower atmosphere. Even in the absence of clouds or refraction, imperfect transmittance leads to a minimum effective thickness of $h_{min} \approx 4$ km in the 10--12$\mu$m opacity window at a spectral resolution of $R=10^3$. Nonetheless, at $R=10^5$, the maximum transmittance at the surface is around 70%. In principle, one can probe the troposphere of an Earth-like planet via high-dispersion transit spectroscopy in the mid-infrared; in practice aerosols and/or refraction likely make this impossible. We simulate the transit spectrum of an Earth-like planet in the TRAPPIST-1 system. We find that a long-term near-infrared campaign with JWST could readily detect CO$_2$ and H$_2$O, establishing the presence of an atmosphere. A mid-IR campaign or longer NIR campaign would be more challenging, but in principle could detect the biosignatures O$_3$ and CH$_4$.
Abstract: 1908.10376
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Full Text: [ PostScript, PDF]
Title:On the orientation of the crescent image of M87*
(Submitted on 27 Aug 2019)
Abstract: The first image of the black hole (BH) M87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to SWW position angles, while the observed direction of the large-scale jet is NWW. Images based on numerical simulations of BH accretion flows suggest that the projected BH spin axis should be oriented SSW. Alternatively, if the spin axis is matched with the jet direction, emission from the SEE sector of the photon ring cannot be accounted for. We explore highly simplified toy models for geometric distribution and kinematics of emitting regions in the Kerr metric, and perform ray tracing to calculate the corresponding images. We strictly assume that (1) the BH spin vector is fixed to the jet axis, (2) the emitting regions are symmetric with respect to the BH spin, (3) the emissivities are isotropic in the local rest frames. Emission from the crescent sector SSE -- SWW can be readily explained in terms of an equatorial ring with outer radius $r_{\rm max} \simeq 6M$ and either circular or plunging geodesic flows, regardless of the value of BH spin. In the case of plane-symmetric polar caps with plunging geodesic flows, the dominant image of the cap located behind the BH is sensitive to the angular momentum of the emitter. Within the constraints of our model, we have not found a viable explanation for the observed brightness of the SEE sector. We suggest that the most likely solution to this problem is that emissivities are not locally isotropic. Polarimetric results from the EHT will be essential to verify the theoretically expected alignment of the BH spin with the large-scale jet.
Abstract: 1908.10305
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Title:Seven years of Tunka-Rex operation
(Submitted on 27 Aug 2019)
Abstract: The Tunka Radio Extension (Tunka-Rex) is a digital antenna array located in the Tunka Valley in Siberia, which measures the radio emission of cosmic-ray air-showers with energies up to EeV. Tunka-Rex is externally triggered by the Tunka-133 air-Cherenkov timing array (during nights) and by the Tunka-Grande array of particle detectors (remaining time). These three arrays comprise the cosmic-ray extension of the Tunka Advanced Instrument for cosmic rays and Gamma Astronomy (TAIGA). The configuration and analysis pipeline of Tunka-Rex have significantly changed over its runtime. Density of the antennas was tripled and the pipeline has become more developed forming now sophisticated piece of reconstruction software. During its lifecycle Tunka-Rex has demonstrated that a cost-effective and full duty-cycle radio detector can reconstruct the energy and shower maximum with a precision comparable to optical detectors. Moreover, it was shown that cosmic-ray instruments, that use different detection techniques and are placed in different locations, can be cross-calibrated via their radio extensions. These results show the prospects of application of the radio technique for future large-scale experiments for cosmic-ray and neutrino detection. For the time being Tunka-Rex has ceased active measurements and focuses on the data analysis and publication of corresponding software and data in an open-access data center with online analysis features. In this report we present the current status of the array and give an overview of the results achieved during these years as well as discuss upcoming improvements in instrumentation and data analysis, which can be applied for the future radio arrays.
Abstract: 1908.10031
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Full Text: [ PostScript, PDF]
Title:Search for very-high-energy emission with HAWC from GW170817 event
(Submitted on 27 Aug 2019)
Abstract: The detection of the gravitational wave GW170817 defined a breakthrough in multi-messenger astronomy. For the first time, a gravitational wave transient detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo interferometer was associated with a faint electromagnetic gamma-ray counterpart reported by the Gamma-ray Burst Monitor (GBM) aboard on the Fermi satellite. GRB 170817A was followed up by an enormous observational campaign covering a large fraction of the electromagnetic spectrum. In this work, we use the data from High Altitude Water Cherenkov (HAWC) gamma-ray observatory to search for very-high-energy (VHE) TeV photons in coincidence with the X-ray emission from GRB 170817A. Since no counts were observed up to $\sim$120 days after the trigger time, we derive and report the corresponding upper limits in the energy range from 1 to 100 TeV. In addition, we extend the analysis to GRBs with similar features proposed by A. von Kienlin.
Abstract: 1908.09973
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Full Text: [ PostScript, PDF]
Title:Giant Elephant Trunks from Giant Molecular Clouds
(Submitted on 27 Aug 2019 (v1), last revised 28 Aug 2019 (this version, v2))
Abstract: We report the discovery of large elephant trunk (ET)-like objects, named giant elephant trunk (GET), of molecular gas in star forming complexes in the Scutum and Norma arms using the $^{12}$CO(J=1-0)-line survey data with the Nobeyama 45-m telescope. In comparison with the CO maps of ETs in M16 as derived from the same data, we discuss physical properties of the GETs. Their lengths are $\sim 20$ to 50 pc, an order of magnitude larger than ETs. GETs show a cometary structure coherently aligned parallel to the galactic plane, and emerge from bow-shaped concave surface of giant molecular clouds (GMC) facing the HII regions, and point down-stream of the gas flow in the spiral arms. The molecular masses of the head clumps are $\sim 10^3 -10^4 M_\odot$, about 3 to 4 times the virial masses, indicating that the clumps are gravitationally stable. Jeans masses calculated for the derived density and assumed kinetic temperature are commonly sub-solar. We suggest that the GET heads are possible birth sites of stellar clusters, similarly to ETs' globules, but at much greater scale. We discuss the origin of the GETs by Rayleigh-Taylor instability due to deceleration of GMCs by low density gas stagnated in the galactic shock waves as well as by pressure of the HII regions.
Abstract: 1908.09945
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Full Text: [ PostScript, PDF]
Title:The sub-TeV transient Gamma-Ray sky: challenges and opportunities
(Submitted on 26 Aug 2019)
Abstract: The detection of gravitational waves and neutrinos from astrophysical sources with gamma-ray counterparts officially started the era of Multi-Messenger Astronomy. Their transient and extreme nature implies that monitoring the VHE sky is fundamental to investigate the non-electromagnetic signals. However, the limited effective area of space-borne instruments prevents observations above a few hundred GeV, while the small field of view and low duty cycle of IACTs make them unsuited for extensive monitoring activities and prompt response to transients. Extensive Air Shower arrays (EAS) can provide a large field of view, a wide effective area and a very high duty cycle. Their main difficulty is the distinction between gamma-ray and cosmic-ray initiated air showers, especially below the TeV range. Here we present some case studies stressing the importance that a new EAS array in the Southern Hemisphere will be able to survey the sky from below 100 GeV up to several TeV. In the energy domain between 100 and 400 GeV we expect the strongest electromagnetic signatures of the acceleration of ultra-relativistic particles in sources like SNRs, blazar jets and gamma-ray bursts, as recently proved by IACT observations. This spectral window is also crucial to understand the Universe opacity to high energy radiation, thus providing constraints on the cosmological parameters. We will discuss the implications of VHE radiation on the mechanisms at work and we will focus on the advantages resulting from the ability to monitor the energy window lying between the domain of space-borne detectors and ground-based facilities.
Abstract: 1908.11849
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Full Text: [ PostScript, PDF]
Title:Describing correlated observations of neutrino and gamma ray flares from the blazar TXS 0506+056 with proton blazar model
(Submitted on 30 Aug 2019)
Abstract: Recent detection of the neutrino event, IceCube-170922A by IceCube observatory from the Blazar TXS 0506+056 in the state of enhanced gamma ray emission indicates for acceleration of cosmic rays in the blazar jet. The non-detection of the broadline emission in the optical spectrum of TXS 0506+056 and other BL Lac objects suggests that external photons emissions are weak and hence photo-meson (p-gamma) interaction may not be a favored mechanism for high energy neutrino production. The lack of broadline signatures also creates doubt about the presence of a high density cloud in the vicinity of the super-massive black hole (SMBH) of TXS 0506+056 and consequently raised question on hadronuclear (pp) interaction interpretation like relativistic jet meets with high density cloud. Here we demonstrate that non-relativistic protons in the proton blazar model, those come into existence under charge neutrality condition of the blazar jet, offer sufficient target matter for pp-interaction with shock accelerated protons and consequently the model can describe consistently the observed high energy gamma rays and neutrino signal from the blazar TXS 0506+056.
Abstract: 1908.10812
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Full Text: [ PostScript, PDF]
Title:Performance study of a Time of Flight Method used for cosmic ray detection
(Submitted on 28 Aug 2019)
Abstract: Time of Flight methods have been rapidly developed and used in many experiments recently for determination of particle direction, identification of particles and energy resolutions. This paper describes a method of time-mark determination on the reconstruction algorithm, based on the sampled signal, used for time-of-flight measurements. This method was developed for distinguishing the signals by fitting to pulse shape which were received from scintillator detector with a silicon photomultiplier readout have been developed for a cosmic ray counter telescope. The method was verified using experimental data taken in the location $40^{o}54'52"N$ and $38^{o}19'26"E$ with the elevation of 30 m above the sea level. The data samples were acquired by the counters which have a scintillator with dimensions of 20$\times$20$\times1.4~cm^3$, optically coupled from one side to silicon photomultiplier, then the signals readout by fast sampling digitizer board Domino Ring Sampler Board version 4. The method can reconstruct each pulse even for multiple events without losing the count within the small time window. Using this method 4.969 ns time-of-flight value were established and the rise times for scintillation counters, named Tile 1 and Tile 2, were measured about $6.27 \pm 0.16~ns$ and $4.979\pm0.165~ns$, respectively.
Abstract: 1908.09824
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Full Text: [ PostScript, PDF]
Title:Cosmic Rays and Magnetic Fields in the Core and Halo of the Starburst M82: Implications for Galactic Wind Physics
(Submitted on 26 Aug 2019)
Abstract: Cosmic rays (CRs) and magnetic fields may be dynamically important in driving large-scale galactic outflows from rapidly star-forming galaxies. We construct two-dimensional axisymmetric models of the local starburst and super-wind galaxy M82 using the CR propagation code GALPROP. Using prescribed gas density and magnetic field distributions, wind profiles, CR injection rates, and stellar radiation fields, we simultaneously fit both the integrated gamma-ray emission and the spatially-resolved multi-frequency radio emission extended along M82's minor axis. We explore the resulting constraints on the gas density, magnetic field strength, CR energy density, and the assumed CR advection profile. In accord with earlier one-zone studies, we generically find low central CR pressures, strong secondary electron/positron production, and an important role for relativistic bremsstrahlung losses in shaping the synchrotron spectrum. We find that the relatively low central CR density produces CR pressure gradients that are weak compared to gravity, strongly limiting the role of CRs in driving M82's fast and mass-loaded galactic outflow. Our models require strong magnetic fields and advection speeds of order ~1000 km/s on kpc scales along the minor axis in order to reproduce the extended radio emission. Degeneracies between the controlling physical parameters of the model and caveats to these findings are discussed.
Abstract: 1908.10865
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Full Text: [ PostScript, PDF]
Title:Feasibility of an Atmospheric Cherenkov Telescope Array to Veto Air Showers for Neutrino Astronomy
(Submitted on 28 Aug 2019)
Abstract: The IceCube Neutrino Observatory has revealed the existence of sources of high-energy astrophysical neutrinos. However, identification of the sources is challenging because astrophysical neutrinos are difficult to separate from the background of atmospheric neutrinos produced in cosmic-ray-induced particle cascades in the atmosphere. The efficient detection of air showers in coincidence with detected neutrinos can greatly reduce those backgrounds and increase the sensitivity of neutrino telescopes. Imaging Air Cherenkov Telescopes (IACTs) are the most sensitive devices for detecting gamma-ray-induced (and cosmic-ray-induced) air showers in the 50 GeV to 50 TeV range, and can therefore be used as background-identifiers for neutrino observatories. This paper describes the feasibility of an array of small scale, wide field-of-view, cost-effective IACTs as an air shower veto for neutrino astronomy. A surface array of 250 to 750 telescopes would significantly improve the performance of a cubic kilometer-scale detector like IceCube, at a cost of a few percent of the original investment. The number of telescopes in the array can be optimized based on astronomical and geometrical considerations.
Abstract: 1908.10874
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Full Text: [ PostScript, PDF]
Title:Characterizing the Nature of the Unresolved Point Sources in the Galactic Center
(Submitted on 28 Aug 2019)
Abstract: The Galactic Center Excess (GCE) of GeV gamma rays can be explained as a signal of annihilating dark matter or of emission from unresolved astrophysical sources, such as millisecond pulsars. Evidence for the latter is provided by a statistical procedure---referred to as Non-Poissonian Template Fitting (NPTF)---that distinguishes the smooth distribution of photons expected for dark matter annihilation from a "clumpy" photon distribution expected for point sources. In this paper, we perform an extensive study of the NPTF on simulated data, exploring its ability to recover the flux and luminosity function of unresolved sources at the Galactic Center. When astrophysical background emission is perfectly modeled, we find that the NPTF successfully distinguishes between the dark matter and point source hypotheses when either component makes up the entirety of the GCE. When the GCE is a mixture of dark matter and point sources, the NPTF may fail to reconstruct the correct contribution of each component. We further study the impact of mismodeling the Galactic diffuse backgrounds, finding that while a dark matter signal could be attributed to point sources in some outlying cases for the scenarios we consider, the significance of a true point source signal remains robust. Our work enables us to comment on a recent study by Leane and Slatyer (2019) that questions prior NPTF conclusions because the method does not recover an artificial dark matter signal injected on actual Fermi data. We demonstrate that the failure of the NPTF to extract an artificial dark matter signal can be natural when point sources are present in the data---with the effect further exacerbated by the presence of diffuse mismodeling---and does not on its own invalidate the conclusions of the NPTF analysis in the Inner Galaxy.
Abstract: 1908.10972
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Full Text: [ PostScript, PDF]
Title:Mysterious Coherence in Several-Megaparsec Scales Between Galaxy Rotation and Neighbor Motion
(Submitted on 28 Aug 2019)
Abstract: In our recent report, observational evidence supports that the rotational direction of a galaxy tends to be coherent with the average motion of its nearby neighbors within 1 Mpc. We extend the investigation to neighbors at farther distances, in order to examine if such dynamical coherence is found even in large scales. The Calar Alto Legacy Integral Field Area (CALIFA) survey data and the NASA-Sloan Atlas (NSA) catalog are used. From the composite map of velocity distribution of 'neighbor' galaxies within 15 Mpc from the CALIFA galaxies, the composite radial profiles of the luminosity-weighted mean velocity of neighbors are derived. These profiles show unexpectedly strong evidence of the dynamical coherence between the rotation of the CALIFA galaxies and the average line-of-sight motion of their neighbors within several Mpc distances. Such a signal is particularly strong when the neighbors are limited to red ones: the luminosity-weighted mean velocity at 1 < D <= 6 Mpc is as large as 30.6+/-10.9 km/s (2.8-sigma significance to random spin-axis uncertainty) for central rotation (R <= Re). In the comparison of several subsamples, the dynamical coherence tends to be marginally stronger for the diffuse or kinematically-well-aligned CALIFA galaxies. For this mysterious coherence in large scales, we cautiously suggest a scenario that it results from a possible relationship between the long-term motion of a large-scale structure and the rotations of galaxies in it.
Abstract: 1908.10885
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Full Text: [ PostScript, PDF]
Title:Time varying polarized gamma-rays from GRB 160821A: evidence for ordered magnetic fields
(Submitted on 28 Aug 2019)
Abstract: GRB 160821A is the third most energetic gamma ray burst observed by the {\it Fermi} gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27} \%$; $5.3 \sigma$ detection), and (ii) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at $3.5 \sigma$ and $3.1 \sigma$ detections respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of $1/\Gamma$, where $\Gamma $ is the Lorentz factor of the outflow.
Abstract: 1908.10431
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Optical polarimetry: Methods, Instruments and Calibration Techniques
(Submitted on 27 Aug 2019)
Abstract: In this chapter we present a brief summary of methods, instruments and calibration techniques used in modern astronomical polarimetry in the optical wavelengths. We describe the properties of various polarization devices and detectors used for optical broadband, imaging and spectropolarimetry, and discuss their advantages and disadvantages. The necessity of a proper calibration of the raw polarization data is emphasized and methods of the determination and subtraction of instrumental polarization are considered. We also present a few examples of high-precision measurements of optical polarization of black hole X-ray binaries and massive binary stars made with our DiPol-2 polarimeter, which allowed us to constrain the sources of optical emission in black hole X-ray binaries and measure orbital parameters of massive stellar binaries.
Abstract: 1908.11159
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Full Text: [ PostScript, PDF]
Title:Summary of the 36th ICRC Gamma-ray Indirect sessions
(Submitted on 29 Aug 2019)
Abstract: At the 36 th ICRC during 11 parallel Gamma-Ray Indirect sessions in total about 70 talks were presented. A few of the plenary highlight talks as well concerned mostly indirect observations of gamma rays. In addition about 140 posters on this topic have been shown in poster sessions. This rapport tries to summarize the results presented in those contributions.
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