Abstracts of Interest
Selected by:
Jose Bellido
Abstract: 1908.03507
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Title:CHIME/FRB Detection of Eight New Repeating Fast Radio Burst Sources
(Submitted on 9 Aug 2019)
Abstract: We report on the discovery of eight repeating Fast Radio Burst (FRB) sources found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. These sources span a dispersion measure (DM) range of 103.5 to 1281~pc~cm$^{-3}$. For all sources, we provide sky coordinates precise to several arcminutes. One repeater, FRB 180916.J0158+65, has a very low DM excess over the inferred Galactic maximum, and a Faraday rotation measure $-114.6 \pm 0.6$~rad~m$^{-2}$, much lower than that of the only other repeater for which this quantity has been measured, FRB 121102. Another of our sources, FRB 181030.J1054+73, has the lowest yet known DM for a repeater, 103.5~pc~cm$^{-3}$, and will be an interesting target for multi-wavelength follow-up once localized. The DM distribution of these repeaters is statistically indistinguishable from that of the first 12 reported CHIME/FRB sources that have thus far not repeated. On the other hand we find evidence that repeater bursts are on average wider than those of CHIME/FRB bursts that have not repeated, suggesting different emission mechanisms. Many, but not all of our repeater events show complex morphologies with downward frequency drifts reminiscent of the first two discovered repeating FRBs. These repeating FRBs will enable interferometric localizations and subsequent host galaxy identifications that will shed new light on the origin and nature of FRBs.
Abstract: 1908.03393
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Title:Proving the outstanding capabilities of IACTs in high time resolution optical astronomy
(Submitted on 9 Aug 2019)
Abstract: Imaging Atmospheric Cherenkov Telescopes (IACTs) are very-large telescopes designed to detect the nanosecond-timescale flashes produced within extended air showers. Because IACTs are sensitive to the Cherenkov light (UV/blue) and use photodetectors with extremely fast time responses, they are also able to perform simultaneous optical observations. The large reflecting areas of these telescopes (larger than 100 m$^2$) makes them well-suited to studying fast optical transient phenomena with timescales ranging from seconds to milliseconds to nanoseconds, and the unique optical design provides a wide field of view monitoring capability with a modest point spread function. VERITAS, with its recently upgraded PMT current monitoring instrumentation, was able to provide the first detection of asteroid occultations with an IACT, resulting in the highest angular resolution measurements for stellar diameters ever taken in the visible band range. Here we explore the feasibility of using this technique to significantly expand the number of stars with directly measured stellar radii, usable for population studies to test stellar evolution modelling or transiting exoplanet radius measurements. A single observatory with a high-speed visible-band photometer with a sensitivity reaching the 13$^{th}$ magnitude could increase the number of directly measured K stars diameters by 50%.
Abstract: 1908.03277
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Title:A Great Successor to the Hubble Space Telescope
(Submitted on 8 Aug 2019)
Abstract: The Hubble Space Telescope (HST) has been the most impactful science-driven mission ever flown by NASA. However, when HST reaches the end of its life, there will be a void due to the loss of some of the science capabilities afforded by HST to astronomers world-wide. The previous 2010 Decadal Survey (DS) noted this void, arguing for the need for a successor to HST with UV capabilities in three separate places in the main report (pp. 190, 203, and 220). The large strategic missions that will follow HST, namely JWST and WFIRST, will continue to spark the interest of the public in space-based astronomy. In order to ensure continued US preeminence in the arena of large space-based astrophysics missions, and a seamless transition after WFIRST, a future flagship mission must be waiting in the wings. Anticipating this need, NASA initiated four large strategic mission concept studies (HabEx, LUVOIR, Lynx, and Origins), which have mature designs, including detailed technology assessments and development plans. Two of these concepts, HabEx and LUVOIR, are responsive to the recommendations of the previous DS regarding a UV-capable mission. Both are more powerful successors to HST, with UV-to-optical capabilities that range from significant enhancements to orders-of-magnitude improvement. At the same time, technological and scientific advances over the past decade only now make it feasible to marry such a mission with one that can search for life outside the solar system. Acknowledging that the constraints that the Astro2020 DS must consider may be difficult to anticipate, the HabEx and LUVOIR studies present eleven different variants, each of which enable groundbreaking science, including the direct imaging and characterization of exoplanets. The HabEx and LUVOIR mission studies offer a full suite of options to the Astro2020 DS, with corresponding flexibility in budgeting and phasing.
Abstract: 1908.03163
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Title:Serendipitous Discovery of PSR J1431-6328 as a Highly-Polarized Point Source with the Australian SKA Pathfinder
(Submitted on 8 Aug 2019)
Abstract: We identified a highly-polarized, steep-spectrum radio source in a deep image with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope at 888 MHz. After considering and rejecting a stellar origin for this source, we discovered a new millisecond pulsar (MSP) using observations from the Parkes radio telescope. This pulsar has period 2.77 ms and dispersion measure 228.27 pc/cm**3. Although this pulsar does not yet appear to be particularly remarkable, the short spin period, wide profile and high dispersion measure do make it relatively hard to discover through traditional blind periodicity searches. Over the course of several weeks we see changes in the barycentric period of this pulsar that are consistent with orbital motion in a binary system, but the properties of any binary need to be confirmed by further observations. While even a deep ASKAP survey may not identify large numbers of new MSPs compared to the existing population, it would be competitive with existing all-sky surveys and could discover interesting new MSPs at high Galactic latitude without the need for computationally-expensive all-sky periodicity searches.
Abstract: 1908.03144
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Title:The high energy universe at ultra-high resolution: the power and promise of X-ray interferometry
(Submitted on 8 Aug 2019)
Abstract: We propose the development of X-ray interferometry (XRI), to reveal the universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 $\mu$as resolution at 10 Å(1.2 keV) and 20 $\mu$as at 2 Å(6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH `shadow'; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within ~100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft `constellation' interferometer would resolve well below 1 $\mu$as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole.
Abstract: 1908.03111
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Title:Characterizing the VHE emission of LS I +61 303 using VERITAS observations
(Submitted on 8 Aug 2019)
Abstract: The TeV gamma-ray binary LS I +61 303, approximately 2 kpc from Earth, consists of a low mass compact object in an eccentric orbit around a massive Be star. LS I +61 303 exhibits modulated VHE gamma-ray emission around its 26.5 days orbit, with strongest TeV emission during its apastron passage (orbital phases {\phi}=0.55-0.65). Multiple flaring episodes with nightly flux variability at TeV energies have been observed since its detection in 2006. GeV, X-ray, and radio emission have been detected along the entire orbit, enabling detailed study of the orbital modulation pattern and its super-orbital period. Previously reported TeV baseline emission and spectral variations may indicate a neutron star flip-flop scenario, in which the binary system switches between accretor and propeller phases at different phases of the orbit.
Since September 2007, VERITAS has observed LS I +61 303 over three additional seasons, accruing 220+ hours of data during different parts of its orbit. In this work, we present a summary of recent and long-term VERITAS observations of LS I +61 303. This analysis includes a discussion of the observed variation of TeV emission during different phases of the orbit, and during different superorbital phases.
Abstract: 1908.02904
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Title:Observing ultra-high energy cosmic rays with prototypes of the Fluorescence detector Array of Single-pixel Telescopes (FAST) in both hemispheres
(Submitted on 8 Aug 2019)
Abstract: The origin and nature of ultra-high energy cosmic rays (UHECRs) are hot topics in the astroparticle physics community. The Fluorescence detector Array of Single-pixel Telescopes (FAST) is a design for a next-generation ground-based UHECR observatory, addressing the requirements for a large-area, low-cost detector suitable for measuring the properties of the highest energy cosmic rays with an unprecedented aperture. We have developed a full-scale prototype consisting of four 200 mm photomultiplier tubes at the focus of a segmented mirror of 1.6 m in diameter. Over the last three years, we have installed three prototypes at the Telescope Array Experiment in Utah, USA. These telescopes have been steadily taking data since installation. We report on preliminary results of the full-scale FAST prototypes, including measurements of UHECRs, and distant ultra-violet lasers used to study the atmospheric transparency. Furthermore, we discuss the installation of an additional identical FAST prototype at the Pierre Auger Observatory in Argentina. Possible benefits to the Telescope Array Experiment and the Pierre Auger Observatory include a comparison of the transparency of the atmosphere above both experiments, a study of the systematic uncertainty associated with their existing fluorescence detectors, and a cross-calibration of their energy and Xmax scales.
Abstract: 1908.02779
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Title:Zee-Burst: A New Probe of Neutrino Non-Standard Interactions at IceCube
(Submitted on 7 Aug 2019)
Abstract: We propose a new way to probe non-standard interactions (NSI) of neutrinos with matter using the ultra-high energy (UHE) neutrino data at current and future neutrino telescopes. We consider the Zee model of radiative neutrino mass generation as a prototype, which allows two charged scalars - one $SU(2)_L$-doublet and one a singlet, both being leptophilic, to be as light as 100 GeV, thereby inducing potentially observable NSI with electrons. We show that these light charged Zee-scalars could give rise to a Glashow-like resonance feature in the UHE neutrino event spectrum at the IceCube neutrino observatory and can probe a sizable fraction of the allowed NSI parameter space in the near future.
Abstract: 1908.02496
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Title:{\it Fermi}-LAT Stacking Analysis Technique: An Application to Extreme Blazars and Prospects for their CTA Detection
(Submitted on 7 Aug 2019)
Abstract: We present a likelihood profile stacking technique based on the {\it Fermi}-Large Area Telescope (LAT) data to explore the $\gamma$-ray characteristics of {\it Fermi}-LAT undetected astrophysical populations. The pipeline is applied to a sample of $\gamma$-ray unresolved extreme blazars, i.e., sources with the highest synchrotron peak frequencies ($\nu_{\rm Syn}^{\rm peak}\geqslant 10^{17}$ Hz), and we report a cumulative $\gamma$-ray detection with more than 32$\sigma$ confidence for 2 degrees of freedom. Comparing the generated stacked $\gamma$-ray spectrum with the sensitivity limits of the upcoming Cherenkov Telescope Array (CTA), we find that the {\it Fermi}-LAT undetected population of such extreme blazars, on average, may remain well below the CTA detection threshold due to their faintness and extragalactic background light (EBL) absorption. However, $\gamma$-ray detected blazars belonging to the same class are promising candidates for CTA observations. The EBL corrected stacked spectra of these sources do not show any softening up to 1 TeV. This finding suggests the inverse Compton peak of extreme blazars to lie above 1 TeV, thus indicating a hard intrinsic TeV spectrum. Our analysis also predicts that at 100 GeV, at least $\sim$10\% of the diffuse extragalactic $\gamma$-ray background originates from the $\gamma$-ray undetected extreme blazars. These results highlight the effectiveness of the developed stacking technique to explore the uncharted territory of $\gamma$-ray undetected astrophysical objects.
Abstract: 1908.02481
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Title:The Effects of the Galactic Magnetic Field on UHECR From Local Sources
(Submitted on 7 Aug 2019)
Abstract: We summarise the results found following a study of the effects that Galactic magnetic fields can have on the propagation of cosmic rays from local extragalactic sources. This study focuses on the coherent structures in the Jansson-Farrar Galactic magnetic field model, namely the: disk field, a toroidal field, and an x-field components. The phenomena of Galactic magnetic field shadowing, source deflection, and Galactic magnetic field tunnel vision are all noted. Attention throughout this study is placed on particles with rigidity around $10^{18.5}$ EV, believed to dominate the cosmic ray spectrum above the ankle. The Galactic magnetic field component predominantly responsible for giving rise to each of these different effects was determined.
Abstract: 1908.02180
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Title:A Condition Monitoring Concept Studied at the MST Prototype for the Cherenkov Telescope Array
(Submitted on 6 Aug 2019)
Abstract: The Cherenkov Telescope Array (CTA) is a future ground-based gamma-ray observatory that will provide unprecedented sensitivity and angular resolution for the detection of gamma rays with energies above a few tens of GeV. In comparison to existing instruments (like H.E.S.S., MAGIC, and VERITAS) the sensitivity will be improved by installing two extended arrays of telescopes in the northern and southern hemisphere, respectively. A large number of planned telescopes (>100 in total) motivates the application of predictive maintenance techniques to the individual telescopes. A constant and automatic condition monitoring of the mechanical telescope structure and of the drive system (motors, gears) is considered for this purpose. The condition monitoring system aims at detecting degradations well before critical errors occur; it should help to ensure long-term operation and to reduce the maintenance efforts of the observatory. We present approaches for the condition monitoring of the structure and the drive system of Medium-Sized Telescopes (MSTs), respectively. The overall concept has been developed and tested at the MST prototype for CTA in Berlin. The sensors used, the joint data acquisition system, possible analysis methods (like Operational Modal Analysis, OMA, and Experimental Modal Analysis, EMA) and first performance results are discussed.
Abstract: 1908.01781
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Full Text: [ PostScript, PDF]
Title:Chandra Spectral and Timing Analysis of Sgr A*'s Brightest X-ray Flares
(Submitted on 5 Aug 2019)
Abstract: We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600 x and 245 x greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology --- it lasts 5.7 ksec ($\sim 2$ hours), with a rapid rise time of 1500 sec and a decay time of 2500 sec. The second flare lasts 3.4 ksec, with rise and decay times of 1700 sec and 1400 sec. These luminous flares are significantly harder than quiescence: the first has a power law spectral index $\Gamma = 2.06\pm 0.14$ and the second has $\Gamma = 2.03\pm 0.27$, compared to $\Gamma = 3.0\pm0.2$ for the quiescent accretion flow. These spectral indices (as well as the flare hardness ratios) are consistent with previously-detected \sgra\ flares, suggesting that bright and faint flares arise from similar physical processes. Leveraging the brightest flare's long duration and high signal-to-noise, we search for intraflare variability and detect excess X-ray power at a frequency of $\nu \approx 3$ mHz, but show that it is an instrumental artifact and not of astrophysical origin. We find no other evidence (at the 95\% confidence level) for periodic or quasi-periodic variability in either flares' time series. We also search for non-periodic excess power but do not find compelling evidence in the power spectrum. Bright flares like these remain our most promising avenue for identifying Sgr A*'s short timescale variability in the X-ray, which may probe the characteristic size scale for the X-ray emission region.
Abstract: 1908.01777
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Title:Unprecedented variability of Sgr A* in NIR
(Submitted on 5 Aug 2019)
Abstract: The electromagnetic counterpart to the Galactic center supermassive black hole, Sgr A*, has been observed in the near-infrared for over 20 years and is known to be highly variable. We report new Keck Telescope observations showing that Sgr A* reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths. In the K$^\prime$ band, Sgr A* reached flux levels of $\sim6$ mJy, twice the level of the previously observed peak flux from $>13,000$ measurements over 130 nights with the VLT and Keck Telescopes. We also observe a factor of 75 change in flux over a 2-hour time span with no obvious color changes between 1.6 $\mu$m and 2.1 $\mu$m. The distribution of flux variations observed this year is also significantly different than the historical distribution. Using the most comprehensive statistical model published, the probability of a single night exhibiting peak flux levels observed this year, given historical Keck observations, is less than $0.3\%$. The probability to observe the flux levels similar to all 4 nights of data in 2019 is less than $0.05\%$. This increase in brightness and variability may indicate a period of heightened activity from Sgr A* or a change in its accretion state. It may also indicate that the current model is not sufficient to model Sgr A* at high flux levels and should be updated. Potential physical origins of Sgr A*'s unprecedented brightness may be from changes in the accretion-flow as a result of the star S0-2's closest passage to the black hole in 2018 or from a delayed reaction to the approach of the dusty object G2 in 2014. Additional multi-wavelength observations will be necessary to both monitor Sgr A* for potential state changes and to constrain the physical processes responsible for its current variability.
Abstract: 1908.01598
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Full Text: [ PostScript, PDF]
Title:Time lag in cosmic-ray modulation and global properties of the Solar Cycle
(Submitted on 5 Aug 2019)
Abstract: When entering the heliosphere, Galactic cosmic rays (GCRs) are influenced by magnetic turbulence and Solar wind disturbances, which cause the so-called "solar modulation" effect. Understanding the time-dependent relationship between the Sun's variability and GCR flux modulation is essential for the investigation of the GCR transport processes in the heliosphere, as well as for the establishment of predictive models of GCR radiation in the interplanetary space. The known anti-correlation between GCR flux and sunspot number appears to be delayed by several months, but the origin of such a time lag is unclear. In this work, we are perform the first global characterization of the time lag evolution over the solar cycles and its energy dependence. We made use of a large collection of time-resolved data, both from space missions and ground based observatories. Since the long-term variation of the GCR flux originates by a combination of several physics processes, the investigation presented here may reveal important aspects of the GCR transport in the heliospheric plasma.
Abstract: 1908.01597
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Full Text: [ PostScript, PDF]
Title:ALMA resolves the hourglass magnetic field in G31.41+0.31
(Submitted on 5 Aug 2019)
Abstract: Context. Submillimeter Array (SMA) 870 micron polarization observations of the hot molecular core G31.41+0.31 revealed one of the clearest examples up to date of an hourglass-shaped magnetic field morphology in a high-mass star-forming region. Aims. To better establish the role that the magnetic field plays in the collapse of G31.41+0.31, we carried out Atacama Large Millimeter/submillimeter Array (ALMA) observations of the polarized dust continuum emission at 1.3 mm with an angular resolution four times higher than that of the previous (sub)millimeter observations to achieve an unprecedented image of the magnetic field morphology. Methods. We used ALMA to perform full polarization observations at 233 GHz (Band 6). The resulting synthesized beam is 0.28"x0"20 which, at the distance of the source, corresponds to a spatial resolution of ~875 au. Results. The observations resolve the structure of the magnetic field in G31.41+0.31 and allow us to study the field in detail. The polarized emission in the Main core of G31.41+0.41is successfully fit with a semi-analytical magnetostatic model of a toroid supported by magnetic fields. The best fit model suggests that the magnetic field is well represented by a poloidal field with a possible contribution of a toroidal component of ~10% of the poloidal component, oriented southeast to northwest at ~ -44 deg and with an inclination of ~-45 degr. The magnetic field is oriented perpendicular to the northeast to southwest velocity gradient detected in this core on scales from 1E3-1E4 au. This supports the hypothesis that the velocity gradient is due to rotation and suggests that such a rotation has little effect on the magnetic field. The strength of the magnetic field estimated in the central region of the core with the Davis-Chandrasekhar-Fermi method is ~8-13 mG and implies that the mass-to-flux ratio in this region is slightly supercritical ...
Abstract: 1908.01544
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Full Text: [ PostScript, PDF]
Title:POSyTIVE -- a GRB population study for the Cherenkov Telescope Array (ICRC-2019)
(Submitted on 5 Aug 2019)
Abstract: One of the central scientific goals of the next-generation Cherenkov Telescope Array (CTA) is the detection and characterization of gamma-ray bursts (GRBs). CTA will be sensitive to gamma rays with energies from about 20 GeV, up to a few hundred TeV. The energy range below 1 TeV is particularly important for GRBs. CTA will allow exploration of this regime with a ground-based gamma-ray facility with unprecedented sensitivity. As such, it will be able to probe radiation and particle acceleration mechanisms at work in GRBs. In this contribution, we describe POSyTIVE, the POpulation Synthesis Theory Integrated project for very high-energy emission. The purpose of the project is to make realistic predictions for the detection rates of GRBs with CTA, to enable studies of individual simulated GRBs, and to perform preparatory studies for time-resolved spectral analyses. The mock GRB population used by POSyTIVE is calibrated using the entire 40-year dataset of multi-wavelength GRB observations. As part of this project we explore theoretical models for prompt and afterglow emission of long and short GRBs, and predict the expected radiative output. Subsequent analyses are performed in order to simulate the observations with CTA, using the publicly available ctools and Gammapy frameworks. We present preliminary results of the design and implementation of this project.
Abstract: 1908.01508
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Full Text: [ PostScript, PDF]
Title:Mass composition of cosmic rays above 0.1 EeV by the Yakutsk array data
(Submitted on 5 Aug 2019)
Abstract: The paper presents the results of the longitudinal development of extensive air showers (X$_{max}$) of ultra-high energies and mass composition of cosmic rays. The measurements of X$_{max}$ are based on data from observations of the Cherenkov radiation at the Yakutsk array for the period 1974-2014. The cascade curves of individual showers and the depth of maximum X$_{max}$ were reconstructed over the energy range 10$^{16}$-5.7$\cdot$10$^{19}$ eV. It is shown that the displacement rate of the parameter dX$_{max}$ / dE in the atmosphere is nonlinear and depends on the energy. Such a feature indicates a change in mass composition, which is confirmed by fluctuations of X$_{max}$ in this energy region. The composition of cosmic rays was determined by interpolation using the QGSJetII-04 model.
Abstract: 1908.01356
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Full Text: [ PostScript, PDF]
Title:Telescope Array 10 Year Composition
(Submitted on 4 Aug 2019)
Abstract: Estimates of the composition of ultra high energy cosmic rays (UHECRs) can be inferred by recording the depth of air shower maximum, $X_{\mathrm{max}}$, for many showers and comparing the distributions to those predicted by Monte Carlo simulations. Traditionally, UHECR composition has relied upon comparison of the first and second moments of the $X_{\mathrm{max}}$ distributions to estimate the compatibility between data and simulations, but with the large UHECR datasets being built the current generation experiments better tests which compare full distributions can be employed. Such tests can be used to understand the accuracy with which UHECR composition can actually be understood at the current level of statistics and quantitatively measure the significance of agreement or disagreement with models in order to reject them. In this paper we present the most recent results of 10 years of Telescope Array hybrid $X_{\mathrm{max}}$ measurements which is found to agree with a predominantly light composition. In previously published results we have demonstrated the agreement of Telescope Array hybrid $X_{\mathrm{max}}$ data with single element models using systematic shifting of the data in order to ensure the shapes of the distributions are being compared. Here we present multi-component source models fit to hybrid $X_{\mathrm{max}}$ data and report on the relative fractions of those sources that best fit the data. Below $10^{19.1}$ eV TA hybrid data is found to be compatible with mixtures composed of predominantly light elements such as protons and helium.
Abstract: 1908.01269
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Full Text: [ PostScript, PDF]
Title:Pointing System for the Large Size Telescopes Prototype of the Cherenkov Telescope Array
(Submitted on 4 Aug 2019)
Abstract: The pointing system of the prototype of the Large Size Telescope (LST-1) for the Cherenkov Telescope Array observatory, should ensure mapping of the gamma-ray image of a point-like source in the Cherenkov camera to the sky coordinates with a precision better than 14 arcseconds. Detailed studies of the telescope deformations are performed in order to disentangle different deformations and quantify their contributions to the miss-pointing, to learn how to correct for them, and finally how to design the system for offline and online pointing corrections. The LST-1 pointing precision system consist of several devices mounted at the center of the dish: Starguider Camera (SG), Camera Displacement Monitor (CDM), two inclinometers, four distance meters, and an Optical Axis Reference Laser (OARL), working together with the LEDs mounted in a circle around the Cherenkov camera. The online pointing corrections are based on a bending model as currently done by existing IACTs. The offline corrections will be performed combining measurements done by the SG and CDM cameras. SG will provide the position of the Cherenkov camera center with respect to the sky coordinates with a precision of 5 arcseconds, while CDM will provide the deviation of the telescope optical axis defined by the OARL spots with respect to the Cherenkov camera center with a precision better than 5 arcseconds. Laboratory measurements on dedicated test benches showed that the required pointing precision can be achieved for SG, CDM and inclinometer.
Abstract: 1908.01232
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Full Text: [ PostScript, PDF]
Title:H.E.S.S. Monitoring of PKS 2155-304 in 2015 and 2016
(Submitted on 3 Aug 2019)
Abstract: PKS 2155-304 is one of the brightest blazar located in Southern Hemisphere, monitored with H.E.S.S. since the first light of the experiment. Here we report multiwavelength monitoring observations collected during the period of 2015-2016 with H.E.S.S.,Fermi-LAT, Swift-XRT, Swift-UVOT, and ATOM. Two years of multiwavelength data with very good temporal coverage allowed to characterize broadband emission observed from the region of PKS 2155-304 and study potential multifrequency correlations. During the period of monitoring, PKS 2155-304 revealed complex multiwavelength variability with two outbursts characterized by completely different multiband properties. The 2015 activity of the blazar is characterized by a flare observed at all wavelengths studied. The broadband emission observed during the outburst is well correlated without any time lags. Contrary to 2015, in 2016, only orphan outburst in the optical and ultraviolet wavelengths was observed. Such an orphan activity is reported for the first time for the blazar PKS 2155-304.
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