1. Intermediate fluence downward terrestrial gamma ray flashes as observed by the Telescope Array Surface DetectorR. U. Abbasi, N. Kieu, P. R. Krehbiel, J. W. Belz, M. M. F. Saba, W. Rison, M. A. Stanley, D. Rodeheffer, D. Mazzucco, Jon Paul Lundquist, 2024, izvirni znanstveni članek Opis: On 11 September 2021, two small thunderstorms developed over the Telescope Array Surface Detector (TASD) that produced an unprecedented number of six downward terrestrial gamma ray flashes (TGFs) within one‐hour timeframe. The TGFs occurred during the initial stage of negative cloud‐to‐ground flashes whose return strokes had increasingly large peak currents up to 223 kA, 147 GeV energy deposit in up to 25 1.2 km‐spaced surface detectors, and intermittent bursts of gamma‐rays with total durations up to 717 s. The analyses are based on observations recorded by the TASD network, complemented by data from a 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high‐speed video camera, and the National Lightning Detection Network. The TGFs of the final two flashes had gamma fluences of and 8, logarithmically bridging the gap between previous TASD and satellite‐based detections. The observations further emphasize the similarity between upward and downward TGF varieties, suggesting a common mechanism for their production.
Ključne besede: Telescope Array Surface Detector, terrestrial gamma ray flashes, 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high‐speed video camera, National Lightning Detection Network, TGF fluence measurement
Objavljeno v RUNG: 23.04.2025; Ogledov: 263; Prenosov: 4
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2. First time-resolved leader spectra associated with a downward terrestrial gamma-ray flash detected at the Telescope Array Surface DetectorN. Kieu, R. U. Abbasi, M. M. F. Saba, J. W. Belz, P. R. Krehbiel, M. A. Stanley, F. J. Gordillo-Vazquez, M. Passas-Varo, T. Warner, Jon Paul Lundquist, 2024, izvirni znanstveni članek Opis: Optical emissions associated with Terrestrial Gamma ray Flashes (TGFs) have recently become important subjects in space‐based and ground‐based observations as they can help us understand how TGFs are produced during thunderstorms. In this paper, we present the first time‐resolved leader spectra of the optical component associated with a downward TGF. The TGF was observed by the Telescope Array Surface Detector (TASD) simultaneously with other lightning detectors, including a Lightning Mapping Array (LMA), an INTerFerometer (INTF), a Fast Antenna (FA), and a spectroscopic system. The spectroscopic system recorded leader spectra at 29,900 frames per second (33.44 s time resolution), covering a spectral range from 400 to 900 nm, with 2.1 nm per pixel. The recordings of the leader spectra began 11.7 ms before the kA return stroke and at a height of 2.37 km above the ground. These spectra reveal that optical emissions of singly ionized nitrogen and oxygen occur between 167 s before and 267 s after the TGF detection, while optical emissions of neutrals (H I, 656 nm; N I, 744 nm, and O I, 777 nm) occur right at the moment of the detection. The time‐dependent spectra reveal differences in the optical emissions of lightning leaders with and without downward TGFs.
Ključne besede: Telescope Array Surface Detector, terrestrial gamma‐ray flashes, time‐resolved tgf leader spectra
Objavljeno v RUNG: 22.04.2025; Ogledov: 279; Prenosov: 2
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3. A 1.8 m class pathfinder Raman LIDAR for the northern site of the Cherenkov Telescope Array Observatory : technical designOtger Ballester, O. Blanch Bigas, Joan Boix, Paolo G. Calisse, Anna Campoy-Ordaz, Sidika Merve Çolak, Darko Kolar, Samo Stanič, Marko Zavrtanik, Miha Živec, 2025, izvirni znanstveni članek Opis: This paper presents the technical design of the pathfinder Barcelona Raman LIDAR (pbrl) for the northern site of the Cherenkov Telescope Array Observatory (Northern site of the Cherenkov Telescope Array Observatory (CTAO-N)) located at the Roque de los Muchachos Observatory (Roque de los Muchachos Observatory (ORM)). The pBRL is developed for continuous atmospheric characterization, essential for correcting high-energy gamma-ray observations captured by Imaging Atmospheric Cherenkov Telescopes (IACTs). The LIDAR consists of a steerable telescope with a 1.8 m parabolic mirror and a pulsed Nd:YAG laser with frequency doubling and tripling. It emits at wavelengths of 355 nm and 532 nm to measure aerosol scattering and extinction through two elastic and Raman channels. Built upon a former Cherenkov Light Ultraviolet Experiment (Cherenkov Light Ultraviolet Experiment (CLUE)) telescope, the pBRL’s design includes a Newtonian mirror configuration, a coaxial laser beam, a near-range system, a liquid light guide and a custom-made polychromator. During a one-year test at the ORM, the stability of the LIDAR and semi-remote-controlled operations were tested. This pathfinder leads the way to designing a final version of a Cherenkov Telescope Array Observatory (CTAO) Raman LIDAR which will provide real-time atmospheric monitoring and, as such, ensure the necessary accuracy of scientific data collected by the CTAO-N telescope array.
Ključne besede: Raman LIDAR, design, aerosols, atmospheric effects, gamma-ray astrophysics, IACTs, polychromator, Liquid Light Guide, Licel, PMT
Objavljeno v RUNG: 19.03.2025; Ogledov: 435; Prenosov: 2
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4. Periodic gamma-ray modulation of the blazar PG 1553+113 confirmed by Fermi-LAT and multiwavelength observationsS. Abdollahi, L. Baldini, G. Barbiellini, R. Bellazzini, B. Berenji, E. Bissaldi, R. D. Blandford, R. Bonino, P. Bruel, Gabrijela Zaharijas, 2024, izvirni znanstveni članek Opis: Abstract
A 2.1 yr periodic oscillation of the gamma-ray flux from the blazar PG 1553+113 has previously been tentatively identified in ∼7 yr of data from the Fermi Large Area Telescope. After 15 yr of Fermi sky-survey observations, doubling the total time range, we report >7 cycle gamma-ray modulation with an estimated significance of 4σ against stochastic red noise. Independent determinations of oscillation period and phase in the earlier and the new data are in close agreement (chance probability <0.01). Pulse timing over the full light curve is also consistent with a coherent periodicity. Multiwavelength new data from Swift X-Ray Telescope, Burst Alert Telescope, and UVOT, and from KAIT, Catalina Sky Survey, All-Sky Automated Survey for Supernovae, and Owens Valley Radio Observatory ground-based observatories as well as archival Rossi X-Ray Timing Explorer satellite-All Sky Monitor data, published optical data of Tuorla, and optical historical Harvard plates data are included in our work. Optical and radio light curves show clear correlations with the gamma-ray modulation, possibly with a nonconstant time lag for the radio flux. We interpret the gamma-ray periodicity as possibly arising from a pulsational accretion flow in a sub-parsec binary supermassive black hole system of elevated mass ratio, with orbital modulation of the supplied material and energy in the jet. Other astrophysical scenarios introduced include instabilities, disk and jet precession, rotation or nutation, and perturbations by massive stars or intermediate-mass black holes in polar orbit.
Ključne besede: gamma rays, AGNs
Objavljeno v RUNG: 13.01.2025; Ogledov: 715; Prenosov: 10
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5. Prospects for annihilating dark matter from M31 and M33 observations with the Cherenkov Telescope ArrayMiltiadis Michailidis, Lorenzo Marafatto, Denys Malyshev, Fabio Iocco, Gabrijela Zaharijas, Olga Sergijenko, Maria Isabel Bernardos, Christopher Eckner, Alexey Boyarsky, Anastasia Sokolenko, Andrea Santangelo, 2023, izvirni znanstveni članek Opis: Abstract
M31 and M33 are the closest spiral galaxies and the largest members (together with the Milky Way) of the Local group, which makes them interesting targets for indirect dark matter searches. In this paper we present studies of the expected sensitivity of the Cherenkov Telescope Array (CTA) to an annihilation signal from weakly interacting massive particles from M31 and M33. We show that a 100 h long observation campaign will allow CTA to probe annihilation cross-sections up to 〈συ〉 ≈ 5·10-25 cm3 s-1 for the τ
+
τ
- annihilation channel (for M31, at a DM mass of 0.3 TeV), improving the current limits derived by HAWC by up to an order of magnitude.
We present an estimate of the expected CTA sensitivity, by also taking into account the contributions of the astrophysical background and other possible sources of systematic uncertainty.
We also show that CTA might be able to detect the extended emission from the bulge of M31, detected at lower energies by the Fermi/LAT.
Ključne besede: dark matter, gamma rays, Cherenkov Telescope Array, i
Objavljeno v RUNG: 13.01.2025; Ogledov: 646; Prenosov: 10
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6. Multi-messenger and transient astrophysics with the Cherenkov Telescope ArrayŽ. Bošnjak, Anthony M. Brown, Alessandro Carosi, M. Chernyakova, Pierre Cristofari, F. Longo, A. López Oramas, M. Santander, Serguei Vorobiov, Danilo Zavrtanik, 2021, drugi sestavni deli Opis: The discovery of gravitational waves, high-energy neutrinos or the very-high-energy counterpart of gamma-ray bursts has revolutionized the high-energy and transient astrophysics community. The development of new instruments and analysis techniques will allow the discovery and/or follow-up of new transient sources. We describe the prospects for the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory, for multi-messenger and transient astrophysics in the decade ahead. CTA will explore the most extreme environments via very-high-energy observations of compact objects, stellar collapse events, mergers and cosmic-ray accelerators.
Ključne besede: multi-messenger astrophysics, gravitational waves, very-high-energy (VHE) gamma rays, cosmic rays, VHE neutrinos, transient astrophysical phenomena, Cherenkov Telescope Array Observatory
Objavljeno v RUNG: 13.01.2025; Ogledov: 663; Prenosov: 7
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7. Probing extreme environments with the Cherenkov Telescope ArrayC. Boisson, Anthony M. Brown, A. Burtovoi, M. Cerruti, M. Chernyakova, T. Hassan, J.-P. Lenain, Marina Manganaro, Serguei Vorobiov, Danilo Zavrtanik, 2021, drugi sestavni deli Opis: The physics of the non-thermal Universe provides information on the acceleration mechanisms in extreme environments, such as black holes and relativistic jets, neutron stars, supernovae or clusters of galaxies. In the presence of magnetic fields, particles can be accelerated towards relativistic energies. As a consequence, radiation along the entire electromagnetic spectrum can be observed, and extreme environments are also the most likely sources of multi-messenger emission. The most energetic part of the electromagnetic spectrum corresponds to the very-high-energy (VHE, E>100 GeV) gamma-ray regime, which can be extensively studied with ground based Imaging Atmospheric Cherenkov Telescopes (IACTs). The results obtained by the current generation of IACTs, such as H.E.S.S., MAGIC, and VERITAS, demonstrate the crucial importance of the VHE band in understanding the non-thermal emission of extreme environments in our Universe. In some objects, the energy output in gamma rays can even outshine the rest of the broadband spectrum. The Cherenkov Telescope Array (CTA) is the next generation of IACTs, which, with cutting edge technology and a strategic configuration of ~100 telescopes distributed in two observing sites, in the northern and southern hemispheres, will reach better sensitivity, angular and energy resolution, and broader energy coverage than currently operational IACTs. With CTA we can probe the most extreme environments and considerably boost our knowledge of the non-thermal Universe.
Ključne besede: black holes, relativistic jets, neutron stars, supernovae, clusters of galaxies, particle acceleration mechanisms, very-high-energy gamma rays, Cherenkov Telescope Array Observatory
Objavljeno v RUNG: 10.01.2025; Ogledov: 672; Prenosov: 4
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8. The Cherenkov Telescope Array sensitivity to the transient skyValentina Fioretti, Christopher Eckner, Gašper Kukec Mezek, Samo Stanič, Serguei Vorobiov, Lili Yang, Gabrijela Zaharijas, Danilo Zavrtanik, Marko Zavrtanik, Lukas Zehrer, 2019, objavljeni znanstveni prispevek na konferenci Opis: The Cherenkov Telescope Array (CTA) will be able to perform unprecedented observations of the transient very high-energy sky. An on-line science alert generation (SAG) pipeline, with a required 30 second latency, will allow the discovery or follow-up of gamma ray bursts (GRBs) and flaring emission from active galactic nuclei, galactic compact objects and electromagnetic counterparts of gravitational waves or neutrino messengers. The CTA sensitivity for very short exposures does not only depend on the technological performance of the array (e.g. effective area, background discrimination efficiency). The algorithms to evaluate the significance of the detection also define the sensitivity, together with their computational efficiency in order to satisfy the SAG latency requirements. We explore the aperture photometry and likelihood analysis techniques, and the associated parameters (e.g. on-source to off-source exposure ratio, minimum number of required signal events), defining the CTA ability to detect a significant signal at short exposures. The resulting CTA differential flux sensitivity as a function of the observing time, obtained using the latest Monte Carlo simulations, is compared to the sensitivities of Fermi–LAT and current-generation IACTs obtained in the overlapping energy ranges.
Ključne besede: very-high-energy gamma rays, Cherenkov Telescope Array (CTA) Observatory, transient astrophysical sources, gamma-ray bursts, active galactic nuclei
Objavljeno v RUNG: 15.11.2024; Ogledov: 943; Prenosov: 10
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9. Monte Carlo study of a single SST-1M prototype for the Cherenkov Telescope ArrayJ. Juryšek, Christopher Eckner, Gašper Kukec Mezek, Samo Stanič, Serguei Vorobiov, Lili Yang, Gabrijela Zaharijas, Danilo Zavrtanik, Marko Zavrtanik, Lukas Zehrer, 2019, objavljeni znanstveni prispevek na konferenci Opis: The SST-1M telescope was developed as a prototype of a Small-Size-Telescope for the Cherenkov Telescope Array observatory and it has been extensively tested in Krakow since 2017. In this contribution we present validation of the Monte Carlo model of the prototype and expected
performance in Krakow conditions. We focus on gamma/hadron separation and mono reconstruction of energy and gamma photon arrival direction using Machine learning methods.
Ključne besede: very-high-energy gamma rays, Cherenkov Telescope Array (CTA) Observatory, Small-Sized Telescopes (SSTs), SST-1M telescope prototype performance, Monte Carlo study
Objavljeno v RUNG: 15.11.2024; Ogledov: 911; Prenosov: 7
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10. The Cherenkov Telescope Array performance in divergent modeA. Donini, Christopher Eckner, Gašper Kukec Mezek, Samo Stanič, Serguei Vorobiov, Lili Yang, Gabrijela Zaharijas, Danilo Zavrtanik, Marko Zavrtanik, Lukas Zehrer, 2019, objavljeni znanstveni prispevek na konferenci Opis: Two of the Key Science Projects of the Cherenkov Telescope Array (CTA) consist in performing a deep survey of the Galactic and Extragalactic sky, providing an unbiased view of the Universe at energies above tens of GeV. To optimize the time spent to perform the Extragalactic survey, a so-called “divergent mode” of the CTA was proposed as an alternative observation strategy to the traditional parallel pointing in order to increase its instantaneous field of view. The search for transient VHE sources would also benefit from an extended field of view. In the divergent mode, each telescope points to a position in the sky that is slightly offset, in the outward direction, from the center of the field of view. In this contribution, we present the first performance estimation from full Monte Carlo simulation of possible CTA divergent mode setups.
Ključne besede: very-high-energy gamma rays, Cherenkov Telescope Array (CTA) Observatory, CTAO Key Science Projects (KSPs), galactic survey KSP, extragalactic survey KSP
Objavljeno v RUNG: 15.11.2024; Ogledov: 928; Prenosov: 6
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