arXiv subjects

A. Chilingarian

Publications and source records attributed to A. Chilingarian.

At least 19 recordsLinked to original sources

arXiv2026

Atmospheric Conditions, Electric Fields, and Thunderstorm Ground Enhancements at Aragats

The Aragats High-Altitude Research Station (3200 m a.s.l.) combines meteorological, atmospheric electric, and particle flux measurements, providing a unique setting for investigating thunderstorm ground enhancements (TGEs) and recent environmental variability. We analyze one-minute observations from 2012 to 2025, first focusing on the meteorological and electric-field conditions controlling TGEs and then on trends in air temperature and daytime solar radiation. From an archive of 610 TGEs, a quality-controlled subset of 284 events with particle flux enhancements larger than 10 percent was selected. Comparisons of May, August, and October show that strong near surface electric fields alone are insufficient for frequent TGEs. May combines strong-field conditions with low cloud bases and shows a pronounced inverse relation between TGE occurrence and cloud-base distance. Thus, TGE occurrence is jointly controlled by thundercloud electrification and cloud-to-detector distance, while the recent warming tendency cannot be attributed to increased measured short-wave radiation. The annual mean air temperature shows a statistically significant positive trend, whereas daytime solar radiation shows a significant negative trend. Thus, warming at Aragats cannot be explained solely by variations in solar radiation.

physics.ao-ph↗

arXiv2026

Constraints on the VHE counterpart of two binary black hole mergers observed by the MAGIC and CTAO LST-1 telescopes

We present very-high-energy gamma-ray observations of two binary black hole merger candidates, GW240615_113620 and GW241125_010116, performed with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the first Large-Sized Telescope of the Cherenkov Telescope Array Observatory's (CTAO LST-1). GW240615_113620 was the best localized event of the fourth observing run of the LIGO-Virgo-KAGRA gravitational waves interferometers. GW241125_010116 was temporally and spatially coincident with a sub-threshold short-duration burst detected with the Swift-Burst Alert Telescope (BAT), the Swift-X-Ray Telescope (XRT) and the Einstein Probe Follow-up X-ray Telescope (FXT). We observed the two events in stereoscopic mode, taking advantage of the improved sensitivity of joint MAGIC+LST-1 observations. No detection was achieved in the GeV-TeV gamma-ray band for any of the two sources. The unfavourable observing conditions of both events posed a challenge for a standard analysis and therefore a non standard analysis was necessary for both objects. Owing to the small localization area and the association with a GRB-like burst respectively, these events represented an unprecedented opportunity to study in details the electromagnetic emission from binary black holes merger events and, in particular, we discussed two theoretical models that predict a detectable gamma-ray emission and the possible future applications.

astro-ph.HE↗

arXiv2026

Direct Observation of Wilsonian Electrons in Thunderstorms

We analyze electron rich Thunderstorm Ground Enhancements (TGEs) detected at Aragats using the SEVAN Light spectrometer, which can separate charged and neutral particles and reconstruct their energy spectra. The events exhibit large electron fluxes, with reconstructed energies extending to 45 MeV. Simultaneous meteorological observations reveal that all events occurred within a remarkably narrow atmospheric regime characterized by exceptionally low cloud-base heights, temperatures near 0{\deg}C, strong electric-field disturbances, and lightning. Comparisons with the complete Aragats TGE archive demonstrate that the direct detection of runaway electrons becomes possible only when the active acceleration region approaches within several tens of meters of detector altitude, allowing electrons to survive atmospheric attenuation. These results provide an experimental explanation for the century long rarity of observations of Wilsonian electrons. The findings establish the atmospheric conditions required for direct observation of runaway electrons and contribute to the understanding of particle acceleration in natural electric fields.

physics.ao-ph↗

arXiv2026

Positron acceleration in thunderstorms

Thunderstorm Ground Enhancements (TGEs) are known manifestations of relativistic runaway electron avalanches (RREAs) developing inside thunderclouds. However, the role of positrons in TGEs and their relationship to thundercloud charge structure remain poorly understood. We report time resolved observations of intense positron fluxes detected at the Aragats Observatory on 16 17 May 2026 during strong thunderstorms. Two of the three events were characterized by a positive near surface electric field (NSEF), graupel precipitation, a low cloud base, and low-to-moderate enhancement of gamma ray and electron fluxes measured by SEVAN and STAND3 detectors. The third event is a classical electron TGE considered for comparative purposes. All events show moderate to strong enhancement of the 511 keV annihilation line, along with enhanced radon progeny gamma ray lines. We observe a temporal separation between the electron and gamma ray TGE peak and the positron flux maximum. To explain these observations, we introduce a dual dipole electrodynamic model. The large scale electron dipole comprises the main negative thundercloud layer and its broad positive mirror charge induced at the Earths surface, producing ordinary TGEs. Simultaneously, a localized positron dipole forms from the LPCR, with its negative mirror charge directly beneath the LPCR footprint. This lower dipole accelerates positrons downward while decelerating electrons entering the same region. These results establish positron TGEs as a new subclass of atmospheric high energy phenomena and provide direct evidence of localized positron acceleration in the lower atmosphere.

physics.ao-ph↗

arXiv2026

Evolution of Thunderstorm Charge Structure Revealed by Particle Composition and Near-Surface Electric Field

Thunderstorm Ground Enhancements (TGEs) provide a direct particle-physics diagnostic of accelerating electric fields within thunderclouds. During 12 15 May 2026, Aragats detectors recorded a compact sequence of TGEs under closely related meteorological and electric field conditions. Ten statistically resolved TGEs were selected for quantitative analysis using STAND3 and SEVAN Light gamma and electron proxy channels. The events were classified into negative (N), transition/disturbed negative (TD N), and positive (P) states based on the stability of the near surface electric field (NSEF). The results show that TGEs occur during both comparatively stable and rapidly evolving charge states. Stable negative events correspond to sustained exposure of the Main Negative (MN) charge region, whereas positive and TD N events reflect Lower Positive Charge Region (LPCR) screening and rapid restructuring of the lower thundercloud dipole during End Of Storm Oscillation (EOSO) like evolution. The analysis demonstrates that the electron/gamma composition depends primarily on the persistence and geometry of the lower accelerating field structure rather than solely on the instantaneous sign or magnitude of the NSEF. Rapid electric field impulses, frequently associated with nearby lightning activity, mark active restructuring of the lower dipole and strongly influence electron transport to the detector level. All analyzed events occurred during persistent low cloud base conditions favorable for electron TGEs at Aragats.

physics.ao-ph↗

arXiv2026

Long-term study of the gamma-ray emission of Cygnus X-3 with MAGIC and Fermi-LAT

Cygnus X-3 is a microquasar composed of a compact object of unknown nature closely orbiting around a Wolf-Rayet star. The particularities of this source make it a unique case among microquasars. This fact, together with its recent establishment as a PeV particle accelerator, makes Cygnus X-3 a very interesting target for the investigation of the physical processes leading to gamma-ray production. In this work, the TeV and GeV gamma-ray emission of Cygnus X-3 is studied in order to determine its origin and constrain the properties of the system. For that purpose, a point-like analysis of 130 h of data taken with the MAGIC telescopes between 2013 and 2024 was performed, which represents the largest available sample for Cygnus X-3 at $\sim$TeV energies. Additionally, contemporary data from Fermi-LAT were also analysed to better contextualize the MAGIC observations. For a more detailed investigation of the source physics, the data were divided into three subsets according to the flaring state of the source and orbital phase. No significant detection of Cygnus X-3 is found between 0.1 and 7 TeV for any of the datasets, and differential and integral flux upper limits are reported over the long-term monitoring of the source. The Fermi-LAT fluxes can be considered compatible with previous results, taking into account the different data samples used across studies. The MAGIC upper limits presented in this work represent the most constraining ones up to date at $\sim$TeV energies. An eventual detection of Cygnus X-3 at these energies would significantly constrain the source properties, and is not unreasonable to expect given that the source has already been detected in both the GeV and PeV regimes during flaring states. Further observations of Cygnus X-3 at energies above tens of GeV would be valuable for this purpose.

astro-ph.HE↗

arXiv2026

MAGIC observations of NGC 4278. The first low-luminosity radio galaxy with compact jets detected at TeV energies

The Large High Altitude Air Shower Observatory (LHAASO) Collaboration has recently reported the first detection at TeV energies of a low-luminosity radio galaxy, NGC 4278. The aim of this work is to investigate the high-energy properties of NGC 4278 during the flaring and subsequent quasi-quiescent states with the Florian Goebel Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. NGC 4278 is located in the field of view of two blazars, 1ES 1215+303 and 1ES 1218+304, previously observed by the MAGIC telescopes. Therefore, we re-analyzed MAGIC observations made between 2010 and 2024 on these sources. We also modeled the broadband spectral energy distribution of the source during and after the flaring state at TeV energies. We did not detect any statistically significant $\gamma$-ray emission from NGC 4278 with MAGIC. The corresponding upper limits obtained using the entire MAGIC dataset ($F_{{\rm UL, }\, >150\, \mathrm{GeV}}=1.5 \times 10^{-12}\, \mathrm{ph \, s^{-1}\, cm^{-2}}$) are consistent with the LHAASO results. The best-fit models obtained for both emission states suggest that the emitting region is strongly particle-dominated, and an efficient acceleration mechanism has to be in action in order to reach TeV energies. The transition between the flaring and quasi-quiescent state cannot be explained by a simple radiative cooling of the emitting particles. The inferred jet power, of the order of $L_{\rm jet}\sim 10^{42}\, \mathrm{erg\,s^{-1}}$, is dominated by the kinetic component in both states and it is in a good agreement with previous, time-averaged observational estimates, supporting the idea that such high-energy flares might be recurrent. The jet, however, remains too weak to break the host-galaxy confinement.

astro-ph.HE↗

arXiv2026

Continued activity of the 25th cycle: largest in 20 years. Ground-level enhancement and Forbush decrease

After a very calm 24th solar activity cycle, the 25th cycle has already seen several interesting events. A Ground Level Enhancement GLE77 was observed on 11 November 2025 following an X5.1 class solar flare. A strong Forbush decrease occurred on 19 and 20 January 2026 during one of the most intense geomagnetic storms of Solar Cycle 25. Events were recorded coherently by the global neutron monitor network and by SEVAN detectors at multiple altitudes. Using spectrometric capabilities, we reconstruct energy spectra of missing neutrons and muons during the FD and compare them with corresponding spectra measured during GLE77. The analysis demonstrates that FD and GLE signatures are intrinsically asymmetric. FDs selectively suppress the preexisting galactic cosmic ray population, whereas GLEs introduce an additional, harder particle component. Neutron and muon channels exhibit markedly different spectral behavior, particularly at higher deposited energies, reflecting their sensitivity to different primary energy ranges. These results show that combined NM and SEVAN observations provide robust, complementary diagnostics of rigidity dependent cosmic ray modulation during extreme heliospheric disturbances.

physics.ao-ph↗

arXiv2025

Solar neutron and muon detection on November 11, 2025: First simultaneous recovery of energy spectra

Ground Level Enhancement (GLE) events provide rare opportunities to study high-energy solar particle acceleration through direct detection of secondary radiation at ground level. On November 11, 2025, the Aragats Solar Neutron Telescope (ASNT) recorded a statistically significant increase in high-energy neutron and muon fluxes associated with an X5.1 flare and the subsequent Solar Energetic Proton (SEP) event. The event displayed a unique dual-peak profile: an initial hard component at 10 28 UT, followed by a softer yet still energetic peak at 10 45 UT. For the first time, we report simultaneous energy spectra of atmospheric neutrons and muons measured in the 10 600 MeV range at Aragats. Broken-power-law fits reveal a clear temporal evolution of acceleration conditions, evidenced by spectral indices declining with energy. These findings highlight the unique capabilities of the ASNT as an instrument for studying extreme solar particle acceleration.

astro-ph.SR↗

arXiv2025

High-Energy Atmospheric Radiation: From Thunderstorm Ground Enhancements to Terrestrial Gamma-Ray Flashes

This work presents a unified conceptual and observational framework that reinterprets these radiation bursts as manifestations of the same runaway processes happening at different atmospheric depths (Dual-stage model, DSM). We review recent results from satellite (ASIM), aircraft (ALOFT), balloon (HELEN), and ground-based (SEVAN and KANAZAWA) experiments to demonstrate the advantages of this integrated approach. This study addresses key contradictions in the field, introduces new classification criteria based on physics rather than detector location, and enhances our understanding of particle acceleration in thunderstorms.

physics.space-ph↗

arXiv2025

The largest Forbush decrease in 20 years: Preliminary analysis of SEVAN network observations

We present a preliminary analysis of the largest Forbush Decrease (FD) observed in solar cycle 25 using SEVAN network data. Triggered by consecutive Earth-directed CMEs on May 30 and 31, 2025, this event produced two distinct FD minima and a prolonged recovery, with significant anisotropies in cosmic ray response across the network. The timing of interplanetary shock arrivals was confirmed by SSC signatures at 05:22 UTC on June 1 and 10:19 UTC on June 2, marking the onset of each FD phase. The depth and temporal structure of the FD varied across SEVAN detectors, providing insights into the complex heliospheric and magnetospheric dynamics during this multi-CME event.

astro-ph.SR↗

arXiv2025

Energy Spectra of Secondary Particles Induced by Solar Energetic Proton Events and Magnetospheric Effects

We investigate the energy spectra of secondary cosmic ray particles associated with two distinct solar events: the magnetospheric effect (ME) of 5 November 2023 and ground-level enhancement (GLE 74) of 11 May 2024. Using data from the SEVAN and Neutron Monitor networks and energy release histograms from particle spectrometers, we reconstruct spectra and identify key differences between ME and GLE. CORSIKA-based simulations reveal that MEs are caused by galactic protons below geomagnetic cutoff rigidities (Rc = 7.1 GV at Aragats) penetrating the magnetosphere during geomagnetic storms, leading to localized flux enhancements at mountain altitudes but not at sea level. In contrast, SEP events initiated by GLEs can involve high-energy solar protons (>10 GeV), producing secondaries that reach sea level at middle latitudes. We present integral energy spectra and spatial correlation of detector responses, demonstrating that SEVAN's energy-resolved data offer new diagnostic tools for identifying hard-spectrum SERs. Our results refine the definition of ME and suggest a strategy for early warning of hazardous solar particle events based on real-time ground-based observations.

physics.ao-ph↗

arXiv2025

Variations of the Near-Surface Electric field measured at Aragats during Geomagnetic Storms

At least two mechanisms effectively transfer interplanetary magnetic field (IMF) disturbances into the atmosphere. First, the inflow of solar wind into the ionosphere at low latitudes significantly enhances the total vertical electron content, increasing atmospheric conductivity. Second, Forbush decreases (FD) reduce the cosmic ray flux by a few percent, lowering ionization levels at middle latitudes and decreasing conductivity. Changes in atmospheric conductivity affect the global electric circuit and atmospheric electric field (AEF). However, to study the response of AEF to geomagnetic storms (GMS), it is necessary to carefully monitor atmospheric conditions before and during storms, as meteorological influences can be much stronger than those of GMS. Charged clouds above detectors, lightning flashes, and abrupt weather changes significantly impact near-surface electric field (NSEF) variations, which serve as a proxy for AEF measured at the Earth's surface. The facilities at Aragats station monitor all environmental parameters on a one-minute timescale. We analyze four GMS events described in previous studies, detailing the corresponding weather conditions to isolate the genuine influence of GMS on NSEF. The GMS of June 22, 2015, and September 8, 2017, occurred under fair-weather conditions, providing clear evidence of GMS influence on NSEF. These events were long-lasting, positive, and modest, ranging from 0.2 to 0.3 kV/m, and coincided with the depletion phase of FD. The sky was clear, no rain was detected, and lightning flashes from previous thunderstorms were more than 20 km from the station. The other two events did not meet favorable weather criteria, and their occurrence during GMS seemed incidental. We identify a feature that may indicate the solar (FD) origin of NSEF enhancement: a dip in the enhanced NSEF during the daytime.

physics.ao-ph↗

arXiv2025

Runaway processes in the upper and lower atmosphere: a change of paradigm

Relativistic Runaway Electron Avalanches (RREA) are central to understanding a spectrum of high-energy atmospheric phenomena, including Terrestrial Gamma-ray Flashes (TGFs), Thunderstorm Ground Enhancements (TGEs), and gamma-ray glows. Despite their common physical origin, these events are often treated separately due to differences in detection methods, duration, and altitude. In this work, we present a unified conceptual and observational framework that reinterprets these radiation bursts as manifestations of the same runaway processes occurring in distinct atmospheric depths. Integrating recent results from satellite (ASIM), aircraft (ALOFT), balloon (HELEN), and ground-based (SEVAN) experiments, we demonstrate consistent spectral and temporal behavior across scales. We propose a rational revision of current terminology and challenge longstanding models that attribute TGFs to lightning leader dynamics. This study resolves key contradictions in the field, establishes new classification criteria based on physics rather than detector location, and reshapes our understanding of particle acceleration in thunderstorms.

physics.ao-ph↗

arXiv2025

Wind-induced Natural Gamma Radiation

During the extreme winter storms of 2024-2025 at Aragats, natural gamma radiation (NGR) increased by more than 1000%, with fluence reaching 2*10^7 gammas/cm^2 over 10 hours and a corresponding dose of 3.26 mSv, 120 times higher than normal background radiation for the same period. This unprecedented radiation surge was detected during dry, electrified snowstorms, exceeding levels explainable by known atmospheric mechanisms, necessitating a significant reassessment of gamma-ray sources in winter storm conditions. These results suggest similar radiation surges may occur in high-altitude and polar regions (Arctic and Antarctic), where strong winds and prolonged snowstorms are common. Understanding radiation surge conditions is essential for refining atmospheric models, improving radiation monitoring, and assessing environmental and climatic impacts in extreme weather conditions.

physics.ao-ph↗

arXiv2025

Very-high-energy gamma-ray detection and long-term multi-wavelength view of the flaring blazar B2 1811+31

Among the blazars whose emission has been detected up to very-high-energy (VHE; 100 GeV < E < 100 TeV) gamma rays, intermediate synchrotron-peaked BL Lacs (IBLs) are quite rare. The IBL B2 1811+31 (z = 0.117) exhibited intense flaring activity in 2020. Detailed characterization of the source emissions from radio to gamma-ray energies was achieved with quasi-simultaneous observations, which led to the first-time detection of VHE gamma-ray emission from the source with the MAGIC telescopes. In this work, we present a comprehensive multi-wavelength view of B2 1811+31 employing data from MAGIC, Fermi-LAT, Swift-XRT, Swift-UVOT and from several optical and radio ground-based telescopes. We investigate the variability, cross-correlations and classification of the source emissions during low and high states. During the 2020 flaring state, the synchrotron peak frequency shifted to higher values and reached the limit of the IBL classification. Variability in timescales of few hours in the high-energy (HE; 100 MeV < E < 100 GeV) gamma-ray band poses an upper limit of 6 x 10^{14} delta_D cm to the size of the emission region responsible for the gamma-ray flare, delta_D being the relativistic Doppler factor of the region. During the 2020 high state, the average spectrum became harder in the X-ray and HE gamma-ray bands compared to the low states. Conversely, during different activity periods, we find harder-when-brighter trends in X rays and a hint of softer-when-brighter trends at HE gamma rays. Gamma-optical correlation indicates the same emission regions dominate the radiative output in both ranges, whereas the levolution at 15 GHz shows no correlation with the flux at higher frequencies. We test one-zone and two-zone synchrotron-self-Compton models for describing the broad-band spectral energy distribution during the 2020 flare and investigate the self-consistency of the proposed scenario.

astro-ph.HE↗

arXiv2025

Cosmic-ray acceleration and escape from supernova remnant W44 as probed by Fermi-LAT and MAGIC

Context. The supernova remnant (SNR) W44 and its surroundings are a prime target for studying the acceleration of cosmic rays (CRs). Several previous studies established an extended gamma-ray emission that is set apart from the radio shell of W44. This emission is thought to originate from escaped high-energy CRs that interact with a surrounding dense molecular cloud complex. Aims. We present a detailed analysis of Fermi-LAT data with an emphasis on the spatial and spectral properties of W44 and its surroundings. We also report the results of the observations performed with the MAGIC telescopes of the northwestern region of W44. Finally, we present an interpretation model to explain the gamma-ray emission of the SNR and its surroundings. Methods. We first performed a detailed spatial analysis of 12 years of Fermi-LAT data at energies above 1 GeV, in order to exploit the better angular resolution, while we set a threshold of 100MeV for the spectral analysis. We performed a likelihood analysis of 174 hours of MAGIC data above 130 GeV using the spatial information obtained with Fermi-LAT. Results. The combined spectra of Fermi-LAT and MAGIC, extending from 100MeV to several TeV, were used to derive constraints on the escape of CRs. Using a time-dependent model to describe the particle acceleration and escape from the SNR, we show that the maximum energy of the accelerated particles has to be ' 40 GeV. However, our gamma-ray data suggest that a small number of lower-energy particles also needs to escape. We propose a novel model, the broken-shock scenario, to account for this effect and explain the gamma-ray emission.

astro-ph.HE↗

arXiv2024

Insights from the first flaring activity of a high-synchrotron-peaked blazar with X-ray polarization and VHE gamma rays

We study a flaring activity of the HSP Mrk421 that was characterized from radio to very-high-energy (VHE; E $>0.1$TeV) gamma rays with MAGIC, Fermi-LAT, Swift, XMM-Newton and several optical and radio telescopes. These observations included, for the first time for a gamma-ray flare of a blazar, simultaneous X-ray polarization measurements with IXPE. We find substantial variability in both X-rays and VHE gamma rays throughout the campaign, with the highest VHE flux above 0.2 TeV occurring during the IXPE observing window, and exceeding twice the flux of the Crab Nebula. However, the VHE and X-ray spectra are on average softer, and the correlation between these two bands weaker that those reported in previous flares of Mrk421. IXPE reveals an X-ray polarization degree significantly higher than that at radio and optical frequencies. The X-ray polarization angle varies by $\sim$100$^\circ$ on timescales of days, and the polarization degree changes by more than a factor 4. The highest X-ray polarization degree reaches 26%, around which a X-ray counter-clockwise hysteresis loop is measured with XMM-Newton. It suggests that the X-ray emission comes from particles close to the high-energy cutoff, hence possibly probing an extreme case of the Turbulent Extreme Multi-Zone model. We model the broadband emission with a simplified stratified jet model throughout the flare. The polarization measurements imply an electron distribution in the X-ray emitting region with a very high minimum Lorentz factor, which is expected in electron-ion plasma, as well as a variation of the emitting region size up to a factor of three during the flaring activity. We find no correlation between the fluxes and the evolution of the model parameters, which indicates a stochastic nature of the underlying physical mechanism. Such behaviour would be expected in a highly turbulent electron-ion plasma crossing a shock front.

astro-ph.HE↗