—Over the last several years, an increase in the amplitude of a seasonal pressure wave and the concentration of heavy radionuclides in the near-surface layer of the atmosphere has been observed in the area of the Baksan Neutrino Observatory of Institute for Nuclear Research in the North Caucasus. This trend is being investigated along with accompanying anomalies in readings recorded by detectors in the fall and winter of 2017, and the winter and spring of 2021. Hypotheses regarding possible causes of these anomalies are considered.

Isotropic diffuse gamma-ray flux in the PeV energy band is an important tool for multimessenger tests of models of the origin of high-energy astrophysical neutrinos and for new-physics searches. So far, this flux has not yet been observed. Carpet-2 is an air-shower experiment capable of detecting astrophysical gamma rays with energies above 0.1 PeV. Here we report the upper limits on the isotropic gamma-ray flux from Carpet-2 data obtained in 1999–2011 and 2018–2022. These results, obtained with the new statistical method based on the shape of the muon-number distribution, summarize Carpet-2 observations as the upgraded installation, Carpet-3, starts its operation.

Density functional CAM-B3LYP/6-311++G(d,p)/SDD quantum chemical calculations of a gold(I) complex with the pentakis(methoxycarbonyl)cyclopentadienyl ligand Ph3PAu[C5(CO2Me)5] and gold(I) complexes with polydentate amidinyltetrakis(methoxycarbonyl)cyclopentadienyl ligands Ph3PAu[N(Ar)C(Ar’)N(Ar)C5(CO2Me)4] were carried out. It was shown that the former system, where the Au atom is bonded to the C(sp3) atom of the cyclopentadiene ring and additionally coordinated by two neighboring ring carbon atoms, exhibits fluxional behavior due to rapid 1,5-sigmatropic shifts of the AuPPh3 group along the perimeter of the five-membered ring occurring with an energy barrier (ΔG≠298 K) of 5.1 kcal mol−1. Contrary to this, all other gold(I) complexes are more stable as ylide isomers, where the gold atom is bonded to the terminal nitrogen atom of the amidinium fragment and additionally coordinated by the π-system of the cyclopentadiene ring. According to the CAM-B3LYP/6-311G(d,p)/SDD calculations, alternative isomers, in which the gold atom is bonded to the carbon atom of the cyclopentadiene ring or to the oxygen atom of the methoxycarbonyl substituent, are much less stable (ΔEZPE = = 12.8−20.0 kcal mol−1).

In the present work we report on the observation of an excess of gamma-ray candidate events in temporal and spatial coincidence with the IceCube high-energy neutrino alert consistent with the origin in the Cygnus Cocoon region. The observations have been performed with Carpet-2, a surface air-shower detector equipped with a large-area muon detector at the Baksan Neutrino Observatory in the Northern Caucasus. As well as we report about the current state of the Carpet-3 facility, which includes a muon detector with an increased area and an expanded surface array. The main aim of the Carpet-3 facility is the registration of cosmic gamma-rays with energy larger than 100 TeV. Moreover, it gives a possibility to carry out research on the composition of primary cosmic rays around the knee. It is planned that the Carpet-3 EAS array will be operation by the end of 2021.

The Carpet-3 array at the Baksan Neutrino Observatory is designed to detect extensive air showers generated by primary cosmic rays of more than 100 TeV in energy. The main aim is to search for cosmic extrahigh-energy gamma rays. In this work, we describe the current state of the facility and the results from searching for extrahigh-energy photons from regions of the localization of candidates for gravitational-wave events detected by LIGO/Virgo during the third observational period.

The Carpet-3 experiment for investigation of gamma radiation with energy above 100 TeV is currently being prepared at the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Science. At present the plastic scintillation counters with a total continuous area of 410 m $${}^{2}$$ are installed in the muon detector (MD) underground tunnel, and they are totally equipped with electronics. The counters’ gains and thresholds have been adjusted. Fifteen modules of shower detectors are placed on the surface of the MD absorber. Ten of them contain 9 standard plastic counters with an area of 1 m $${}^{2}$$ each. Also 24 modules without counters are arranged on the territory of the array. These modules will accomplish a surface part of the Carpet-3 array. The preliminary estimates show that the new array will have the best sensitivity to the flux of primary gamma rays with energy in region 100 TeV $$-$$ 1 PeV. The increased area of the surface part of the array gives a possibility to increase the area of location of the shower axis, thereby increasing the statistics of detected events and decreasing the energy threshold for primary cosmic radiation. The Carpet-3 experiment will start data taking at the end of 2021.

3-Allyl-3-methyl-1,2-diphenylcyclopropene (1) and 3-allyl-2-methyl-1,3-diphenylcyclopropene (2) were shown by the DFT B3LYP/6-311++G(d,p) method to interconvert with the energy barriers ΔE≠ZPE(1→2) = 35.6 kcal mol−1 and ΔE≠ZPE(2→1) = 31.9 kcal mol−1via the Cope rearrangement to form an intermediate. This barrier is capable of transiting to stable 1,2-diphenyl-6-methyltricyclo[2.2.0.0]hexane with the barrier of ΔE≠ZPE = 11.8 kcal mol−1via synchronous processes of five-membered ring inversion and ring closure. The calculated higher barriers for the alternative concerted 1,3- and 3,3-sigmatropic shifts of the allyl group (ΔE≠ZPE = 39.9–57.0 kcal mol−1) indicate the energetic preference of the aforementioned pathway in accord with the experimental data.

The Baksan Neutrino Observatory setups are currently performing search for neutrino and ultra-high-energy gamma-ray counterparts of the events on the Gamma-ray Coordinates Network. The Gamma-ray Coordinates Network is a system for distributing alerts from gamma-ray bursts, transients. Muon neutrinos and antineutrinos with energies above 1 GeV are registered with the Baksan Underground Scintillation Telescope. Ultra-high-energy gamma rays are registered with the ‘‘Carpet-2’’ setup. Registration of events and analysis of alerts occurs in near-real time. Alerts from Swift BAT, Fermi-GBM, Fermi-LAT, INTEGRAL, IceCube, HAWC are used. This work presents the description of the alert processing program and preliminary results.

The Carpet-3 air shower array is being built at the Baksan Neutrino Observatory, which is located near Mount Elbrus (North Caucasus) at an altitude of 1700 m above sea level. The main aim of the experiment is gamma-ray astronomy in the energy range above 100 TeV to search for diffuse gamma radiation and sources and to study the generation mechanisms of this radiation. The paper provides an overview of the current state of the experiment, as well as its prospects.

The mechanisms of origin of ultrahigh-energy gamma radiation are poorly studied. One way to find out is to search for temporal and directional coincidences of high-energy galactic neutrinos with photons of similar energies. The results of such a search could provide indications of the hadronic origin of this radiation. In this paper, we report on the search for photons with energies above 300 TeV in coincidence with high–energy neutrinos. The searches of ultrahigh–energy gammas were carried out at the Carpet–2 EAS array, using three years of data taking.

Galactic sites of acceleration of cosmic rays to energies of order 10 15 eV and higher, dubbed PeVatrons, reveal themselves by recently discovered gamma radiation of energies above 100 TeV. However, joint gamma-ray and neutrino production, which marks unambiguously cosmic-ray interactions with ambient matter and radiation, was not observed until now. In 2020 November, the IceCube neutrino observatory reported an ∼150 TeV neutrino event from the direction of one of the most promising Galactic PeVatrons, the Cygnus Cocoon. Here we report on the observation of a 3.1 σ (post-trial) excess of atmospheric air showers from the same direction, observed by the Carpet–2 experiment and consistent with a few months flare in photons above 300 TeV, in temporal coincidence with the neutrino event. The fluence of the gamma-ray flare is of the same order as that expected from the neutrino observation, assuming the standard mechanism of neutrino production. This is the first evidence for the joint production of high-energy neutrinos and gamma-rays in a Galactic source.

Reversible non-degenerate 3,3-sigmatropic shifts of the allyl group along the perimeter of the five-membered ring occurring with energy barriers ΔG°≠ = 28.5–30.2 kcal/mol (o-dichlorobenzene-d4) have been detected in the allyl derivatives of 5-methyl-1,2,3,4-tetramethoxycarbonylcyclopentadiene by NMR method. Using DFT B3LYP/6-311++G(d,p) method, it has been shown that degenerate migrations of the allyl group in the related 5-allyl-1,2,3,4,5-pentamethoxycarbonylcyclopentadiene should occur via 3,3-sigmatropic shift through transition states with conformation of a six-membered ring (chair or boat, with close barriers ΔG°≠ = 27.4 or 27.7 kcal/mol, respectively). The simulated higher barrier of alternative 1,5-sigmatropic shifts of the allyl group (ΔG°≠ = 30.8 kcal/mol) indicates the energy preference of the migrations via 3,3-shifts.

Some unexpected sporadic increases of an environmental radioactive background have been recorded at mountain level at Baksan Neutrino Observatory (BNO, 1700 m above sea level) using electron-neutron detectors (en-detectors), which could be explained by radioactive aerosol enhancements. The large area inorganic scintillator en-detectors developed for cosmic ray study are continuously monitoring environmental thermal neutron fluxes at various geophysical conditions. Application of the pulse shape discrimination method allows us to select and separately measure both thermal neutrons and radioactive beta-decay nuclides being products of radon decays in air (mostly Rn-222 and Rn-220). There are two en-detector setups running now at BNO, one deep underground while another one at surface. Both installations had recorded some strange sporadic increases of radioactive nuclides in air. In this paper, we present results and the most probable explanation of the significant increases joint by radioactive aerosols production but caused by different reasons: Baksan river floods or nearby underground experiment with powerful Cr-51 radioactive source.

We report on the search of astrophysical gamma rays with energies in the 100 TeV to several PeV range arriving in directional and temporal coincidence with public alerts from HAWC (TeV gamma rays) and IceCube (neutrinos above ∼ 100TeV). The observations have been performed with the Carpet-2 air-shower detector at the Baksan Neutrino Observatory, working in the “photon-friendly” mode since 2018. Photon candidate showers are selected by their low muon content. No significant excess of the photon candidates have been observed, and upper limits on gamma-ray fluences associated with the alerts are obtained. For events with good viewing conditions, the Carpet-2 effective area for photons is of the order of the IceCube effective area for neutrinos of the same energy, so the constraints start to probe the production of neutrinos in fast flares of Galactic sources.

Recently two collaborations (Tibet AS gamma and HAWC) almost simultaneously reported about their measurements of ultra-high-energy (> 100 TeV) gamma rays from the Crab Nebula. The Baksan air shower array (BASA) recorded a burst from this very source as long ago as more than 20 years. At the moment the new experiment on gamma ray astronomy (Carpet-3 project) started at Baksan, and it is appropriate to recall old results of BASA to be compared with the new results, already published and expected.

Early results of the search for E γ > 1 PeV cosmic photons from point sources with the data of Carpet–2 , an air-shower array equipped with a 175 m 2 muon detector, are presented. They include 95% CL upper limits on PeV photon fluxes from stacked directions of high-energy IceCube neutrino events and from four predefined sources, Crab, Cyg X-3, Mrk 421 and Mrk 501. An insignificant excess of events from Mrk 421 will be further monitored. Prospects of the use of the upgraded installation, Carpet–3 (410 m 2 muon detector), scheduled to start data taking in 2019, for searches of E γ > 100 TeV photons, are briefly discussed.