Search

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.

During a Cygnus X-3 campaign many years ago the Carpet air shower array of the Baksan Neutrino Observatory had recorded two bursts from two different sources: from the Crab Nebula on February 23, 1989 and from Cygnus X-3 on October 14–16, 1985. The characteristics of the bursts were quite different, as well as types of the source objects. The first of these bursts was discussed in some detail after registration of regular flares of gamma rays with energies of several hundred MeV by the satellite gamma-ray telescopes FermiLAT and AGILE. The second burst was not discussed in recent time, and in this paper attention is attracted to it in connection with the new results of the Tibet ASγ collaboration.

An experiment for measuring the flux of cosmic diffuse gamma rays with energies higher than 100 TeV (the Carpet-3 project) is under preparation at the Baksan Neutrino Observatory. The experiment suggests to increase areas of both the muon detector and the surface shower detectors. The experiment’s sensitivity to the showers initiated by primary gamma rays is estimated in this paper for different configurations of the array. In addition, preliminary estimates of the upper limit on diffuse gamma rays with energy above 700 TeV are presented. They are derived from experimental data of the Carpet-2 array (previous version of the experiment) for the net exposure of 9.2 years of observation.

New physical effects accompanying thunderstorms are studied at the Baksan Neutrino Observatory with the Carpet air shower array. Techniques are developed that allow the potential difference in the troposphere to be estimated using data on the variations in secondary cosmic rays. Remote digital video cameras continuously observe the night sky above the array. The means of analysis is described, and some experimental evidence is presented for correlations between the high-altitude glow and variations in the detected particle flux and global disturbances of the geomagnetic field. Examples of the possible influence of seismic activity on the dynamics of thunderstorms are presented.

Carpet-2 is an air-shower array at Baksan Valley, Russia, equipped with a large-area (175 m^2) muon detector, which makes it possible to separate primary photons from hadrons. We report the first results of the search for primary photons with energies E_\gamma>1 PeV, directionally associated with IceCube high-energy neutrino events, in the data obtained in 3080 days of Carpet-2 live time., Comment: 6 pages, 2 figures. V2: minor copy-editing changes. Accepted by JETP Letters

A short review of some experiments carried out at the Baksan Neutrino Observatory during the entire time of its existence is given. The main emphasis is on the results of the very first experiments conducted at Baksan and on those that were the world-best, at least for a short period of time (or still remain so).

An experiment for measuring the flux of cosmic gamma rays with energy above 100 TeV is currently being prepared at the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences. For this purpose, based on the Carpet-2 shower array which has been operating for many years, the Carpet-3 multipurpose shower array is being constructed. In particular, the preparation of the experiment supposes an increase in the area of the muon detector (MD) and ground part of the array. For ground modules of the Carpet-3 array, a scintillation detector with a liquid scintillator in a plastic package has been elaborated. In this work, descriptions of the Carpet-3 array and working characteristics of the new scintillation detector are presented.

An experiment for measuring the flux of diffuse cosmic gamma rays with energies above 100 TeV (the Carpet-3 experiment) is now being prepared at the Baksan Neutrino Observatory, Institute for Nuclear Research, Russian Academy of Sciences. The preparation of the experiment entails an increase in the area both of the muon detector and of the surface part of the air shower array. This work presents an estimate of the experiment sensitivity to showers generated by primary gamma quanta for different configurations of the array. According to experimental data of the Carpet-2 array for 9.2 years of pure data acquisition time, preliminary estimates for the upper limit of the diffuse cosmic gamma ray flux with energies above 930 PeV have been obtained.

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 November 2020, 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-sigma (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., 7 pages, 3 figures, AASTeX. V2: minor changes; Appendix with more analysis details added. Version accepted by Astrophys. J. Letters. V3: title corrected (text unchanged), matches the published version

An experiment for measuring the flux of cosmic diffuse gamma rays with energies above 100 TeV (the Carpet-3 air shower array) is now being prepared at the Baksan Neutrino Observatory, Institute for Nuclear Research, Russian Academy of Sciences. The preparations entail a substantial increase of the areas of both the muon detector and the surface air shower array. The experiment’s sensitivity to showers generated by primary gamma rays is estimated for different configurations of the array. In addition, preliminary estimates of the upper limit on the flux of diffuse gamma rays with energies higher than 1.3 PeV, derived using experimental data from the old Carpet-2 array, are presented for a net exposure time of 9.2 years.

An array of scintillators tightly packed in the horizontal plane and designed for recording secondary particles produced by cosmic rays is capable of probing the angular dependence of intensity disturbances caused by electric fields of a thunderstorm atmosphere. The basic parameters of thunderstorm activity are estimated using a specific example of variation in intensity in different ranges of energy release in the detectors. These parameters include the characteristic radius of the active region (5 km), its total charge (30 C), the accumulated electric field energy (6 GJ), and the potential difference in the stratosphere above the active thunderstorm region (375 MV).

Thunderstorms have been studied at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) since the summer of 2013 using a measuring instrumentation complex. During the period of observation, three events of continuous night-sky airglow correlating with the disturbances in the flux of energetic particles have been recorded during thunderstorms. It is established that there is interaction between a thunderstorm’s front and the precipitation of protons from the Earth’s radiation belt into the atmosphere due to seismic activity.

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 Carpet air shower array of the Baksan Neutrino Observatory is in operation for a long time, and it was modernized more than once. A short review of important results obtained with this array is given, as well as future prospects, since at the moment the new project called Carpet-3 is in progress, using the old array as its basis. The purpose of the new project is to substantially increase the muon detector area (quite soon up to 400 m2 and later up to 600 m2). This improvement will allow one to reach a very good sensitivity to diffuse cosmic photons by selecting muon-poor showers. The energy range where the new experiment will be competitive with other experiments of gamma-ray astronomy is near and below 100 TeV.

The recently published result of GRAPES-3 (a strong thunderstorm muon intensity change) and conclusions made on its basis by this collaboration are discussed and compared with the Baksan observations of muon variations during thunderstorms performed much earlier. It is demonstrated that generally both experiments are quite consistent as far as characteristic patterns of events are concerned, as well as conclusions about direct connection between amplitude of muon intensity disturbance and electric field strength in the atmosphere. However, the GRAPES-3 conclusion about gigavolt potentials associated with their event is doubtful and seems to contradict the fundamental limits on electric fields that can exist in the atmosphere.

Recently two collaborations (Tibet ASγ 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.

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 ~100 TeV). 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., 4 pages paper + 5 pages Supplemental Material. V2: references added, version accepted by JETP Letters

Early results of the search for E_\gamma>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_\gamma>100 TeV photons, are briefly discussed., Comment: 5 pages, 2 figures, webofc style. Presented at the VLVNT-2018 workshop, Dubna, October 2-4, 2018. V2: minor copy-editing changes

An experiment for measuring the flux of gamma rays of cosmic origin with energy above 100 TeV (the Carpet-3 experiment) is currently being prepared at the Baksan Neutrino Observatory. The experimental setup suggests substantial increase of the areas of both muon detector and surface air shower array. We present some results of calculations of selection efficiency of air showers from primary gamma rays for different configurations of the array. It is demonstrated that by increasing the muon detector area up to 615 m$^2$ (the maximum possible value) one can reach with the Carpet-3 the world-best sensitivity to 100 TeV gamma rays. The preliminary values of upper limits on the flux of cosmic diffuse gamma rays with energy higher than 930 TeV are also presented, derived from experimental data of the Carpet-2 shower array for a net exposure time of 9.2 years.

An experiment for measuring the flux of cosmic gamma rays with energy above 100 TeV is currently being prepared at the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences. At present the plastic scintillation counters with a total continuous area of 410 m2 are installed in the muon detector (MD) underground tunnels, and they are totally equipped with electronics. Six modules of shower detectors have been already placed on the surface of the MD absorber. In each of them 9 standard plastic scintillation counters are placed with an area of 1 m2 each. These modules are components of the ground surface part of Carpet-3 shower array. The data acquisition system for this array is almost ready. Expected flux limits for cosmic diffuse gamma rays of galactic origin in this work is close to existing experimental limits in the range lower than about 5 PeV, and with improved sensitivity of Carpet-3 one can hope to detect this flux or at least to obtain the best upper limit.

Processes that occur during thunderstorms accompanied by anomalous disturbances in cosmic rays are comprehensively investigated using the Carpet air shower array at the Baksan Neutrino Observatory. The data collected so far indicate the discovery of a new type of electric discharge: runaway electron break-down in the near-threshold regime. A glow in the stratosphere corresponding to this discharge is detected, as is stabilization of geomagnetic micro-pulsations. Both are demonstrated using one event as an example.

In the course of an experiment studying variations of cosmic rays during thunderstorms at the Baksan Neutrino Observatory evidence has been obtained in favor of existence of a new type of large-scale electric discharge in thunderstorm atmosphere: a slow runaway electron breakdown in the near-threshold regime. Continuous glow of the stratosphere region above thunderclouds is discovered, correlating with anomalous disturbances in intensity of secondary cosmic rays detected on the ground.

Characteristics of the flares of gamma rays detected from the Crab Nebula by the AGILE and Fermi-LAT satellite instruments are compared with those of a gamma ray burst recorded by several air shower arrays on February 23, 1989 and with one recent observation made by the ARGO-YBJ array. It is demonstrated that though pulsar-periodicity and energy spectra of emissions at 100 MeV (satellite gamma ray telescopes) and 100 TeV (EAS arrays) are different, their time structures seem to be similar. Moreover, maybe the difference between “flares” and “waves” recently found in the Crab Nebula emission by the AGILE team also exists at ultra-high energies.

Strong variations of the intensity of secondary cosmic rays during thunderstorms are found to be accompanied in some cases by very clear pulsations of the geomagnetic field. The experiment is carried out in the Baksan Valley, North Caucasus, the Carpet air shower array being used as a particle detector. Magnetic field measurements are made with high-precision magnetometers located deep underground in the tunnel of the Baksan Neutrino Observatory, several kilometers apart from the air shower array., 8 pages, 2 figures

It was discovered that strong variations in the intensity of secondary cosmic rays observed during thunderstorms and earlier interpreted as a consequence of the cyclic generation of electrons and positrons in a strong electric field are accompanied in some cases by well-pronounced pulsations of the geomagnetic field. An experiment was performed with the CARPET array for studying extensive air showers of cosmic rays. This array, located in the Baksan Valley (North Caucasus), was used as a particle detector. Magnetic measurements were made using a high-precision magnetic variation station located deep underground in a tunnel of the Baksan Neutrino Observatory at a distance of about 4 km from the CARPET array.

A method of reconstructing the declination of galactic cosmic ray anisotropy is described, and its results are presented. The method is based on analysis of delay distributions in symmetrically arranged detectors of an air shower array, and it represents a modification of the crossed telescopes method. It is shown that the declination of the true anisotropy vector is close to 60° (i.e., this vector lies approximately within the galactic plane). Because of this, the true degree of anisotropy of galactic cosmic rays is severalfold higher than the first harmonic of intensity in the sidereal time (the quantity measured directly), and it equals about 0.2%.

Energy spectra (estimated experimentally and derived by numerical modeling) are analyzed for the most significant events of anomalous increases of the soft component of secondary cosmic rays detected during thunderstorms in Baksan Valley (North Caucasus) in the period 2003–2007. The estimates of experimental energy spectra are made using the counting rates in two neighboring ranges of energy release in scintillation detectors, 10–17 MeV and 17–30 MeV.

The results of measuring the diurnal cosmic-ray intensity variations in the energy range 1–100 TeV are discussed. Whereas the phase of the first harmonic of the sidereal daily wave directly determines the phase (right ascension) of the cosmic-ray anisotropy vector, the amplitude and declination of the true anisotropy cannot be reconstructed directly from the amplitude of the first harmonic. However, they can be determined by invoking data on the zero harmonic. The results of some recent experiments purporting to measure the cosmic-ray anisotropy with a particularly high accuracy are shown to be interpreted erroneously.

The upper limit of transformation of the equilibrium electron-photon spectrum (E > 10 MeV) is obtained for the case of strong fields by solving the one-dimensional kinetic equation. Comparison with the experimental data from the Carpet array at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) shows that variations in the intensity of the electron-photon component are observed during thunderstorms, which cannot be explained only by transformation of the spectrum in strong thunderstorm fields. One has to hypothesize some process of particle production in the energy range below ∼30 MeV.

The transformation of the energy spectrum of the electron-positron component of cosmic rays in the energy range 10–100 MeV in the near-Earth electric field of thunderstorm atmosphere has been estimated analytically by solving the kinetic equation for secondary cosmic rays in the one-dimensional case. The solution obtained adequately describes the observed variations in the soft component in moderate electric fields (±7 kV m−1).

The present-day status of the problem of searching for primary cosmic gamma rays at energies above 100 TeV is discussed, as well as a proposal for a new experiment in this field. It is shown that an increase of the area of the muon detector of the Carpet-2 air shower array up to 410 square meters, to be realized in 2016, will make this array quite competitive with past and existing experiments, especially at modest energies. Some preliminary results of measurements made with smaller area of the muon detector are presented together with estimates of expected results to be obtained with the coming large-area muon detector., 7 pages. To appear in Proceedings of The International Workshop on Quark Phase Transition in Compact Objects and Multimessenger Astroronomy: Neutrino Signals, Supernovae and Gamma-Ray Bursts (Nizhny Arkhyz - Terskol, October 7-14, 2015)

We present the data on correlations of the intensity of the soft component of cosmic rays with the local electric field of the near-earth atmosphere during thunderstorm periods at the Baksan Valley (North Caucasus, 1700 m a.s.l.). The large-area array for studying the extensive air showers of cosmic rays is used as a particle detector. An electric field meter of the ‘electric mill’ type (rain-protected) is mounted on the roof of the building in the center of this array. The data were obtained in the summer seasons of 2000–2002. We observe strong enhancements of the soft component intensity before some lightning strokes. At the same time, the analysis of the regression curve ‘intensity versus field’ discovers a bump at the field sign that is opposite to the field sign corresponding to acceleration of electrons. It is interpreted as a signature of runaway electrons from the region of the strong field (with opposite sign) overhead.

Recent experiments with the Fermi-LAT and AGILE satellite gamma-ray telescopes, in which bursts of gamma-rays from the Crab Nebula were detected in the energy range of ∼100 MeV, have aroused keen interest, if not a sensation. However, data on a possible burst in the Crab Nebula at much higher energies of ∼100 TeV were published more than 20 years ago. Characteristics of transient and stationary fluxes of gamma rays from the Crab Nebula in various energy ranges are discussed in this work, and it is shown that the old data obtained at ultra-high energies is reasonably consistent with the latest pattern of the source’s burst activity.

The strong variations in cosmic rays observed by the Carpet air shower array at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) during thunderstorms are shown to have different origins. One type of event in particular is discussed: when disturbances in the intensities of the muon and electron-photon components of cosmic rays take place simultaneously. The correlations of these disturbances with variations in the geomagnetic field are presented and interpreted as a manifestation of a single physical process in the upper atmosphere. Details of this process are discussed.

A review of the present-day situation in the experimental studies of variations of cosmic rays during thunderstorms is given. It is shown that these experiments can be a useful tool for studying very interesting physical effects in the electrically disturbed atmosphere. The main emphasis is laid on the experimental verification of the runaway breakdown theory and on the role of cosmic rays in the processes of lightning initiation.

We present the data on short-term variations of the intensity of secondary cosmic rays during thunderstorm periods. The effect is studied separately for the soft (electrons) and hard (muons) components of cosmic rays, their intensities being correlated with the near-earth electric field using a second-order polynomial. It is shown that the linear and quadratic terms are dominant for the soft and hard components, respectively. In addition, some irregular enhancements of the soft component just before lightning discharges are observed giving evidence in favor of some particle acceleration process. They can be considered as an experimental confirmation of the runaway breakdown theory developed for the process of lightning initiation.

At present an experiment for measuring the flux of cosmic diffuse gamma rays with energy higher than 100 TeV (experiment "Carpet-3") is being prepared at the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences. The preparation of the experiment implies considerable enlargement of the area of both muon detector and surface part of the shower array. At the moment the plastic scintillation counters with a total continuous area of 410 m2 are installed in the muon detector (MD) underground tunnels, and they are totally equipped with electronics. Adjusting of the counters and their electronic circuits is in progress. Six modules of shower detectors (out of twenty planned to be installed) have already been placed on the surface of the MD absorber. A new liquid scintillation detector is developed for modules of the ground –surface part of the array, whose characteristics are presented. It is shown that the "Carpet-3" air shower array will have the best sensitivity to the flux of primary gamma rays with energies in the range 100TeV – 1PeV, being quite competitive in gamma-ray astronomy at such energies.

An analysis is made of experimental data on variations in muons with a threshold of 100 MeV, detected by the CARPET air shower array in the Baksan Valley during thunderstorms of 2008. It is shown that with using a special criterion for selecting statistically significant variations (0.2%), muon intensity variations are observed in the majority of cases. Their characteristic duration is ∼8 min. Their amplitudes can have different polarities and do not exceed 1%.

Using a modified GEANT4 code, we calculated the probabilities for cosmic ray electrons to become runaway particles in an electric field with a strength ranging from 0 to 5 kV/cm. About 10 million trajectories were calculated for particles with initial energies of 0.1 to 100 MeV. Instead of a single separatrix determining the region of runaway electrons, a set of curves connecting points with equal probabilities of a runaway regime (from 0 to 100%) was obtained. It was shown that not only is the threshold for a runaway regime probabilistic in character, but the critical field as well. This must be considered when calculating variations in the intensity of cosmic rays during thunderstorms.