arXiv subjects

O. Kalashev

Publications and source records attributed to O. Kalashev.

At least 19 recordsLinked to original sources

arXiv2026

The performance of the TA$\times$4 surface detector array: 4.3 years of the first-half expansion

The Telescope Array (TA) experiment aims to reveal the origin of ultra-high-energy cosmic rays (UHECRs) by observing air showers using surface detectors (SDs), which spread over an area of approximately 700 km$^2$, and fluorescence detectors (FDs) viewing the skies above the SD array. The TA experiment has been observing UHECRs since 2008, and has reported an indication of clustering in the arrival directions of cosmic-ray events with energy greater than 57 EeV. To improve the exposure for anisotropy studies of UHECRs, the TA$\times$4 upgrade was designed to expand the observational area by approximately 2,000 km$^2$ with 500 additional SDs. Half of the planned upgrade, consisting of 257 SDs, was completed, and the newly installed array began operation in 2019. In addition to the expanded SD array, two FD stations were constructed for the TA$\times$4 experiment. In this paper, we present a study of the performance of the expanded SD array, including the energy resolution, angular resolution, and effective aperture, over the first 4.3 years of data acquisition. While the effective aperture varied initially due to changing detector states, it has stabilized since June 2023 with more than 90% operational SDs. Furthermore, a new inter-tower trigger system was implemented to connect six new communication towers to form two geographically separated arrays, increasing the effective aperture. The time variation of this effective aperture, the resulting total exposure of approximately 3,500 km$^2$ sr yr, and a comparison with the original TA SD array are presented to demonstrate the performance of the expanded array.

astro-ph.IM↗

arXiv2026

Cosmic ray mass composition measurement in the energy range from $10^{16.5}$ eV to $10^{18.5}$ eV observed with the TALE hybrid detector

We report on the cosmic ray mass composition measured by the Telescope Array Low-energy Extension (TALE) hybrid detector. The TALE detector consists of a fluorescence detector (FD) station with 10 FD telescopes located at the Telescope Array (TA) Middle Drum FD Station (itself made up of 14 FD telescopes), and a surface detector (SD) array of scintillators. The array consists of 40 SDs with 400 m spacing and 40 SDs with 600 m spacing. In this paper, we present results on the measurement of the depth of shower maxima ($X_\mathrm{max}$) in the energy range from $10^{16.5}$ eV to $10^{18.5}$ eV collected over five years of the TALE hybrid detector. The $X_\mathrm{max}$ distributions were analyzed and compared with Monte Carlo simulations of proton, helium, nitrogen, and iron primaries, using the QGSJet II-04 hadronic interaction model. Our results indicate that the elongation rate of the mean $X_\mathrm{max}$, which is defined as the slope of $\langle X_\mathrm{max} \rangle$ versus cosmic ray energy, exhibits a break around $10^{17}$ eV. Up to this energy, the composition becomes increasingly heavy, characterized by a growing dominance of heavy nuclei and a steadily decreasing fraction of light primaries. Beyond this energy, the proton fraction increases significantly with energy. These findings suggest a transition from Galactic to extra-Galactic cosmic ray sources around the so-called second knee.

astro-ph.HE↗

arXiv2025

Searching for EeV photons with Telescope Array Surface Detector and neural networks

Ultra-high-energy photons play an important role in probing astrophysical models and beyond-Standard-Model scenarios. We report updated limits on the diffuse photon flux using Telescope Array's Surface Detector data collected over 14 years of operation. Our method employs a neural network classifier to effectively distinguish between proton-induced and photon-induced events. The input data include both reconstructed composition-sensitive parameters and raw time-resolved signals registered by the Surface Detector stations. To mitigate biases from Monte Carlo simulations, we fine-tune the network with a subset of experimental data. The number of observed photon candidates is found to be consistent with the expected hadronic background, yielding upper limits on photon flux $\Phi_\gamma(E_\gamma > 10^{19} \text{eV}) < 2.3 \cdot 10^{-3} $, and $\Phi_\gamma(E_\gamma > 10^{20} \text{eV}) < 3.0 \cdot 10^{-4} $ $ (\text{km}^2 \cdot \text{sr} \cdot \text{yr})^{-1} $.

astro-ph.GA↗

arXiv2025

Neutrino emission and corona heating induced by high-energy proton interactions in Seyfert galaxies

Recent detection of very-high-energy neutrino emission from Seyfert type active galactic nuclei (AGN) provides a new insight into the physics of the AGN central engines. We notice that if high-energy protons responsible for neutrino emission are accelerated close to the surface of the accretion disk, the neutrino flux may have no unambiguously identifiable electromagnetic counterpart. This is because the electromagnetic power released in interactions of high-energy protons would only contribute to the heating of the disk surface and corona above the disk, rather than escape from the source. Given that the heat deposited in the corona is released in the hard X-ray range we notice that there still might be an "indirect" electromagnetic counterpart of the neutrino signal: the hard X-ray flux variability may be strongly or weakly correlated with the neutrino flux variations, depending on the importance of the high-energy proton heating in the disk/corona heat balance. If heating by high-energy protons provides a sizable contribution to the overall corona heating rate, the overall flux of diffuse GeV neutrino background from Seyfert galaxies may be comparable to the X-ray background flux and the high-energy tail of this background can provide a sizable contribution to the astrophysical neutrino flux in the TeV band.

astro-ph.HE↗

arXiv2024

Mass composition of ultra-high energy cosmic rays from distribution of their arrival directions with the Telescope Array

We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array experiment (TA) with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extra-galactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields (GMF). The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E > 100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as $2 \times 10^{-5}$ Mpc$^{-3}$, that is the conservative lower limit for the source number density.

astro-ph.HE↗

arXiv2024

Isotropy of cosmic rays beyond $10^{20}$ eV favors their heavy mass composition

We report an estimation of the injected mass composition of ultra-high energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extra-galactic magnetic fields the results are consistent with a relatively heavy injected composition at E ~ 10 EeV that becomes lighter up to E ~ 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extra-galactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

astro-ph.HE↗

arXiv2024

Observation of Declination Dependence in the Cosmic Ray Energy Spectrum

We report on an observation of the difference between northern and southern skies of the ultrahigh energy cosmic ray energy spectrum with a significance of ${\sim}8σ$. We use measurements from the two largest experiments$\unicode{x2014}$the Telescope Array observing the northern hemisphere and the Pierre Auger Observatory viewing the southern hemisphere. Since the comparison of two measurements from different observatories introduces the issue of possible systematic differences between detectors and analyses, we validate the methodology of the comparison by examining the region of the sky where the apertures of the two observatories overlap. Although the spectra differ in this region, we find that there is only a $1.8σ$ difference between the spectrum measurements when anisotropic regions are removed and a fiducial cut in the aperture is applied.

astro-ph.HE↗

arXiv2023

First High-speed Video Camera Observations of a Lightning Flash Associated with a Downward Terrestrial Gamma-ray Flash

In this paper, we present the first high-speed video observation of a cloud-to-ground lightning flash and its associated downward-directed Terrestrial Gamma-ray Flash (TGF). The optical emission of the event was observed by a high-speed video camera running at 40,000 frames per second in conjunction with the Telescope Array Surface Detector, Lightning Mapping Array, interferometer, electric-field fast antenna, and the National Lightning Detection Network. The cloud-to-ground flash associated with the observed TGF was formed by a fast downward leader followed by a very intense return stroke peak current of -154 kA. The TGF occurred while the downward leader was below cloud base, and even when it was halfway in its propagation to ground. The suite of gamma-ray and lightning instruments, timing resolution, and source proximity offer us detailed information and therefore a unique look at the TGF phenomena.

physics.ao-ph↗

arXiv2023

Modelling the propagation of very-high-energy gamma rays with the CRbeam code: Comparison with CRPropa and ELMAG codes

Very-high-energy gamma rays produce electron positron pairs in interactions with low-energy photons of extragalactic background light during propagation through the intergalactic medium. The electron-positron pairs generate secondary gamma rays detectable by gamma-ray telescopes. This secondary emission can be used to detect intergalactic magnetic fields (IGMF) in the voids of large-scale structure. A new gamma-ray observatory, namely, Cherenkov Telescope Array (CTA), will provide an increase in sensitivity for detections of these secondary gamma-ray emission and enable the measurement of its properties for sources at cosmological distances. The interpretation of the CTA data, including detection of IGMF and study of its properties and origins, will require precision modeling of the primary and secondary gamma-ray fluxes. We asses the precision of the modeling of the secondary gamma-ray emission using model calculations with publicly available Monte-Carlo codes CRPropa and ELMAG and compare their predictions with theoretical expectations and with model calculations of a newly developed CRbeam code. We find that model predictions of different codes differ by up to 50% for low-redshift sources, with discrepancies increasing up to order-of-magnitude level with the increasing source redshifts. We identify the origin of these discrepancies and demonstrate that after eliminating the inaccuracies found, the discrepancies between the three codes are reduced to 10% when modeling nearby sources with z~0.1. We argue that the new CRbeam code provides reliable predictions for spectral, timing and imaging properties of the secondary gamma-ray signal for both nearby and distant sources with z~1. Thus, it can be used to study gamma-ray sources and IGMF with a level of precision that is appropriate for the prospective CTA study of the effects of gamma-ray propagation through the intergalactic medium.

astro-ph.HE↗

arXiv2022

Deep learning method for identifying mass composition of ultra-high-energy cosmic rays

We introduce a novel method for identifying the mass composition of ultra-high-energy cosmic rays using deep learning. The key idea of the method is to use a chain of two neural networks. The first network predicts the type of a primary particle for individual events, while the second infers the mass composition of an ensemble of events. We apply this method to the Monte-Carlo data for the Telescope Array Surface Detectors readings, on which it yields an unprecedented low error of 7% for 4-component approximation. We also discuss the problems of applying the developed method to the experimental data, and the way they can be resolved.

astro-ph.IM↗

arXiv2021

Observation of Variations in Cosmic Ray Single Count Rates During Thunderstorms and Implications for Large-Scale Electric Field Changes

We present the first observation by the Telescope Array Surface Detector (TASD) of the effect of thunderstorms on the development of cosmic ray single count rate intensity over a 700 km$^{2}$ area. Observations of variations in the secondary low-energy cosmic ray counting rate, using the TASD, allow us to study the electric field inside thunderstorms, on a large scale, as it progresses on top of the 700 km$^{2}$ detector, without dealing with the limitation of narrow exposure in time and space using balloons and aircraft detectors. In this work, variations in the cosmic ray intensity (single count rate) using the TASD, were studied and found to be on average at the $\sim(0.5-1)\%$ and up to 2\% level. These observations were found to be both in excess and in deficit. They were also found to be correlated with lightning in addition to thunderstorms. These variations lasted for tens of minutes; their footprint on the ground ranged from 6 to 24 km in diameter and moved in the same direction as the thunderstorm. With the use of simple electric field models inside the cloud and between cloud to ground, the observed variations in the cosmic ray single count rate were recreated using CORSIKA simulations. Depending on the electric field model used and the direction of the electric field in that model, the electric field magnitude that reproduces the observed low-energy cosmic ray single count rate variations was found to be approximately between 0.2-0.4 GV. This in turn allows us to get a reasonable insight on the electric field and its effect on cosmic ray air showers inside thunderstorms.

astro-ph.HE↗

arXiv2021

Evidence for Declination Dependence of the Ultrahigh Energy Cosmic Ray Spectrum in the Northern Hemisphere

Telescope Array (TA) is the largest experiment in the Northern Hemisphere studying ultrahigh energy cosmic rays. TA measurements of the cosmic ray spectrum using the surface detector have the best statistical power in the experiment, and observe the ankle of the spectrum and the high energy cutoff. When the data are divided into two declination bands, above and below 24.8 degrees, the cutoff appears at $10^{19.64 \pm 0.04}$ ($10^{19.84 \pm 0.02}$) eV in the lower (higher) band, an energy difference of 58\%. The global significance of the difference is 4.3 standard deviations. The lack of an instrumental cause of this difference implies it is astrophysical in nature.

astro-ph.HE↗

arXiv2021

Indications of a Cosmic Ray Source in the Perseus-Pisces Supercluster

The Telescope Array Collaboration has observed an excess of events with $E \ge 10^{19.4} ~{\rm eV}$ in the data which is centered at (RA, dec) = ($19^\circ$, $35^\circ$). This is near the center of the Perseus-Pisces supercluster (PPSC). The PPSC is about $70 ~{\rm Mpc}$ distant and is the closest supercluster in the Northern Hemisphere (other than the Virgo supercluster of which we are a part). A Li-Ma oversampling analysis with $20^\circ$-radius circles indicates an excess in the arrival direction of events with a local significance of about 4 standard deviations. The probability of having such excess close to the PPSC by chance is estimated to be 3.5 standard deviations. This result indicates that a cosmic ray source likely exists in that supercluster.

astro-ph.HE↗

arXiv2021

Surface detectors of the TAx4 experiment

Telescope Array (TA) is the largest ultrahigh energy cosmic-ray (UHECR) observatory in the Northern Hemisphere. It explores the origin of UHECRs by measuring their energy spectrum, arrival-direction distribution, and mass composition using a surface detector (SD) array covering approximately 700 km$^2$ and fluorescence detector (FD) stations. TA has found evidence for a cluster of cosmic rays with energies greater than 57 EeV. In order to confirm this evidence with more data, it is necessary to increase the data collection rate.We have begun building an expansion of TA that we call TAx4. In this paper, we explain the motivation, design, technical features, and expected performance of the TAx4 SD. We also present TAx4's current status and examples of the data that have already been collected.

astro-ph.IM↗

arXiv2020

Sensitivity reach of gamma-ray measurements for strong cosmological magnetic fields

A primordial magnetic field with the strength in the 1-10 pG range can resolve the tension between different measurements of the Hubble constant and provide an explanation for the excess opacity in the 21 cm line at redshift $15<z<20$, if it is present during the recombination and reionization epochs. This field can also survive in the voids of the large-scale Structure in the present day universe. We study the sensitivity reach of the gamma-ray technique for measurement of such relatively strong cosmological magnetic field using deep exposure(s) of the nearest hard spectrum blazar(s) with CTA telescopes. We show that the gamma-ray measurement method can sense the primordial magnetic field with a strength of up to $10^{-11}$~G. Combination of the cosmic microwave background and gamma-ray constraints can thus sense the full range of possible cosmological magnetic fields to confirm or rule out their relevance to the problem of the origin of cosmic magnetic fields, as well as their influence on recombination and reionization epochs.

astro-ph.CO↗

arXiv2020

Observations of the Origin of Downward Terrestrial Gamma-Ray Flashes

In this paper we report the first close, high-resolution observations of downward-directed terrestrial gamma-ray flashes (TGFs) detected by the large-area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud-to-ground and low-altitude intracloud flashes, and that the IBPs are produced by a newly-identified streamer-based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark-like transient conducting events (TCEs) within the fast streamer system, and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub-pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub-pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

physics.ao-ph↗

arXiv2020

Search for Large-scale Anisotropy on Arrival Directions of Ultra-high-energy Cosmic Rays Observed with the Telescope Array Experiment

Motivated by the detection of a significant dipole structure in the arrival directions of ultrahigh-energy cosmic rays above 8 EeV reported by the Pierre Auger Observatory (Auger), we search for a large-scale anisotropy using data collected with the surface detector array of the Telescope Array Experiment (TA). With 11 years of TA data, a dipole structure in a projection of the right ascension is fitted with an amplitude of 3.3+- 1.9% and a phase of 131 +- 33 degrees. The corresponding 99% confidence-level upper limit on the amplitude is 7.3%. At the current level of statistics, the fitted result is compatible with both an isotropic distribution and the dipole structure reported by Auger.

astro-ph.HE↗

arXiv2020

Evidence for a Supergalactic Structure of Magnetic Deflection Multiplets of Ultra-High Energy Cosmic Rays

Evidence for a large-scale supergalactic cosmic ray multiplet (arrival directions correlated with energy) structure is reported for ultra-high energy cosmic ray (UHECR) energies above 10$^{19}$ eV using seven years of data from the Telescope Array (TA) surface detector and updated to 10 years. Previous energy-position correlation studies have made assumptions regarding magnetic field shapes and strength, and UHECR composition. Here the assumption tested is that, since the supergalactic plane is a fit to the average matter density of the local Large Scale Structure (LSS), UHECR sources and intervening extragalactic magnetic fields are correlated with this plane. This supergalactic deflection hypothesis is tested by the entire field-of-view (FOV) behavior of the strength of intermediate-scale energy-angle correlations. These multiplets are measured in spherical cap section bins (wedges) of the FOV to account for coherent and random magnetic fields. The structure found is consistent with supergalactic deflection, the previously published energy spectrum anisotropy results of TA (the hotspot and coldspot), and toy-model simulations of a supergalactic magnetic sheet. The seven year data post-trial significance of this supergalactic structure of multiplets appearing by chance, on an isotropic sky, is found by Monte Carlo simulation to be 4.2$σ$. The ten years of data post-trial significance is 4.1$σ$. Furthermore, the starburst galaxy M82 is shown to be a possible source of the TA Hotspot, and an estimate of the supergalactic magnetic field using UHECR measurements is presented.

astro-ph.HE↗