arXiv subjects

A. Korochkin

Publications and source records attributed to A. Korochkin.

11 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

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 $Φ_γ(E_γ> 10^{19} \text{eV}) < 2.3 \cdot 10^{-3} $, and $Φ_γ(E_γ> 10^{20} \text{eV}) < 3.0 \cdot 10^{-4} $ $ (\text{km}^2 \cdot \text{sr} \cdot \text{yr})^{-1} $.

astro-ph.GA↗

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

Displacement of ultra-high-energy cosmic ray source images by the intergalactic magnetic field: the cases of Cen A and M83

The standard assumption about the influence of the turbulent intergalactic magnetic field (IGMF) on the images of ultra-high-energy cosmic rays (UHECR) sources is that the latter are formed in a random walk mode in the deflection angle. As a result, the images are symmetrically broadened to angular scales proportional to the IGMF strength and the square root of its correlation length. We demonstrate that when the size of the emitting region is smaller than the correlation length of the IGMF, a new focusing regime emerges. In this regime, significant deviations from the standard random walk approximation occur even when the distance between the source and the observer exceeds several tens of IGMF correlation lengths. The angular size of the resulting images is typically smaller than predicted by random walk, and the IGMF causes a systematic shift of the entire image away from the true source direction. This introduces additional uncertainty in the search for UHECR sources. We show that the excess observed by Pierre Auger Observatory in the direction of Cen A can be explained by the contribution of M83, provided that the image shift occurs as predicted by some models of the Galactic magnetic field (GMF) and that the IGMF plays a minor role due to its low strength and short coherence length. Alternatively, Cen A may indeed be the true source of the excess, as certain realizations of the IGMF can compensate the deflection caused by the GMF.

astro-ph.HE↗

arXiv2024

Images of the Ultra-High Energy Cosmic Rays from Point Sources

Our latest paper investigates the effects of UHECR propagation in a turbulent intergalactic magnetic field in the small-angle scattering regime, specifically focusing on the non-trivial caustic-like pattern that arises with strong deviation from isotropy. In this paper, we explore the effect of the observer's position on the measurement of source flux at a given distance. We examine three types of source locations, characterized by the density of cosmic rays from a given source at the observation point, which we call knots, filaments and voids. We also investigate the energy spectrum in these different cases and present simulated images of the source as it appears on the observer's telescope after propagation in the combination of intergalactic and Galactic magnetic fields. We show that hot spots in the UHECR data can arrive due to combined distortions of source images on the intergalactic and Galactic magnetic fields. Also the fact that flux of most nearby sources is diluted in the voids affects source population studies.

astro-ph.HE↗

arXiv2023

Constraint on intergalactic magnetic field from Fermi/LAT observations of the "pair echo" of GRB 221009A

Delayed "pair echo" signal from interactions of very-high-energy gamma rays in the intergalactic medium can be used for detection of the inter-galactic magnetic field (IGMF). We use the data of Fermi/LAT telescope coupled with LHAASO observatory measurements to confirm the presence of IGMF along the line of sight to the gamma-ray burst GRB221009A. Comparing the Fermi/LAT measurements with the expected level of the pair echo flux, set by the multi-TeV LHAASO detection, we derive a lower bound $10^{-19}$ G on the IGMF with correlation length $l$ larger than 1 Mpc, improving as $l^{-1/2}$ for shorter correlation lengths. This provides an independent verification of existence of a lower bound on IGMF in the voids of the Large Scale Structure, previously derived from the observations of active galactic nuclei.

astro-ph.HE↗

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

Caustic-like Structures in UHECR Flux after Propagation in Turbulent Intergalactic Magnetic Fields

UHECR propagation in a turbulent intergalactic magnetic field in the small-angle scattering regime is well understood for propagation distances much larger than the field coherence scale. The diffusion theory doesn't work and unexpected effects may appear for propagation over smaller distances, from a few and up to 10-20 coherence scales. We study the propagation of UHECRs in this regime, which may be relevant for intermediate mass UHECR nuclei and nG scale intergalactic magnetic fields with 1 Mpc coherence scale. We found that the trajectories form a non-trivial caustic-like pattern with strong deviation from isotropy. Thus, measurements of the flux from a source at a given distance will depend on the position of the observer.

astro-ph.HE↗

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↗

arXiv2019

Search for spectral features in extragalactic background light with gamma-ray telescopes

Cumulative optical and infrared emission from galaxies accumulated over cosmological time scales, the extragalactic background light (EBL), could be probed by complementary techniques of direct observations and source counting in the visible and infrared as well as via its imprint on the signal of distant active galactic nuclei in gamma rays. We compare the visible and infrared measurements with the gamma-ray constraints and study if the discrepancies of the measurements with different methods could be due to the presence of features in the EBL spectrum that are localised in the micron wavelength range. We combined data on time-averaged spectra of selected blazars that were obtained by Fermi and ground-based gamma-ray telescopes. We also modelled the effect of absorption on EBL while allowing for the existence of a previously unaccounted spectral feature. We show that a previously reported 'excess' in EBL flux in the $\sim 1$~micron wavelength range is consistent with gamma-ray measurements, that is, if the excess has the form of a narrow feature of the width $δλ< λ$ and an overall flux of up to 15 nW/(m$^2$ sr) above the 'minimal' EBL, which is estimated from the visible and infrared source counts. Such 'bump-like' spectral features could originate, for example, from decaying dark-matter particles, or either axions or peculiar astrophysical processes in the course of star-formation history. We discuss the possibilities for the search of spectral features in the EBL with the Cherenkov Telescope Array (CTA).

astro-ph.HE↗

arXiv2019

Search for decaying eV-mass axion-like particles using gamma-ray signal from blazars

Decaying axion-like particles (ALP) with masses in the eV range which might occupy dark matter halos of the Milky Way and other galaxies produce a characteristic "bump" feature in the spectrum of extragalactic background light (EBL). This feature leaves an imprint on the gamma-ray spectra of distant extragalactic sources. We derive constraints on the ALP coupling to photons based on analysis of spectra of very-high-energy gamma-ray loud blazars. We combine gamma-ray spectral measurements by Fermi/LAT and Cherenkov telescopes and fit a model in which the intrinsic source spectrum is modified by pair production on photons produced by ALP decays. We constrain the amplitude of gamma-ray flux suppression by this effect. We find that the combined Fermi/LAT and VERITAS data set for the source 1ES 1218+304 currently provides the tightest constraint on ALP-two-photon coupling which is complementary to the constraints imposed by non-observation of excess energy loss in Horizontal Branch stars, by the high-resolution spectroscopic observations of galaxy clusters with optical telescopes and by the searches of ALP signal with CERN Solar Axion Telescope. Our analysis favours existence of a bump in the EBL spectrum which could be produced by ALPs in the mass range 2-3 eV and axion-photon coupling $\sim 10^{-10}$ GeV$^{-1}$. We discuss possibilities for verification of this hint with deeper Cherenkov telescope observations of large number of blazars with current generation instruments and with the Cherenkov Telescope Array.

hep-ph↗