arXiv subjects

T. A. Dzhatdoev

Publications and source records attributed to T. A. Dzhatdoev.

At least 19 recordsLinked to original sources

arXiv2025

The blazar PKS 0605-085 as the origin of the KM3-230213A ultra high energy neutrino event

The KM3Net Collaboration has recently reported on the observation of a remarkable event KM3-230213A that could have been produced by an ultra high energy cosmic neutrino. The origin of this event is still unclear. In particular, the cosmogenic neutrino scenario is not favoured due to the non-observation of a similar event by the IceCube detector, and most galactic scenarios are disfavoured as well. We show that the blazar PKS 0605-085 is a viable source of the KM3-230213A event. In particular, even though this blazar is located at 2.4$^{\circ}$ from the KM3-230213A event, the association between the blazar and the event is not unlikely due to a sizable direction systematic uncertainty of $\approx 1.5^{\circ}$ reported by the KM3Net Collaboration. Furthermore, we show that the observation of a $\approx$72 PeV neutrino from PKS 0605-085 is entirely possible given that a $\approx$7.5 PeV neutrino could have been observed from another blazar TXS 0506+056. Finally, we consider $γ$-ray constraints on the number of observable neutrino events and show that for the case of the external photon field production mechanism these constraints could be relaxed due to the often-neglected effect of the isotropisation of the hadronically-produced electrons in the magnetic field of the blob. We encourage further multi-wavelength observations of the blazar PKS 0605-085.

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arXiv2025

Carpet-3 detection of a photon-like air shower with estimated primary energy above 100 TeV in a spatial and temporal coincidence with GRB 221009A

The brightest cosmic gamma-ray burst (GRB) ever detected, GRB 221009A, was accompanied by photons of very high energies. These gamma rays may be used to test both the astrophysical models of the burst and our understanding of long-distance propagation of energetic photons, including potential new-physics effects. Here we present the observation of a photon-like air shower with the estimated primary energy of $300^{+43}_{-38}$ TeV, coincident (with the chance probability of $\sim 9\cdot 10^{-3}$) with the GRB in its arrival direction and time. Making use of the upgraded Carpet-3 muon detector and new machine learning analysis, we estimate the probability that the primary was hadronic as $\sim 3 \cdot 10^{-4}$. This is the highest-energy event ever associated with any GRB.

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arXiv2022

Upper limits on the isotropic diffuse flux of cosmic PeV photons from Carpet-2 observations

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.

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arXiv2022

Constraints on the extragalactic magnetic field strength from blazar spectra based on 145 months of Fermi-LAT observations

Properties of the extragalactic magnetic field (EGMF) outside of clusters and filaments of the large-scale structure are essentially unknown. The EGMF could be probed with $γ$-ray observations of distant (redshift $z > 0.1$) blazars. TeV $γ$ rays from these sources are strongly absorbed on extragalactic background light photons; secondary electrons and positrons produce cascade $γ$ rays with the observable flux dependent on EGMF parameters. We put constraints on the EGMF strength using 145 months of Fermi-LAT observations of the blazars 1ES 1218+304, 1ES 1101-232, and 1ES 0347-121, and imaging atmospheric Cherenkov telescope observations of the same sources. We perform a series of full direct Monte Carlo simulations of intergalactic electromagnetic cascades with the ELMAG 3.01 code and construct a model of the observable spectra inside the point spread functions of the observing instruments for a range of EGMF strengths. We compare the observed spectra with the models for various values of the EGMF strength $B$ and calculate the exclusion statistical significance for every value of $B$. We find that the values of the EGMF strength $B \le 10^{-17}$ G are excluded at a high level of the statistical significance $Z > 4 σ$ for all the four options of the intrinsic spectral shape considered (power-law, power-law with exponential cutoff, log-parabola, log-parabola with exponential cutoff). The value of $B = 10^{-16}$ G is not excluded; it is still a viable option of the EGMF strength. These results were obtained for the case of steady sources.

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arXiv2022

Modelling the persistent low-state $γ$-ray emission of the PKS 1510-089 blazar with electromagnetic cascades initiated in hadronuclear interactions

Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive due to a high value of the energy threshold of proton interaction with photon fields inside the source. However, broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons "visible" in $γ$ rays via hadronuclear interactions. In the present paper we study the persistent $γ$-ray emission of the FSRQ PKS 1510-089 in its low state utilizing the publicly-available Fermi-LAT data, as well as using the spectrum measured with the MAGIC imaging atmospheric Cherenkov telescopes. We find an indication for an excess of $γ$ rays at the energy range $\gtrsim 20$ GeV with respect to a simple baseline log-parabolic intrinsic spectral model. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the Cloudy code. To our knowledge, this is the first calculation of electromagnetic cascade spectrum inside a blazar based on a direct calculation of the photon field spectrum with a spectral synthesis code.

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arXiv2021

Observation of photons above 300 TeV associated with a high-energy neutrino from the Cygnus region

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.

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arXiv2021

Observable spectral and angular distributions of $γ$-rays from extragalactic ultrahigh energy cosmic ray accelerators: the case of extreme TeV blazars

Ultrahigh energy protons and nuclei from extragalactic cosmic ray sources initiate intergalactic electromagnetic cascades, resulting in observable fluxes of $γ$-rays in the GeV-TeV energy domain. The total spectrum of such cascade $γ$-rays of hadronic nature is significantly harder than the one usually expected from blazars. The spectra of some sources known as "extreme TeV blazars" could be well-described by this "intergalactic hadronic cascade model" (IHCM). We calculate the shape of the observable point-like spectrum, as well as the observable angular distibution of $γ$-rays, for the first time taking into account the effect of primary proton deflection in filaments and galaxy clusters of the extragalactic magnetic field assuming the model of Dolag et al. (2005). We present estimates of the width of the observable $γ$-ray angular distribution derived from simple geometrical considerations. We also employ a hybrid code to compute the observable spectral and angular distributions of $γ$-rays. The observable point-like spectrum at multi-TeV energies is much softer than the one averaged over all values of the observable angle. The presence of a high-energy cutoff in the observable spectra of extreme TeV blazars in the framework of the IHCM could significantly facilitate future searches of new physics processes that enhance the apparent $γ$-ray transparency of the Universe (for instance, $γ\rightarrow ALP$ oscillations). The width of the observable angular distribution is greater than or comparable to the extent of the point spread function of next-generation $γ$-ray telescopes.

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arXiv2020

Can we constrain the extragalactic magnetic field from very high energy observations of GRB 190114C?

Primary very high energy $γ$-rays from $γ$-ray bursts (GRBs) are partially absorbed on extragalactic background light (EBL) photons with subsequent formation of intergalactic electromagnetic cascades. Characteristics of the observable cascade $γ$-ray signal are sensitive to the strength and structure of the extragalactic magnetic field (EGMF). GRB 190114C was recently detected with the MAGIC imaging atmospheric Cherenkov telescopes, for the first time allowing to estimate the observable cascade intensity. We inquire whether any constraints on the EGMF strength and structure could be obtained from publicly-available $γ$-ray data on GRB 190114C. We present detailed calculations of the observable cascade signal for various EGMF configurations. We show that the sensitivity of the Fermi-LAT space $γ$-ray telescope is not sufficient to obtain such constraints on the EGMF parameters. However, next-generation space $γ$-ray observatories such as MAST would be able to detect pair echoes from GRBs similar to GRB 190114C for the EGMF strength below 10^{-17}--10^{-18} G.

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arXiv2020

Intergalactic electromagnetic cascade component of observable blazar emission

Secondary γ-rays from intergalactic cascades may contribute to observable spectra of blazars, also modifying observable angular and temporal distributions. In this paper we briefly review basic features of intergalactic electromagnetic cascade physics, suggest a new approximation for γ-ray mean free path, consider angular patterns of magnetically broadened cascade emission, and present an example of a fit to the observable blazar spectrum.

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arXiv2019

The SPHERE-2 detector for observation of extensive air showers in 1 PeV -- 1 EeV energy range

The SPHERE-2 balloon-borne detector designed for extensive air shower (EAS) observations using EAS optical Vavilov-Cherenkov radiation (``Cherenkov light''), reflected from the snow-covered surface of Lake Baikal is described. We briefly discuss the concept behind the reflected Cherenkov light method, characterize the conditions at the experimental site and overview the construction of the tethered balloon used to lift the SPHERE-2 telescope above the surface. This paper is mainly dedicated to a detailed technical description of the detector, including its optical system, sensitive elements, electronics, and data acquisition system (DAQ). The results of some laboratory and field tests of the optical system are presented.

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arXiv2019

Spatial and temporal structure of EAS reflected Cherenkov light signal

A compact device lifted over the ground surface might be used to observe optical radiation of extensive air showers (EAS). Here we consider spatial and temporal characteristics of Vavilov-Cherenkov radiation ("Cherenkov light") reflected from the snow surface of Lake Baikal, as registered by the SPHERE-2 detector. We perform detailed full direct Monte Carlo simulations of EAS development and present a dedicated highly modular code intended for detector response simulations. Detector response properties are illustrated by example of several model EAS events. The instrumental acceptance of the SPHERE-2 detector was calculated for a range of observation conditions. We introduce the concept of "composite model quantities", calculated for detector responses averaged over photoelectron count fluctuations, but retaining EAS development fluctuations. The distortions of EAS Cherenkov light lateral distribution function (LDF) introduced by the SPHERE-2 telescope are understood by comparing composite model LDF with the corresponding function as would be recorded by an ideal detector situated at the ground surface. We show that the uncertainty of snow optical properties does not change our conclusions, and, moreover, that the expected performance of the SPHERE experiment in the task of cosmic ray mass composition study in the energy region $\sim$10 PeV is comparable with other contemporary experiments. Finally, we compare the reflected Cherenkov light method with other experimental techniques and briefly discuss its prospects.

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arXiv2018

Intergalactic $γ$-ray propagation: basic ideas, processes, and constraints

We review extragalactic $γ$-ray propagation models with emphasis on the electromagnetic (EM) cascade process in the magnetized expanding Universe. We consider cascades initiated by primary protons of ultra-high energy accelerated by blazars and show that the observable spectrum is similar to the universal spectrum of a purely EM cascade. We also present a detailed calculation of the observable angular distribution for the case of EM cascades developing from relatively nearby (<20 Mpc) sources. Finally, we calculate the point-like source differential sensitivity of a novel liquid Argon time projection chamber $γ$-ray telescope and show that its sensitivity is several times better than the Fermi LAT sensitivity in the 100 MeV -- 100 GeV energy range.

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arXiv2017

The intergalactic electromagnetic cascade solution for the anomalies from $γ$-ray blazar observations

Recent progress in very high energy (VHE, E >100 GeV) $γ$-ray observations, together with advances in the extragalactic background light (EBL) modelling, allows to search for new phenomena such as $γ$-axion-like particle ($γ\rightarrow$ ALP) oscillation and to explore the extragalactic magnetic field (EGMF) strength and structure. These studies are usually performed by searching for some deviation from the so-called absorption-only model, that accounts for only primary photon absorption on the EBL and adiabatic losses. In fact, there exist several indications that the absorption-only model is incomplete. We present and discuss the intergalactic electromagnetic cascade model (IECM) --- the simplest model that allows to coherently explain all known anomalies. This model has a number of robust signatures that could be searched for with present and future instruments. The IECM model may serve as a new background template, allowing to make future searches for $γ\rightarrow$ ALP oscillation more robust. A detailed account of our calculations is available in astro-ph/1609.01013v2 (A\&A,in print).

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arXiv2017

Electromagnetic cascade masquerade: a way to mimic $γ$--axion-like particle mixing effects in blazar spectra

Context. Most of the studies on extragalactic γ-ray propagation performed up to now only accounted for primary gamma-ray absorption and adiabatic losses ("absorption-only model"). However, there is growing evidence that this model is oversimplified and must be modified in some way. In particular, it was found that the intensity extrapolated from the optically-thin energy range of some blazar spectra is insufficient to explain the optically-thick part of these spectra. This effect was interpreted as an indication for γ-axion-like particle (ALP) oscillation. On the other hand, there are many hints that a secondary component from electromagnetic cascades initiated by primary γ-rays or nuclei may be observed in the spectra of some blazars. Aims. We study the impact of electromagnetic cascades from primary γ-rays or protons on the physical interpretation of blazar spectra obtained with imaging Cherenkov telescopes. Methods. We use the publicly-available code ELMAG to compute observable spectra of electromagnetic cascades from primary γ-rays. For the case of primary proton, we develop a simple, fast and reasonably accurate hybrid method to calculate the observable spectrum. We perform the fitting of the observed spectral energy distributions (SEDs) with various physical models: the absorption-only model, the "electromagnetic cascade model" (for the case of primary γ-rays), and several versions of the hadronic cascade model (for the case of primary proton). We distinguish the following species of hadronic cascade models: 1) "basic hadronic model", where it is assumed that the proton beam travels undisturbed by extragalactic magnetic field and that all observable γ-rays are produced by primary protons through photohadronic processes with subsequent development of electromagnetic cascades /abridged

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arXiv2017

Testing models of extragalactic $γ$-ray propagation using observations of extreme blazars in GeV and TeV energy ranges

We briefly review contemporary extragalactic γ-ray propagation models. It is shown that the Extragalactic Magnetic Field (EGMF) strength and structure are poorly known. Strict lower limits on the EGMF strength in voids are of order 10^{-17}--10^{-20} G, thus allowing a substantial contribution of a secondary component generated by electromagnetic cascades to the observable spectrum. We show that this "electromagnetic cascade model" is supported by data from imaging Cherenkov telescopes and the Fermi LAT detector.

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arXiv2017

On conservative models of "the pair-production anomaly" in blazar spectra at Very High Energies

For some blazars, the gamma-ray absorption features due to pair-production on the Extragalactic Background Light (EBL) are fainter than expected. The present work reviews the main models that could explain this paradox, with emphasis on conservative ones, that do not include any new physics. The models that are intrinsic to the source, do allow a very hard primary spectrum, but fail to explain a regular redshift dependence of the anomaly starting energy. The model that includes a contribution from secondary photons produced by cosmic rays (CR) near the Earth seems to require a well collimated CR beam, what is hard to achieve. Finally, the model with secondary photons produced in electromagnetic (EM) cascades initiated by primary gamma-rays is considered. In principle, it allows to decrease the statistical significance of the anomaly and, while requiring quite low EGMF strength B, does not contradict to most contemporary constraints on the B value. Additionally, it is shown that the recently observed correlation between directions to hard gamma-ray sources and voids in the Large Scale Structure is a natural feature of the EM cascade model.

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arXiv2015

Event-by-event study of CR composition with the SPHERE experiment using the 2013 data

We present an event-by-event study of cosmic ray (CR) composition with the reflected Cherenkov light method. The fraction of CR light component above 5 PeV was reconstructed using the 2013 run data of the SPHERE experiment which observed optical Vavilov-Cherenkov radiation of extensive air showers, reflected from snow surface of Lake Baikal. Additionally, we discuss a possibility to improve the elemental groups separability by means of multidimensional criteria.

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arXiv2015

Results and prospects on registration of reflected Cherenkov light of EAS from cosmic particles above 10^{15} eV

We give an overview of the SPHERE experiment based on detection of reflected Vavilov-Cherenkov radiation (Cherenkov light) from extensive air showers in the energy region E>10^{15} eV. A brief history of the reflected Cherenkov light technique is given; the observations carried out with the SPHERE-2 detector are summarized; the methods of the experimental datasample analysis are described. The first results on the primary cosmic ray all-nuclei energy spectrum and mass composition are presented. Finally, the prospects of the SPHERE experiment and the reflected Cherenkov light technique are given.

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