arXiv subjects

Rajat K. Dey

Publications and source records attributed to Rajat K. Dey.

16 recordsLinked to original sources

arXiv2025

Search for UHE gamma photons in cosmic rays by studying the geomagnetic influence on air-shower muons

A major challenge in ground-based ultra-high-energy gamma-ray observations remains in discriminating sporadic gamma-ray signals from a huge background of cosmic-ray events. To achieve good discrimination power of gamma rays against protons, an unconventional approach is presented that exploits the geomagnetic influence on air shower muons in Monte Carlo shower events. A recognizable polar asymmetry in the positive and negative muon distributions is noticed after transforming the impact coordinates of each muon from the observer plane onto the shower front plane, plus a judicious ignoring of the minimal attenuation of muons in the space between the two planes. A couple of observables, $d_{\text{max}}$ and $N_{μ; \text{IQS}}^{\text{tr.}}$, can be extracted from the formation of two densely populated positive and negative muon zones in each shower. Both observables demonstrate excellent gamma/proton discrimination power, with efficiencies of $93.2\%$ and $98.6\%$. This analysis permitted the usage of muon detectors just covering a small area of $\simeq{1.4}\%$ of a square-km array for ultra-high-energy gamma-ray observations. It is expected that the present and future gamma-ray observatories will implement the current approach.

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arXiv2025

Broadband Spectral Analysis of 1ES2344+514: A Multi-epoch study

We present the results of the multi-epoch broadband spectral study of 1ES 2344+514 and study the evolution of physical parameters. We used nearly simultaneous data obtained from 2017 June 6 to 2022 August 6 (MJD 57910 -- 59797) in optical, UV, X-ray and $γ$-ray wavebands from various instruments including Swift-UVOT, Swift-XRT, NuSTAR, AstroSat (SXT and LAXPC), Fermi-LAT, and TeV flux from MAGIC. During 2017 July, 1ES 2344+514 appeared to be in the highest flaring state compared to other periods. We used the 0.5 -- 7.0 keV and 3.0 -- 20.0 keV data, respectively, from SXT and LAXPC of AstroSat and also 0.3 -- 8.0 keV and 3.0 -- 79.0 keV data, respectively, from Swift-XRT and NuSTAR. A joint fit between SXT and LAXPC, and between Swift-XRT and NuSTAR has been done for constraining the synchrotron peak. A clear shift in the synchrotron peak has been observed, which included 1ES 2344+514 in the HSP BL Lac family. A `harder-when-brighter' trend is observed in X-rays, and the opposite trend, i.e., `softer-when-brighter', is seen in the $γ$-rays. The multi-epoch broad-band spectral energy distributions (SEDs) of this source were built and studied to get an idea of the radiation processes. The SEDs were fitted using a steady-state leptonic one-zone synchrotron+SSC model, and the fitted parameters of the emission region are consistent with those of other TeV BL Lacs. In this study, we found a weak correlation tendency between bolometric luminosity and magnetic field (B), as well as between bolometric luminosity and the break Lorentz factor ($γ_{break}$).

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arXiv2024

A new basic air shower observable sensitive to the cosmic-ray elemental mass

Based on a Monte Carlo simulation study of vertical extensive air showers (EAS) at the KASCADE location we introduce a new simple observable $η_{ρ{(45;310)}}$ (in short $η_ρ$) - the ratio between two lateral electron densities of an EAS measured at two well-defined radial distances indicated by the characteristic radial feature of the local age parameter (LAP). Our analyses of simulated data generated by the Group I nuclei with cosmic-ray elemental masses, $A_{\text{actual}}=1,4,12,16,24,32,40,56$, observed a double correlation of $η_ρ$ with, (i) the lateral shower age $s_{\text{av}}$, and (ii) $A_{\text{actual}}$ of the EAS initiating primary particle. Applying the first correlation to a new set of simulated showers initiated by the Group II nuclei with $A_{\text{actual}}=7,14,20,28,40,52$, the average difference between the lateral shower age $s_{\text{av}}$ obtained directly from the conventional LAP approach and the estimated one $s_{\text{est}}$ is found to be close to $0$. The second correlation involving $η_ρ$ with $A_{\text{actual}}$ applied to the set of such showers estimates the average atomic mass $<\ln{A_{\text{est}}}>$ of their shower initiating primaries, utilizing the $η_ρ$s. Results of the work revealed that the method of estimating the lateral shower age is almost high-energy interaction model-independent. A certain level of model dependence is albeit found in the case of $<\ln{A_{\text{est}}}>$ estimation.

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arXiv2023

Sagittarius $\mathrm{A}^{\star}$ as a plausible source candidate for PeV neutrinos

We propose that the recently observed diffuse neutrinos by IceCube with energies above 1 PeV might have originated from Sagittarius $\mathrm{A}^{\star}$ located in the galactic disk. This implies that the astrophysical settings of Sagittarius $\mathrm{A}^{\star}$ need to accelerate hadronic cosmic rays to energies of $\sim 100$ PeV or more. Then, the hadronic emission scenario argues that this galactic neutrino source is also a PeV gamma-ray source. Recent observation of galactic diffuse PeV gamma-rays with energies $\sim 1$ PeV by Large High Altitude Air Shower Observatory has also advocated this conjecture. In the present paper, we demonstrate that if protons are accelerated to energies of $\sim 100$ PeV or more as reported by Osmanov {\it et al.} (Astrophys. J. {\bf 835} 164:2017) in Sagittarius $\mathrm{A}^{\star}$ environment, then they might generate PeV neutrinos and gamma rays through cosmic rays-gas/interstellar matter ({\it e.g.} $pp$) interactions. We estimate theoretically the diffuse neutrino flux due to back-to-back charged pion decays and the accompanying gamma-ray flux from neutral pion decays. These results suggest that a fraction ($\simeq 1\%$) of the PeV diffuse neutrino flux observed by IceCube can be explained by the neutrino emission from Sagittarius $\mathrm{A}^{\star}$. Upcoming IceCube Gen2 and Cherenkov telescope array could be able to test our scenario for PeV neutrino and gamma-ray emissions from the only known galactic Pevatron Sagittarius $\mathrm{A}^{\star}$ with CR energies more than $100$ PeV.

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arXiv2023

The shape of the electron and muon lateral distribution functions of extensive air showers

The lateral density data obtained for different secondaries of an extensive air shower (EAS) from an array of detectors are usually described by some suitable lateral density functions (LDFs). Analyzing non-vertical simulated EASs generated with the CORSIKA code, it is found that the lateral and polar density distributions of electrons and muons are asymmetric in the ground plane. It means that typical expressions for symmetric lateral density functions (SLDFs) (\emph{e.g.} the Nishimura-Kamata-Greisen function) are inadequate to reconstruct the lateral and polar dependencies of such asymmetric electron or muon densities accurately. In order to provide a more consistent LDF for non-vertical shower reconstruction in the ground plane, the paper considers the issue of the modification of the SLDF analytically. The asymmetry arising from additional attenuation and correction of the positional coordinates (radial and polar) of cascade particles causes a gap length between the center of concentric equidensity ellipses and the EAS core. A toy function is introduced as a basic LDF to describe the asymmetric lateral and polar density distributions of electrons or muons of EASs, thereby predicting the gap length parameter. Consequently, the desired LDF describing the asymmetric density distributions of electrons and muons of EASs has emerged. We compare results from detailed simulations with the predictions of the analytical parametrization. The LDF derived in this work is found to be well-suited to reconstruct EASs in the ground plane directly.

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arXiv2022

Probing the mass composition of primary cosmic rays from the effect of the geomagnetic field on EAS muons: A simulation study

The distribution of the muon content of highly inclined Monte Carlo cosmic ray showers is affected by the influence of Earth's geomagnetic field. It is found that the shapes of the positive and negative muon distributions get affected/modified by the influence of the Earth's geomagnetic field. Such a correlation between the earth's geomagnetic activity and the cosmic ray (CR) air shower muons is found sensitive to the primary cosmic ray mass composition.

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arXiv2022

Behaviour of the lateral shower age of cosmic ray extensive air showers

Some simple arguments are introduced for a possible explanation of the behavior of the lateral shower age of proton-initiated showers. The corresponding analytical treatment based on the proposed argument is then illustrated. Using the Monte Carlo simulation (MC) code CORSIKA, we have validated how the different characteristics associated with the lateral shower age predicted in the present analytical parametrization, can be understood. The lateral shower age of a proton-initiated shower and its correlations with the lateral shower ages of electron- and neutral pion-initiated showers supports the idea that the result of superposition of several electromagnetic sub-showers initiated by neutral pions might produce the lateral density distribution of electrons of a proton initiated shower. It is also noticed with the simulated data that the stated feature still persists even in the local shower age representation.

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arXiv2022

Diffuse flux of PeV neutrinos from centrifugally accelerated protons in active galactic nuclei

Evidence for high-energy astrophysical PeV neutrinos has been found in the IceCube experiment from an analysis with a 7.5 year (2010 - 2017) data. Active galactic nuclei (AGN) are among the most prominent objects in the universe, and are widely speculated to be emitters of ultra-high-energy (UHE) cosmic rays with proton domination. Based on the standard two-step LLCD mechanism of particle acceleration, a transformation of energy occurs from AGN's central super-massive black hole (SMBH) rotation to high-energy protons. Protons can be accelerated up to $\sim 0.1$ EeV energies and above, and might generate PeV neutrinos in the energy range $1$--$10$~ PeV through plausible hadronic interactions. The theoretically estimated revised extragalactic diffuse muon neutrino flux employing the "luminosity-dependent density evolution (LDDE)" model for the AGN luminosity function (LF) is found consistent with the IceCube level if only a fraction, $6.56\%$ of the total bolometric luminosity (BL) of AGN is being realizable to power the PeV neutrinos. In the $Λ$~CDM cosmological framework with the LDDE modeled LF and photon index distribution, about $5.18\%$ of the total BL is enough to power the IceCube neutrinos.

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arXiv2021

Probing the magnitude of asymmetries in the lateral density distribution of electrons in EAS

The lateral density distributions (LDD) of inclined cosmic ray air shower are asymmetric and the corresponding iso-density contours are of increasing eccentric ellipses with zenith angles of different showers. The polar asymmetry of the iso-density contours introduces a significant shift of the EAS core, which is quantitatively expressed as a gap length (GL) parameter between the EAS core and the center of the modified density pattern consisting of several equi-density ellipses. The LDD of EAS particles is usually approximated by a particular type of lateral density function (LDF) which is generally assumed to be polar symmetric about the EAS axis, and cannot describe the asymmetric LDDs accurately. A polar angle-dependent modified lateral density function of EASs has been derived analytically by considering the effect of attenuation of EAS particles in the atmosphere. From the simulation studies, it has been found that the GL manifests sensitivity to the cosmic ray mass composition. The cosmic ray mass sensitivity of the lateral shower age is also re-examined by applying the modified LDF to the simulated data.

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arXiv2021

Explaining the characteristics of lateral shower age of cosmic ray extensive air showers

A simple analytical argument is proposed for a possible explanation of the characteristics of the lateral shower age ($s$) of proton ($p$)/nuclei-initiated showers. The analytical argument states that lateral density distribution (LDD) of electrons of a $p$-initiated shower is due to superposition of several electromagnetic (EM) sub-showers developed at a very early stage in the atmosphere from the decay of neutral pions ($π^{0}$s). Thanks to the superposition property of the electron LDD in a $p$ shower, a plausible analytical parametrization has been worked out by giving well represented analytic function for the electron LDDs of $p$- and $π^{0}$-initiated showers. Based on cosmic ray extensive air shower simulations, we have validated how the various characteristics of $s$ can be understood in the context of the present analytical argument. The $s$ parameter of a $p$ shower and its correlations with the shower ages of electron- and $π^{0}$-initiated showers supports the idea that the result of superposition of several EM sub-showers initiated by $π^{0}$s with varied energies at a very early stage might produce the LDD of electrons of a $p$ shower. It is also noticed with the simulated data that the stated feature still persists concerning the notion of the local shower age parameter.

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arXiv2021

Newly born extragalactic millisecond pulsars as efficient emitters of PeV neutrinos

The origins of the diffuse flux of cosmogenic PeV neutrinos detected in the IceCube experiment during 2010 - 2017 remain unidentified. A population of extragalactic newly born fast spinning pulsars are investigated as possible candidates for generating the PeV energy scale neutrinos. A two-step mechanism of particle acceleration is adopted for transferring energy from star rotation to high energy electrons. Electrons might be boosted up to $\approx 0.01$ EeV energies and above, and produce ultra-high-energy (UHE) neutrinos and gamma rays when these electrons interact with low energy positrons and soft radiations in the acceleration zone. The theoretically derived extragalactic diffuse muon neutrino flux in the energy range [1-10] PeV is found consistent with the IceCube level if only a fraction ($η_{k} \approx 0.31\%$) of the total bolometric luminosity of pulsars are transferred to power the PeV neutrinos. Using the above value of the cosmic ray electron loading parameter $η_{k}$, the diffuse gamma ray flux from inverse Compton scattering off UHE electrons in the soft radiation field might be predicted from the model.

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arXiv2020

Local shower age and segmented slope parameters of lateral density distributions of cosmic ray shower particles

The paper describes lateral density distributions of electrons and muons of cosmic ray extensive air showers (EAS) in the energy regime of the KASCADE experiment. Potential EAS observables are extracted while some suitable lateral distribution functions are introduced to describe these lateral density distributions of simulated/observed cosmic ray showers. Mass-sensitivity of these parameters are demonstrated particularly for the KASCADE electron and muon distributions data. Our studies indicate that the KASCADE data favours an idea of gradual change of cosmic ray mass composition from lighter concentration to heavier concentration in terms of these parameters.

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arXiv2020

Searching for correlations of geomagnetic activities with high-energy EAS muons

The paper aims to explore the asymmetry of the muon content of non-vertical and very high energy Monte Carlo showers due to the influence of Earth's geomagnetic field. Simulations have shown that the geomagnetic field modifies the trajectories of muons in a shower producing a polar asymmetry in the density/number of positive and negative muons in the shower front plane. The asymmetry is quantified by a transverse separation between the positive and negative muons barycentric positions through opposite quadrants across the shower core. The dependence of this transverse muon barycenter separation (TMBS) on polar position shows a clear maximum at a position that is correlated with the primary composition and geomagnetic activities. It is noticed that the maximum TMBS parameter exhibits sensitivity to any transient weakening of Earth's magnetic shield caused by geomagnetic storm originated from bursting solar processes. Obtained simulation results are quite important to design any possible new experiment based on these features of muons in extensive air showers.

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arXiv2017

PeV neutrinos from local magnetars

The paper estimates the flux of PeV neutrinos from magnetar polar caps, assuming that ions/protons are injected, and accelerated in these regions and interact with the radiative background. The present study takes into account the effect of the photon splitting mechanisms that should modify the radiative background, and enhance the neutrino flux at PeV energies, with a view to explain the PeV neutrino events detected in IceCube. The results indicate that in near future, a possibility of any significant excess of neutrino events from a magnetar in Milky Way is extremely low.

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arXiv2016

Slope of the lateral density function of extensive air showers around the knee region as an indicator of shower age

Analyzing simulated extensive air shower (EAS) events generated with the Monte Carlo code CORSIKA, this paper critically studies the characteristics of lateral distribution of electrons in EAS around the knee energy region of the energy spectrum of primary cosmic rays. The study takes into account the issue of lateral shower age parameter as indicator of the stage of development of showers in the atmosphere. The correlation of lateral shower age parameter with other EAS observables is examined, using simulated data in the context of its possible use in a multi-parameter study of EAS, with a view to obtaining information about the nature of the shower initiating primaries at sea level EAS experiments. It is shown that the observed slope of the lateral density function in the 3-dimensional plot at least for the KASCADE data supports the idea of a transition from light to heavy mass composition around the knee.

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arXiv2016

Searching for PeV neutrinos from photomeson interactions in magnetars

In this paper we estimate the flux of PeV neutrinos and gamma-rays from magnetar polar caps, assuming that ions/protons are injected, and accelerated in these regions and interact with the radiative background. The present study takes into account the effect of the photon splitting mechanisms that should modify the radiative background, and enhance the neutrino and gamma-ray fluxes at PeV energies, with a view to explain the PeV neutrino events detected in IceCube. The results indicate that in near future, possibility of any significant excess of neutrino events from a magnetar in Milky Way is extremely low. Further, we suggest that the simultaneous observation of neutrinos and gamma-rays at Earth from expanded 'Gen2' IceCube detector and/or High Altitude Water Cherenkov Observatory would provide opportunities to explore the possible origin of very high energy neutrinos and gamma-rays.

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