arXiv subjects

Swaraj Pratim Sarmah

Publications and source records attributed to Swaraj Pratim Sarmah.

8 recordsLinked to original sources

arXiv2026

Reconstructing the Auger UHECR Dipole: A Hybrid Analysis of 4LAC AGNs and Nearby Starburst Galaxies

The large-scale dipole anisotropy observed by the Pierre Auger Observatory above $8$~EeV provides important clues about the origin of ultra-high-energy cosmic rays (UHECRs). In this work, we investigate whether gamma-ray active galactic nuclei (AGNs) from the Fermi-LAT Fourth AGN Catalog (4LAC) can explain the observed dipole and examine the contribution of nearby starburst galaxies (SBGs). Incorporating source fluxes, redshift distributions, GZK attenuation, and a mixed-composition framework, we demonstrate that single-source populations of AGNs and SBGs alone fail to reproduce the observed Auger dipole. To resolve this, we construct a hybrid AGN+SBG model. The best-fit solution for the $8$--$16$~EeV interval requires a $23$\% AGN contribution and $45$\% SBG contribution, which gives a dipole amplitude of $5.03$\% while maintaining the dipole direction of ($104.1^\circ$, $-25.1^\circ$) with angular separation of $7.10^\circ$. In the other energy intervals also, we find that the nearby SBGs increasingly dominate the anisotropic component while AGNs provide a subdominant contribution alongside a quasi-isotropic background.

astro-ph.HE↗

arXiv2026

Study of Flat Spectrum Radio Quasars and BL Lacertae Objects as Sources of Diffusive Ultra High-Energy Cosmic Rays

We examine whether Flat Spectrum Radio Quasars (FSRQs) and BL Lacertae objects (BL Lacs) can act as plausible astrophysical sources of diffuse ultra-high-energy cosmic rays (UHECRs). Using realistic luminosity-dependent density evolution (LDDE) functions derived from observed gamma-ray luminosity functions for FSRQs and BL Lacs, we calculate the redshift evolution of the cosmic ray source population through integrated luminosity functions. The diffuse UHECRs flux from these sources is modelled by propagating nuclei through extragalactic space, including energy losses from interactions with cosmic photon backgrounds. The resulting UHECRs spectra are compared with observational data from the Pierre Auger Observatory and the Telescope Array, with fluxes normalised at reference energies. We also construct illustrative full skymaps of the integral UHECRs flux from $10$ EeV to $100$ EeV using the \textsc{HEALPix} framework, assuming a uniform distribution of synthetic sources and including energy dependent magnetic diffusion. Our results show that although both FSRQs and BL Lac objects can reproduce the observed UHECR flux, the FSRQ scenario is disfavoured by anisotropy and inconsistent source density, while BL Lacs remain consistent with observations.

astro-ph.HE↗

arXiv2025

Modelling Cosmic Rays Flux with Pierre Auger and Telescope Array Data in $\boldsymbol{f(R)}$ and $\boldsymbol{f(Q)}$ Theories of Gravity

We investigate the effects of the magnetic field and the redshift on the propagation of galactic and extragalactic cosmic rays (CRs) in a modified theory of gravity (MTG) and an alternative theory of gravity (ATG) framework. For this purpose, we consider the $f(R)$ gravity and the $f(Q)$ gravity theories. We utilise these two MTG and ATG to compute the density enhancement factor of CRs as a function of the magnetic field and the redshift. For this work, we take the magnetic field strength from $1-100$ nG, while $0-2.5$ for the redshift. For each of these parameters, we take $100$ bins within their considered range for the computation. The enhancement parameter for the mixed composition of heavy nuclei up to Fe is also taken into account for this work. Further, we compute the $E^3$ magnified diffusive flux for $150$ sources separated by a distance $d_\text{s}$ for the different cosmological models. For the fitting with observational data from the Pierre Auger Observatory (PAO) and Telescope Array (TA), we parameterized some set that consists of the redshift $z$, the separation distances $d_\text{s}$ between the $150$ sources, and the maximum cutoff energy $E_{\text{max}}$. For each case, a residue plot and $χ^2$ value are also added to check the goodness of fitting. A comparative analysis of considered models has been performed in each of the cases along with the $Λ$CDM model. The considered MTG and ATG models produce notable and fascinating outcomes, with the $f(Q)$ model showing the highest CR density enhancement and the lowest reduced chi-squared value when fitted to PAO and TA data of UHECRs flux. This research provides new perspectives on the connections between high-energy physics, astrophysics, and cosmology.

astro-ph.HE↗

arXiv2025

Analysing Ultra High Energy Cosmic Rays' Anisotropy in $\boldsymbol{f(R, T)}$ Gravity Theory

In this study, we investigated the anisotropy of diffusive Ultra-High Energy Cosmic Rays (UHECRs) by employing three cosmological models: two models from the $f(R, T)$ gravity theory and the other is the standard $Λ$CDM model. The primary objective of this work was to ascertain the role of the $f(R, T)$ gravity theory in comprehending the anisotropy of UHECRs without implicitly endorsing the conventional cosmology. We parameterized the magnetic field and the source distance in anisotropy calculations to align well with the observational data from the Pierre Auger Observatory for all the cosmological models. An uncertainty band is presented along with the $χ^2$ test for all cosmological models to demonstrate the goodness of fitting. Our findings revealed that the amplitude of the anisotropy is highly sensitive to these cosmological models. Notably, the $f(R, T)$ models exhibited a lower amplitude of anisotropy (i.e., more isotropy), while the $Λ$CDM model predicted a comparatively higher amplitude at most of the energies considered.

astro-ph.HE↗

arXiv2025

Propagation of UHE cosmic rays in the light of f(R) gravity power-law model

While the origins of ultra-high energy (UHE) cosmic rays (CRs) remain shrouded in uncertainty, several important milestones have been reached in recent years in the experimental study of CRs with energy above 1018 eV. Within the vast expanse of intergalactic space, turbulent magnetic fields (TMFs) are believed to pervade, and these fields could exert a significant influence on the journey of UHECRs across the expanding Universe, which is currently undergoing acceleration. Thus, it is imperative to incorporate these considerations into our theoretical framework to gain a deeper understanding of the empirical observations related to UHECRs. In light of this, our research delves into the impact of UHE particle diffusion in the presence of TMFs, all within the context of the f(R) gravity power-law model. Based on this f(R) model, we explore the diffusive behavior of UHECR protons, particularly focusing on their density enhancement throughout their propagation and their energy spectrum. We found that the f(R) gravity model considered here plays an effective role in the propagation of CRs and the results have lain within our range of interest. Also, we compare our results for flux with observational data like the Pierre Auger Observatory (PAO) and Telescope Array (TA).

astro-ph.HE↗

arXiv2025

Magnetic Suppression of Cosmic Rays' Flux in $\boldsymbol{f(R)}$ and $\boldsymbol{f(Q)}$ Theories of Gravity

We investigate the effects of magnetic diffusion on the spectrum of ultra-high energy cosmic rays (UHECRs) from a cosmological perspective. To this end, we consider two modified theories of gravity (MTGs), namely, the $f(R)$ gravity and a symmetric teleparallel gravity, also known as $f(Q)$ gravity. Utilizing these two MTGs, we calculate the suppression in the flux of UHECRs for a collection of sources. Non-evolution (NE) and cosmic star formation rate (SFR) scenarios have been considered in our calculation of the suppression factor. This study also includes a mixed composition scenario involving the nuclei upto iron (Fe). Furthermore, we provide a parameterization of the suppression factor for the proton and also for the mixed compositions within the $f(R)$ and $f(Q)$ theories, considering both NE and SFR scenarios. The influence of the turbulent magnetic field on the suppression factor is also incorporated in our work. Comparative analysis of all our results with the standard $Λ$CDM model reveals significant effects of MTGs on the suppression factor that the $f(R)$ power-law model predicts the lowest suppression factor, while the $f(Q)$ model predicts the highest, and interestingly the results from the standard model fall within the range predicted by these two cosmological models.

astro-ph.HE↗

arXiv2024

Anisotropies of Diffusive Ultra-high Energy Cosmic Rays in $f(R)$ Gravity Theory

Understanding the anisotropy of ultra high-energy cosmic rays (UHECRs) is crucial for unraveling the origins and propagation mechanisms of these enigmatic particles. In this work, we studied the dipolar anisotropy of UHECRs in the diffusive regime by considering three cosmological models: the standard $Λ$CDM model, $f(R)$ gravity power-law model and the Starobinsky model. This work aims to see the role of the $f(R)$ gravity theory in understanding the anisotropy of UHECRs without condoning the standard cosmology. We found that the amplitude of the dipolar anisotropy is sensitive to these cosmological models, with the $f(R)$ power-law model predicting the largest amplitude, while the $Λ$CDM model predicting the smallest amplitude at most of the energies in the range considered. The predicted amplitude of the Starobinsky model lies within the range of the $Λ$CDM one. This work not only provides a way for exploration of UHECRs anisotropy within different cosmological contexts but also may pave the way for new avenues of research at the intersection of high-energy astrophysics.

astro-ph.HE↗

arXiv2024

Propagation and Fluxes of Ultra High Energy Cosmic Rays in f(R) Gravity Theory

We study the effect of diffusion of ultra-high energy cosmic ray (UHECR) protons in the presence of turbulent magnetic fields in the light of the $f(R)$ theory of gravity. The $f(R)$ theory of gravity is a successful modified theory of gravity in explaining the various aspects of the observable Universe including its current state of expansion. Here, we consider the two most studied $f(R)$ gravity models, viz., the power-law model and the Starobinsky model. For these two models, the diffusive character of the propagation of UHECR protons is explored in terms of their density enhancement (a measure of how the density of UHECRs changes due to their diffusion and interactions in the intergalactic medium). Ankle, instep, and Greisen-Zatsepin-Kuzmin cutoff are spectral characteristics that extragalactic UHECRs acquire when they propagate through the cosmic microwave background. All these characteristics are analysed through the diffusive flux as well as its modification factor. We compare the UHECR protons spectra calculated for the considered $f(R)$ gravity models with the available data of the Telescope Array and Pierre Auger Observatory. Both models of $f(R)$ gravity predict energy spectra of UHECRs with all experimentally observed features, which lay well within the range of combined data of both experiments. However, this work is only to investigate the possible effects of $f(R)$ gravity theory on the UHECRs propagation, using pure proton composition as a simplified case study. At this stage, our results cannot be used to favour or disfavour the $f(R)$ cosmology over the $Λ$CDM cosmology.

hep-th↗