arXiv subjects

Abhijit Roy

Publications and source records attributed to Abhijit Roy.

At least 19 recordsLinked to original sources

arXiv2026

Contest between diverging k-vectors and astigmatism in an aberrated randomly scattered light

The impact of astigmatism on randomly scattered light, here speckles which are generated from a ground glass (GG) plate, is investigated by studying the change in the shape of the intensity correlation function (ICF) of the speckles for different amount of astigmatism introduced to it by rotating a lens at different angles. It is observed that the shape of the ICF, which reflects the average shape of the speckles, changes from circular to elliptical with the change of the tilt angle of the lens (theta), and the variation of length of two axes of the ICF with theta is completely different in nature. It is shown that these different natures are observed due to a contest between the diverging k-vectors of the speckles and the astigmatism introduced by the tilted lens system. It is also found that the diverging k-vectors resist the effect of astigmatism on the speckles, and after a certain theta, the impact of astigmatism becomes prominent. The contest between the diverging k-vectors and astigmatism in an aberrated speckle pattern are confirmed by further experimental studies and are discussed in detail. This study will surely be helpful in designing different imaging systems, where scattered light plays an important role.

physics.optics↗

arXiv2026

The Origin of Multi-TeV Gamma-rays in LHAASO J0341+5258 via Cosmic Ray Illumination of Molecular Clouds

We investigate the origin of the ultra-high-energy $\gamma$-ray emission detected by the Large High Altitude Air Shower Observatory (LHAASO) from the source LHAASO J0341+5258, which has not yet been associated with any known astrophysical object within the detector's field of view. The observed UHE emission is modelled within two independent frameworks: initially with a time-dependent, source-independent hadronic scenario implemented with the numerical package GAMERA, where particles propagate through the interstellar medium and subsequently interact with the molecular gas observed in the region and finally with an analytical description of the interaction between the accelerated cosmic-ray population from a supernova remnant (SNR) and the molecular gas around it. The relevant parameter space for the hypothetical past SNR is explored using the observed TeV $\gamma$-rays. We show that, for physically plausible source-cloud separations and propagation timescales, GAMERA provides a first-order approximation to the spectral modifications induced by particle transport and yields an adequate fit to the ultra-high-energy $\gamma$-ray data. Within the framework of our analytical approach, we demonstrated that the observed GeV emission can plausibly originate from the SNR itself, while the TeV emission detected by LHAASO can be consistently interpreted as arising from particles that have escaped from the SNR and are illuminating nearby molecular clouds. We invoke a spatio-temporally evolved SNR-molecular cloud interaction scenario to account self-consistently for the entire $\gamma$-ray spectrum from GeV to TeV energies. Despite the remaining uncertainty regarding the nature of the acceleration source, we conclude that the TeV emission detected by LHAASO can be consistently interpreted within the framework of an illumination scenario.

astro-ph.HE↗

arXiv2026

Multi-TeV $\gamma$-ray candidates from GRB 221009A: a downturn in the intrinsic $\gamma$-ray spectrum, an echo of the prompt emission phase, and intergalactic electromagnetic cascades

The detection of $\gamma$-ray candidates up to the energy of $\approx$13 TeV from the exceptionally bright $\gamma$-ray burst GRB 221009A by the Large High Altitude Air-Shower Observatory (LHAASO) has raised considerable interest in the astrophysical community. The $\gamma$-ray dataset resulting from the LHAASO observations allows one to reconstruct the intrinsic spectrum of GRB 221009A with an unprecedented precision. This intrinsic spectrum reveals a downturn at the energy of several TeV (statistical significance $\gt 5 \sigma$), i.e. the reconstructed intensity is below the intensity expected for a power-law spectrum. We show that a significant TeV $\gamma$-ray component may be produced by neutrons from photohadronic interactions inside the fireball. These neutrons escape the fireball and interact with the surrounding matter, giving rise to a flux of electrons and positrons, eventually resulting in an observable flux of GeV--TeV synchrotron photons -- a high energy "echo" of the GRB prompt emission phase. Finally, we show that at multi-TeV energies the contribution of $\gamma$ rays from intergalactic electromagnetic cascades initiated by primary ultra high energy protons is severely limited during the early afterglow phase for the typical magnetic field strength in the intergalactic filaments above 1 nG.

astro-ph.HE↗

arXiv2026

Constraints on GRB Jet Properties from IceCube Upper Limits: Insights from GRB 221009A and GRB 240825A

The IceCube neutrino telescope has provided upper limits on neutrino emission from gamma ray bursts. These constraints provided by the IceCube detector have been instrumental in investigating the properties of the GRB jet and its emission models. During the prompt phase of gamma ray burst emission, intense radiation components are generated that interact with the shock-accelerated particles within the jet. We study various GRB emission models, such as the internal shock model, the photospheric models, and also include a model-independent case. Based on these models, we calculate the neutrino fluence using the photo-hadronic interaction process. We estimate the bulk Lorentz factor using the well-known correlations between prompt phase observables, which is then used to calculate the emission site for the model-dependent scenarios. For GRB 221009A, we find that a low baryon loading scenario is consistent with the IceCube upper limits; however, for GRB 240825A, a higher value of baryon loading is preferred. Also, the values of the microphysical parameters $\epsilon_e$ and $\epsilon_B$ for GRB 240825A are lower by factors of approximately 10 and 100, respectively, compared to those of GRB 221009A. Further, using neutrino upper limits for these two sources, we estimate the lower limits on the dissipation radius for our models. The current TeV PeV upper limits for GRB 221009A are already useful for constraining parameter space for the BPH and MPH models.

astro-ph.HE↗

arXiv2025

Hadronic Emissions from the Microquasar V4641 Sgr, SS433, and its implications in the Diffuse Galactic Emission

Microquasars (MQs) are Galactic binary systems, consisting of a star and a compact object, a neutron star or a stellar mass black hole, which accretes matter from its companion star and gives rise to relativistic jets. Recent detection of very-high-energy (VHE; $E \gtrsim 100,\text{GeV}$) and ultra-high-energy (UHE; $E \gtrsim 100,\text{TeV}$) gamma-rays by LHAASO, HAWC and HESS from the MQ V4641 Sgr and SS 433 suggests them as Galactic PeVatrons. In this work, we studied a hadronic origin of the observed TeV-PeV gamma-ray emission from these MQs. We considered the hadronic scenario where the gamma-rays are produced by the interaction of relativistic protons in the MQ jet with the stellar wind. We fitted our model with observed data and constrained physical parameters like the hadronic jet power fraction, the proton spectral index, the maximum proton energy and the jet bulk Lorentz factor. Our best-fit model shows hard proton spectra ($1.84-2.44$) and maximum proton energies between 1 and 5 PeV. We also estimated the all-flavor neutrino fluxes corresponding to the gamma-ray fluxes from the hadronic model and found that V4641 sgr can be detected by next-generation neutrino telescopes like KM3NeT-ARCA and TRIDENT. Furthermore, we modeled a synthetic population of Galactic MQs and estimated their contribution to the diffuse TeV-PeV gamma-ray flux. For the inner Galaxy PSR contribution dominates in the range 10-100 TeV, and above 100 TeV diffused cosmic ray interactions with the molecular clouds is most dominant. We find that a population $\sim 14$ MQs is required to explain the LHAASO data above 100 TeV. For the outer Galaxy, we show that MQs are the dominant class of sources, and we constrain their population $\sim$14. Our findings strongly suggest that MQs are efficient particle accelerators, contributing to Galactic PeVatrons and potential multimessenger sources in our Galaxy.

astro-ph.HE↗

arXiv2025

Trends in Open Access Academic Outputs of State Agricultural Universities in India: Patterns from OpenAlex

Purpose: The study examines the Open Access (OA) landscape of Indian state agricultural universities, focusing on OA growth, leading institutions, prolific authors, preferred sources, funding, APC usage, and trending topics. It aims to identify research gaps, guide future research, and support policymakers in developing effective OA policies Design/methodology/approach The experiment utilized the OpenAlex database to collect global open access (OA) publications from Indian state agricultural universities over the past ten years (2014-2023). Using the Research Organization Registry ID, 97,536 publications were extracted. Data analysis was performed with OpenRefine, and ArcGIS 10.8 and Microsoft Excel were used for visualization. Findings: The global OA research output from state agricultural universities amounted to 65,889 publications across five OA categories: Green OA (7.35%), Diamond OA (6.74%), Gold OA (57.27%), Hybrid OA (9.24%), and Bronze OA (19.41%). Notably, 78.34% of articles were published in 864 low-impact domestic journals. Tamil Nadu Agricultural University produced the most publications in Gold, Diamond, Hybrid, and Bronze OA categories, while Punjab Agricultural University excelled in Green OA and received the highest funding, incurring the most article processing charges (APCs). Collaborative research focusing on agricultural policies, rice water management, soil fertility, and crop productivity had a greater impact. Originality/value The experiment is the first effort to evaluate the OA global academic research outputs of Indian state agriculture universities. The findings offer institutions, state governments, and funding agencies the opportunity to prioritise open-access publishing to promote sustainable agricultural research. Research limitations/implications The study is limited to the publications data indexed in the OpenAlex database.

cs.DL↗

arXiv2025

Direct Visualization of Temperature-Induced Phase Separation of Completely Miscible Au-Pd Alloy by In-Situ TEM

In-situ transmission electron microscopy (TEM) studies reveal key insights into the structural and chemical evolution of nanoparticles (NPs) under external stimuli like heating and biasing, which is critical for evaluating their suitability in chemical reactions and their tendency towards forming novel NP systems. In this study, starting from a core@shell Au nanotriangle (AuNT)@Pd nanostructure, the formation of a phase-separated bi-metallic Au-Pd NP system at high temperature is reported, despite the fact that Au and Pd are miscible in the entire composition and temperature range. In-situ TEM heating of bare AuNT@Pd core@shell structures up to 1000{\deg}C has been performed. Between 400{\deg}C and 800{\deg}C, an initial alloy formation has been observed. It is also noted that higher initial loading of Pd increases the melting temperature of the bi-metallic system. However, the most important observation is the separation of the nanostructure into Au and Pd phases at temperatures higher than 850{\deg} C for high Pd doping. The extent of Pd separation depends on the amount of initial Pd loading. A Janus Au-Pd nanostructure is formed at the end of the thermal treatments at 1000{\deg} C. The phase-separated NP is observed to be highly stable and could be clearly beneficial for various applications, particularly in catalytic processes.

cond-mat.mtrl-sci↗

arXiv2024

Gamma-rays and Neutrinos from Giant Molecular Cloud Populations in the Galactic Plane

The recent IceCube detection of significant neutrino flux from the inner Galactic plane has provided us valuable insights on the spectrum of cosmic rays in our Galaxy. This flux can be produced either by a population of Galactic point sources or by diffused emission from cosmic ray interactions with the interstellar medium or by a mixture of both. In this work, we compute diffused gamma-ray and neutrino fluxes produced by a population of giant molecular clouds (GMCs) in our Galaxy, assuming different parametrizations of the Galactic diffused cosmic ray distribution. In particular, we take into account two main cases: (I) constant cosmic ray luminosity in our Galaxy, and (II) space-dependent cosmic ray luminosity, based on the supernovae distribution in our Galaxy. For Case-I, we found that the neutrino flux from GMCs is a factor of $\sim 10$ below compared to $π^0$ and KRA$_γ$ best-fitted models of IceCube observations at $10^5$ GeV. Instead, for Case-II the model can explain up to $\sim 90 \%$ of the neutrino flux at that energy. Moreover, for this last scenario IceCube detector could be able to detect neutrino events from the Galactic centre regions. We then calculated gamma-ray and neutrino fluxes from individual GMCs and noticed that several current and future Cherenkov telescopes and neutrino observatories have the right sensitivities to study these objects. In particular, very neutrino-bright region such as Aquila Rift is favourable for detection by the IceCube-Gen2 observatory.

astro-ph.HE↗

arXiv2024

Experimental study of $α$-induced reactions on $^{113}$In for astrophysical $p$-process

Neutron deficient nuclei from $^{74}$Se$-^{196}$Hg are thought to be produced by $γ$-induced reactions ($γ$,n), ($γ$,p) and ($γ,α$) processes. The relatively high abundance of $^{113}$In odd A $p$-nuclei has inspired to study its production processes. As reaction with $γ$-beam is difficult to perform in the laboratory, $γ$-induced reaction rate is calculated from the inverse reaction data employing reciprocity theorem. Stacked foil activation method was used to measure the $^{113}$In($α,γ$) and $^{113}$In($α$, n) reactions cross-section near the astrophysical energies. Theoretical statistical model calculations were performed with different nuclear input parameters and compared with the experimental results. An appropriate $α$-optical potential has been identified from the ($α,γ$) and ($α$, n) fitting, which provides the major source of uncertainty in the statistical model calculations. The other nuclear input parameters like level density, and $γ$-ray strength function were also constrained for theoretical calculations. $^{113}$In($α,γ$)$^{117}$Sb and $^{117}$Sb($α,γ$)$^{113}$In reaction rates were calculated using best-fitted input parameters.

nucl-ex↗

arXiv2023

Quasi 1D Nanobelts from the Sustainable Liquid Exfoliation of Terrestrial Minerals for Future Martian based Electronics

The sky is the limit with regards to the societal impact nanomaterials can have on our lives. However, in this study we show that their potential is out of this world. The planet Mars has an abundant source of calcium sulfate minerals and in our work, we show that these deposits can be the basis of transformative nanomaterials to potentially support future space endeavors. Through a scalable eco-friendly liquid processing technique performed on two common terrestrial gypsum, our simple method presented a cost-efficient procedure to yield the commercially valuable intermediate phase of gypsum, known as bassanite. Through the liquid exfoliation of bassanite powders, suspensions of large aspect ratio anhydrite nanobelts with long-term stability were characterized through scanning electron microscopy and Raman spectroscopy. Transmission electron microscopy showed nanobelts to have a mesocrystal structure, with distinct nanoparticle constituents making up the lattice. Unexpectedly, anhydrite nanobelts had remarkable electronic properties, namely a bandgap that was easily tuned between semiconducting (~2.2 eV) and insulating (~4 eV) behaviors through dimensional control measured via atomic force microscopy. To demonstrate the application potential of our nanobelts; optoelectronic, electrochemical and nanocomposite measurements were made. For the hydrogen evolution reaction and mechanical reinforcement, selenite-based anhydrite nanobelts displayed superlative performances.

physics.app-ph↗

arXiv2023

Interpreting the GeV-TeV Gamma-Ray Spectra of Local Giant Molecular Clouds using GEANT4 Simulation

Recently, the Fermi-LAT gamma-ray satellite has detected six Giant Molecular Clouds (GMCs) located in the Gould Belt and the Aquila Rift regions. In half of these objects (Taurus, Orion A, Orion B), the observed gamma-ray spectrum can be explained using the Galactic diffused Cosmic Ray (CR) interactions with the gas environments. In the remaining three GMCs (Rho Oph, Aquila Rift, Cepheus), the origin of the gamma-ray spectrum is still not well established. We use the GEometry ANd Tracking (GEANT4) simulation framework in order to simulate gamma-ray emission due to CR/GMC interaction in these three objects, taking into account the gas density distribution inside the GMCs. We find that propagation of diffused Galactic CRs inside these GMCs can explain the Fermi-LAT detected gamma-ray spectra. Further, our estimated TeV-PeV fluxes are consistent with the HAWC upper limits, available for the Aquila Rift GMC. As last step, we compute the total neutrino flux estimated for these GMCs and compare it with the IceCube detection sensitivity.

astro-ph.HE↗

arXiv2022

AQuaMoHo: Localized Low-Cost Outdoor Air Quality Sensing over a Thermo-Hygrometer

Efficient air quality sensing serves as one of the essential services provided in any recent smart city. Mostly facilitated by sparsely deployed Air Quality Monitoring Stations (AQMSs) that are difficult to install and maintain, the overall spatial variation heavily impacts air quality monitoring for locations far enough from these pre-deployed public infrastructures. To mitigate this, we in this paper propose a framework named AQuaMoHo that can annotate data obtained from a low-cost thermo-hygrometer (as the sole physical sensing device) with the AQI labels, with the help of additional publicly crawled Spatio-temporal information of that locality. At its core, AQuaMoHo exploits the temporal patterns from a set of readily available spatial features using an LSTM-based model and further enhances the overall quality of the annotation using temporal attention. From a thorough study of two different cities, we observe that AQuaMoHo can significantly help annotate the air quality data on a personal scale.

cs.CY↗

arXiv2021

Simulation of Cosmic Rays in the Earth's Atmosphere and Interpretation of Observed Counts in an X-ray Detector at Balloon Altitude Near Tropical Region

The study of secondary particles produced by the cosmic-ray interaction in the Earth's atmosphere is very crucial as these particles mainly constitute the background counts produced in the high-energy detectors at balloon and satellite altitudes. In the present work, we calculate the abundance of cosmic-ray generated secondary particles at various heights of the atmosphere by means of a Monte Carlo simulation and use this result to understand the background counts in our X-ray observations using balloon-borne instruments operating near the tropical latitude (geomagnetic latitude: $\sim 14.50^{\circ}$ N). For this purpose, we consider a 3D description of the atmospheric and geomagnetic field configurations surrounding the Earth, as well as the electromagnetic and nuclear interaction processes using Geant4 simulation toolkit. Subsequently, we use a realistic mass model description of the detector under consideration, to simulate the counts produced in the detector due to secondary cosmic-ray particles.

astro-ph.EP↗

arXiv2021

Monte Carlo Simulation of CRAND Protons Trapped at Low Earth Orbits

The Cosmic Ray Albedo Neutron Decay (CRAND) is believed to be the principal mechanism for the formation of inner proton radiation belt -- at least for relatively higher energy particles. We implement this mechanism in a Monte Carlo simulation procedure to calculate the trapped proton radiation at the low Earth orbits, through event-by-event interaction of the cosmic ray particles in the Earth's atmosphere and their transportation in the magnetosphere. We consider the generation of protons from subsequent decay of the secondary neutrons from the cosmic ray interaction in the atmosphere and their transport (and/or trapping) in the geomagnetic field. We address the computational challenges for this type of calculations and develop an optimized algorithm to minimize the computation time. We consider a full 3D description of the Earth's atmospheric and magnetic-field configurations using the latest available models. We present the spatial and phase-space distribution of the trapped protons considering the adiabatic invariants and other parameters at the low Earth orbits. We compare the simulation results with the trapped proton flux measurements made by PAMELA experiment at low Earth orbit and explain certain features observed by the measurement.

astro-ph.EP↗

arXiv2021

Background model of Phoswich X-ray detector on board small balloon

We performed a detailed modelling of the background counts observed in a phoswich scintillator X-ray detector at balloon altitude, used for astronomical observations, on board small scientific balloon. We used Monte Carlo simulation technique in Geant4 simulation environment, to estimate the detector background from various plausible sources. High energy particles and radiation generated from the interaction of Galactic Cosmic Rays with the atmospheric nuclei is a major source of background counts (under normal solar condition) for such detectors. However, cosmogenic or induced radioactivity in the detector materials due to the interaction of high energy particles and natural radioactive contamination present in the detector can also contribute substantially to the detector background. We considered detailed 3D modelling of the earth's atmosphere and magnetosphere to calculate the radiation environment at the balloon altitude and deployed a proper mass model of the detector to calculate the background counts in it. The calculation satisfactorily explains the observed background in the detector at 30 km altitude (atmospheric depth: 11.5 $g/cm^{2}$) during the balloon flight experiment from a location near 14.5$^{\circ}$N geomagnetic latitude.

astro-ph.IM↗

arXiv2021

Extensive study of radiation dose on human body at aviation altitude through Monte Carlo simulation

The diverse near-Earth radiation environment due to cosmic rays and solar radiation has direct impact on human civilization. In the present and upcoming era of increasing air transfer, it is important to have precise idea of radiation dose effects on human body during air travel. Here, we calculate the radiation dose on the human body at the aviation altitude, also considering the shielding effect of the aircraft structure, using Monte Carlo simulation technique based on Geant4 toolkit. We consider proper 3D mathematical model of the atmosphere and geomagnetic field, updated profile of the incoming particle flux due to cosmic rays and appropriate physics processes. We use quasi-realistic computational phantoms to replicate the human body (male/female) for the effective dose calculation and develop a simplified mathematical model of the aircraft (taking Boeing 777--200LR as reference) for the shielding study. We simulate the radiation environment at the flying altitude (considering geomagnetic latitude in the range of 45-50$^{\circ}$), as well as at various locations inside the fuselage of the aircraft. Then, we calculate the dose rates in the different organs for both male and female phantoms, based on latest recommendations of International Commission on Radio logical Protection. This calculation shows that the sex-averaged effective dose rate in human phantom is 5.46 $μ$Sv/h, whereas, if we calculate weighted sum of equivalent dose contributions separately in female and male body: total weighted sum of equivalent dose rate received by the female phantom is 5.72 $μ$Sv/h and that by the male phantom is 5.20 $μ$Sv/h. From the simulation, we also calculate the numerous cosmogenic radionuclides produced inside the phantoms through activation or spallation processes which may induce long-term biological effects.

physics.med-ph↗

arXiv2021

Generalized theory of spatial coherence for superposition of two speckle patterns with polarization diversity

A generalized theory of spatial coherence for superposition of two speckle patterns with polarization diversity is presented. The presented theory deals with superposition in different scenarios i.e. superposition of two fully correlated, partially correlated and completely uncorrelated speckle patterns, and describes the effect on the spatial coherence-polarization (CP) property of the superposed random field from the study of the spatial degree of coherence and degree of polarization. The change in the spatial CP property with the polarization diversity is studied for different correlation factors of the constituent random fields, and a modulation of the spatial CP property is observed, when the constituent speckle patterns are not fully correlated. The effect of the average intensity ratio of the two random fields on the spatial CP property is also studied. Thus, this theoretical study completes the case of superposition of two speckle patterns with polarization diversity and its effect on the spatial CP property.

physics.optics↗

arXiv2021

Spatial statistics of superposition of two uncorrelated speckle patterns with polarization diversity

A detailed theoretical and experimental study on the effect of the superposition of uncorrelated speckle patterns with polarization diversity on the spatial statistics of the superposed speckle pattern is presented. It is shown that depending on the mutual orientation of the polarization vectors of the constituent speckle patterns, the maximum degree of coherence (DoC) and degree of polarization (DoP) of the superposed speckle pattern changes between a maximum and minimum value in a sinusoidal fashion. Moreover, the average intensity ratio of the constituent speckle patterns is also found to be affecting these variations. A study of the change in the visibility of the two-point intensity correlation function also reveals a sinusoidal nature of the variation and its dependence on the ratio of the average intensity, which are found to be similar to the variations of the maximum DoC and DoP. A detailed study on the changes in the normalized probability density function is also performed for better understanding of the effect on the spatial statistics.

physics.optics↗