arXiv subjects

P. Tiwari

Publications and source records attributed to P. Tiwari.

9 recordsLinked to original sources

arXiv2026

Neural-network reconstruction of THz transmission spectra using electrically tunable AlGaN/GaN plasmonic-crystal analyzer

We demonstrate machine learning (ML) based reconstruction of terahertz transmission spectra using an electrically tunable grating-gate AlGaN/GaN plasmonic-crystal analyzer. The analyzer encodes the transmission spectrum into a voltage-dependent intensity, which is then inverted by an ML algorithm. A feedforward neural network trained on a synthetic dataset is validated experimentally on four samples in standard Fourier Transform Infrared (FTIR) mode and in direct (fixed-mirror) acquisition mode. The network achieves a mean square error (MSE) of the reconstruction of 0.015 in FTIR mode and 0.038 in direct mode, correctly identifying six out of seven ground-truth resonances in each mode. Against a first-difference Tikhonov regularization baseline, the mean reconstruction error is reduced 3.6 times in FTIR mode and 1.55 times in direct mode, with fewer spurious peaks and lower peak-position errors. Voltage-tunable plasmonic filtering combined with neural-network inversion establishes an interferometer-free architecture for THz spectral reconstruction.

physics.optics↗

arXiv2026

Detection of TeV emission during early afterglow from poorly localized GRBs with ground based IACTs

Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe. While space-based instruments have extended GRB observations up to energies of $\sim$100 GeV, the detection of very-high-energy (VHE; $E>100$ GeV) emission from ground-based telescopes, especially during prompt or/and the early afterglow phase, remains challenging. These difficulties arise from the rapid temporal decay of GRB afterglows, strong attenuation by the extragalactic background light (EBL), observational latency, and the typical poor sky localization provided by MeV-detectors such as Fermi/GBM. In this work, we investigate the prospects for detecting TeV ($\sim$100 GeV--1 TeV) emission from poorly localized GRBs by adopting optimized follow-up strategies based on rapid tiling of large localization regions. We simulate a realistic population of GRBs informed by more than fifteen years of Fermi/GBM and Swift/XRT detections and recent progresses in the afterglow emission modeling. Using these simulations, we evaluate the detectability of GRB early afterglows by the next-generation Imaging Atmospheric Cherenkov Telescopes, equipped with larger field-of-view (FoV), as a function of latency, exposure time, and observational strategy. Our strategy can significantly enhance the detection rate; for instruments such as ASTRI and LACT, it increases by up to a factor of two compared to strategies limited to well-localized (Swift-like) events. For CTAO, our proposed approach provides up to four VHE detections per year.

astro-ph.HE↗

arXiv2025

From X-rays to High-Energy Gamma-rays: A Comprehensive Multi-Wavelength Study of Early Gamma-Ray Burst Afterglows

Gamma-ray Bursts (GRBs) generate powerful relativistic jets that inject a large amount of energy into their surrounding environment, producing blast waves that accelerate particles to high energies. The GRB afterglow radiation provides a powerful means to investigate the microphysics of relativistic shocks and to probe the medium surrounding the progenitor of the burst. In this study, we present a comprehensive multiwavelength analysis of 31 GRBs observed between 2008 and 2024 from the Neil Gehrels Swift Observatory (X-ray Telescope and Burst Alert Telescope) and the Fermi Large Area Telescope, covering photon energies from 0.3 keV to 300 GeV. Our goal is to characterize the broadband spectral properties of GRB afterglows in soft X-rays, hard X-rays, and high-energy gamma rays. We investigate correlations between spectral shape and energy output across different parts of the spectrum. The observed emission is modeled using a forward shock scenario that includes both synchrotron and synchrotron self-Compton (SSC) radiation losses. The results favor an SSC-dominated radiation model in a wind-like medium, consistent with expectations for long-duration GRBs. Crucially, this work provides new benchmarks for the microphysical parameters governing the emission, particularly indicating a notably low magnetic energy fraction, which refines previous estimates. By modeling broadband data, this study offers one of the most detailed SSC analyses in a wind-like environment to date. Notably, our results naturally account for the comparable energy output observed in both the soft X-ray and TeV bands, consistent with the previously detected TeV-GRBs.

astro-ph.HE↗

arXiv2025

A mathematical model of HPAI transmission between dairy cattle and wild birds with environmental effects

Highly pathogenic avian influenza (HPAI), particularly H5N1, poses an increasing threat at the wildlife--livestock--environment interface, with recent detections in dairy cattle motivating multi-host transmission models. We develop and analyse a deterministic compartmental model for HPAI transmission among cattle, wild birds, and a shared contaminated environment. The model combines an SEIR structure for cattle with an SIR framework for wild birds and includes environmentally mediated transmission. We establish positivity and boundedness, characterise the disease-free and endemic equilibria, and derive the basic reproduction number $\mathcal R_0$ as the spectral radius of a reduced next-generation matrix representing coupled transmission pathways. We also establish threshold and global stability properties of the equilibria. Numerical simulations illustrate extinction, persistence, and parameter-dependent disease dynamics. Global sensitivity analysis identifies cattle recovery, cattle-to-cattle transmission, environmental viral decay, and environmental transmission as the principal determinants of $\mathcal R_0$ within the parameter ranges considered. Response-surface and intervention analyses indicate that reducing cattle-to-cattle transmission produces the greatest reduction in transmission potential and peak cattle infection, while increasing cattle recovery provides an additional effective pathway. Reducing environmental transmission alone has a comparatively small effect under the baseline parameterisation. Within the model assumptions and parameter ranges considered, the findings provide qualitative insights into the relative effects of transmission pathways and potential control measures.

q-bio.PE↗

arXiv2023

Cosmology from LOFAR Two-metre Sky Survey Data Release 2: Angular Clustering of Radio Sources

Covering $\sim$5600 deg$^2$ to rms sensitivities of $\sim$70$-$100 $\mu$Jy beam$^{-1}$, the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS-DR2) provides the largest low-frequency ($\sim$150 MHz) radio catalogue to date, making it an excellent tool for large-area radio cosmology studies. In this work, we use LoTSS-DR2 sources to investigate the angular two-point correlation function of galaxies within the survey. We discuss systematics in the data and an improved methodology for generating random catalogues, compared to that used for LoTSS-DR1, before presenting the angular clustering for $\sim$900,000 sources $\geq$$1.5$ mJy and a peak signal-to-noise $\geq$$7.5$ across $\sim$$80\%$ of the observed area. Using the clustering we infer the bias assuming two evolutionary models. When fitting {angular scales of $0.5 \leq\theta<5\,\deg$, using a linear bias model, we find LoTSS-DR2 sources are biased tracers of the underlying matter, with a bias of $b_{C}= 2.14^{+0.22}_{-0.20}$ (assuming constant bias) and $b_{E}(z=0)= 1.79^{+0.15}_{-0.14}$ (for an evolving model, inversely proportional to the growth factor), corresponding to $b_E= 2.81^{+0.24}_{-0.22}$ at the median redshift of our sample, assuming the LoTSS Deep Fields redshift distribution is representative of our data. This reduces to $b_{C}= 2.02^{+0.17}_{-0.16}$ and $b_{E}(z=0)= 1.67^{+0.12}_{-0.12}$ when allowing preferential redshift distributions from the Deep Fields to model our data. Whilst the clustering amplitude is slightly lower than LoTSS-DR1 ($\geq$2 mJy), our study benefits from larger samples and improved redshift estimates.

astro-ph.CO↗

arXiv2023

Cosmology from LOFAR Two-metre Sky Survey Data Release 2: Cross-correlation with the cosmic microwave background

We combine the LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) second data release (DR2) catalogue with gravitational lensing maps from the Cosmic Microwave Background (CMB) to place constraints on the bias evolution of LoTSS radio galaxies, and on the amplitude of matter perturbations. We construct a flux-limited catalogue, and analyse its harmonic-space cross-correlation with CMB lensing maps from Planck, $C_\ell^{g\kappa}$, as well as its auto-correlation, $C_\ell^{gg}$. We explore the models describing the redshift evolution of the large-scale radio galaxy bias, discriminating between them through the combination of both $C_\ell^{g\kappa}$ and $C_\ell^{gg}$. Fixing the bias evolution, we then use these data to place constraints on the amplitude of large scale density fluctuations. We report the significance of the $C_\ell^{g\kappa}$ signal at a level of $26.6\sigma$. We determine that a linear bias evolution of the form $b_g(z) = b_{g,D} / D(z)$, where $D(z)$ is the growth rate, is able to provide a good description of the data, and measure $b_{g,D} = 1.41 \pm 0.06$ for a sample flux-limited at $1.5\,{\rm mJy}$, for scales $\ell < 250$ for $C_\ell^{gg}$, and $\ell < 500$ for $C_\ell^{g\kappa}$. At the sample's median redshift, we obtain $b(z = 0.82) = 2.34 \pm 0.10$. Using $\sigma_8$ as a free parameter, while keeping other cosmological parameters fixed to the Planck values, we find fluctuations of $\sigma_8 = 0.75^{+0.05}_{-0.04}$. The result is in agreement with weak lensing surveys, and at $1\sigma$ difference with Planck CMB constraints. We also attempt to detect the late-time integrated Sachs-Wolfe effect with LOFAR, but with the current sky coverage, the cross-correlation with CMB temperature maps is consistent with zero. Our results are an important step towards constraining cosmology with radio continuum surveys from LOFAR and other future large radio surveys.

astro-ph.CO↗

arXiv2021

$^{25}$Si $β^+$-decay spectroscopy

$β$-decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line. To search for new $β$-delayed protons and $γ$ rays of $^{25}$Si to investigate the properties of $^{25}$Al excited states. $^{25}$Si $β$ decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and $γ$ rays emitted in the decay were detected simultaneously. A Monte Carlo method was used to model the Doppler broadening of $^{24}$Mg $γ$-ray lines caused by nuclear recoil from proton emission. Shell-model calculations using two newly developed universal \textit{sd}-shell Hamiltonians, USDC and USDI, were performed. The most precise $^{25}$Si half-life to date has been determined. A new proton branch at 724(4)~keV and new proton-$γ$-ray coincidences have been identified. Three $^{24}$Mg $γ$-ray lines and eight $^{25}$Al $γ$-ray lines are observed for the first time in $^{25}$Si decay. The first measurement of the $^{25}$Si $β$-delayed $γ$ ray intensities through the $^{25}$Al unbound states is reported. All the bound states of $^{25}$Al are observed to be populated in the $β$ decay of $^{25}$Si. Several inconsistencies between the previous measurements have been resolved, and new information on the $^{25}$Al level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of $^{25}$Na and the shell-model calculations. The measured excitation energies, $γ$-ray and proton branchings, log~$ft$ values, and Gamow-Teller transition strengths for the states of $^{25}$Al populated in the $β$ decay of $^{25}$Si are in good agreement with the shell-model calculations, offering gratifyingly consistent insights into the fine nuclear structure of $^{25}$Al.

nucl-ex↗

arXiv2020

Low-energy $^{23}$Al $β$-delayed proton decay and $^{22}$Na destruction in novae

The radionuclide $^{22}$Na is a target of $γ$-ray astronomy searches, predicted to be produced during thermonuclear runaways driving classical novae. The $^{22}$Na(p,$γ$)$^{23}$Mg reaction is the main destruction channel of $^{22}$Na during a nova, hence, its rate is needed to accurately predict the $^{22}$Na yield. However, experimental determinations of the resonance strengths have led to inconsistent results. In this work, we report a measurement of the branching ratios of the $^{23}$Al $β$-delayed protons, as a probe of the key 204--keV (center-of-mass) $^{22}$Na(p,$γ$)$^{23}$Mg resonance strength. We report a factor of 5 lower branching ratio compared to the most recent literature value. The variation in $^{22}$Na yield due to nuclear data inconsistencies was assessed using a series of hydrodynamic nova outburst simulations and has increased to a factor of 3.8, corresponding to a factor of $\sim$2 uncertainty in the maximum detectability distance. This is the first reported scientific measurement using the Gaseous Detector with Germanium Tagging (GADGET) system.

nucl-ex↗

arXiv2017

Fabrication of plasmonic surface relief gratings for the application of band-pass filter in UV-Visible spectral range

The measured experimental results of optical diffraction of 10, 5 and 3.4 micrometer period plasmonic surface relief grating are presented for the application of band-pass filter in visible spectral range. Conventional scanning electron microscopic (SEM) is used to fabricate the grating structures on the silver halide based film (substrate) by exposing the electron beam in raster scan fashion. Morphological characterization of the gratings is performed by atomic force microscopy (AFM) shows that the period, height and profile depends on the line per frame, beam spot, single line dwell time, beam current, and accelerating voltage of the electron beam. Optical transmission spectra of 10 micrometer period grating shows a well-defined localized surface plasmon resonance (LSPR) dip at ~366 nm wavelength corresponding to gelatin embedded silver nanoparticles of the grating structure. As the period of the grating reduces LSPR dip becomes prominent. The maximum first order diffraction efficiency (DE) and bandwidth for 10 micrometer period grating are observed as 4% and 400 nm in 350 nm to 800 nm wavelength range respectively. The DE and bandwidth are reduced up to 0.03% and 100 nm for 3.4 micrometer period grating. The profile of DE is significantly flat within the diffraction bandwidth for each of the gratings. An assessment of the particular role of LSPR absorption and varied grating period in the development of the profile of first order DE v/s wavelength are studied. Fabrication of such nano-scale structures in a large area using conventional SEM and silver halide based films may provide the simple and efficient technique for various optical devices applications.

physics.app-ph↗