arXiv subjects

Pankaj Jain

Publications and source records attributed to Pankaj Jain.

At least 19 recordsLinked to original sources

arXiv2026

Radio Galaxies detection and characterization using deep learning techniques

Future radio telescopes will generate data volumes that are increasingly difficult to analyse using traditional statistical methods, motivating the adoption of machine-learning techniques. In this work, we present YOLO-Chars (YOLO-based Detection and Characterisation of Radio Sources), a two-stage deep-learning framework for the automated detection and characterisation of radio galaxies in survey images. The framework is developed and evaluated using the Square Kilometre Array Science Data Challenge 1 (SKA SDC1) dataset. In the first stage, customised YOLO-based multi-scale detection models are used to localise compact and extended sources across large sky maps. In the second stage, a dedicated source-characterisation network estimates the physical properties of the detected sources. We focus on three key parameters: flux density, angular size, and position angle. Our results show that YOLO-Chars achieves competitive detection and characterisation performance on the SKA SDC1 benchmark, demonstrating its potential as a scalable framework for next-generation radio continuum surveys.

astro-ph.IM↗

arXiv2026

Complex Energy-Dependent Behaviour of Quasi-Periodic Oscillation Observed in GRS 1915+105

We present complex energy-resolved properties of quasi-periodic oscillations (QPOs) in the black hole X-ray binary GRS 1915+105 using an observation from the LAXPC instrument onboard AstroSat. Power density spectra (PDSs) are constructed in multiple energy bands and modeled with multi-Lorentzian components to investigate the energy dependence of QPO properties. The QPO frequency shows a modest increase with energy. Dynamic PDS analysis does not reveal clear evidence for time-dependent evolution of the QPO frequency, suggesting that the observed frequency shift is not primarily driven by temporal variability. We perform simultaneous fitting of energy-resolved PDSs and find that a model in which the QPO feature is described by two Lorentzian components provides a better fit. The two components exhibit different evolution in fractional root mean square amplitude as a function of energy. We further examine the phase-lag properties by simultaneously modeling the PDS and the real and imaginary parts of the cross-spectrum and find distinct phase-lag behavior for the two components. Overall, these results indicate that the apparent energy-dependent evolution of the QPO feature may be a result of the presence of more than one variability component.

astro-ph.HE↗

arXiv2026

Correlations Between kHz QPOs and Spectral Parameters from Time-Resolved Spectro-Temporal Analysis of 4U 1728-34

We present a time-resolved analysis of the persistent emission in 4U 1728--34 using AstroSat observations from 2016 to 2019. We detect kilohertz quasi-periodic oscillations (kHz QPOs) during all epochs, with centroid frequencies ranging from $\sim 350$ to $1180~\mathrm{Hz}$, although some detections are of lower significance ($< 3\sigma$). We model the simultaneous spectra from the Soft X-ray Telescope and the Large Area X-ray Proportional Counter using a combination of an absorbed disk component (diskbb), a blackbody component (bbodyrad), a thermal Comptonization model (thcomp), and a broad Gaussian line. From the diskbb parameters, we estimate the accretion rate and find that all observations fall into two accretion regimes, namely AR1 and AR2, with accretion rates of $\sim 3 \times 10^{16}~\mathrm{g\,s^{-1}}$ and $\sim 7 \times 10^{16}~\mathrm{g\,s^{-1}}$, respectively. Interestingly, we find that for AR1, the lower kHz QPO frequency ($\nu_{\mathrm{L}}$) is always $< 500~\mathrm{Hz}$, while for AR2 it is $\gtrsim 500~\mathrm{Hz}$. We found that the spectral index showed no clear correlation with $\nu_{\mathrm{L}}$. For AR1, the coronal electron temperature ($kT_{\mathrm{e}}$) and optical depth ($\tau$) are $\sim 10~\mathrm{keV}$ and $\sim 5$, respectively. In contrast, for AR2, $kT_{\mathrm{e}}$ decreases to $\sim 3~\mathrm{keV}$ and $\tau$ increases to $\sim 12$, showing correlations with $\nu_{\mathrm{L}}$, with Spearman's rank correlation coefficients of $-0.78$ and $0.71$, respectively. The transition of the spectral parameters at $\nu_{\mathrm{L}} \sim 500~\mathrm{Hz}$ indicates the existence of a critical QPO frequency governed or influenced by the accretion state of the source.

astro-ph.HE↗

arXiv2026

Constraining Spin and Inclination Angle of XTE J2012+381 using AstroSat and NICER

We present a spectral analysis of a black hole X-ray binary XTE J2012+381 during its 2022 outburst, using data from NICER and AstroSat. Combining data from NICER, LAXPC20, and SXT, we extract energy spectra covering the 0.7-10.0 keV range. We model the energy spectra using a series of physical models and find that a reflection-Comptonization model provides the best fit. Given the uncertainties in the black hole mass and source distance, we investigate the stability of the inferred spectral parameters by systematically varying the black hole mass (7.26, 11, and 16.5 M$_\odot$), source distance (3.3, 5.4, and 7.5 kpc), and spectral hardening factor (1.5, 1.7, and 1.9). We find that, across most combinations of these parameters, the spin solutions consistently lie in the high-spin regime, spanning values between $\sim$0.67 and $\sim$0.998, with only a limited subset of configurations favoring lower spins. In contrast, the disk inclination angle remains well constrained over the majority of the explored parameter space, typically ranging between $\sim$50{\deg} and $\sim$65{\deg}. Only a few parameter combinations yield higher inclination values.

astro-ph.HE↗

arXiv2026

Experimental Determination of Slow-Neutron Detection Efficiency and Background Discrimination in Mixed Radiation Fields Using Differential CR-39 Track Detectors

Measuring slow neutrons is difficult when the radiation field also contains charged particles and fast neutrons, especially when the radiation composition is not known in advance. In this work, we present a tested method to measure slow neutron fluence using CR-39 solid state nuclear track detectors. Two detectors are used together: a boron coated CR-39 detector and an uncoated CR-39 detector.The uncoated detector records tracks from charged particles and fast neutrons but does not respond to slow neutrons. The boron coated detector additionally detects charged particles produced when slow neutrons react with boron and generate lithium and alpha particles. Subtracting the track density of the uncoated detector from that of the boron coated detector provides a reliable and conservative measure of slow neutrons.Experiments using a reference thermal neutron source show that the difference between the two detectors increases linearly with exposure time. Statistical analysis gives a slow neutron equivalent track rate of 5.84 plus or minus 0.18 tracks per minute, clearly different from zero. The slope of this response is used to determine the detection efficiency of the boron coated detector. The uncoated detector measures the background caused by fast neutron leakage from the source. These results show that boron coated CR-39 detectors cannot be used alone for accurate slow neutron measurements. Reliable neutron fluence determination requires the simultaneous use of an uncoated detector. The difference between the two detectors provides a correct estimate of the thermal neutron flux in mixed radiation fields and where conventional neutron detectors cannot be used.

nucl-ex↗

arXiv2026

Probing the morphology of the Gum Nebula through pulsar observables and a novel distance estimation method

Various existing models of the Gum Nebula differ significantly in their parameters and suggested origins, which can be independently tested for consistency with data on some key observables of pulsars in the direction of the nebula. Our analysis of such data on the Vela pulsar, assuming a dominant scattering region in its foreground, suggests that the fractional distance of the scatterer is $0.89 \pm 0.01$, and for the given distance of the Vela pulsar, it translates to $254 \pm 16$ pc. Using independent distances of ten pulsars, we suggest a refined description of the Gum Nebula electron density model with its basic morphology similar to that used in the YMW16 model, which now provides better estimates of pulsar distances in these directions. In our new Gum Nebula model, as expected, the Vela pulsar would be behind the frontal edge of the Gum shell, which was intriguingly located in front of the nebula in the YMW16 model. We also present a new technique to better constrain the pulsar distances using their dispersion measure and temporal broadening simultaneously, and find that it is less affected by the uncertainties in the Galactic electron density distribution models. Notably, the new approach shows that the expected temporal broadening as a function of trial distance does not follow a monotonic increasing trend, but instead exhibits oscillations at regions of enhanced electron density. This behaviour is expected, as the method employs the integral form of temporal broadening with the appropriate weighting kernel, leading to more reliable estimates.

astro-ph.HE↗

arXiv2026

CR-39 track detector signatures of slow neutron like signals in Heavy-water electrolysis

We report reproducible track-detector signals consistent with slow neutron capture events, recorded in D$_2$O electrolysis involving D-Pd deposited on Pt cathode. Sensitivity to slow neutrons was achieved using boron-coated CR-39 (BCR) detectors, which register charged particle tracks arising from the $^{10}$B$(n,\alpha)^{7}$Li reaction. These detectors were positioned adjacent to identically prepared uncoated CR-39 control detectors (CCR), which are effectively insensitive to slow neutrons and serve to quantify background contributions from charged particles and fast neutrons under the present experimental conditions. A reproducible differential detector signature (BCR $>$ CCR) would thus indicative of slow neutron fluences. Across multiple independent D$_2$O electrolysis experiments in $0.25~\mathrm{T}$ field, the BCR exhibited significantly excess track signals relative to CCRs. Under these conditions, the observed differential response corresponds to an inferred detector-equivalent slow neutron flux of approximately $(6.7 \pm 0.2)~\mathrm{cm^{-2},s^{-1}}$. Removal of the magnetic field resulted in a reduction of the differential signal by a factor of $\sim6$, indicating a strong empirical dependence on the applied field. In contrast, H$_2$O electrolysis performed under otherwise identical conditions produced no measurable differential detector response, establishing the necessity of deuterated electrochemical conditions for the observed effect. The results are reported strictly as detector signatures consistent with slow neutron capture and do not assert any theoretical explanation. Instead, this work establishes a control verified and detector validated experimental protocol for detecting low flux slow neutrons, and provides empirical constraints relevant to slow neutron studies in experiments involving metal-deuteride systems.

physics.ins-det↗

arXiv2025

Evolution of the 2021 Outburst of GX 339-4 with AstroSat

We present a comprehensive study of the 2021 outburst of GX 339-4 using AstroSat observations in the hard-intermediate (HIMS) and soft-intermediate states (SIMS). Spectral and timing analyses across these states suggest that during the SIMS, unabsorbed flux (0.1-3 keV), inner disc temperature, and "apparent" inner disc radius do not change, suggesting the stability of the disc. In the SIMS, the photon index decreases from 2.1 to 1.7, indicating spectral hardening. The power density spectra (PDS) suggest the presence of quasi-periodic oscillations (QPOs) in the HIMS and SIMS. The QPO frequency evolves from 0.1 Hz to 0.2 Hz in the HIMS, and further to 5.7 Hz in the SIMS. We also observe a decrease in QPO frequency from 5.7 Hz to 4.5 Hz during the SIMS. We discuss the evolution of the QPO, fractional root mean square (rms) amplitude, and time-lag spectra. We discover that variations in disc normalization, disc temperature, and coronal heating rate can reproduce the observed rms and lag spectra with a time delay between them.

astro-ph.HE↗

arXiv2025

Enhancement of Alpha Decay due to Medium Effects

We study the effect of medium on radioactive alpha decay and other similar decays. The initial state in these type of decays is a quasi-bound state with energy greater than zero. Such a state has very large amplitude in the nuclear region and is exponentially suppressed at larger distances. The decay rate of such states is known to decrease rapidly with decrease in the Q-value. Here we study such a decay within a medium. We assume a simple spherically symmetric repulsive potential model for the medium. This models the cumulative effect of all nuclei in the medium which at short distances present repulsive Coulomb interaction. We find that as the Q-value becomes very small, the medium effects lead to a substantial enhancement in rate. In contrast, for large Q-values, the medium effects are negligible. We briefly comment on application to real systems and the experimental implications of this result.

nucl-th↗

arXiv2025

CMB Hemispherical Power Asymmetry from Early Phase of Inflation

We investigate the hemispherical power asymmetry observed in the CMBR by attributing it to an early inhomogeneous phase of cosmic expansion. Unlike the conventional assumption of a perfectly isotropic and homogeneous pre-inflationary Universe, we introduce a small inhomogeneous perturbation, treated within a perturbative framework. Our analysis builds on previously developed empirical models of inhomogeneous primordial power spectrum models based on dipole modulation. Using in-in formalism, we compute two-point correlations directly from the metric and demonstrate that, at leading order, this introduces a direction-dependent power spectrum that breaks rotational symmetry and naturally selects a preferred direction, relating observed violation of the cosmological principle to inflationary power spectra arising from scalar field fluctuations. Additionally, we find that this framework produces correlations between multipoles separated by $\Delta l=1$, leading to distinctive signatures in the multipole space. Furthermore, we constrain the parameters of the inhomogeneous perturbation using observed PR4 \texttt{Commander} CMB data.

astro-ph.CO↗

arXiv2025

VLBA astrometry of PSRs B0329+54 and B1133+16: Improved pulsar distances and comparison of global ionospheric models

Very long baseline interferometry (VLBI) astrometry is used to determine the three-dimensional position and proper motion of astronomical objects. A typical VLBI astrometric campaign generally includes around ten observations, making it challenging to characterise systematic uncertainties. Our study on two bright pulsars, B0329+54 and B1133+16, involves analysis of broadband Very Long Baseline Array (VLBA) data over $\sim30$ epochs (spanning approximately $3.5\, {\rm years}$). This extended dataset has significantly improved the precision of the astrometric estimates of these pulsars. Our broadband study suggests that, as expected, the primary contribution to systematic uncertainties in L-band VLBI astrometry originates from the ionosphere. We have also assessed the effectiveness of the modified TEC (total electron content) mapping function, which converts vertical TEC to slant TEC, in correcting ionospheric dispersive delays using global TEC maps. The parallax and proper motion obtained from the multiple data sets, calibrated using the traditional and the modified TEC mapping functions, are consistent. However, the reduced chi-square values from least-squares fitting and precision of the fitted astrometric parameters show no significant improvement, and hence, the effectiveness of the new TEC mapping function on astrometry is unclear. For B0329+54, the refined parallax estimate is $0.611^{+0.013}_{-0.013}\, {\rm mas}$, with best-fit proper motion of $\mu_{\alpha} = 16.960^{+0.011}_{-0.010}\, {\rm mas\, yr^{-1}}$ in R.A. and and $\mu_{\delta} = -10.382^{+0.022}_{-0.022}\, {\rm mas\, yr^{-1}}$ in Dec. For B1133+16, the new estimated parallax is $2.705^{+0.009}_{-0.009}\, {\rm mas}$, with proper motions of $\mu_{\alpha} = -73.777^{+0.008}_{-0.008}\, {\rm mas\, yr^{-1}}$ and $\mu_{\delta} = 366.573^{+0.019}_{-0.019}\, {\rm mas\, yr^{-1}}$.

astro-ph.HE↗

arXiv2025

Extracting local velocity from cosmic dipole using simulations

Our velocity with respect to the cosmic frame of rest leads to a dipole in the number count distribution of galaxies. The dipole depends on the source spectrum, which is usually assumed to be a power law, $S(\nu) \propto \nu^{-\alpha}$ and on the flux dependence of the number density of sources. The latter is also generally assumed to be a power law, parametrised with exponent $x$. The velocity can be extracted from the observed dipole once the two parameters $x$ and $\alpha$ are known. The standard procedure uses the mean value of $\alpha$ across the entire sample, and the parameter $x$ is inferred by fitting the cumulative number count, $\frac{dN}{d\Omega}(>S_*) \propto S_*^{-x}$, near the flux limit $S_*$ of the survey. Here, we introduce a simulation procedure to extract the velocity which directly uses the $\alpha$ values of each source rather than their mean and does not rely on the functional form of the cumulative number count near the flux limit. We apply this to the quasar sample in CatWISE2020 data and find that the final results differ from the standard procedure by approximately one sigma.

astro-ph.CO↗

arXiv2025

Matter Dipole and Hubble Tension due to Large Wavelength Perturbations

We theoretically analyze the dipole anisotropy observed in the quasar distribution from the CatWISE2020 catalog. The catalog data shows a peak around $z\approx 1$, suggesting the presence of a large-scale dipole component. We explore the possibility that this dipole could be driven by primordial density fluctuations from modes that were superhorizon at the time of CMB decoupling but have since entered the horizon and become subhorizon. In particular, we consider the impact of adiabatic modes with wave numbers $k$ in the range $(10^{-4} - 4 \times 10^{-3})~\mathrm{Mpc}^{-1} $, corresponding to wavelength scales of several Gpc. Such modes can create large-scale density variations, likely causing anisotropies in the distribution of matter and, as a result, affecting the number density of observed quasars. We find that these can lead to a significant contribution to the dipole for sources up to redshifts of about 1, but are unable to explain the observed dipole. We also demonstrate that a superhorizon curvature perturbations mode, with a comoving wavenumber $k\lesssim0.3H_0$ can lead to a significant enhancement in the locally inferred Hubble constant. This effect offers a viable explanation for the observed discrepancy between local and CMB inferred measurements of $H_0$.

astro-ph.CO↗

arXiv2025

Discovery of a new flaring class in GRS 1915+105 using AstroSat

Highly variable Black Hole X-ray Binary (BHXB) GRS~1915+105 has shown many flaring classes when the source oscillates between the High Soft state (HS) and the Hard Intermediate state (HIMS) with a transition time of less than 10 s. The X-ray flux is anti-correlated with hardness ratio (HR2) during these X-ray flaring classes. We have analyzed Astrosat/LAXPC \& SXT data and report here a new X-ray flaring class named $\eta$ class when the source oscillates between two HS states (the power-law index is always greater than 4) with transition time around 50 s. The X-ray flux is correlated with hardness ratio. This class is quasi-regular, and we have detected High Frequency Quasi Periodic Oscillations (HFQPOs) around 70 Hz during this new flaring class. The accretion rate changes by a factor of three over the burst cycle. We report here the results of our extensive study of spectral and timing characteristics of this new class.

astro-ph.HE↗

arXiv2024

Exploring the broadband spectral and timing characteristics of GRS 1915+105 with AstroSat and NICER observations

In this study, we undertake a spectral-timing analysis of the black hole X-ray binary source GRS 1915+105 using simultaneous observations carried out by AstroSat (LAXPC and SXT) and NICER in 2017. The source showed two flux levels (high and low), whose energy spectra can be described by the thermal comptonization of disk photons. The spectral parameters obtained by the joint fitting of SXT/LAXPC and NICER/LAXPC were consistent. The power density spectra from LAXPC and NICER revealed a broad, prominent feature at approximately 2 Hz. The energy dependence of the fractional R.M.S. and time lag of this feature cannot be explained by only variations of coronal spectral parameters. Instead, a model where the coronal heating rate varies first and induces a change in the disk temperature and inner radius can explain the variation. Our results underline the importance of simultaneous observations by AstroSat and NICER and highlight the need for more sophisticated models to explain the spectral-temporal behavior of black hole systems.

astro-ph.HE↗

arXiv2024

Reaction of deuteron with a heavy nucleus at low energies

We extend the recently proposed mechanism for inducing low energy nuclear reactions (LENR) to compute the reaction rate of deuteron with a heavy nucleus. The process gets dominant contribution at second order in the time dependent perturbation theory and is assisted by a resonance. The reaction proceeds by breakdown of deuteron into a proton and a neutron due to the action of the first perturbation. In the second, nuclear perturbation, the neutron gets captured by the heavy nucleus. Both perturbations are assumed to be electromagnetic and lead to the emission of two photons, one at each vertex. The heavy nucleus is taken to be ${}^{58}$Ni although many other may be considered.The reaction rate is found to be very small unless assisted by some special conditions. In the present case we assume the presence of a nuclear resonant state. In the presence of such a state we find that the reaction rate is sufficiently large to be observable in laboratory even at low energies.

nucl-th↗

arXiv2024

Low Energy Nuclear Reactions Through Weak Interactions

We consider the possibility that low-energy nuclear reactions (LENR) arise due to the conversion of proton to neutron through weak interactions. The resulting neutron forms a short-lived virtual state, which then gets captured by another nucleus through photon emission. This whole process happens under the framework of second-order perturbation theory. We find that the rate of this process is negligibly small in most cases. However, in the presence of a resonance, the rate can be substantial and observable.

nucl-th↗

arXiv2024

Color Dependence of Dipole in CatWISE2020 Data

The signal of dipole anisotropy in quasar number counts is studied using the CatWISE2020 catalog in various color bins. It is found that the dipole signal differs significantly in two color bins, namely, $1.1>W1-W2\ge 0.8$ and $1.4>W1-W2>1.1$. The color bin $1.4>W1-W2>1.1$ appears strongly contaminated, with possibly Galactic contributions and is unreliable for extracting the signal of cosmological dipole. The source of this contamination has not been identified and cannot be attributed to known emissions within the galaxy. Removing this contaminated color bin leads to a strong dipole signal with a direction significantly different from that obtained from full data. If we interpret this dipole as due to our local motion, the extracted velocity turns out to be $900\pm 113$ Kms$^{-1}$, which deviates from the CMB dipole velocity with approximately $4.7$ sigma significance.

astro-ph.CO↗