arXiv subjects

Surajit Kalita

Publications and source records attributed to Surajit Kalita.

At least 19 recordsLinked to original sources

arXiv2026

Structural quasi-universality in highly magnetized differentially rotating neutron stars

Universal relations among macroscopic properties of neutron stars provide a powerful framework to probe their internal structures while minimizing uncertainties associated with the equation of state (EoS). Although such relations have been extensively studied for uniformly rotating stars, their extension to differentially rotating and strongly magnetized configurations remains largely unexplored. We systematically investigate equilibrium configurations for a wide range of EoSs, rotation profiles, and magnetic field strengths. We establish a generalized quasi-universal relation between the moment of inertia and compactness that remains remarkably insensitive to the underlying EoS across uniformly rotating, differentially rotating, and strongly magnetized configurations. For sequences with fixed angular momentum, the normalized moment of inertia exhibits a quasi-universal dependence on compactness, with deviations primarily due to magnetic field strength and degree of differential rotation. We derive analytic expressions for these dependencies, enabling a unified phenomenological model applicable over a wide range of stellar configurations. As astrophysical applications, we quantify the systematic bias in magnetar luminosity estimates and the rotational kinetic energy of post-merger remnant of GW170817. These results extend quasi-universal relations beyond the standard assumptions of uniform rotation and weak magnetization, providing a robust framework for interpreting observations of highly magnetized and differentially rotating neutron stars, and enabling more reliable constraints on their astrophysical properties.

astro-ph.HE↗

arXiv2026

Understanding constraints on primordial mass black holes made of dark matter using fast radio bursts

In recent decades, a multitude of modified gravity theories have been proposed to address a variety of cosmological and astrophysical problems. While many of these theories remain viable, observational constraints on their parameters are increasingly stringent. Fast Radio Bursts (FRBs), in particular, have emerged as powerful probes of cosmology and fundamental physics. This study investigates the implications of a generic modified gravity theory for gravitational lensing by FRBs. By analyzing the dataset of CHIME/FRBs, we constrain the fraction of dark matter composed of primordial black holes within this theoretical framework. Furthermore, we demonstrate that modified gravity introduces a screening effect on gravitational lensing, analogous to the scattering effect of plasma on light rays.

astro-ph.CO↗

arXiv2026

Binary White Dwarfs as Gravitational Wave Sources for LISA

Gravitational waves (GWs) have proven to be a powerful probe of compact binary populations. In the millihertz frequency range accessible to Laser Interferometer Space Antenna LISA, binary white dwarfs (BWDs) are expected to constitute a dominant source, forming both individually resolvable signals and an unresolved Galactic background. In this work, we construct a Milky Way like population model and calculate the GW background from unresolved Galactic BWD in the LISA sensitivity range, with particular emphasis on exploring and constraining uncertainties in binary stellar evolution. We employ COMPAS binary population synthesis framework to generate synthetic populations of BWD in the Milky Way. Various physically motivated evolution prescriptions and initial model parameters are used to study diverse population of BWDs. From these populations, we construct the GW background and investigate the dependence of the background spectrum on the assumptions on binary analysis. We discuss the possibility of constraints on binary evolution that LISA GW observations may yield. We find that the shape and amplitude of the background are sensitive to key binary evolution parameters like common envelope evolution and mass-transfer efficiencies. Variations in these assumptions lead to measurable differences in the predicted background spectrum. Our results demonstrate that LISA observations of the unresolved BWD background have the potential to constrain binary evolution models. This highlights the importance of GW background modelling as a complementary tool for studying the formation and evolution of compact binaries in the Milky Way.

astro-ph.SR↗

arXiv2026

Identifying highly magnetized white dwarfs: A dimensionality reduction framework for estimating magnetic fields

Magnetic fields play a crucial role in compact object physics, particularly in white dwarfs (WDs), where high densities can sustain strong magnetic fields. Observations have revealed magnetized WDs (MWDs) with surface fields reaching approximately $10^9\rm\,G$, although high-field MWDs are fewer in number in current catalogs owing to their intrinsic faintness and limitations in conventional electromagnetic surveys. In this study, we apply unsupervised machine learning (ML) techniques to systematically analyze a sample of hydrogen-atmosphere (DA) WDs. Using Uniform Manifold Approximation and Projection (UMAP) for dimensionality reduction and Density-Based Spatial Clustering of Applications with Noise (DBSCAN) for cluster identification, we classify distinct subpopulations within the DA WD sample. Each cluster exhibits unique intrinsic properties such as mass, surface gravity, temperature, and age. Our analysis further reveals that these subgroups effectively differentiate MWDs from non-magnetic or weakly magnetic counterparts. Moreover, utilizing a set of previously confirmed MWDs, we estimate the field strengths of all other MWDs lacking magnetic field measurements. This study underscores the effectiveness of ML-based approaches in astrophysical discovery, particularly detecting magnetized compact objects when direct measurements are unavailable.

astro-ph.SR↗

arXiv2026

Population synthesis of double white dwarfs: evolutionary effects on system properties

Double white dwarf (DWD) binaries are natural outcomes of binary stellar evolution and key sources for future space-based gravitational wave (GW) observatories such as Laser Interferometer Space Antenna (LISA). We investigate how different binary interaction channels shape the physical and orbital properties of DWD systems, focusing on component masses, orbital separations, core compositions, and mass transfer rates. Using the binary population synthesis code COMPAS, we evolve $10^7$ binaries with physically motivated initial distributions of binary parameters. Our simulations reproduce the strong bimodality in the final orbital separations, including a pronounced deficit of systems around $100-500 \rm\,R_\odot$, arising from distinct evolutionary pathways: wide DWDs predominantly originate from stable Roche lobe overflow (RLOF), while close DWDs form through unstable RLOF leading to at least one common envelope (CE) phase. Moreover, we show that the core compositions of WDs provide a powerful tracer of evolutionary history: He-core WDs are strongly concentrated in close systems, whereas CO-core WDs span the full separation range and exhibit a small mass gap in wide binaries. We further identify a correlation between the donor mass transfer rate and the final orbital separation, highlighting the impact of non-conservative mass transfer on the resulting orbital configuration of DWD systems. These results underscore the links among evolutionary channels, chemical composition, and mass transfer rates; thereby provide a unique framework for interpreting Gaia DWD samples and forecasting the joint electromagnetic and GW population accessible to LISA.

astro-ph.SR↗

arXiv2026

Testing the cosmic distance-duality relation with localized fast radio bursts: a cosmological model-independent study

We test the Etherington cosmic distance-duality relation (CDDR), by comparing Type Ia supernova (SNIa) luminosity-distance information from the Pantheon+ compilation with an angular-diameter-distance reconstructed from localized Fast Radio Bursts (FRBs). The core of our methodology is a data-driven reconstruction from FRBs using artificial neural networks (ANNs): we infer a smooth mean extragalactic dispersion-measure relation and use its redshift derivative to recover $H(z)$ and hence $D_\mathrm{A}^{\rm FRB}(z)$ without assuming a parametric form for the expansion history. Possible deviations from CDDR are parameterized through three one-parameter models of $η(z)\equiv D_\mathrm{L}/[(1+z)^2D_\mathrm{A}]$. We implement two complementary likelihoods: (i) a direct approach using individual SNIa with the full Pantheon+ covariance, and (ii) a machine-learning approach in which we reconstruct the SN Hubble diagram on the FRB redshift grid, propagating SN and FRB uncertainties into non-diagonal covariance matrices via Monte Carlo and bootstrap realizations. Within the FRB reconstruction, we anchor the mean extragalactic dispersion measure at $z=0$, which yields a data-driven constraint on the average host/near-source contribution $\mathrm{DM}_{\rm host}=128.8\pm 34.1\,\mathrm{pc\,cm^{-3}}$ ($3σ$ of statistical confidence). We find that both likelihood implementations give consistent posteriors and no statistically significant evidence for departures from CDDR at the current precision.

astro-ph.CO↗

arXiv2026

A MeerKAT search for persistent radio sources towards twenty-five localised Fast Radio Bursts

The discovery of persistent radio sources (PRSs) associated with repeating fast radio bursts (FRBs) has shed light on the immediate environments and possible progenitors of these FRBs. The confirmed PRSs may support the theory that FRB progenitors are compact central engines, whilst the non-detections suggest diversity of FRB's local environment. We perform a subarcsecond-resolution MeerKAT search at 1.28 GHz on 25 well-localised FRB positions provided by ASKAP and MeerTRAP. We detect 14 radio sources and provide flux upper limits for 12 non-detections (both these numbers include a source that was detected during two epochs of observation, and not detected during one epoch, adding up to 26). One radio source shows variability as seen in flux variations over three epochs of observation. Archival optical data reveal excesses in the direction of 13 detected radio sources. Similarly for four sources in the X-ray band, with one possibly being a high-energy signature of a radio galaxy core. Since we cannot definitively classify our detected radio sources as PRSs, future high-resolution observations with e-MERLIN will be required to resolve the radio emission and pronounce on the presence of compact PRSs associated with the 14 detected sources presented here.

astro-ph.HE↗

arXiv2025

Over-Luminous Type Ia Supernovae and Standard Candle Cosmology

Type Ia supernovae (SNe\,Ia) serve as crucial cosmological distance indicators because of their empirical consistency in peak luminosity and characteristic light curve decline rates. These properties facilitate them to be standardized candles for the determination of the Hubble constant ($H_0$) within late-time universe cosmology. Nevertheless, a statistically significant difference persists between $H_0$ values derived from early and late-time measurements, a phenomenon known as the Hubble tension. Furthermore, recent observations have identified a subset of over-luminous SNe\,Ia, characterized by peak luminosities exceeding the nominal range and faster decline rates. These discoveries raise questions regarding the reliability of SNe\,Ia as standard candles for measuring cosmological distances. In this article, we present the Bayesian analysis of 15 over-luminous SNe\,Ia and show that they yield a lower $H_0$ estimate due to the increase in their absolute magnitude. This investigation potentially represents a step toward addressing the Hubble tension.

astro-ph.CO↗

arXiv2025

Revealing Limitation in the Standard Cosmological Model: A Redshift-Dependent Hubble Constant from Fast Radio Bursts

A major issue in contemporary cosmology is the persistent discrepancy, known as the Hubble tension, between the Hubble constant ($H_0$) estimates from local measurements and those inferred from early-Universe observations under the standard $Λ$ cold dark matter ($Λ$CDM) paradigm. Recent advances have identified fast radio bursts (FRBs), a class of extragalactic phenomena observable at considerable redshifts, as a promising observational tool for probing late-time cosmology. In this study, we incorporate two complementary methodologies, machine learning algorithms and Bayesian analysis, on a set of localized FRBs to rigorously test the consistency of the $Λ$CDM model at late cosmic epochs. Our results reveal a statistically significant redshift-dependent variation of $H_0$ when using separate priors on baryon density parameters $Ω_\mathrm{b}$ or $Ω_\mathrm{b}h^2$, indicating contradiction to the core postulate of $Λ$CDM. However, when the priors are combined, this redshift dependence disappears, yielding a consistent estimate of $H_0$. We further validate that the redshift dependency of $H_0$ can be removed within the more flexible framework of $w_0w_a$CDM model even without combining the priors. These findings highlight that the redshift evolution of $H_0$ is not merely an artifact of the standard model but an indication of a deeper inadequacy in the $Λ$CDM model, supporting the need for a more flexible cosmological framework.

astro-ph.CO↗

arXiv2025

Refining fundamental constants with white dwarfs: machine learning informed constraints on fine-structure constant and proton-to-electron mass ratio

We explore the potential variation of two fundamental constants, the fine-structure constant $α$ and the proton-to-electron mass ratio $μ$, within the framework of modified gravity theories and finite-temperature effects. Utilising high-precision white dwarf observations from the Gaia-DR3 survey, we construct a robust mass--radius relation using a Bayesian-inspired machine learning framework. This empirical relation is rigorously compared with theoretical predictions derived from scalar-tensor gravity models and temperature-dependent equations of state. Our results demonstrate that both underlying gravitational theory and temperature substantially influence the inferred constraints on $α$ and $μ$. We obtain the strongest constraints as $|Δα/α|=2.10^{+32.56}_{-39.26}\times10^{-7}$ and $|Δμ/μ|=1.61^{+37.16}_{-34.67}\times10^{-7}$ for modified gravity parameter $γ\simeq -3.69\times10^{13}\,\mathrm{cm}^2$, while for the finite temperature case, these are $|Δα/α|=1.60^{+37.31}_{-35.42}\times10^{-7}$ and $|Δμ/μ|=1.23^{+37.02}_{-35.71}\times10^{-7}$ for $T \simeq 1.1 \times 10^7\rm\, K$. These findings yield tighter constraints than those reported in earlier studies and underscore the critical roles of gravitational and thermal physics in testing the constancy of fundamental parameters.

astro-ph.SR↗

arXiv2025

Fast Radio Bursts as probes of the late-time universe: a new insight on the Hubble tension

Fast Radio Bursts (FRBs) are bright radio transient events, a subset of which have been localized to their host galaxies. Their high dispersion measures offer valuable insights into the ionized plasma along their line of sight, enabling them to serve as probes of cosmological parameters. One of the major challenges in contemporary cosmology is the Hubble tension -- an unresolved discrepancy between two independent methods of determining the Universe's expansion rate, yielding differing values for the Hubble constant. In this study, we analyze a sample of 64 extragalactic, localized FRBs observed by various telescopes, employing Bayesian analysis with distinct likelihood functions. Our findings suggest that FRBs serve as tracers of the Hubble constant in the late-time Universe. Notably, our results exhibit smaller error bars compared to previous studies, and the derived Hubble constant with 1$σ$ error bars no longer overlap with those obtained from early-Universe measurements. These results underscore the continuing tension between early- and late-time measurements of the Hubble constant.

astro-ph.CO↗

arXiv2024

Novel understanding of Cosmological Phenomena using Fast Radio Bursts

Fast radio bursts (FRBs) offer unique probes of diverse cosmological phenomena due to their characteristic properties, including short duration timescale and high dispersion measure. This study investigates two distinct theoretical frameworks: the Gertsenshtein-Zel'dovich (GZ) mechanism for ultra-high-frequency gravitational waves (GWs) and fraction of dark matter in primordial mass black holes. We explore the hypothesis that ultra-high-frequency GWs could be responsible for FRB generation. Consequently, the detection of continuous GWs signal from the vicinity of an FRB by current or future detectors would disfavour merger-based FRB formation models and lend significant credence to the GZ theory, which postulates the existence of high-frequency GWs. Moreover, we examine the effects of modified gravity on the gravitational lensing of FRBs and thereby put constraints on the fraction of primordial mass black holes made up of dark matter. Our analysis suggests that modified gravity introduces a screening effect on lensing, analogous to the scattering effect by plasma on light rays. We further discuss the expected detection rates of FRBs as well as lensed FRBs with upcoming radio telescopes, primarily HIRAX.

astro-ph.CO↗

arXiv2024

Constraining Fundamental Constants with Fast Radio Bursts: Unveiling the Role of Energy Scale

Understanding physical mechanisms relies on the accurate determination of fundamental constants, although inherent limitations in experimental techniques introduce uncertainties into these measurements. This paper explores the uncertainties associated with measuring the fine-structure constant ($α$) and the proton-to-electron mass ratio ($μ$) using observed fast radio bursts (FRBs). We select 50 localized FRBs to quantify the effects of varying this fundamental coupling on the relation between dispersion measure and redshift. By leveraging independent measurements of dispersion measures and redshifts of these FRBs, we constrain the uncertainties in $α$ and $μ$ approximately to $Δα/α=1.99\times 10^{-5}$ and $Δμ/μ=-1.00\times 10^{-5}$ within the standard $Λ$CDM cosmological framework. Remarkably, these constraints improve nearly an order-of-magnitude when considering a dynamical dark energy model. This investigation not only yields one of the most stringent constraints on $α$ and $μ$ to date but also emphasizes the criticality of accounting for the energy scale of the system when formulating constraints on fundamental parameters.

gr-qc↗

arXiv2024

Effects of modified gravity on microscopic properties and cooling timescale of white dwarfs

There are currently two open questions in white dwarf physics: why are massive dwarfs observed less often in astronomical surveys, and why have not any super-Chandrasekhar white dwarfs been found despite the discovery of more than a dozen peculiar, overly-luminous type Ia supernovae in about a couple of decades? According to different research, magnetic fields appear to somewhat resolve these issues, but stability remains a concern. For the first time, we investigate how modified gravity affects the specific heat of electrons and ions, the crystallization process, and the cooling mechanism in white dwarfs. We demonstrate it for the Ricci-based gravity. We show that massive white dwarfs fade faster and conclude that it could be a physical reason, apart from the presence of high magnetic fields, both for finding fewer massive white dwarfs and the lack of direct detection of super-Chandrasekhar white dwarfs.

astro-ph.SR↗

arXiv2023

Gravitational Lensing in Modified Gravity: A case study for Fast Radio Bursts

Over the last few decades, a plethora of modifications to general relativity have been proposed to solve a host of cosmological and astrophysical problems. Many modified gravity models are now ruled out with further astrophysical observations; some theories are still viable, with, at best, bounds on their parameters set by observations to date. More recently, observations of Fast Radio Bursts have proven to be remarkably powerful tools to constrain cosmology and fundamental physics. In this work, we consider a generic modified gravity theory and consider the implications for gravitational lensing with Fast Radio Bursts. We use a set of Fast Radio Burst observations to constrain the fraction of dark matter made up of primordial black holes in such a theory. We further show that modified gravity adds a screening effect on gravitational lensing similar to the case when there is plasma in the path of the light ray acting as a scattering screen.

gr-qc↗

arXiv2023

Effect of temperature on measurement of fundamental constants using white dwarfs in Gaia-EDR3 survey

Fundamental constants are crucial for comprehending physical mechanisms, but their measurements contain uncertainties due to experimental limitations. We investigate the impact of system temperature on these uncertainties using nearby white dwarfs observed in the Gaia Early Data Release 3 (EDR3) survey. Using the structures of these white dwarfs, we show that the variation in system temperature can affect the accuracy of measurements for fundamental parameters such as the fine-structure constant and the proton-to-electron mass ratio. This exploration emphasizes the importance of considering the energy of a system while putting bounds on the values of fundamental constants.

astro-ph.SR↗

arXiv2023

Constraining fundamental parameters in modified gravity using Gaia-DR2 massive white dwarf observation

Various experiments and observations have led researchers to suggest different bounds on fundamental constants like the fine-structure constant and the proton-to-electron mass ratio. These bounds differ mostly due to the energy scale of the systems where the experiments are performed. In this article, we obtain bounds on these parameters in the modified gravity context using the Gaia-DR2 massive white dwarf data and show that the bounds alter as the gravity theory changes. This exploration not only indicates strong support for non-negligible influences of modified gravity in astrophysical scenarios in high-density regimes but also reveals that the bounds on fundamental parameters can be much stronger under alternate gravity theories.

gr-qc↗

arXiv2023

Metric-affine effects in crystallization processes of white dwarfs

We analyze the effects of modified gravity on specific heats of electrons and ions, Debye temperature, crystallization process, and cooling mechanism in white dwarfs. We derive the Lane-Emden-Chandrasekhar equation and relate it to the cooling process equations for Palatini $f(R)$ gravity. Moreover, for the first time in the literature, we show that the gravity model plays a crucial role not only in the mass and size of the white dwarf, but also affects their internal properties. We further demonstrate that modified gravity can decrease the cooling age significantly.

gr-qc↗