arXiv subjects

Banibrata Mukhopadhyay

Publications and source records attributed to Banibrata Mukhopadhyay.

At least 19 recordsLinked to original sources

arXiv2026

Super-Chandrasekhar white dwarfs by the evolution of magnetized main-sequence stars: New mass-limits from STARS simulation

We explore the time evolution of the magnetized main-sequence stars (MSs) to magnetized non-rotating white dwarfs (WDs) with a series of models, capturing modified Hertzsprung-Russell diagram. We use the Cambridge Stellar evolution code, STARS, with modifications by introducing magnetic effects and cooling. We investigate the possible existence of stable, magnetized, massive, super-Chandrasekhar, approximately spherical WDs, away from Chandrasekhar's mass-radius curve. These we refer to as B-WDs. They are inferred from the observations of peculiar over-luminous type Ia supernovae, e.g. SN2003fg. We show that the classical effects of magnetic fields significantly impact MSs and their time evolution to WDs. Moreover, the magnetic field could be decisive in explaining the unusually large radius observed in some sub-Chandrasekhar WDs, e.g. SDSS J123204.19+522548.2, along with the finite temperature effect. We further investigate the stability of magnetized WDs, both below and above the Chandrasekhar limit and confirm the possibility of super-Chandrasekhar B-WDs with new mass limits that depend on the magnetic field geometry, mass accretion from a companion and cooling rates. Therefore, while the "Chandrasekhar limit" may be sacrosanct, its value severely depends on the underlying physics, here the magnetic field.

astro-ph.SR↗

arXiv2026

Analog Gravity in Magneto-Viscous Fluids: Enhanced Analog Hawking Temperature in Accretion Disks

We present a three-dimensional visco-magnetoacoustic framework for analog gravity in magnetohydrodynamic (MHD) flows. While standard fluid dissipation typically breaks the Lorentzian signature of acoustic metrics, we demonstrate that evaluating wave perturbations in the eikonal limit alongside the Shakura-Sunyaev $α$-viscosity prescription preserves a well-defined effective spacetime geometry for the fast magnetoacoustic mode. The slow-magnetoacoustic mode and Alfvén mode do not admit a non-degenerate metric. To investigate analog horizon thermodynamics, we utilize astrophysical accretion disks as background media, specifically modeling numerical magnetized advective accretion flows around rotating black holes and analytical advection-dominated inflow-outflow solutions (ADIOS). Standard self-similar ADIOS models strictly enforce a constant Mach number, precluding horizon formation. We therefore introduce a magnetic field perturbation that breaks self-similarity, generates a dynamic Mach number, and enables the formation of a visco-magnetoacoustic horizon. By evaluating the spontaneous phonon emission at these horizons, we reveal that the analog Hawking temperature is highly sensitive to the magnetic field topology; the spatial orientation of the background magnetic gradients dictates whether the Hawking radiation is amplified or suppressed. Furthermore, we find that increasing the viscosity parameter leads to a monotonic increase in the analog Hawking temperature.

gr-qc↗

arXiv2026

Hawking Radiation in $f(\mathcal{R})$ Gravity: Survival of lighter black holes

Einstein's theory of general relativity (GR) has been remarkably successful in describing gravitational phenomena. However, several open questions in modern cosmology and astrophysics (e.g. inflation, dark energy) suggest the need for extensions or modifications to this framework. Modified gravity (MGR) theories, including scalar-tensor models and higher-dimensional approaches, attempt to address these gaps while maintaining consistency with established experimental tests. This work investigates Hawking radiation within an $f(\mathcal{R})$ gravity theory, with $\mathcal{R}$ being scalar curvature, focusing on its implications for primordial black holes (PBHs) as potential dark matter (DM) candidates. Our analysis reveals that PBHs evaporate slowly in MGR compared to GR predictions. Specifically, we find that non-rotating black holes with masses $\sim 5 \times 10^{13}$ g or lower would have survived by the present epoch, depending on the MGR parameter-a mass threshold approximately at least ten times smaller than in GR. This retarded evaporation timeline imposes relaxed new constraints on the viability of PBHs as DM constituents, thereby reshaping the landscape of possible solutions to the DM problem. This motivates further investigation into alternative gravitational theories and their cosmological consequences.

gr-qc↗

arXiv2026

Probing magnetized accretion disk-jet systems: stellar mass to supermassive black holes

Ubiquitous nature of accretion disks and associated jets in modern astrophysics is extreme for black holes. The current state-of-the-art of black hole activities lies with modeling of underlying general relativistic magnetohydrodynamic (GRMHD) flows. These simulations have shown the importance of magnetic fields in the generation of outflows/jets and the overall dynamical evolution of the accretion flow. They also reveal critical insights into mechanisms that influence accretion dynamics, jet formation and stability. This further sheds light on the underlying magnetic field configurations based on magnetic field saturation leading to Standard and Normal Evolution: SANE, and Magnetically Arrested Disk: MAD. By employing SKA's high-resolution imaging and sensitivity, we can directly compare simulation outcomes with observational data, validating our models and enhancing our understanding of these phenomena. Key to this investigation is the examination of magnetic fields and their associated polarization signatures. Comparing the observational data from SKA with GRMHD simulations will facilitate a deeper analysis of the polarization properties, which can reveal the magnetic field geometry and dynamics in these extreme environments. The VLBI capabilities of SKA will prove instrumental in understanding jet morphologies and spectra of these systems due to its high spatial resolution. Collating these observations with GRMHD simulations will lead to a better understanding of the jet generation mechanisms and their interaction with ambient medium. By integrating advanced GRMHD simulations with SKA's capabilities, we aim to bridge theoretical predictions and observations, ultimately contributing to a more comprehensive understanding of the behavior of accreting black holes and their jets.

astro-ph.HE↗

arXiv2026

Probing Neutron Star Interiors and the Properties of Cold Ultra-dense Matter with the SKAO

Matter inside neutron stars is compressed to densities several times greater than nuclear saturation density, while maintaining low temperatures and large asymmetries between neutrons and protons. Neutron stars, therefore, provide a unique laboratory for testing physics in environments that cannot be recreated on Earth. To uncover the highly uncertain nature of cold, ultra-dense matter, discovering and monitoring pulsars is essential, and SKAO will play a crucial role in this endeavour. In this chapter, we will present the current state-of-the-art in dense matter physics and dense matter superfluidity, and discuss recent advances in measuring global neutron star properties (masses, moments of inertia, and maximum rotation frequencies) as well as non-global observables (pulsar glitches and free precession). We will specifically highlight how radio observations of isolated neutron stars and those in binaries -- such as those performed with SKAO in the near future -- inform our understanding of ultra-dense physics and address in detail how SKAO's telescopes unprecedented sensitivity, large-scale survey and sub-arraying capabilities will enable novel dense matter constraints. We will also address the potential impact of dark matter and modified gravity models on these constraints and emphasise the role of synergies between SKAO and other facilities, specifically X-ray telescopes and next-generation gravitational wave observatories.

astro-ph.HE↗

arXiv2026

Spectral analysis of magnetized advective accretion flows around rotating black holes

The spectra of an accretion disk around black holes are the basic diagnostic tool to enlighten the underlying flows and then black holes. Accretion flows around black holes, however, are controlled by parameters like the magnetic field, spin of the black hole, accretion rate and temperature of the flow. These quantities affect the (magneto)hydrodynamics of the flow thus consequently lead to variations in the spectrum. We first consider numerical steady state magnetohydrodynamic (MHD) solutions of magnetized accretion flows around black holes to study the dependence of the spectra on these disk properties. The spectrum exhibits strong dependence on the spin of the black hole, accretion rate, magnetic field and the electron temperature of the flow. Variations in these quantities influence the emission peaks and overall luminosity, which can be a tell-tale sign to extract physics of observed spectra. We further validate our results with general relativistic MHD (GRMHD) simulations using the standard and normal evolution (SANE) and magnetically arrested disk (MAD) vector potentials. We consider two black hole spins ($a=0.5$ and $a=0.9375$) to model the magnetic field configurations and study the resulting spectra by comparing MAD and SANE results. We find a large difference in the bolometric luminosities and the location of the emission peaks between SANE and MAD flows. Certain properties of the spectra, like, the ratio of synchrotron radiation to synchrotron self-Comptonization peaks in SANE and MAD, show drastically distinct features. The overall luminosity combined with such metrics can distinguish the magnetic field characteristics in astrophysical systems.

astro-ph.HE↗

arXiv2026

GRMHD Simulations of Magnetized Accretion Disk/Jet: Variabilities of Black Holes and Spectral Energy Distributions in Magnetic States

We perform three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of a near-maximally spinning black hole (spin parameter, a = 0.998) with varying initial magnetic field geometries, systematically exploring the parameter space connecting magnetically arrested disk (MAD), intermediate (INT), and standard and normal evolution (SANE) accretion states. The magnetic flux threading the black hole horizon emerges as the fundamental state variable controlling jet efficiency, flow magnetization, and radiative output across all three states. We introduce complementary diagnostics-broadband spectral energy distributions spanning radio through hard X-ray frequencies and time-resolved X-ray light curves-that together connect simulation dynamics directly to multiwavelength observables. The radiative output follows a clear MAD > INT > SANE hierarchy in time-averaged luminosity, mean X-ray emission, as well as variability. Furthermore, MAD exhibits the highest fractional variability through quasi-periodic magnetic flux eruption events, and INT and SANE show moderate variability driven by episodic reconnection and stochastic MRI turbulence, respectively. Scaling to GRS 1915+105, Cyg X-1, and HLX-1, we demonstrate that all twelve temporal classes of GRS 1915+105 map naturally onto our three magnetic states, Cyg X-1's persistent hard state is reproduced by a sustained INT configuration, and HLX-1's extreme luminosities arise through efficient Blandford-Znajek extraction in MAD states scaled to higher black hole mass.

astro-ph.HE↗

arXiv2026

Possible fractal nature of accretion flows in MAD and SANE simulations: Implications to GRS 1915+105

The general relativistic magnetohydrodynamic (GRMHD) simulations are widely used to study accretion disk and jet dynamics around a black hole. Despite strong observational evidences for intrinsically nonlinear behavior, the interpretations of GRMHD simulation results, more precisely the underlying timeseries, have not been well-explored by nonlinear timeseries analysis. In this work, we characterize the jet and disk dynamics of different GRMHD simulated flows using the nonlinear timeseries analysis. As diagnostic tools, we consider Higuchi fractal dimension (HFD), Hurst Index (H) and spectral slope. We implement them for two model disk frameworks: magnetically arrested disk (MAD) and standard and normal evolution (SANE), across a range of black hole spins with the Kerr parameter spanning from -0.9375 to 0.9375. We simulate the disk/jet systems by two well-documented codes: HARMPI and BHAC, and obtain, respectively, low and high temporally resolved timeseries data. For both jet and disk dynamics, MADs are characterized by higher HFD, lower H and flatter spectral slopes than SANEs. High HFD in MAD could be due to its intermittent variability and indicates that it has lesser long-range temporal correlations than SANE. Moreover, HFD in MAD decreases with spin magnitude owing to increase in collimated, hence ordered, jets. However, in SANE, it increases with spin for positive ones due to interplay of winds and jets. Extending our analysis to observations, we attempt to segregate the classes of black hole: GRS 1915+105, into MAD- and SANE-like clusters based on their spectral properties extracted from X-ray data. The mean HFD of MAD-like cluster is higher than SANE-like cluster, thus, corroborating with the simulation results. Our work highlights the role of nonlinear timeseries analysis to understand the underlying dynamics of accretion flows and their connection to magnetic regulation.

astro-ph.HE↗

arXiv2026

Anisotropic hybrid stars: Interplay of superconductivity and magnetic field leading to gravitational waves

Neutron stars, at their cores, are highly dense and, thus, are expected to have a number of exotic processes. This includes a possible phase transition to deconfined quark matter at the core, leading to a hybrid star. The quark matter is expected to additionally be color superconducting. The physics of superconductivity plays an important role in understanding the high density matter in the interiors of neutron/hybrid stars. At their high densities, additionally, both proton superconductivity and neutron superfluidity are expected. We study the effect of superconducting (quark/proton) matter, along with the internal magnetic field, leading to pressure anisotropy within hybrid stars. We aim to probe the effect of superconductivity, especially from color superconducting quarks, in hybrid star structure. We propose new phenomenological model anisotropy profiles within a one-dimensional framework. We model quark matter using the vector interaction enhanced Bag model, and hadron matter with the DD2 equation of state. A Maxwell construction joins both phases. We further investigate the possible observational signatures of these hybrid stars. These include mass enhancement and continuous gravitational waves, possibly arising from the anisotropy induced deformation, helping us further constrain our model and its physical parameters.

astro-ph.HE↗

arXiv2026

Lepton asymmetry leading to baryogenesis by primordial black holes

Baryogenesis remains an unresolved problem in cosmology, with existing mechanisms facing significant caveats. We show that the effects of primordial black holes (PBHs) on neutrinos produce the lepton asymmetry $\sim 10^{-10}$ which subsequently produces the baryon asymmetry. We consider the Dirac Lagrangian in curved spacetime in local coordinates exhibiting Hermitian pseudo-vector and non-Hermitian vector terms. These terms lead to energy splitting between weakly interacting neutrinos and antineutrinos, resulting in their unequal number densities and hence a lepton asymmetry. While the non-Hermitian effect leads to a non-conserved total probability of neutrinos, the leptogenesis due to gravitational effects of a PBH could be significant until the nucleosynthesis era. This in turn produces baryon asymmetry from the symmetry of lepton and baryon numbers via the sphaleron process in the electro-weak era. We show that in the most conservative scenario, the PBHs of mass $\sim 10^{12}$ g and spin $\sim 0.01$ produce the observed baryogenesis at temperature 130 GeV, when such PBHs are available abundantly. However, massive PBHs also could produce the observed asymmetry, assuming the non/anti-Hermitian vector couplings for neutrino and anti-neutrino get canceled from the Lagrangian, leading the system to be Hermitian.

hep-ph↗

arXiv2026

Tests of general relativity in pseudo-Newtonian approach

We investigate the extent to which pseudo-Newtonian gravitational potentials can reproduce classic tests of general relativity without resorting to full general relativistic formalisms. This is useful for the researchers seeking intuitive insight into relativistic gravity. Focusing on the perihelion precession of Mercury, gravitational redshift, and gravitational light bending, we derive analytical expressions for orbital precession and demonstrate that, with suitable physically acceptable parameters, pseudo-Newtonian approaches can accurately reproduce the observed perihelion advance and gravitational redshift. However, we confirm that no single potential consistently captures all relativistic effects. In particular, while certain parameters yield agreement with general relativity for planetary motion and redshift, they fail to reproduce gravitational lensing over a broad range of impact parameters. Our results highlight both the usefulness and limitations of pseudo-Newtonian methods in modeling gravitational phenomena. Although the pseudo-Newtonian approach cannot serve as universal substitutes for general relativity, especially in strong-field regimes, it provides valuable semi-analytical insight and pedagogical simplicity. Our results indicate the usefulness of the pseudo-Newtonian approach to uncover more complicated phenomena involved with strong field gravity in possibly modification to general relativity.

gr-qc↗

arXiv2026

Pseudo-Newtonian potential for accretion disks in a modified gravity spacetime around the black hole and underlying properties

We construct a pseudo-Newtonian potential (PNP) corresponding to a rotating black hole solution in a modified gravity (MGR) framework using a metric-based prescription. The motivation is to enable realistic accretion disk studies in MGR, where full relativistic MHD simulations remain computationally prohibitive. Effective potentials and the underlying Newtonian-like forces are derived for both massive and massless particles in the equatorial plane, relevant for disk dynamics. The reliability of the PNP is tested by comparing key orbital properties -- marginally stable, marginally bound, photon orbits and energies at marginally stable orbit radii -- with exact MGR predictions. The PNP reproduces the marginally stable and photon orbits exactly, while marginally bound orbits and specific energies deviate by less than about 7-10%. The influence of the MGR parameter on particle dynamics and effective potentials is analyzed, revealing non-trivial departures from simple Newtonian intuition. The study demonstrates that the proposed PNP accurately captures essential spatial properties of MGR spacetime and provides an efficient, physically consistent tool for investigating accretion phenomena and strong-gravity astrophysics beyond general relativity.

gr-qc↗

arXiv2026

Continuous Gravitational Waves from Supersoft X-ray Sources: Promising Targets for deci-Hz Detectors

Supersoft X-ray sources (SSSs) host white dwarfs (WDs) accreting at rates that sustain steady nuclear burning, driving rapid mass growth, radial contraction, and magnetic field amplification. Angular-momentum transfer from the accretion disk naturally spins up the WD, while the amplified internal magnetic field induces a non-axisymmetric deformation in presence of a misaligned rotation. Such WDs emits continuous gravitational waves (CGWs). We model the coupled evolutions of stellar mass, spin, and magnetic structure in accreting WDs in SSSs with MESA, and compute the resulting quadrupolar deformation with the Einstein-Maxwell solver XNS. We show that WDs in SSSs, particularly near the end of thermal timescale mass transfer and close to the Chandrasekhar mass limit, produce CGWs predominantly in the deci-Hz band accessible to planned detectors such as DECIGO, BBO, Deci-Hz, ALIA, and LGWA, and are distinguishable from other Galactic CGW sources such as AM CVn systems, detached double WDs, and isolated WDs. Well studied SSSs such as CAL 83 and RX J0019+2156 can be detectable, enabling targeted CGW measurements that directly probe WD's internal magnetic fields and rotation, while blind searches can reveal hundreds of obscured SSSs otherwise missed in soft X-rays and map the hidden population of accreting, rapidly rotating, magnetized WDs in nearby galaxies. A CGW detection from WDs in SSSs could also identify potential pre-explosion Type Ia progenitors.

astro-ph.HE↗

arXiv2026

GRMHD simulations of black hole accretion variabilities: Implications to hard state X-ray binary transients

Using high-resolution general relativistic magnetohydrodynamic (GRMHD) simulations, we investigate accretion flows around spinning black holes and identify three distinct accretion states. Our results suggest the origin of the complex phenomenology observed across the black hole mass spectrum as the interplay between magnetic and gravitational fields. The magnetically arrested disk (MAD) state, characterized by strong magnetic fields (plasma-$β<< 1$), exhibits powerful jets, highly variable accretion, and significant sub-Keplerian motion. On the other hand, weakly magnetized disks (plasma-$β>> 1$), known as the standard and normal evolution (SANE) state, show steady accretion with primarily winds. An intermediate state bridges the gap between MAD and SANE regimes, with moderate magnetic support (plasma-$β\sim 1$) producing mixed outflow morphologies and complex variability. This unified framework has many implications including its possible connection to extreme variability of GRS 1915+105, particularly in its hard spectral states. It also suggests the possible origin of steady jets of Cyg X-1 and the unusually high luminosities (even super-Eddington based on stellar mass black hole) of HLX-1 without requiring super-Eddington mass accretion rates. Our simulations reveal a hierarchy of timescales that explain the rich variety of variability patterns, with magnetic processes driving transitions between states. Comparing two with three dimensional simulations demonstrates that while quantitative details differ, the qualitative features distinguishing different accretion states remain robust.

astro-ph.HE↗

arXiv2026

Topology of the Superconducting Heart of Neutron Stars: Effects of Microphysics and Gravitational-Wave Signatures

We present a general-relativistic study of the distribution of proton superconductivity in strongly magnetized neutron stars (NSs), using the XNS code to solve the coupled Einstein-Maxwell equations. We investigate equilibrium configurations with both toroidal and poloidal magnetic field geometries and incorporate complex many-body effects through microscopically derived proton pairing gaps. The models employ equations of state (EoS) obtained from microscopic many-body theory - including realistic two- and three-body nuclear interactions - as well as from relativistic mean-field approaches. We compare superconducting topologies across our collection of EoS and explore the influences of magnetic field geometry in stellar models parameterized by central density. Our models confirm the absence of $S$-wave superconductivity in the inner core and, importantly, reveal that non-superconducting regions exhibit complex three-dimensional geometries: doughnut-shaped for toroidal fields and prolate-shaped for poloidal fields -- spatial structures that are inherently absent in one-dimensional analyses. We also compute magnetic deformations and ellipticities for several millisecond pulsars (MSPs), estimating their continuous gravitational wave strain. While these MSPs remain undetectable by current detectors, next-generation instruments such as the Einstein Telescope and Cosmic Explorer may detect their signals, opening an observational window into internal superconductivity and internal magnetic field of NSs, as well as the fundamental microphysics of dense matter.

astro-ph.HE↗

arXiv2025

Probing neutron star interiors and the properties of cold ultra-dense matter with the SKAO

Matter inside neutron stars is compressed to densities several times greater than nuclear saturation density, while maintaining low temperatures and large asymmetries between neutrons and protons. Neutron stars, therefore, provide a unique laboratory for testing physics in environments that cannot be recreated on Earth. To uncover the highly uncertain nature of cold, ultra-dense matter, discovering and monitoring pulsars is essential, and the SKA will play a crucial role in this endeavour. In this paper, we will present the current state-of-the-art in dense matter physics and dense matter superfluidity, and discuss recent advances in measuring global neutron star properties (masses, moments of inertia, and maximum rotation frequencies) as well as non-global observables (pulsar glitches and free precession). We will specifically highlight how radio observations of isolated neutron stars and those in binaries -- such as those performed with the SKA in the near future -- inform our understanding of ultra-dense physics and address in detail how SKAO's telescopes unprecedented sensitivity, large-scale survey and sub-arraying capabilities will enable novel dense matter constraints. We will also address the potential impact of dark matter and modified gravity models on these constraints and emphasise the role of synergies between the SKA and other facilities, specifically X-ray telescopes and next-generation gravitational wave observatories.

astro-ph.HE↗

arXiv2025

Continuous gravitational waves from magnetized white dwarfs: Quantifying the detection plausibility by LISA

White dwarfs (WDs) are frequently observed to have strong magnetic fields up to $10^9$ G and expected to have a possible internal field as high as $\sim 10^{14}$ G. High internal fields can significantly deform a WD's equilibrium structure, generating a quadrupole moment. If the rotation axis is misaligned with the magnetic axis, the deformation can lead to the emission of continuous gravitational waves (CGWs). We examine the potential for detecting CGWs from magnetized WDs with future space-based detectors such as LISA, ALIA, DECIGO, Deci-Hz, BBO and TianQin. We model the field-induced deformation and compute the resulting GW strain, incorporating amplitude decay due to angular momentum loss from electromagnetic and gravitational radiation. This sets a timescale for detection -`active timescale' of $10^{5-6}$ yr, requiring observation while the object remains sufficiently young. Our results suggest that LISA could detect a few dozens of highly magnetized WDs across the Galaxy during its mission. As a specific case, we investigate ZTF J1901+1458- a compact, massive, fast-rotating, and strongly magnetized WD with spin period $\sim416$ s and inferred surface field $\sim10^{9}$ G. We find that this object would be detectable by LISA with four years of continuous data. This highlights the potential of CGW observations to probe magnetic field structure in WDs and their role in type Ia supernova progenitors.

astro-ph.HE↗

arXiv2025

Probing the quantum speed limit and entanglement in flavor oscillations of neutrino-antineutrino system in curved spacetime

We consider a spinning primordial black hole (PBH) described by the Kerr metric in Kerr-Schild polar coordinates. We derive an analytical expression for the four-vector gravitational potential in the underlying Hermitian Dirac Hamiltonian using these coordinates. This gravitational potential introduces an axial vector term in the Dirac equation in curved spacetime. We find that the magnitudes of the temporal and spatial components of the four-vector gravitational potential are significantly affected by the angle of the position vector of the spinor with respect to the spin axis of the PBH, its radial distance from the PBH, and the strength of the specific angular momentum of the PBH. These potentials modify the effective mass matrix of the neutrino-antineutrino system and significantly affect the transition probabilities during the flavor oscillation of the neutrino-antineutrino system. We then use the transition probability to investigate the quantum speed limit time bound ratio for the two-flavor oscillation of the neutrino-antineutrino system in curved spacetime. This helps us estimate how quickly the initial neutrino flavor state evolves over time under the influence of the gravitational field. Finally, we discuss quantum correlations such as entanglement entropy during the two-flavor oscillation of the neutrino-antineutrino system near a spinning PBH.

gr-qc↗