arXiv subjects

Arun Mathew

Publications and source records attributed to Arun Mathew.

13 recordsLinked to original sources

arXiv2025

Supermassive Neutron Stars in Starobinsky Gravity with Causal Hybrid Stellar Matter

We investigate the stellar structure of neutron stars in the framework of Starobinsky gravity, characterized by a quadratic correction to the Einstein-Hilbert action, $f(R) = R + \alpha R^2$. In order to {\em preserve causality throughout\/} the star, we adopt a two-phase hybrid construction for the stellar matter, in which the core region consists of deconfined quark matter described by the MIT bag model, while the outer layers are composed of hadronic matter represented by unified equations of state such as SLy4, BSk20, and BSk21. Within this framework, we derive the modified field equations with static spherical symmetry, and numerically integrate the corresponding Tolman-Oppenheimer-Volkoff (TOV) equations with the chosen hybrid equations of state. Our analyses show that, unlike in general relativity, the Ricci scalar remains nonzero outside the stellar surface, and gradually falls to zero beyond 50 km, while the stellar surface remains within 10--12 km for the hybrid equations of state considered. This extended Ricci scalar profile arises from the extra degree of freedom (scalaron) inherent in Starobinsky gravity, which also contributes to the gravitational mass outside the star, causing the ADM mass measured at infinity to exceed the stellar mass at the surface. Nevertheless, the key physical relationships, such as the mass-central density and mass-radius curves, remain consistent with what is expected physically. Notably, we find that the maximum stable mass of neutron stars increases with the Starobinsky parameter $\alpha$, with the combined MIT-BSk21 model supporting an ADM mass of up to $2.07 \, M_\odot$ for $\alpha = 10\,r_g^2$. This theoreical limit for a nonrotating neutron star suggests that a rotating configuration could reach mass thresholds in the range of $2.48$ to $2.59 \, M_\odot$, considering that rapid rotation can enhance the maximum mass by approximately 20--25\%.

astro-ph.HE↗

arXiv2025

Thermal emission from bow shocks. III. Variable diffuse X-ray emission from stellar-wind bow shocks driven by dynamical instabilities

X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the ISM by quantifying mixing at the interface between hot and warm gas phases. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study convergence properties and assess robustness of predicted observables from simulations. A suite of 2D and 3D HD and MHD simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission. Resolving the Kelvin-Helmholtz (KH) instability at the wind-ISM contact discontinuity is crucial for obtaining converged results and for predicting X-ray emission and the properties of the hot shocked wind. When sufficient spatial resolution is used, we measure time variation of X-ray emission of at least an order of magnitude on a timescale comparable to the advection timescale of the wake downstream from the bow shock. Good correspondence is found between 2D and 3D simulations with comparable resolution, and 3D simulations can achieve the required resolution with reasonable computing resources. Development of the KH instability is inhibited for shear flows parallel to the ISM magnetic field, compared with what is seen in the perpendicular direction, resulting in synthetic IR emission maps of bow shocks that are smooth when seen from one perspective but show strong distortions from another. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energy-transfer rates between hot and warm gas phases of the ISM. Dynamical instabilities at the wind-ISM interface are a crucial ingredient in determining the properties of the hot-gas phase in stellar bow-shocks, in particular to capture its time dependence.

astro-ph.HE↗

arXiv2024

A multi-ion non-equilibrium solver for ionised astrophysical plasmas with arbitrary elemental abundances

While many astrophysical plasmas can be modelled successfully assuming ionisation and thermal equilibrium, in some cases this is not appropriate and a non-equilibrium approach is required. In nebulae around evolved stars the local elemental abundances may also strongly vary in space and time. Here we present a non-equilibrium multi-ion module developed for the fluid-dynamics code \textsc{pion}, describing the physical processes included and demonstrating its capabilities with some test calculations. A non-equilbrium ionisation solver is developed that allows arbitrary elemental abundances for neutral and ionised (but not molecular) gas, for the elements H, He, C, N, O, Ne, Si, S and Fe. Collisional ionisation and recombination, photoionisation and charge-exchange reactions are included, and ion-by-ion non-equilibrium radiative cooling is calculated based on the instantaneous ion fractions of each element. Element and ion mass-fractions are advected using passive scalars, operator-split from the microphysical processes. The module is validated by comparing with equilibrium and non-equilibrium calculations in the literature. Effects of charge exchange on ion abundances in cooling plasmas are discussed. Application to modelling shocks and photoionised H~\textsc{ii} regions is demonstrated. The time-dependent expansion of a Wolf-Rayet nebula is studied, including photoionisation and collisional processes, and spectral-line luminosities calculated for non-equilibrium and equilibrium plasma states. The multi-ion module enables simulation of ionised plasmas with spatially varying elemental abundances using self-consistent ion abundances and thermal evolution. This allows prediction of spectral lines in UV, optical, IR and X-ray even in cases where the plasma is out of ionisation equilibrium.

astro-ph.GA↗

arXiv2024

Maximal mass of the neutron star with a deconfined quark core

The nature of equation of state for the matter in the neutron star plays an important role in determining its maximal mass. In addition, it must comply with the condition of causality. Noting that the central density of a maximally massive neutron star is well above the nuclear saturation density, a deconfined quark core in the central region is motivated in this paper. We analyze this scenario by employing the MIT bag model to represent the core region and one of the unified equations of state for the region outside the core. Such combination is found to solve the problem of causality violation. In each case of the combined equations of state, the radial profile of $\rho r^2$ displays a peak and dominant contribution to the total mass of the star comes from the region around the peak value of $\rho r^2$, whereas the contribution is small from the regions near the center and the surface. This peak occurs in the region of hadronic matter for the combinations considered in this paper. Importantly, we find that the position of the peak in $\rho r^2$ is well-correlated with the maximal mass -- the highest value of $1.98\ M_\odot$ obtains for the case with the peak occurring farthest from the center. This gravitational threshold being obtained for a non-rotating neutron star, we expect the threshold to lie well above 2 $ M_\odot$ for a rapidly rotating neutron star, that may explain the existance of massive pulsars from recent astronomical observations.

astro-ph.HE↗

arXiv2023

Inverse Compton cooling of thermal plasma in colliding-wind binaries

The inverse-Compton effect (IC) is a widely recognized cooling mechanism for both relativistic and thermal electrons in various astrophysical environments, including the intergalactic medium and X-ray emitting plasmas. Its effect on thermal electrons is however frequently overlooked in theoretical and numerical models of colliding-wind binaries (CWB). In this article, we provide a comprehensive investigation of the impact of IC cooling in CWBs, presenting general results for when the photon fields of the stars dominate the cooling of the thermal plasma and when shocks at the stagnation point are expected to be radiative. Our analysis shows that IC cooling is the primary cooling process for the shocked-wind layer over a significant portion of the relevant parameter space, particularly in eccentric systems with large wind-momentum ratios, e.g., those containing a Wolf-Rayet and O-type star. Using the binary system WR 140 as a case study, we demonstrate that IC cooling leads to a strongly radiative shocked wind near periastron, which may otherwise remain adiabatic if only collisional cooling was considered. Our results are further supported by 2D and 3D simulations of wind-wind collisions. Specifically, 3D magnetohydrodynamic simulations of WR 140 show a significant decrease in hard-X-ray emission around periastron, in agreement with observations but in contrast to equivalent simulations that omit IC cooling. A novel method is proposed for constraining mass-loss rates of both stars in eccentric binaries where the wind-collision zone switches from adiabatic to radiative approaching periastron. IC scattering is an important cooling process in the thermal plasma of CWBs.

astro-ph.HE↗

arXiv2020

Primordial reheating in $f(R)$ cosmology by spontaneous decay of scalarons

We employ a viable $f(R)$ gravity model capable of giving an inflationary phase in order to study the subsequent reheating phase due to particle creation at the expense of energy in the scalaron field. Since quantum mechanics is expected to play a dominant role in particle creation, we formulate a plausible scenario of reheating obeying Heisenberg's uncertainty principle that imposes constraints on the particles created in the configuration space. We show that, so long as the energy available in the scalaron field is sufficient to populate the entire configuration space, the energy density of the particles grows, attaining a maximum value giving an efficient reheating. Beyond this maximum, the available energy becomes insufficient to populate the entire configuration space leading to a declining energy density. We further find that there is a negligible growth of energy density in the inflationary phase that lasts for $\sim 10^7 \, t_{\rm P}$, although particles are constantly created in this phase. The subsequent reheating phase spans for $\sim10^{11} \, t_{\rm P}$ and it begins with a well-defined preheating stage lasting for $\sim 10^{5} \, t_{\rm P}$, making a cross-over to a thermilization regime. The temperature at the beginning of the thermilization is found to be $T_{\rm th}\sim 10^{12}$ GeV, whereas the reheating temperature is estimated as $T_{r}\sim10^{13}$ GeV. Importantly, these estimates follow from a single parameter, the scalaron mass, $M\sim10^{-5} \, M_{\rm P}$.

gr-qc↗

arXiv2020

Non-uniform FIR Digital Filter Bank for Hearing Aid Application Using Frequency Response Masking Technique: A Review

Hearing aid is an electroacoustic device used to selectively amplify the audio sounds with an aim to make speech more intelligible for a hearing impaired person. Filter bank is one of the important parts of digital hearing aid where the sub band gains of each filter can be tuned to compensate an individuals unique hearing loss pattern. As the human perception is based on the logarithmic scale, nonuniform filter bank outperforms uniform filter bank. The main advantage of nonuniform filer bank is that it requires less number of sub-band filters, hence resulted in low hardware complexity and cost. Much effort has been devoted to design these nonuniform filter banks for hearing aid applications. This paper aimed to provide a review of previous researches based on nonuniform finite impulse response (FIR) digital filter bank for hearing aid application using frequency response masking (FRM) technique. By reviewing filter banks, we try to find the difference between fixed and variable band filter bank and to give an insight about which method is more suitable for matching most common types of hearing loss. Papers which involved methods of design, theoretical computation and simulation results of filter bank have been reviewed.

cs.SD↗

arXiv2020

Stellar structure of quark stars in a modified Starobinsky gravity

We propose a form of gravity-matter interaction given by $\omega RT$ in the framework of $f(R,T)$ gravity and examine the effect of such interaction in spherically symmetric compact stars. Treating the gravity-matter coupling as a perturbative term on the background of Starobinsky gravity, we develop a perturbation theory for equilibrium configurations. For illustration, we take the case of quark stars and explore their various stellar properties. We find that the gravity-matter coupling causes an increase in the stable maximal mass which is relevant for recent observations on binary pulsars.

gr-qc↗

arXiv2020

Existence of Chandrasekhar's limit in GUP white dwarfs

Various recent theoretical investigations suggest that gravitational collapse of white dwarfs is withheld for arbitrarily high masses if the equation of state is described by the generalized uncertainty principle (GUP). There have been a few attempts to restore the Chandrasekhar limit but they are found to be inadequate from different perspectives and some of them led to unphysical mass-radius relations. In this paper, we rigorously resolve this problem by analyzing the dynamical instability in general relativity. We confirm the existence of Chandrasekhar's limit as well as stable mass-radius curves that behave consistently with astronomical observations. Moreover, this stability analysis suggests gravitational collapse beyond the Chandrasekhar limit signifying the possibility of compact objects denser than white dwarfs.

gr-qc↗

arXiv2018

Prospect of Chandrasekhar's limit against modified dispersion relation

Newtonian gravity predicts the existence of white dwarfs with masses far exceeding the Chandrasekhar limit when the equation of state of the degenerate electron gas incorporates the effect of quantum spacetime fluctuations (via a modified dispersion relation) even when the strength of the fluctuations is taken to be very small. In this paper, we show that this Newtonian "super-stability" does not hold true when the gravity is treated in the general relativistic framework. Employing dynamical instability analysis, we find that the Chandrasekhar limit can be reassured even for a range of high strengths of quantum spacetime fluctuations with the onset density for gravitational collapse practically remaining unaffected.

gr-qc↗

arXiv2018

Noncommutative dispersion relation and mass-radius relation of white dwarfs

The equation of state of the electron degenerate gas in a white dwarf is usually treated by employing the ideal dispersion relation. However, the effect of quantum gravity is expected to be inevitably present and when this effect is considered through a non-commutative formulation, the dispersion relation undergoes a substantial modification. In this paper, we take such a modified dispersion relation and find the corresponding equation of state for the degenerate electron gas in white dwarfs. Hence we solve the equation of hydrostatic equilibrium and find that this leads to the possibility of the existence of excessively high values of masses exceeding the Chandrasekhar limit although the quantum gravity effect is taken to be very small. It is only when we impose the additional effect of neutronization that we obtain white dwarfs with masses close to the Chandrasekhar limit with nonzero radii at the neutronization threshold. We demonstrate these results by giving the numerical estimates for the masses and radii of $^4He$ and $^{12}C$, and $^{16}O$ white dwarfs.

gr-qc↗

arXiv2017

Effect of minimal length uncertainty on the mass-radius relation of white dwarfs

Generalized uncertainty relation that carries the imprint of quantum gravity introduces a minimal length scale into the description of space-time. It effectively changes the invariant measure of the phase space through a factor $(1+\beta \mathbf{p}^2)^{-3}$ so that the equation of state for an electron gas undergoes a significant modification from the ideal case. It has been shown in the literature (Rashidi 2016) that the ideal Chandrasekhar limit ceases to exist when the modified equation of state due to the generalized uncertainty is taken into account. To assess the situation in a more complete fashion, we analyze in detail the mass-radius relation of Newtonian white dwarfs whose hydrostatic equilibria are governed by the equation of state of the degenerate relativistic electron gas subjected to the generalized uncertainty principle. As the constraint of minimal length imposes a severe restriction on the availability of high momentum states, it is speculated that the central Fermi momentum cannot have values arbitrarily higher than $p_{\rm max}\sim\beta^{-1/2}$. When this restriction is imposed, it is found that the system approaches limiting mass values higher than the Chandrasekhar mass upon decreasing the parameter $\beta$ to a value given by a legitimate upper bound. Instead, when the more realistic restriction due to inverse $\beta$-decay is considered, it is found that the mass and radius approach the values close to $1.45$ M$_{\odot}$ and $600$ km near the legitimate upper bound for the parameter $\beta$. On the other hand, when $\beta$ is decreased sufficiently from the legitimate upper bound, the mass and radius are found to be approximately $1.46$ M$_{\odot}$ and $650$ km near the neutronization threshold.

gr-qc↗

arXiv2014

General Relativistic Calculations for White Dwarf Stars

The mass-radius relations for white dwarf stars are investigated by solving the Newtonian as well as Tolman-Oppenheimer-Volkoff (TOV) equations for hydrostatic equilibrium assuming the electron gas to be non-interacting. We find that the Newtonian limiting mass of $1.4562M_\odot$ is modified to $1.4166M_\odot$ in the general relativistic case for $^4_2$He (and $^{12}_{\ 6}$C) white dwarf stars. Using the same general relativistic treatment, the critical mass for $^{56}_{26}$Fe white dwarf is obtained as $1.2230M_\odot$. In addition, departure from the ideal degenerate equation of state (EoS) is accounted for by considering Salpeter's EoS along with the TOV equations yielding slightly lower values for the critical masses, namely $1.4081M_{\odot}$ for $^4_2$He, $1.3916M_{\odot}$ for $^{12}_{\ 6}$C and $1.1565M_{\odot}$ for $^{56}_{26}$Fe white dwarfs. We also compare the critical densities for gravitational instability with the neutronization threshold densities to find that $^4_2$He and $^{12}_{\ 6}$C white dwarf stars are stable against neutronization with the critical values of $1.4081M_\odot$ and $1.3916M_{\odot}$, respectively. However the critical masses for $^{16}_{\ 8}$O, $^{20}_{10}$Ne, $^{24}_{12}$Mg, $^{28}_{14}$Si, $^{32}_{16}$S and $^{56}_{26}$Fe white dwarf stars are lower due to neutronization. Corresponding to their central densities for neutronization thresholds, we obtain their maximum stable masses due to neutronization by solving the TOV equation coupled with the Salpeter EoS.

astro-ph.SR↗