arXiv subjects

K. Yavuz Ekşi

Publications and source records attributed to K. Yavuz Ekşi.

At least 19 recordsLinked to original sources

arXiv2026

Impact of Anisotropy on Neutron Star Structure and Curvature

We investigate the impact of pressure anisotropy on the structural and geometric properties of neutron stars within general relativity, focusing primarily on the phenomenological Bowers-Liang (BL) model, and comparing selected results with a quasi-local prescription. Using the SLy equation of state, we explore how anisotropic stresses modify global observables such as the mass-radius relation, moment of inertia, compactness, and tidal deformability over a broad range of anisotropy parameters. We find that moderate positive anisotropy can increase the maximum supported mass up to approximately $2.4\;M_\odot$ and enhance stellar compactness by up to $20\%$ relative to isotropic configurations, while remaining broadly consistent with current NICER and gravitational-wave constraints. To probe the internal gravitational field, we compute curvature invariants including the Ricci scalar, the Ricci tensor contraction, the Kretschmann scalar, and the Weyl scalar. We show that curvature measures directly tied to the matter distribution exhibit a strong sensitivity to anisotropy, whereas the Weyl curvature remains comparatively insensitive, reflecting its role as a measure of the free gravitational field. Within the phenomenological BL framework, the maximum compactness increases with anisotropy and reaches values as high as $\mathcal{C}_{\max}\approx 0.25$-$0.38$ for $λ_{\rm BL}\in[-4,+4]$, although the physical realizability of such highly compact configurations depends sensitively on the underlying anisotropy mechanism. A comparison with the quasi-local model highlights the strong model dependence of anisotropic effects, underscoring both the potential significance and the limitations of phenomenological anisotropy prescriptions in modeling strong-field neutron-star interiors.

gr-qc↗

arXiv2023

Evolution of fallback discs around millisecond magnetars: effect of supercritical accretion on GRB afterglows

Some models of gamma-ray bursts (GRBs) invoke nascent millisecond magnetars as the central engine and address the X-ray afterglows with the interaction of magnetar magnetospheres with fallback discs. We study the evolution of fallback discs interacting with the millisecond magnetars. Initially, the accretion rate in the fallback disc is very high, well above the rate required for the Eddington limit. The inner parts of such a disc, even if it is cooling by the neutrino emission, get spherical due to the radiation pressure, which regulates the mass accretion rate within the spherization radius. Such a disc can not penetrate the light cylinder radius for the typical magnetic fields, and the initial spin frequencies invoked for the magnetars. As a result of the auto-regulation of the accretion flow, the fallback disc can not interact directly with the magnetar's magnetosphere within the first few days. This has implications for the fallback disc models of GRB afterglows since the accretion and propeller luminosities, in the presence of radiation pressure, are too low to address the typical luminosities of X-ray afterglows.

astro-ph.HE↗

arXiv2023

On the minimum spin period of accreting pulsars

The distribution of the spin frequencies of neutron stars in low-mass X-ray binaries exhibits a cut-off at 730 Hz, below the break-up frequency (mass-shedding limit) of neutron stars. The absence of the sub-millisecond pulsars presents a problem, given that these systems are older than the spin-up timescale. We confront models of disc-magnetosphere interaction near torque equilibrium balanced by the torque due to gravitational wave emission. We note that field lines penetrating the disc beyond the inner radius reduce the maximum rotation frequency of the star, a result well-known since the seminal work of Ghosh & Lamb. We show that the polar cap area corresponds to about half the neutron star surface area at the cut-off frequency if the inner radius is slightly smaller than the corotation radius. We then include the change in the moment of inertia of the star due to the accretion of mass and find that this effect further reduces the maximum rotation frequency of the star. Finally, we include the torque due to gravitational wave emission and calculate its contribution to the torque equilibrium. Our results suggest that all three processes are significant at the cut-off frequency, and all of them must be considered in addressing the absence of sub-millisecond pulsars.

astro-ph.HE↗

arXiv2023

On the hysteresis effect in transitions between accretion and propeller regimes

Some observations and numerical simulations of disc-magnetosphere interaction show that accretion can proceed in the propeller regime. When the Alfvén radius is beyond the corotation radius, matter climbs up to the high latitudes where the Alfvén surface is inside the equilibrium surface and can accrete. We calculate the fraction of the mass flux in the disc that can accrete onto the neutron star depending on the fastness parameter and the inclination angle between rotation and magnetic axis. We find that, for a narrow range of the fastness parameter, the Alfvén and the equilibrium surfaces intersect at two different critical latitudes. While the system is transiting from the propeller to the accretion regime (the initial rise of an outburst), the disc is already thick and the part of the disc between these two critical latitudes cannot accrete. In transitions from the accretion to the propeller regime (decay of an outburst), the disc is thin, hence, full accretion of matter proceeds until the Alfvén radius moves beyond the equilibrium radius at the disc-midplane. Therefore, the accretion regime commences at a smaller fastness parameter than it ceases. As a result, the transition from the propeller to the accretion regime occurs at a luminosity higher than the transition from the accretion to the propeller regime. We discuss the implications of our results for spectral transitions exhibited by low-mass X-ray binaries.

astro-ph.HE↗

arXiv2022

GRMHD simulations of accreting neutron stars I: nonrotating dipoles

We study the general-relativistic dynamics of matter being accreted onto and ejected by a magnetised and nonrotating neutron star. The dynamics is followed in the framework of fully general relativistic magnetohydrodynamics (GRMHD) within the ideal-MHD limit and in two spatial dimensions. More specifically, making use of the numerical code BHAC, we follow the evolution of a geometrically thick matter torus driven into accretion by the development of a magnetorotational instability. By making use of a number of simulations in which we vary the strength of the stellar dipolar magnetic field, we can determine self-consistently the location of the magnetospheric (or Alfvén) radius $r_{\rm msph}$ and study how it depends on the magnetic moment $μ$ and on the accretion rate. Overall, we recover the analytic Newtonian scaling relation, i.e. $r_{\rm msph} \propto B^{4/7}$, but also find that the dependence on the accretion rate is very weak. Furthermore, we find that the material torque correlates linearly with the mass-accretion rate, although both of them exhibit rapid fluctuations. Interestingly, the total torque fluctuates drastically in strong magnetic field simulations and these unsteady torques observed in the simulations could be associated with the spin fluctuations observed in X-ray pulsars.

astro-ph.HE↗

arXiv2022

Intra-pulse variability induced by plasmoid formation in pulsar magnetospheres

Pulsars show irregularities in their pulsed radio emission that originate from propagation effects and the intrinsic activity of the source. In this work, we investigate the role played by magnetic reconnection and the formation of plasmoids in the pulsar wind current sheet as a possible source of intrinsic pulse-to-pulse variability in the incoherent, high-energy emission pattern. We used a two-dimensional particle-in-cell simulation of an orthogonal pulsar magnetosphere restricted to the plane perpendicular to the star spin axis. We evolved the solution for several tens of pulsar periods to gather a statistically significant sample of synthetic pulse profiles. The formation of plasmoids leads to strong pulse-to-pulse variability in the form of multiple short, bright subpulses, which appear only on the leading edge of each main pulse. These secondary peaks of emission are dominated by the dozen plasmoids that can grow up to macroscopic scales. They emerge from the high end of the hierarchical merging process occurring along the wind current layer. The flux of the subpulses is correlated with their width in phase. Although the full-scale separation is not realistic, we argue that the simulation correctly captures the demographics and the properties of the largest plasmoids, and therefore of the brightest subpulses. The prediction of subpulses at specific pulse phases provides a new observational test of the magnetic reconnection scenario as the origin of the pulsed incoherent emission. High-time-resolution observations of the Crab pulsar in the optical range may be the most promising source to target for this purpose.

astro-ph.HE↗

arXiv2022

A precessing magnetar model for GLEAM-X J162759.5-523504.3

We propose a precessing transient magnetar model for the recently discovered radio source GLEAM-X J162759.5-523504.3. We identify the observed period of $\sim 1$ ks as the precession period of the magnetar deformed due to its strong ($B_ϕ \sim 10^{16}$ G) toroidal field. The resulting deformation of order $10^{-4}$ implies a spin period of $P_{\rm s} = 0.1$ s. Assuming a strong dipole field of $B_{\rm d} \sim 10^{14}$ G we predict a period derivative of $\dot{P}_{\rm s}\sim 10^{-11}$ s/s. We also predict that the precession period of the magnetar can be observed in the hard X-ray band just as the other three galactic magnetars exhibit precession.

astro-ph.HE↗

arXiv2021

Ricci-Determinant Gravity: Dynamical Aspects and Astrophysical Implications

The Palatini gravitational action is enlarged by an arbitrary function $f(X)$ of the determinants of the Ricci tensor and the metric, $X=|\textbf{det}.R|/|\textbf{det}.g|$. The resulting Ricci-determinant theory exhibits novel deviations from general relativity. We study a particular realization where the extension is characterized by the square-root of the Ricci-determinant, $f(X)=λ_\text{Edd}\sqrt{X}$, which corresponds to the famous Eddington action. We analyze the obtained equations for a perfect fluid source and show that the affine connection can be solved in terms of the energy density and pressure of the fluid through an obtained disformal metric. As an application, we derive the hydrostatic equilibrium equations for relativistic stars and inspect the significant effects induced by the square-root of the Ricci tensor. We find that an upper bound on $λ_{\rm Edd}$, at which deviations from the predictions of general relativity on neutron stars become prominent, corresponds to the hierarchy between the Planck and the vacuum mass scales. The Ricci-determinant gravity that we propose here is expected to have interesting implications in other cosmological domains.

gr-qc↗

arXiv2021

Nonlinear superfluidity and time-delay based chaotic spin-down in pulsars

We investigate the chaotic spin-down behavior seen from some pulsars in terms of the nonlinear superfluid dynamics. To this end, we numerically solve the set of equations for the superfluid-normal matter system whose coupling is mediated by creep of the vortex lines. We show that glitch perturbations which introduce a time-delay to the steady-state dynamics leave behind a remnant in the third time derivative of the rotational phase. This time-delay induces a hyper-chaotic spin-down for pulsars. We find that glitch-induced changes in the rotational parameters lead to non-closing cyclic patterns in the time-delayed phase difference diagram. We observe that the number of cycles, $N$, in the diagram results from $N+1$ glitches occurred in total observation time.

astro-ph.HE↗

arXiv2020

The initial evolution of millisecond magnetars: an analytical solution

Millisecond magnetars are often invoked as the central engine of some gamma-ray bursts (GRBs), specifically the ones showing a plateau phase. We argue that an apparent plateau phase may not be realized if the magnetic field of the nascent magnetar is in a transient rapid decay stage. Some GRBs that lack a clear plateau phase may also be hosting millisecond magnetars. We present an approximate analytical solution of the coupled set of equations describing the evolution of the angular velocity and the inclination angle between rotation and magnetic axis of a neutron star in the presence of a co-rotating plasma. We also show how the solution can be generalized to the case of evolving magnetic fields. We determine the evolution of the spin period, inclination angle, magnetic dipole moment and braking index of six putative magnetars associated with GRB 091018, GRB 070318, GRB 080430, GRB 090618, GRB 110715A, GRB 140206A through fitting, via Bayesian analysis, the X-ray afterglow light curves by using our recent model [Şaşmaz Muş et al. 2019]. We find that within the first day following the formation of the millisecond magnetar, the inclination angle aligns rapidly, the magnetic dipole field may decay by a few times and the braking index varies by an order of magnitude.

astro-ph.HE↗

arXiv2019

The First Day in the Life of a Magnetar: Evolution of the Inclination Angle, Magnetic Dipole Moment and Braking Index of Millisecond Magnetars During Gamma-Ray Burst Afterglows

The afterglow emission of some gamma-ray bursts (GRBs) show a shallow decay (plateau) phase implying continuous injection of energy. The source of this energy is very commonly attributed to the spin-down power of a nascent millisecond magnetar. The magnetic dipole radiation torque is considered to be the mechanism causing the spin-down of the neutron star. This torque has a component working for the alignment of the angle between rotation and magnetic axis, i.e., inclination angle, which has been neglected in modelling GRB afterglow light curves. Here, we demonstrate the evolution of the inclination angle and magnetic dipole moment of nascent magnetars associated with GRBs. We constrain the initial inclination angle, magnetic dipole moment and rotation period of seven magnetars by modelling the seven long-GRB afterglow light curves. We find that, in its first day, the inclination angle of a magnetar decreases rapidly. The rapid alignment of the magnetic and rotation axis may address the lack of persistent radio emission from mature magnetars. We also find that in three cases the magnetic dipole moments of magnetars decrease exponentially to a value a few times smaller than the initial value. The braking index of nascent magnetars, as a result of the alignment and magnetic dipole moment decline, is variable during the afterglow phase and always greater than three.

astro-ph.HE↗

arXiv2019

Neutron Star Structure in the Presence of Nonminimally Coupled Scalar Fields

We study the structure of neutron stars in scalar-tensor theories for the nonminimal coupling of the form $(1+κξϕ^{2})\cal R$. We solve the hydrostatic equilibrium equations for two different types of scalar field potentials and three different equations of state representative of different degrees of stiffness. We obtain the mass-radius relations of the configurations and determine the allowed ranges for the term $ξϕ^2$ at the center of the star and spatial infinity based on the measured maximum value of the mass for neutron stars and the recent constraints on the radius coming from gravitational wave observations. Thus we manage to limit the deviation of the model from general relativity. We examine the possible constraints on the parameters of the model and compare the obtained restrictions with the ones inferred from other cosmological probes that give the allowed ranges for the coupling constant only. In the case of the Higgs-like potential, we also find that the central value for the scalar field cannot be chosen arbitrarily but it depends on the vacuum expectation value of the field. Finally, we discuss the effect of the scalar field potential on the mass and the radius of the star by comparing the results obtained for the cases considered here.

gr-qc↗

arXiv2018

Ultra-luminous X-ray sources as neutron stars propelling and accreting at super-critical rates in high-mass X-ray binaries

Ultra-luminous X-ray sources (ULXs) are off-nuclear point sources in nearby galaxies with luminosities well exceeding the Eddington limit for stellar-mass objects. It has been recognized after the discovery of pulsating ULXs (PULXs) that a fraction of these sources could be accreting neutron stars in high-mass X-ray binaries (HMXBs) though the majority of ULXs are lacking in coherent pulsations. The earliest stage of some HMXBs may harbor rapidly rotating neutron stars propelling out the matter transferred by the massive companion. The spin-down power transferred by the neutron-star magnetosphere to the accretion disk at this stage can well exceed the Eddington luminosities and the system appears as a non-pulsating ULX. In this picture, PULXs appear as super-critical mass-accreting descendants of non-pulsating ULXs. We present this evolutionary scenario within a self-consistent model of magnetosphere-disk interaction and discuss the implications of our results on the spin and magnetic field of the neutron star.

astro-ph.HE↗

arXiv2018

Constraint on energy-momentum squared gravity from neutron stars and its cosmological implications

Deviations from the predictions of general relativity due to energy-momentum squared gravity (EMSG) are expected to become pronounced in the high density cores of neutron stars. We derive the hydrostatic equilibrium equations in EMSG and solve them numerically to obtain the neutron star mass-radius relations for four different realistic equations of state. We use the existing observational measurements of the masses and radii of neutron stars to constrain the free parameter, $α,$ that characterizes the coupling between matter and spacetime in EMSG. We show that $-10^{-38}\,\mathrm{cm^{3}/erg}<α<+10^{-37}\,\mathrm{cm^{3}/erg}$. Under this constraint, we discuss what contributions EMSG can provide to the physics of neutron stars, in particular, their relevance to the so called \textit{hyperon puzzle} in neutron stars. We also discuss how EMSG alters the dynamics of the early universe from the predictions of the standard cosmological model. We show that EMSG leaves the standard cosmology safely unaltered back to $t\sim 10^{-4}$ seconds at which the energy density of the universe is $\sim 10^{34}\,\mathrm{erg\,cm^{-3}}$.

gr-qc↗

arXiv2017

Relativistic stars in Starobinsky gravity with the matched asymptotic expansions method

We study the structure of relativistic stars in $\mathcal{R}+α\mathcal{R}^{2}$ theory using the method of matched asymptotic expansion to handle the higher order derivatives in field equations arising from the higher order curvature term. We find solutions, parametrized by $α$, for uniform density stars. We obtain the mass-radius relations and study the dependence of maximum mass on $α$. We find that $M_{\max}$ is almost linearly proportional to $α$. For each $α$ the maximum mass configuration has the biggest compactness parameter ($η= GM/Rc^2$), and we argue that the general relativistic stellar configuration corresponding to $α=0$ is the least compact among these.

gr-qc↗

arXiv2017

Constraints on the disc-magnetosphere interaction in accreting pulsar 4U 1626--67

Using the spin and flux evolution of the accreting pulsar 4U 1626$-$67 across the 2008 torque reversal, we determine the fastness parameter dependence of the dimensionless torque acting on the pulsar. We find that the dimensionless torque is qualitatively different from the existing models: it is concave-up across the torque equilibrium whereas the existing torque models predict a concave-down (convex) relation with the fastness parameter. We show that the dimensionless torque has a cubic dependence on the fastness parameter near the torque equilibrium. We also find that the torque can not attain large values away from the equilibrium, either in the positive or the negative side, but saturates at limited values. The spin-down torque can attain a 2.5 times larger magnitude at the saturation limit than the spin-up torque. From the evolution of the frequency of quasi-periodic oscillations of 4U 1626$-$67 across the torque reversal of 1990, we determine the critical fastness parameter corresponding to torque equilibrium to be $ω_{\rm c} \simeq 0.75$ within the framework of the beat frequency model and boundary region model for reasonable values of the model parameters. We find that the disc magnetosphere interaction becomes unstable when the inner radius approaches the corotation radius as predicted by some models, though with a longer timescale. We also find that there is an unstable regime that is triggered when the fastness parameter is 0.8 times the critical fastness parameter ($ω=0.6$ for $ω_{\rm c} \simeq 0.75$) possibly associated with an instability observed in numerical simulations.

astro-ph.HE↗

arXiv2017

On the new braking index of PSR B0540-69: further support for magnetic field growth of neutron stars following submergence by fallback accretion

The magnetic fields of the nascent neutron stars could be submerged in the crust by rapid fallback accretion and could diffuse to the surface later in life. According to this field burial scenario young pulsars may have growing magnetic fields, a process known to result with less-than-three braking indices; larger braking indices implying longer field-growth time-scales. A nascent neutron star with a larger kick velocity would accrete less amount of matter leading to a shallower burial of its field and a more rapid field growth. Such an inverse relation between the field growth time-scale inferred from the braking indices and space velocity of pulsars was claimed in the past as a prediction of the field-burial scenario. With a braking index of $n\sim 2$ and large space velocity PSR~B0540--69 was then an outlier in the claimed relation. The object recently made a transition to a rapid spin-down state accompanied by a low braking index. This new braking index implies a much shorter time-scale for the field growth which is consistent with the high space velocity of the object, in better agreement with the claimed relation. This observation lends support to the field burial scenario and implies that the growth of the magnetic field does not proceed at a constant pace but is slowed or completely halted at times. The slow spin-down state associated with the high braking index before 2011 which lasted for at least about 30 years was then such an episode of slow field growth.

astro-ph.HE↗

arXiv2016

Moment of inertia of neutron star crust in alternative and modified theories of gravity

The glitch activity of young pulsars arises from the exchange of angular momentum between the crust and the interior of the star. Recently, it was inferred that the moment of inertia of the crust of a neutron star is not sufficient to explain the observed glitches. Such estimates are presumed in the Einstein's general relativity in describing the hydrostatic equilibrium of neutron stars. The crust of the neutron star has a space-time curvature of 14 orders of magnitude larger than that probed in solar system tests. This makes gravity the weakest constrained physics input in the crust related processes. We calculate the ratio of the crustal to the total moment of inertia of neutron stars in the scalar-tensor theory of gravity and the non-perturbative $f({\cal R})={\cal R}+ a {\cal R}^2$ gravity. We find for the former that the crust to core ratio of the moment of inertia does not change significantly from what is inferred in general relativity. For the latter we find that the ratio increases significantly from what is inferred in general relativity in the case of high mass objects. Our results suggest that the glitch activity of pulsars may be used to probe gravity models, although the gravity models explored in this work are not appropriate candidates.

gr-qc↗