arXiv subjects

Sercan Çıkıntoğlu

Publications and source records attributed to Sercan Çıkıntoğlu.

11 recordsLinked to original sources

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

Magnetically Threaded Thin Disks in the Presence of the Quadrupole Magnetic Field

Neutron stars might have multipole magnetic fields as implied by recent observations of pulsars. The presence of the quadrupole field might have an effect on the interaction between the disc and the neutron star depending on the location of the inner radius of the disc and the strength of the quadrupole field. For a quadrudipole stellar field, we calculate the toroidal field generated within the disc, the magnetospheric radius and the torque exerted onto the star. Also, we deduce the effect of the rotation of the star on the magnetospheric radius which is relevant even for pure dipole magnetic fields.

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↗

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↗

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↗

arXiv2018

Vacuum solutions around spherically symmetric and static objects in the Starobinsky model

The vacuum solutions around a spherically symmetric and static object in the Starobinsky model are studied with a perturbative approach. The differential equations for the components of the metric and the Ricci scalar are obtained and solved by using the method of matched asymptotic expansions. The presence of higher order terms in this gravity model leads to the formation of a boundary layer near the surface of the star allowing the accommodation of the extra boundary conditions on the Ricci scalar. Accordingly, the metric can be different from the Schwarzschild solution near the star depending on the value of the Ricci scalar at the surface of the star while matching the Schwarzschild metric far from the star.

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↗

arXiv2016

Inclination angle and braking index evolution of pulsars with plasma-filled magnetosphere: application to high braking index of PSR J1640-4631

The recently discovered rotationally powered pulsar PSR J1640-4631 is the first to have a braking index measured, with high enough precision, that is greater than three. An inclined magnetic rotator in vacuum or plasma would be subject not only to spin-down but also to an alignment torque. The vacuum model can address the braking index only for an almost orthogonal rotator that is incompatible with the single peaked pulse profile. The magnetic dipole model with the corotating plasma predicts braking indices between $3-3.25$. We find that the braking index of $3.15$ is consistent with two different inclination angles, $18.5\pm 3$ degrees and $56 \pm 4$ degrees. The smaller angle is preferred given the pulse profile has a single peak and the radio output of the source is weak. We infer the change in the inclination angle to be at the rate $-0.23$ degrees per century, three times smaller in absolute value than the rate recently observed for the Crab pulsar.

astro-ph.HE↗