arXiv subjects

Ayse Ulubay

Publications and source records attributed to Ayse Ulubay.

4 recordsLinked to original sources

arXiv2026

Binary and neutron star evolution in low-mass X-ray binaries on the evolutionary tracks of accreting millisecond X-ray pulsars

Neutron star low-mass X-ray binaries (LMXBs) are the progenitors of millisecond pulsars. In these systems, old neutron stars (NSs) can be spun up during a long-lasting accretion phase. The discovery of accreting millisecond X-ray pulsars (AMXPs) and transitional millisecond pulsars has provided key observational insights into the connection between millisecond pulsars and LMXBs. In this work, we have investigated both the binary system and the individual NS evolution leading to AMXP properties. We use MESA to analyse the binary evolution of LMXBs, following three distinct evolutionary tracks defined by the AMXP donor types. We find that while the magnetic braking index may affect the mass-transfer history, the initial orbital period is the most influential parameter that shapes the overall binary evolution. We use the mass accretion histories estimated from these binary simulations to study the rotational evolution of NSs employing the model that can account for torque-luminosity relations and the lack of X-ray pulses from most of these systems. With reasonable model parameters, our model results are in agreement with the typical properties of AMXPs. For these AMXP sources from each evolutionary track, we have shown that the model can reproduce the NS and binary properties simultaneously. Finally, we discuss the time-scales of different evolutionary paths, as well as the conditions under which these systems could be detectable at various stages of their evolution.

astro-ph.HE↗

arXiv2024

On the lack of X-ray pulsation in most neutron star low-mass X-ray binaries

We have investigated whether the lack of X-ray pulsations from most neutron star (NS) low-mass X-ray binaries (LMXBs) could be due to the extension of their inner disc to the NS surface. To estimate the inner disc radii, we have employed the model, recently proposed to account for the torque reversals of LMXBs. In this model, the inner disc radius depends on the spin period as well as the dipole moment and the mass inflow rate of the disc. Our model results indicate that most LMXBs have mass accretion rates above the minimum critical rates required for the inner disc to reach down to the NS surface and thereby quench the pulsed X-ray emission. For most sources X-ray pulsations are allowed when the period decreases below a certain critical value. For the same parameters, the model is also consistent with the observed X-ray luminosity ranges of the individual accreting millisecond X-ray pulsars (AMXPs). The paucity of AMXPs compared to the majority population of non-pulsing LMXBs is explained, as well as the fact that AMXPs are transient sources.

astro-ph.HE↗

arXiv2021

On the torque reversals of 4U 1626--67

We have investigated the detailed torque-reversal behavior of 4U 1626--67 in the framework of the recently developed comprehensive model of the inner disk radius and torque calculations for neutron stars accreting from geometrically thin disks. The model can reproduce the torque -- X-ray luminosity relation across the torque reversals of 4U 1626--67. Our results imply that: (1) rotational equilibrium is reached when the inner disk radius equals the co-rotation radius, $r_\mathrm{co}$, while the conventional Alfven radius is greater than and close to $r_\mathrm{co}$, (2) both spin-up and spin-down torques are operating on either side of torque reversal, (3) with increasing accretion rate the spin-up torque associated with accretion onto the star gradually dominates the spin-down torque exerted by the disk. The torque reversals are the natural outcome of transitions between the well-defined weak-propeller and spin-up phases of the star with a stable geometrically thin accretion disk.

astro-ph.HE↗

arXiv2020

Magnetorotational Instability in Diamagnetic, Misaligned Protostellar Discs

In the present study, we addressed the question of how the growth rate of the magnetorotational instability is modified when the radial component of the stellar dipole magnetic field is taken into account in addition to the vertical component. Considering a fiducial radius in the disc where diamagnetic currents are pronounced, we carried out a linear stability analysis to obtain the growth rates of the magnetorotational instability for various parameters such as the ratio of the radial-to-vertical component and the gradient of the magnetic field, the Alfvenic Mach number and the diamagnetization parameter. Our results show that the interaction between the diamagnetic current and the radial component of the magnetic field increases the growth rate of the magnetorotational instability and generates a force perpendicular to the disc plane which may induce a torque. It is also shown that considering the radial component of the magnetic field and taking into account a radial gradient in the vertical component of the magnetic field causes an increase in the magnitudes of the growth rates of both the axisymmetric ($m=0$) and the non-axisymmetric ($m=1$) modes.

astro-ph.SR↗