arXiv subjects

Unal Ertan

Publications and source records attributed to Unal Ertan.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of Calvera and Descendants of Calvera-like Central Compact Objects

Calvera (1RXS J141256.0+792204) is an isolated neutron star recently classified as a central compact object (CCO) after its association with the supernova remnant (SNR) candidate G118.4+37.0. In this work, we investigate the long-term evolution and descendants of Calvera and the CCOs with similar initial conditions in the fallback disc model. We show that the observed spin period, period derivative, and X-ray luminosity of Calvera can be reproduced simultaneously with a magnetic field strength of $\simeq 4 \times 10^{10}$ G at the pole of the neutron star at an age of $\sim 10^4$ yr which is consistent with the estimated SNR age of the source. In the model, the lack of ordinary radio pulsations is due to ongoing mass accretion on to the star. From our simulations, we estimate that the source will become an ordinary radio pulsar (RP) after the termination of the accretion. The source will spin down under the weak dipole torque alone as an RP for a very long time ($\gtrsim 10^8$ yr) after the inactivation of the disc. From our simulations, we find that most of the CCOs with initial conditions similar to those of Calvera also become RPs which remain above or close to the upper border of the pulsar death valley after the inactivation of their discs. In the period-period derivative diagram, there is indeed a cluster of RPs in the region where the descendants of Calvera-like CCOs are estimated to be located in our model.

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arXiv2026

The Torques Acting on Accreting Millisecond X-Ray Pulsars in the Outburst and Quiescent States, and During the Long-Term Evolution

Accreting millisecond X-ray pulsars (AMXPs) are transient X-ray sources likely to be in the final evolutionary phases of low-mass X-ray binaries (LMXBs). In this work, we have investigated the torque and X-ray luminosity variations of five AMXPs during outburst and quiescence, using a model previously employed to explain some typical behaviours of LMXBs. Most of these systems spin up in the outburst state and spin down in quiescence, while they slow down on the long term. We have obtained reasonable results with the model in agreement with these observations. We have found that the torques produced by the disc-magnetosphere interaction, the magnetic dipole radiation, and the mass accretion are compatible with the X-ray luminosity and rotational properties of the sources in their outburst and quiescent states, and during the resultant long-term evolution. Nevertheless, our results are not sufficient to rule out the spin-down contribution of the gravitational radiation torques due to significant timing noise and uncertainties about the bolometric corrections for X-ray luminosity during the outbursts of some sources.

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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.

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arXiv2024

Long-term Evolutionary Links Between the Isolated Neutron Star Populations

We have investigated the evolutionary connections of the isolated neutron star (NS) populations including radio pulsars (RPs), anomalous X-ray pulsars (AXPs), soft gamma repeaters (SGRs), dim isolated NSs (XDINs), ``high-magnetic-field'' RPs (``HBRPs''), central compact objects (CCOs), rotating radio transients (RRATs), and long-period pulsars (LPPs) in the fallback disc model. The model can reproduce these NS families as a natural outcome of different initial conditions (initial period, disc mass, and dipole moment, $μ$) with a continuous $μ$ distribution in the $\sim 10^{27} - 5 \times 10^{30}$ G cm$^3$ range. Results of our simulations can be summarised as follows: (1) A fraction of ``HBRPs'' with relatively high $μ$ evolve into the persistent AXP/SGR properties, and subsequently become LPPs. (2) Persistent AXP/SGRs do not have evolutionary links with CCOs, XDINs, and RRATs. (3) For a wide range of $μ$, most RRATs evolve passing through RP or ``HBRP'' properties during their early evolutionary phases. (4) A fraction of RRATs which have the highest estimated birth rate seem to be the progenitors of XDINs. (5) LPPs, whose existence was predicted by the fallback disc model, are the sources evolving in the late stage of evolution before the discs become inactive. These results provide concrete support to the ideas proposing evolutionary connections between the NS families to account for the ``birth-rate problem'', the discrepancy between the cumulative birth rate estimated for these systems and the core-collapse supernova rate.

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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.

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arXiv2023

Typical X-ray Outburst Light Curves of Aql X-1

We show that a typical X-ray outburst light curve of Aql X-1 can be reproduced by accretion onto the neutron star in the frame of the disk instability model without invoking partial accretion or propeller effect. The knee and the subsequent sharp decay in the X-ray light curve can be generated naturally by taking into account the weak dependence of the disk aspect ratio, $h/r$, on the disk mass-flow rate, $\dot{M}_\mathrm{in}$, in the X-ray irradiation flux calculation. This $\dot{M}_\mathrm{in}$ dependence of $h/r$ only slightly modifies the irradiation temperature profile along the hot disk in comparison to that obtained with constant $h/r$. Nevertheless, this small difference has a significant cumulative effect on the hot disk radius leading to a much faster decrease in the size of the hot disk, and thereby to a sharper decay in the X-ray outburst light curve. The same model also produces the long-term evolution of the source consistently with its observed outburst recurrence times and typical light curves of Aql X-1. Our results imply that the source accretes matter from the disk in the quiescent state as well. We also estimate that the dipole moment of the source $μ\lesssim 4 \times 10^{26}$ G cm$^3~$($B \lesssim 4 \times 10^{8}$ G at the surface).

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arXiv2022

Evolution of the long-period pulsar PSR J0901-4046

The fallback disc model predicted that anomalous X-ray pulsars (AXPs) and soft-gamma repeaters (SGRs) will evolve to isolated long period pulsars before the discovery of the first two long-period pulsars (LPPs) this year. Unlike normal radio pulsars, LPPs show transient pulsed-radio epochs with unusual and variable pulse shapes, similar to the radio behaviour of the few radio emitting AXP/SGRs. We show that the present properties of the recently discovered second LPP, PSR J0901-4046 ($P \simeq 76$ s), are obtained as a result of evolution in interaction with a fallback disc, as we had already shown for the first discovered LPP, GLEAM-X J162759.5-523504.3 ($P \simeq 1091$ s). While there is only an upper limit to the period derivative, $\dot{P}$, of GLEAM-X J162759.5-523504.3, the $\dot{P}$ of the PSR J0901-4046 has already been measured, providing better constraints for the evolutionary models. The model can produce the source properties with a dipole moment $μ\simeq 10^{30}$ G cm$^3$. The results are not sensitive to the initial pulsar period. Our results indicate that PSR J0901-4046 went through an AXP/SGR epoch at an age of a few $10^4$ yr, and is $\sim (6 - 8) \times 10^5$ yr old at present.

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arXiv2022

Evolution of the long-period pulsar GLEAM-X J162759.5-523504.3

The long-period ($P = 1091$ s) of the recently discovered pulsar GLEAM-X J162759.5-523504.3 can be attained by neutron stars evolving with fallback discs and magnetic dipole moments of a few $10^{30}$ G cm$^3$ at ages greater than $\sim 2 \times 10^5$ yr consistently with the observational upper limits to the period derivative, $\dot{P}$, and the X-ray luminosity, $L_X$, of the source. The current upper limits for $\dot{P}$ allow two alternative present states: (1) The disc is still active with ongoing accretion at a low rate such that the accretion luminosity is much less than the neutron star's cooling luminosity, which in turn is below the upper limit for $L_X$. In this scenario the spin-down will continue at $\dot{P} \sim 10^{-10}$ s s$^{-1}$ until the disc becomes inactive; the final period will be $P \sim$ a few $10^3$ s. (2) The disc is already inactive, there is no accretion. In this case the period evolution has leveled off to the observed value in the final period range. The remaining, very weak, dipole torque sustaining asymptotic spin-down at $\dot{P} \sim 4 \times 10^{-18}$ s s$^{-1}$. Long periods $P \sim$ a few $10^3$ s were predicted for the final states of soft gamma repeaters and anomalous X-ray pulsars with relatively strong dipole fields in earlier work with the fallback disc model.

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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.

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arXiv2021

The minimum rotation period of millisecond pulsars

A simple and natural explanation for the minimum period of millisecond pulsars follows from a correlation between the accretion rate and the frozen surface dipole magnetic field resulting from Ohmic diffusion through the neutron star crust in initial stages of accretion in low mass X-ray binaries.

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arXiv2020

On the Torque Reversals of Accreting Neutron Stars

We have extended the analytical model proposed earlier to estimate the inner disk radius of accreting neutron stars in the strong-propeller (SP) phase, and the conditions for the transitions between the strong and weak propeller (WP) phases (Ertan 2017, 2018) to the WP (accretion with spin-down) and the spin-up (SU) phases, and the torque reversals during the WP/SU transitions. The model can account for some basic observed properties of these systems that are not expected in conventional models: (1) accretion on to the star at low X-ray luminosities and the transitions to the SP phase (no accretion) at critical accretion rates much lower than the rate required for the spin-up/spin-down transition, (2) ongoing accretion throughout a large range of accretion rates while the source is spinning down (WP phase), and (3) transitions between the spin-up and spin-down phases with comparable torque magnitudes, without substantial changes in the mass-flow rate. Our results indicate that the magnitudes of the torques on either side of the torque reversal have a ratio similar for different systems independently of their spin periods, magnetic dipole moments and accretion rates during the transitions. Estimated torque reversal properties in our model are in agreement with the observed torque reversals of 4U 1626--67.

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arXiv2020

On the long-term evolution of rotating radio transients

Investigation of the long-term evolution of rotating radio transients (RRATs) is important to understand the evolutionary connections between the isolated neutron star populations in a single picture. The X-ray luminosities of RRATs (except one source) are not known. In the fallback disc model, we have developed a method to estimate the dipole field strengths of RRATs without X-ray information. We have found that RRATs could have dipole field strengths, $B_0$, at the poles ranging from $\sim 7 \times 10^9$ G to $\sim 6 \times 10^{11}$ G which fill the gap between the $B_0$ ranges of central compact objects (CCOs) and dim isolated neutron stars (XDINs) estimated in the same model. In our model, most of RRATs are evolving at ages $(\sim 2 - 6) \times 10^5$ yr, much smaller than their characteristic ages, such that, cooling luminosities of a large fraction of relatively nearby RRATs could be detected by the eROSITA all-sky survey. Many RRATs are located above the upper border of the pulsar death valley with the fields inferred from the dipole-torque formula, while they do not show strong, continuous radio pulses. The $B_0$ values estimated in our model, place all RRATs either into the death valley or below the death line. We have tentatively proposed that RRATs could be the sources below their individual death points, and their short radio bursts could be ignited by the disc-field interaction occasionally enhancing the flux of open field lines through the magnetic poles. We have also discussed the evolutionary links between CCOs, RRATs and XDINs.

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arXiv2020

Is PSR J0726-2612 a dim isolated neutron star progenitor?

The rotational properties and X-ray luminosity of PSR J0726-2612 are close to those of dim isolated neutron stars (XDINs). It was proposed that the source could be the first XDIN with observable pulsed radio emission. We have investigated the long-term evolution of the source to test this possibility in the fallback disc model. Reasonable model curves that can account for the evolution of PSR J0726-2612 consistently with its radio pulsar property are similar to those of high-B radio pulsars with dipole field strength $B_0 \sim$ a few $\times 10^{12}~$G at the pole of the star. In the same model, XDINs are estimated to have relatively weak fields ($B_0 \lesssim 10^{12}~$G) locating them well below the pulsar death line. From the simulations, we estimate that PSR J0726-2612 is at an age of $t \sim 5 \times 10^4~$yr, and will achieve the rotational properties of a normal radio pulsar within $\sim 10^5~$yr, rather than the XDIN properties.

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arXiv2018

Long-term evolution of RRAT J1819-1458

At present, J1819-1458 is the only rotating radio transient (RRAT) detected in X-rays. We have studied the long-term evolution of this source in the fallback disc model. The model can reproduce the period, period derivative and X-ray luminosity of J1819-1458 simultaneously in the accretion phase at ages $\sim 2 \times 10^5$ yr. We obtained reasonable model curves with a magnetic dipole field strength $B_0 \sim 5 \times 10^{11}$ G on the pole of the neutron star, which is much weaker than the field inferred from the dipole-torque formula. With this $B_0$ and the measured period, we find J1819-1458 below and close to the radio pulsar death line. Our results are not sensitive to initial period, and the source properties can be produced with a large range of disc masses. Our simulations indicate that J1819-1458 is evolving towards the properties of dim isolated neutron stars at later phases of evolution. This implies a close evolutionary link between RRATs and dim isolated neutron stars. For other RRATs with measured period derivatives and unknown X-ray luminosities, we have estimated the lower limits on the $B_0$ values in the fallback disc model. These limits allow a dipole field distribution for RRATs that could fill the $B_0$ gap between the estimated $B_0$ ranges of dim thermal isolated neutron stars and central compact objects in the same model.

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arXiv2018

Accretion And Propeller Torque In The Spin-down Phase Of Neutron Stars: The Case Of Transitional Millisecond Pulsar PSR J1023+0038

The spin-down rate of PSR J1023+0038, one of the three confirmed transitional millisecond pulsars, was measured in both radio pulsar (RMSP) and X-ray pulsar (LMXB) states. The spin-down rate in the LMXB state is only about 27 % greater than in the RMSP state (Jaodand et al. 2016). The inner disk radius, r_in, obtained recently by Ertan (2017) for the propeller phase, which is close to the co-rotation radius, r_co, and insensitive to the mass-flow rate, can explain the observed torques together with the X-ray luminosities, Lx . The X-ray pulsar and radio pulsar states correspond to accretion with spin-down (weak propeller) and strong propeller situations respectively. Several times increase in the disk mass-flow rate takes the source from the strong propeller with a low Lx to the weak propeller with a higher Lx powered by accretion on to the star. The resultant decrease in r_in increases the magnetic torque slightly, explaining the observed small increase in the spin-down rate. We have found that the spin-up torque exerted by accreting material is much smaller than the magnetic spin-down torque exerted by the disk in the LMXB state.

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arXiv2018

Central Compact Objects: some of them could be spinning up?

Among confirmed central compact objects (CCOs), only three sources have measured period and period derivatives. We have investigated possible evolutionary paths of these three CCOs in the fallback disc model. The model can account for the individual X-ray luminosities and rotational properties of the sources consistently with their estimated supernova ages. For these sources, reasonable model curves can be obtained with dipole field strengths $\sim$ a few $\times 10^9$ G on the surface of the star. The model curves indicate that these CCOs were in the spin-up state in the early phase of evolution. The spin-down starts, while accretion is going on, at a time $t \sim 10^3 - 10^4$ yr depending on the current accretion rate, period and the magnetic dipole moment of the star. This implies that some of the CCOs with relatively long periods, weak dipole fields and high X-ray luminosities could be strong candidates to show spin-up behavior if they indeed evolve with fallback discs.

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arXiv2017

Rotational and X-ray luminosity evolution of high-B radio pulsars

In continuation of our earlier work on the long-term evolution of the so-called high--B radio pulsars (HBRPs) with measured braking indices, we have investigated the long-term evolution of the remaining five HBRPs for which braking indices have not been measured yet. This completes our source-by-source analyses of HBRPs in the fallback disc model that was also applied earlier to anomalous X-ray pulsars (AXPs), soft gamma repeaters (SGRs), and dim isolated neutron stars (XDINs). Our results show that the X-ray luminosities and the rotational properties of these rather different neutron star populations can be acquired by neutron stars with fallback discs as a result of differences in their initial conditions, namely the initial disc mass, initial period and the dipole field strength. For the five HBRPs, unlike for AXPs, SGRs and XDINs, our results do not constrain the dipole field strengths of the sources. We obtain evolutionary paths leading to the properties of HBRPs in the propeller phase with dipole fields sufficiently strong to produce pulsed radio emission.

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arXiv2017

On the evolution of high-B radio pulsars with measured braking indices

We have investigated the long-term evolutions of the high-magnetic field radio pulsars (HBRPs) with measured braking indices in the same model that was applied earlier to individual anomalous X-ray pulsars (AXPs), soft gamma repeaters (SGRs) and dim isolated neutron stars (XDINs). We have shown that the rotational properties (period, period derivative and braking index) and the X-ray luminosity of individual HBRPs can be acquired simultaneously by the neutron stars evolving with fallback discs. The model sources reach the observed properties of HBRPs in the propeller phases, when pulsed radio emission is allowed, at ages consistent with the estimated ages of the supernova remnants of the sources. Our results indicate that the strength of magnetic dipole fields of HBRPs are comparable to and even greater than those of AXP/SGRs and XDINs, but still one or two orders of magnitude smaller than the values inferred from the magnetic dipole torque formula. The possible evolutionary paths of the sources imply that they will lose their seemingly HBRP property after about a few 10^4 yr, because either their rapidly decreasing period derivatives will lead them into the normal radio pulsar population, or they will evolve into the accretion phase switching off the radio pulses.

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