arXiv subjects

Kazım Yavuz Ekşi

Publications and source records attributed to Kazım Yavuz Ekşi.

10 recordsLinked to original sources

arXiv2026

Vortex-reconnection energy bounds in Bose-Einstein-condensed and superfluid dark matter halos

Bose-Einstein-condensed (BEC) and superfluid dark-matter (SFDM) halos can contain coherent, wave-supported cores whose angular momentum is carried by quantized vortices. When vortices form a tangle, reconnections convert part of the vortex kinetic energy into dark phonons, density waves, Kelvin waves, and vortex loops, providing a microscopic channel by which vortex structure can affect halo-core evolution. We estimate the dynamical importance of this channel by combining the local Gross-Pitaevskii (GP) reconnection law with a halo-scale vortex-line density calibrated against Schrödinger-Poisson (SP) simulations. Vortex reconnections cannot appreciably restructure a standard SP halo core under the fiducial assumptions: for a $m=10^{-22}$ eV, $r_c=1$ kpc soliton and a one-percent energy-transfer efficiency per event, an unforced network transfers at most $1.3\%$ of the core virial energy in 10 Gyr. Reconnection converts ordered vortex energy into sound and smaller-scale excitations; we bound the cumulative transfer using a spectral scale measured in published SP halo simulations. To connect the result with measured systems, we match the standard soliton profile to half-light masses derived with a single mass estimator for eight classical Milky Way dwarf spheroidals. Assigning the measured aperture mass to the soliton gives ceilings of $0.98-4.52\%$ at the Mocz line-density reference and $0.061-3.19\%$ for a modest-rotation normalization. The largest value is a factor of 22 below the core virial-energy scale and limits the fixed-structure velocity change to $2.3\%$. The spectral reference represents fewer than one projected vortex crossing, so these are continuum upper limits; a vortex-free core has no reconnection heating. Reconnections remain worth testing because they are unavoidable when a tangle exists.

astro-ph.GA↗

arXiv2026

On the origin of the strong internal magnetic fields of central compact objects

Central compact objects are radio-quite young neutron stars associated with supernova remnants. They have relatively small dipole fields, $B_{\rm p} \sim 10^{10}\,{\rm G}$ as inferred from their spin parameters. X-ray observations and theoretical arguments imply the presence of stronger internal magnetic fields. We argue that the dipole fields of these objects are very close to what they had inherited from the \textit{core} of the progenitor by flux conservation and their small initial rotation frequency does not allow for the $α$-process to enhance their poloidal fields. Although a full-fledged dynamo process can not proceed, relatively strong toroidal magnetic fields, $B_ϕ\sim 10^{13}\,{\rm G}$, can be generated from the seed poloidal fields via the $Ω$-effect in the proto-neutron star stage. We present a simplistic model for these processes and further speculate that the reason why these objects are born relatively slow-rotating is that they were not spun-up by acquiring angular momentum from the fallback matter.

astro-ph.HE↗

arXiv2026

The period clustering of magnetars and X-ray dim isolated neutron stars

The spin periods of magnetars and X-ray dim isolated neutron stars (XDINS) cluster within a remarkably narrow range. Using the current sample of 30 magnetars with measured periods (ranging from 0.33 to 11.78 s) and 8 XDINS (ranging from 3.45 to 12.76 s), we utilize the point-likelihood technique to constrain the birth and final periods of these sources, assuming a steady-state population. Employing a general braking law characterized by a constant braking index $n$, we find that for $n > 2$ the final (cut-off) period of magnetars is constrained to $P_f \simeq 11.8 - 12.0$ s and XDINS to $P_f \simeq 12.8 - 14.9$ s, at the 95 per cent confidence level, while the birth periods remains largely unconstrained for dipole spin-down ($n=3$) as in earlier work. The slight increase in the upper cutoff from $\sim$12 to $\sim$15 s over two decades of discoveries of new sources, yielding a threefold increase in the known magnetar population, and the extension of the minimum period to $\sim 0.33$ s strongly support a physical origin for this clustering. We discuss this result in the context of magnetic-field-decay models and fallback-disc torque-equilibrium scenarios. The combined magnetar and XDINS sample (38 sources) yields the tightest constraints on $P_f\simeq 12.8-12.9$ s, for $n=3$, suggesting possible evolutionary connections between these populations and pointing toward a common physical mechanism that terminates the observable phase of these neutron stars at periods near 14 s.

astro-ph.HE↗

arXiv2026

Evolution of proto-neutron stars to pulsars, magnetars and central compact objects

Some young neutron stars, the magnetars, have ultra-strong magnetic fields, yet their inferred birth rate is comparable to the core-collapse supernova rate, challenging scenarios that require rare, extreme conditions. We propose that this discrepancy can be reconciled if both pulsars and magnetars pass through a dynamo process during the proto-neutron star (PNS) phase. We employ a shear-driven $α$--$Ω$ dynamo model that includes PNS contraction. The dynamo generically produces toroidal-dominated fields set mainly by the $Ω$-effect. The evolution of the poloidal field is first dominated by flux conservation during collapse and then by the $α$-effect. The saturated toroidal field depends strongly on the initial value of the shear, with a threshold at $q_0 \simeq 0.23$; below this, the poloidal field remains near the value obtained by the flux-conservation ($\approx 2.5\times10^{10}\,{\rm G}$). For the shortest initial periods, the model leads to magnetar-like strengths ($B_{\rm p} \simeq 10^{15}\,{\rm G}$, $B_ϕ\simeq 10^{16}\,{\rm G}$), while for the slower rotators it yields ordinary pulsar fields ($B_{\rm p} \simeq 10^{12}\,{\rm G}$, $B_ϕ\simeq 10^{14}\,{\rm G}$). We also argue that the central compact objects can acquire toroidal fields amplified solely by the $Ω$-effect; lacking the $α$-effect, their poloidal fields are not shaped by the dynamo effect.

astro-ph.HE↗

arXiv2020

Stimulated-emission based model of fast radio bursts

Fast radio bursts (FRBs) are bright, short-duration radio transients with very high brightness temperatures implying highly coherent emission. We suggest that the FRBs are caused by the self-focusing of an electron beam interacting with an ambient plasma right beyond the light cylinder radius of a neutron star. The magnetic field at the light cylinder radius is relatively high which can accommodate both young Crab-like systems and old millisecond pulsars addressing the diverse environments of FRBs. At the first stage, the intense pulsed-beam passing through the background plasma causes instabilities such that the trapped particles in local Buneman-type cavitons saturate the local field. The beam is then radially self-focused due to the circular electric field developed by the two-stream instability which leads to Weibel instability in the transverse direction. Finally, the non-linear saturation of the Weibel instability results in the self-modulational formation of solitons due to plasmoid instability. The resonant solitary waves are the breather-type solitons hosting relativistic particles with self-excited oscillations. The analytical solutions obtained for non-linear dispersion and solitons suggest that, near the current sheets, the relativistic bunches are accelerated/amplified by klystron-like structures due to self-excited oscillations by the induced local electric field. Boosted coherent radio emission propagates through a narrow cone with strong focusing due to radial electric field and magnetic pinching. The non-linear evolution of solitons and the stimulated emission are associated with the Buneman instability and the possibility of the presence of nanosecond shots in FRBs are investigated.

astro-ph.HE↗

arXiv2019

Ultra-luminous X-ray sources as super-critical propellers

We study the evolution of newborn neutron stars in high-mass X-ray binaries interacting with a wind-fed super-Eddington disk. The inner disk is regularized to a radiation-dominated quasi-spherical configuration for which we calculate the inner radius of the disk, the total luminosity of the system and the torque acting on the neutron star accordingly, following the evolution of the system through the ejector and early propeller stages. We find that the systems with $B \gtrsim 10^{13}$ G pass through a short ($\sim 20\,{\rm yr}$) ejector stage appearing as supernova impostors followed by a propeller stage lasting $\sim 10^3\,{\rm yr}$. In the super-critical propeller stage the system is still bright ($L\sim 10^{40}\,{\rm erg\, s^{-1}}$) due to the spindown power and therefore appears as an ultra-luminous X-ray source (ULX). The system evolves into pulsating ULX (PULX) when the neutron star spins down to a period ($P\sim 1$ s) allowing for accretion onto its surface to commence. Systems with lower magnetic fields, $B \sim 10^{11}$ G, pass through a long ($10^5\,{\rm yr}$) super-critical propeller stage with luminosities similar to those of the ultra-luminous super-soft sources (ULS), $L \lesssim 10^{40}\,{\rm erg\, s^{-1}}$. The equilibrium periods of these systems in the accretion stage is about $10\,{\rm ms}$, which is much smaller than the typical period range of PULX observed to date. Such systems could have a larger population, but their pulsations would be elusive due to the smaller size of the magnetosphere. Our results suggest that the ULS and some nonpulsating ULX are rapidly spinning and highly magnetized young neutron stars at the super-critical propeller stage.

astro-ph.HE↗

arXiv2017

Partial accretion in the propeller stage of low mass X-ray binary Aql X--1

Aql X--1 is one of the most prolific low mass X-ray binary transients (LMXBTs) showing outbursts almost annually. We present the results of our spectral analyses of RXTE/PCA observations of the 2000 and the 2011 outbursts. We investigate the spectral changes related to the changing disk-magnetosphere interaction modes of Aql X--1. The X-ray light curves of the outbursts of LMXBTs typically show phases of fast rise and exponential decay. The decay phase shows a "knee" where the flux goes from the slow decay to the rapid decay stage. We assume that the rapid decay corresponds to a weak propeller stage at which a fraction of the inflowing matter in the disk accretes onto the star. We introduce a novel method for inferring, from the light curve, the fraction of the inflowing matter in the disk that accretes onto the NS depending on the fastness parameter. We determine the fastness parameter range within which the transition from the accretion to the partial propeller stage is realized. This fastness parameter range is a measure of the scale-height of the disk in units of the inner disk radius. We applied the method to a sample of outbursts of Aql X--1 with different maximum flux and duration times. We show that different outbursts with different maximum luminosity and duration follow a similar path in the parameter space of accreted/inflowing mass flux fraction versus fastness parameter.

astro-ph.HE↗

arXiv2017

On the morphology of outbursts of accreting millisecond X-ray pulsar Aquila X-1

We present the X-ray light curves of the last two outbursts --2014 & 2016-- of the well known accreting millisecond X-ray pulsar (AMXP) Aquila X-1 using the monitor of all sky X-ray image (MAXI) observations in the $2-20$ keV band. After calibrating the (MAXI) count rates to the all-sky monitor (ASM) level, we report that the 2016 outburst is the most energetic event of Aql X-1, ever observed from this source. We show that 2016 outburst is a member of the long-high class according to the classification presented by Güngör et al. with $\sim68$ cnt/s maximum flux and $\sim60$ days duration time and the previous outburst, 2014, belongs to the short-low class with $\sim 25$ cnt/s maximum flux and $\sim 30$ days duration time. In order to understand differences between outbursts, we investigate the possible dependence of the peak intensity to the quiescent duration leading to the outburst and find that the outbursts following longer quiescent episodes tend to reach higher peak energetic.

astro-ph.HE↗

arXiv2017

The maximum dipole moments of neutron stars implied by the Barrow-Gibbons conjecture

The maximum magnetic moment to angular momentum conjecture, recently posed by Barrow \& Gibbons, is in tension with the generally accepted parameters of magnetars. According to the conjecture the dimensionless Schuster-Wilson-Blacket number, $cμ/J\sqrt{G}$ where $μ$ is the magnetic moment and $J$ is the angular momentum, should be of order unity. A resolution is to assume that not only the low-magnetic field magnetars, but all magnetars have their super-critical fields in the higher multipoles. This requires the presence of external torques other than the magnetic dipole torque, such as from fallback disks or winds, on these systems to yield the measured high spin-down rates.

astro-ph.HE↗

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↗