arXiv subjects

Kadri Yakut

Publications and source records attributed to Kadri Yakut.

13 recordsLinked to original sources

arXiv2026

Multipolar Neutrino Radiation in Binary Neutron Star Mergers: Angular Structure, Rotational Variability, and Implications for Electron Fraction

The angular structure and temporal variability of neutrino emission from binary neutron star mergers are characterized using fully general-relativistic simulations with energy-integrated M1 neutrino transport across a representative set of equations of state, total masses, and mass ratios. The angle-dependent neutrino energy flux is extracted on a spherical surface outside the remnant and decomposed into spherical harmonics to quantify its multipolar content and evolution. Following the initial post-merger transient, the neutrino radiation flux approaches an axisymmetric configuration dominated by a strong quadrupolar component, producing persistent polar flux enhancement and equatorial suppression due to torus shadowing. The dipolar contribution remains subdominant, indicating the absence of sustained one-sided emission. The degree of anisotropy increases with mass asymmetry and for softer equations of state, reflecting the compactness and morphology of the remnant--disk system. Superimposed on this time-averaged geometry, coherent azimuthal modulations associated with the $m=1$ mode are identified. Fourier analysis reveals a characteristic frequency of $\sim 0.6$--$0.7$ kHz, consistent with differential rotation in the remnant and inner disk layers, indicating a dynamical coupling between rotational structure and neutrino emission variability. Finally, we quantify how the same quadrupole-dominated radiation geometry induces a latitude-dependent equilibrium electron fraction. The polar material is driven close to the neutrino-equilibrium target, whereas equatorial material remains systematically more neutron-rich and below equilibrium.

astro-ph.HE↗

arXiv2026

A Binary-Based Reassessment of the Age and Stellar Properties of NGC 7789 Using Twelve Binary Components

We present a binary-based reassessment of the age of the intermediate-age open cluster NGC 7789, together with well-constrained stellar parameters for twelve components in six SB2 systems, including two eclipsing binaries. Our analysis employs a unified modelling framework that combines radial-velocity orbits, TESS light curves, and blue-to-IR spectral energy distributions (SEDs), providing a robust alternative to traditional isochrone-based age determinations. By adopting common cluster-wide parameters (age, distance, and line-of-sight extinction) when solving for the stellar parameters of the binary components, we obtain a coherent set of masses, radii, effective temperatures, and luminosities for all twelve stars. The combined SED, eclipsing-binary, and radial-velocity analysis yields a well-constrained cluster age of $1.26 \pm 0.09$ Gyr and an extinction of $A_V = 0.90 \pm 0.05$ mag, while remaining consistent with the Gaia DR3 distance of $d \simeq 2.06$ kpc used as an external prior. An independent Gaia DR3 astrometric analysis gives a distance of $2082 \pm 142$ pc and confirms the membership of all six systems. The twelve binary components occupy the turnoff and subgiant regions of the cluster, enabling stringent evolutionary tests: in the radius--mass, radius--temperature, and temperature--mass diagrams, they show excellent agreement with modern stellar evolution models for the derived cluster parameters. NGC 7789 thus serves as a valuable benchmark for multi-observable, binary-based age determinations in open cluster studies.

astro-ph.SR↗

arXiv2026

ESO Expanding Horizon White Paper: Revealing the properties of matter at supranuclear densities with gravitational waves

Understanding dense matter under extreme conditions is one of the most fundamental puzzles in modern physics. Complex interactions give rise to emergent, collective phenomena. While nuclear experiments and Earth - based colliders provide valuable insights, much of the quantum chromodynamics phase diagram at high density and low temperature remains accessible only through astrophysical observations of neutron stars, neutron star mergers, and stellar collapse. Astronomical observations thus offer a direct window to the physics on subatomic scales with gravitational waves presenting an especially clean channel. Next-generation gravitational - wave observatories, such as the Einstein Telescope, would serve as unparalleled instruments to transform our understanding of neutron star matter. They will enable the detection of up to tens of thousands of binary neutron star and neutron star - black hole mergers per year, a dramatic increase over the few events accessible with current detectors. They will provide an unprecedented precision in probing cold, dense matter during the binary inspiral, exceeding by at least an order of magnitude what current facilities can achieve. Moreover, these observatories will allow us to explore uncharted regimes of dense matter at finite temperatures produced in a subset of neutron star mergers, areas that remain entirely inaccessible to current instruments. Together with multimessenger observations, these measurements will significantly deepen our knowledge of dense nuclear matter.

astro-ph.IM↗

arXiv2025

Binary Black Hole Mergers: Spin and mass ratio effects on gravitational waveforms

We present a comprehensive parameter-space study of binary black hole (BBH) mergers using the SEOBNRv4\_opt waveform model. Our analysis spans $\sim 10^6$ simulated waveforms across a broad range of mass ratios $ q = \frac{m_1}{m_2} \in [1.0, 2.0] $ and aligned spin configurations. We investigate the influence of these parameters on remnant properties, including final spin ($χ_f$), fractional mass loss ($M_{\mathrm{FL}}$), and peak gravitational-wave strain ($h_{\max}$). By systematically analyzing the trends across four distinct spin alignments (PP, PN, BP, BN), we identify non-monotonic behaviors and turning points in $M_{\mathrm{FL}}$ and $χ_f$ as functions of $q$, highlighting subtle dynamical effects that are not explicitly emphasized in commonly used remnant fitting formulae. While confirming known correlations from numerical relativity, our results offer new insights into parameter interactions and waveform morphology, with implications for BBH population studies and remnant characterization. Across all configurations studied, the fractional mass loss due to gravitational-wave emission ranges between 2\% and 9.5\%, depending on the mass ratio and spin alignment. This work may also aid in understanding the spin and mass distributions of the more massive black holes formed post-merger, thereby contributing to future remnant-based astrophysical inference.

astro-ph.HE↗

arXiv2023

General Relativistic Simulations of High-Mass Binary Neutron Star Mergers: rapid formation of low-mass stellar black holes

Almost a hundred compact binary mergers have been detected via gravitational waves by the LIGO-Virgo-KAGRA collaboration in the past few years providing us with a significant amount of new information on black holes and neutron stars. In addition to observations, numerical simulations using newly developed modern codes in the field of gravitational wave physics will guide us to understand the nature of single and binary degenerate systems and highly energetic astrophysical processes. We here presented a set of new fully general relativistic hydrodynamic simulations of high-mass binary neutron star systems using the open-source Einstein Toolkit and LORENE codes. We considered systems with total baryonic masses ranging from 2.8 $M_\odot$ to 4.0 $M_\odot$ and used the SLy equation of state. We analyzed the gravitational wave signal for all models and reported potential indicators of systems undergoing rapid collapse into a black hole that could be observed by future detectors like the Einstein Telescope and the Cosmic Explorer. The properties of the post-merger black hole, the disk and ejecta masses, and their dependence on the binary parameters were also extracted. We also compared our numerical results with recent analytical fits presented in the literature and provided parameter-dependent semi-analytical relations between the total mass and mass ratio of the systems and the resulting black hole masses and spins, merger frequency, BH formation time, ejected mass, disk mass, and radiated gravitational wave energy.

astro-ph.HE↗

arXiv2023

Fundamental Physics Opportunities with the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT), which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable.

astro-ph.HE↗

arXiv2020

Photometric study of selected X-ray binaries

We present results of a long-term photometric multicolor optical monitoring project of selected low-mass and high-mass X-ray binaries carried out at the TÜBİTAK National Observatory (TUG). New long-term $VRI$ multicolor observations of three selected X-ray binaries with neutron star components (HZ Her, ScoX-1, SAX J2103.5+4545) were observed between 2015 and 2019 with the TUG 60-cm telescope. The light variations of the systems are presented and discussed.

astro-ph.SR↗

arXiv2020

Photometric study of close binary stars in the M35, M67, and M71 Galactic clusters

We obtained new multicolour photometry of close binary stars in the young open cluster M35, the solar-age open cluster M67, and the globular cluster M71. New observations have been carried out at the TÜBİTAK National Observatory (TUG) by using the 100cm (T100) telescope. We present observational results for eclipsing binary systems in the selected Galactic clusters. New accurate light curves for 2MASS J19532554 + 1851175, 2MASS J19533427 + 1844047, 2MASS J06092044 + 2415155, and AH Cnc were obtained. The light curves were analysed and we derived some of the orbital parameters of the systems.

astro-ph.SR↗

arXiv2019

Absolute properties of RU Cnc revisited: An active RS CVn-type eclipsing binary with a red giant branch and a main sequence components

We present observations and analysis of an RS CVn-type double-lined eclipsing binary system, RU Cnc. The system has been observed for over a century. The high-quality long-cadence \emph{Kepler} K2 C5 and C18, newly obtained observations, and two radial velocity curves were combined and analyzed simultaneously assuming multi-spot model. The masses, radii and luminosities of the component stars are precisely obtained as $M_\textrm{c} = 1.386\pm0.044\, M_{_\odot}$, $M_\textrm{h} = 1.437 \pm 0.046\, M{_\odot}$, $R_\textrm{h} = 2.39\pm 0.07\, R{_\odot}$, $R_\textrm{c} = 5.02 \pm 0.08\, R{_\odot}$, $L_\textrm{h} = 11.4\pm 1.2\, L{_\odot}$, $L_\textrm{c} = 12.0 \pm 1.0\, L{_\odot}$ and with a separation of $\textrm{a} = 27.914 \pm 0.016\, R{_\odot}$. The distance of the system is determined to be $380\pm 57\,$ pc which is consistent with the Gaia DR2 result. Long-term detailed period variation analysis of the system indicate a period decrease of $7.9\times10^{-7}$ days per year. The results suggest the cooler component to be on the red giant branch (RGB) and the hotter one to be still on the main sequence.

astro-ph.SR↗

arXiv2016

Models for Sixty Double-Lined Binaries containing Giants

The observed masses, radii and temperatures of 60 medium- to long-period binaries, most of which contain a cool, evolved star and a hotter less-evolved one, are compared with theoretical models which include (a) core convective overshooting, (b)mass loss, possibly driven by dynamo action as in RS CVn binaries, and (c) tidal friction, including its effect on orbital period through magnetic braking. A reasonable fit is found in about 42 cases, but in 11 other cases the primaries appear to have lost either more mass or less mass than the models predict, and in 4 others the orbit is predicted to be either more or less circular than observed. Of the remaining 3 systems, two ($γ$ Per and HR 8242) have a markedly `over-evolved' secondary, our explanation being that the primary component is the merged remnant of a former short-period sub-binary in a former triple system. The last system (V695 Cyg) defies any agreement at present. Mention is also made of three other systems (V643 Ori, OW Gem and V453 Cep), which are relevant to our discussion.

astro-ph.SR↗

arXiv2013

The low-mass interacting binary system OO Aql revisited: a new quadruple system

In this study we present photometric and spectroscopic variation analysis and orbital period study of a low-mass interacting system OO Aql. Simultaneous solution of the light and radial velocity curves provide us a determination of new set of stellar physical parameters for the primary and the secondary companion as M$_{1}$ = 1.05(2) M$_{\odot}$, M$_{2}$ = 0.89(2) M$_{\odot}$, R$_{1}$ = 1.38(2) R$_{\odot}$, R$_{2}$ = 1.28(2) R$_{\odot}$, $\log{(L_1/L_{\odot})} = 0.258$ and $\log{(L_2/L_{\odot})} = 0.117$ and the separation of the components were determined a = 3.333(16) R$_{\odot}$. Newly obtained parameters yield the distance of the system as 136(8) pc. Analyses of the mid-eclipse times indicate a period increase of $\frac{P}{\dot{P}}=4\times 10^{7}$ yr that can be interpreted in terms of the mass transfer $\frac{dM}{dt}=5\times 10^{-8}$ M$_{\odot}$/yr from the less massive component to the more massive component. Our new solution confirmed that OO Aql is a multiple system in the form of AB + C + D. We found initial astrophysical parameters for the component of the system and its current age to be 8.6 Gyr using nonconservative stellar evolution model (EV-TWIN code).

astro-ph.SR↗

arXiv2012

The Interacting Early-Type Binary V382 Cyg

We present photometric and spectroscopic data analysis and orbital period study of an early-type interacting binary system V382 Cyg by using all the available data. We made a simultaneous light and radial velocity curve solution. The derived physical parameters of the primary and secondary stellar components are $M_{1}$ = 27.9(5) $M_{\odot}$, $M_{2}$ = 20.8(4) $M_{\odot}$, $R_{1}$ = 9.7(2) $R_{\odot}$, $R_{2}$ = 8.5(2) $R_{\odot}$, $\log{(L_1/L_{\odot})} = 5.152(20)$ and $\log{(L_2/L_{\odot})} = 4.954(19)$ while the separation of the components is {\it a} = 23.4 $R_{\odot}$. Newly obtained parameters yield the distance of the system as 1466(76) pc. Analyses of the mid-eclipse times indicate a period increase of $\frac{dP}{dt}=4.2(1)\times 10^{-7}$ days/yr that can be interpreted in terms of the high mass transfer ($\frac{dM}{dt}=6.1(5)\times 10^{-6}$ $M_{\odot}$/yr) from the less massive component to the more massive component. Finally we modelled the evolution of the components using nonconservative codes and discussed the obtained results. The age of the binary system is estimated as 3.85 Myr.

astro-ph.SR↗

arXiv2008

Variation of the Light and Period of the Magnetic Cataclysmic Variable Am Her

Ground-based long-term optic variability of AM Her, covering the period between 2003-2008, has been conducted to study the features seen in both low and high states of the system. Low-state analysis shows the presence of short-term, low-amplitude light variations of about 0.02-0.03 mag with a mean power time between 16 s and 226 s. Brightness variations on the order of 0.7--2 mag, which could be due to the stellar activity of the component in the system, are also detected. A total of 30 years times of minimum light given in the literature are combined with nine times of minima obtained in this study. We represented the (observed--calculated) diagram by a parabolic curve and also by two broken lines. Under the assumption of a parabolic variation, we estimate an increase in period, dP/dt=7.5(1.2)x10^{-9} days yr^{-1}, with a mass transfer rate of dM/dt = 8(2)x10^{-9} M_{\odot} yr^{-1}, in agreement with the previous findings by a different method.

astro-ph↗