arXiv subjects

Konstantinos N. Gourgouliatos

Publications and source records attributed to Konstantinos N. Gourgouliatos.

At least 19 recordsLinked to original sources

arXiv2026

Centrifugal instability of compressible flows and the hydrodynamic stability of accretion disks

A recent analysis of the centrifugal instability in the case of pressure-supported compressible relativistic rotation, with application to astrophysical jets, yielded a generalisation of the famous Rayleigh criterion for Newtonian flows. According to this criterion, the centrifugal instability is strongly affected by the flow Mach number, and not only in the relativistic fluid dynamics but also in its Newtonian limit. To validate the Newtonian version of this criterion, we performed axisymmetric numerical simulations of non-relativistic transonic rotating flows which are stable according to the original Rayleigh criterion but can be either stable or unstable according to the new one. The results of computer simulations are found to be in perfect agreement with the theory. The hydrodynamic stability of accretion disks is often explained by referring to the original Rayleigh criterion, even if their rotation is highly supersonic. To clarify the matter, we analysed the hydrodynamic stability of flows rotating about central compact object and derived an instability criterion that retains the explicit dependence on the flow Mach number. This criterion turns out to be equivalent to the standard Solberg-Høiland criterion, which does not involve the Mach number. The same applies to the case of pressure-supported rotation, where the role of gravity is played by the centrifugal force.

astro-ph.HE↗

arXiv2026

Magnetic plasmoid explosions in the context of magnetar giant flares and fast radio bursts

Magnetar flares are highly energetic and rare events in which intense X and γ-ray emission is released from strongly magnetised neutron stars. The events are also accompanied by mass ejection from the neutron star. Fast radio bursts are short and intense pulses of coherent radio emission. Their large dispersion measures support an extragalactic origin. While their exact origin still remains elusive, a substantial number of models associates them with strong magnetic field and high-energy relativistic plasma found in the vicinity of magnetars. There is growing evidence that some fast radio bursts are associated with flare-type events from magnetars. We provide a set of configurations describing a relativistic, spherical, magnetic plasmoid explosion. We proceed by solving the equations of relativistic magnetohydrodynamics for a system that expands while maintaining its internal equilibrium. We find the interdependent relation between pressure, mass density, Lorentz factor, and magnetic field that determines the detailed properties of the solutions. A dichotomy of solutions exists that correspond to higher and lower density and thermal pressure compared to the external one. For stronger toroidal magnetic fields, the maximum permitted expansion velocity becomes lower than the weaker toroidal fields. These solution classes can be applied to magnetar giant flares and fast radio bursts. Those that corresponding to over densities and higher pressure can be associated with magnetar flares, and those corresponding to underdensities can be relevant to fast radio bursts that correspond to magnetically dominated events with low mass loading. [Abridged]

astro-ph.HE↗

arXiv2026

Equation of State of Highly Asymmetric Neutron-Star Matter from Liquid Drop Model and Meson Polytropes

We present a unified description of dense matter and neutron-star structure based on simple but physically motivated models. Starting from the thermodynamics of degenerate Fermi gases, we construct an equation of state for cold, catalyzed matter by combining relativistic fermion statistics with the liquid drop model of nuclear binding. The internal stratification of matter in the outer crust is described by $β$-equilibrium, neutron drip and a gradual transition to supranuclear matter. Short-range repulsive interactions inspired by Quantum Hadrodynamics are incorporated at high densities in order to ensure stability and causality. The resulting equation of state is used as input to the Tolman--Oppenheimer--Volkoff equations, yielding self-consistent neutron-star models. We compute macroscopic stellar properties including the mass-radius relation, compactness and surface redshift that can be compared with recent observational data. Despite the simplicity of the underlying microphysics, the model produces neutron-star masses and radii compatible with current observational constraints from X-ray timing and gravitational-wave measurements. This work demonstrates that physically transparent models can already capture the essential features of neutron-star structure and provide valuable insight into the connection between dense-matter physics and astrophysical observables while they can also be used as easy to handle models to test the impact of more complicated phenomena and variations in neutron stars.

nucl-th↗

arXiv2025

Probing fundamental physics using compact astrophysical objects

It is well known that alternative theories to the Standard Model allow -- and sometimes require -- fundamental constants, such as the fine-structure constant, $α$, to vary in spacetime. We demonstrate that one way to investigate these variations is through the Mass-Radius relation of compact astrophysical objects, which is inherently affected by $α$ variations. We start by considering the model of a polytropic white dwarf, which we perturb by adding the $α$ variations for a generic class of Grand Unified Theories. We then extend our analysis to neutron stars, building upon the polytropic approach to consider more realistic equations of state, discussing the impact of such variations on mass-radius measurements in neutron stars. We present some constraints on these models based on current data and also outline how future observations might distinguish between extensions of the Standard Model.

hep-ph↗

arXiv2025

Logarithmic Separable Solutions of Force-Free Magnetic Fields in Plane-Parallel and Axial Symmetry

This work introduces a systematic method for identifying analytical and semi-analytical solutions of force-free magnetic fields with plane-parallel and axial symmetry. The method of separation of variables is used, allowing the transformation of the non-linear partial differential equation, corresponding to force-free magnetic fields, to a system of decoupled ordinary differential equations, which nevertheless, are in general non-linear. It is then shown that such solutions are feasible for configurations where the electric current has a logarithmic dependence to the magnetic field flux. The properties of the magnetic fields are studied for a variety of physical parameters, through solution of the systems of the ordinary differential equations for various values of the parameters. It is demonstrated that this new logarithmic family of solutions has properties that are highly distinct from the known linear and non-linear equations, as it allows for bounded solutions of magnetic fields, for periodic solutions and for solutions that extend to infinity. Possible applications to astrophysical fields and plasmas are discussed as well as their use in numerical studies, and the overall enrichment of our understanding of force-free configurations.

astro-ph.HE↗

arXiv2024

Revisiting thermoelectric effects in the crust of neutron stars

Context. Large thermal variations have been observed in neutron stars that typically are not aligned with density gradients. Such terms may activate the Biermann battery effect, leading to thermoelectric interactions and to the generation of electromotive force. Aims. We aim to identify the possible impact of a temperature anisotropy on the crust of a neutron star can have in the evolution of its magnetic field, through the thermoelectric terms. Methods. We consider a neutron star crust with large temperature gradients, associated with long-lived hot spots, described by a localized Gaussian-type function. We simulate the interplay between the battery term and the Hall and Ohmic evolution numerically for axisymmetric systems. Results. The results indicate that for crust temperatures of $\sim$$10^9$ K the toroidal field can be amplified up to $\sim$$10^{14}-10^{15}$ G near the points of maximum temperature gradients, and it locally changes the architecture of the poloidal field lines. For internal crustal temperatures of $\sim$$10^8$ K, the temperature gradient generates fields that are lower by about two orders of magnitude. In these cases, saturation is achieved after some hundred thousand years, after which the battery and Ohmic dissipation balance each other, whereas the Hall drift contributes comparatively little to the final field strength, but it can affect its structure. Conclusions. We conclude that the thermoelectric effect can impact the overall magnetic field evolution, provided that the thermal gradient is maintained for a sufficiently long time. Neutron stars endowned with moderate-strength magnetic fields may be affected by the thermoelectric effect if the hotspots survive for timescales of a few kiloyears.

astro-ph.HE↗

arXiv2024

A new solution of the pulsar equation

We present the first new type of solution of the pulsar equation since 1999. In it, the whole magnetosphere is confined inside the light cylinder and an electrically charged layer wraps around it and holds it together. The reason this new solution has never been obtained before is that all current time-dependent simulations are initialized with a vacuum dipole configuration that extends to infinity, thus their final steady-state solution also extends to infinity. Under special conditions, such a confined configuration may be attained when the neutron star first forms in the interior of a collapsing star during a supernova explosion, or when it accretes from an external wind or disk from a donor star. It is shown that this new maximally closed non-decelerating solution is the limit of a continuous sequence of standard magnetospheres with open and closed field lines when the amount of open field lines gradually drops to zero. The minimum energy solution in this sequence is a standard magnetosphere in which the closed field line region extends up to about 80% of the light cylinder. We estimate that the released energy when the new solution transitions to the minimum energy one is enough to power a fast radio burst.

astro-ph.HE↗

arXiv2024

Combined magnetic field evolution in neutron star cores and crusts: Ambipolar diffusion, Hall effect and Ohmic dissipation

Neutron star magnetic field evolution is mediated through the Hall effect and Ohmic dissipation in the crust while ambipolar diffusion is taking place in the core. These effects have been studied in detail in either part of the star, however, their combined, simultaneous evolution and interplay has not been explored in detail yet. Here, we present simulation results of the simultaneous evolution of the magnetic field in the core due to ambipolar diffusion and the crust due to Hall effect and Ohmic decay, under the assumption of axial symmetry. We find that a purely poloidal field generates a toroidal field in the crust, due to the Hall effect, that sinks into the core. A purely toroidal field remains toroidal and spreads into the core and the crust. Finally, for a mixed poloidal-toroidal field, the north-south symmetry is broken due to the Hall effect in the crust, however, ambipolar diffusion, tends to restore it. We examine the role of ambipolar diffusion to the magnetic field decay and we compare the rate of the conversion of magnetic field energy into heat, finding that it enhances the magnetic field decay in neutron stars.

astro-ph.HE↗

arXiv2023

The Laser Interferometer Space Antenna mission in Greece White Paper

The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is a gravitational wave observatory in space designed to detect sources emitting in the millihertz band. LISA is an ESA flagship mission, currently entering the Phase B development phase. It is expected to help us improve our understanding about our Universe by measuring gravitational wave sources of different types, with some of the sources being at very high redshifts $z\sim 20$. On the 23rd of February 2022 we organized the 1$^\mathrm{st}$ {\it LISA in Greece Workshop}. This workshop aimed to inform the Greek scientific and tech industry community about the possibilities of participating in LISA science and LISA mission, with the support of the Hellenic Space Center (HSC). In this white paper, we summarize the outcome of the workshop, the most important aspect of it being the inclusion of $15$ Greek researchers to the LISA Consortium, raising our total number to $22$. At the same time, we present a road-map with the future steps and actions of the Greek Gravitational Wave community with respect to the future LISA mission.

gr-qc↗

arXiv2023

Constraining supermassive primordial black holes with magnetically induced gravitational waves

Primordial black holes (PBHs) can answer a plethora of cosmic conundra, among which the origin of the cosmic magnetic fields. In particular, supermassive PBHs with masses $M_\mathrm{PBH}>10^{10} M_\odot$ and furnished with a plasma-disk moving around them can generate through the Biermann battery mechanism a seed primordial magnetic field which can later be amplified so as to provide the magnetic field threading the intergalactic medium. In this work, we derive the gravitational wave (GW) signal induced by the magnetic anisotropic stress of such a population of magnetised PBHs. Interestingly enough, by using GW constraints from Big Bang Nucleosynthesis (BBN) and an effective model for the galactic/turbulent dynamo amplification of the magnetic field, we set a conservative upper bound constraint on the abundances of supermassive PBHs at formation time, $Ω_\mathrm{PBH,f}$ as a function of the their masses, namely that $Ω_\mathrm{PBH,f}\leq 2.5\times 10^{-10}\left(\frac{M}{10^{10}M_\odot}\right)^{45/22}$. Remarkably, these constraints are comparable, and, in some mass ranges, even tighter compared to the constraints on $Ω_\mathrm{PBH,f}$ from large-scale structure (LSS) probes; hence promoting the portal of magnetically induced GWs as a new probe to explore the enigmatic nature of supermassive PBHs.

astro-ph.CO↗

arXiv2023

Primordial magnetic field generation via primordial black hole disks

Large scale primordial magnetic fields (PMFs) threading the intergalactic medium are observed ubiquitously in the Universe playing a key role in the cosmic evolution. Their origin is still debated constituting a very active field of research. In the present article, we propose a novel natural ab initio mechanism for the origin of such PMFs through the portal of supermassive primordial black holes (PBHs) forming between the Big Bang Nucleosynthesis and the recombination era. In particular, by considering PBHs furnished with a locally isothermal disk we study the generation of a Biermann battery induced seed magnetic field (MF) due to the vortexlike motion of the primordial plasma around the black hole. Finally, by considering monochromatic PBH mass distributions and deriving the relevant MF power spectrum we make a conservative estimate for the seed PMF in intergalactic scales and at redshift $z=30$, when typical galaxies are considered to form, which reads as $B\simeq 10^{-30}\mathrm{G}\left(\frac{\ell_\mathrm{R}}{10^6}\right)^2\left(\frac{M_\mathrm{PBH}}{10^{14}M_\odot}\right)^{5/2}$, where $M_\mathrm{PBH}$ is the PBH mass and $\ell_\mathrm{R}\equiv R_\mathrm{d}/R_\mathrm{ISCO}$, is the ratio of the radius of the disk, $R_\mathrm{d}$ over the radius of the innermost stable circular orbit, $R_\mathrm{ISCO}$. Interestingly enough, by requiring to seed a PMF of the order of $10^{-30}\mathrm{G}$ necessary to give rise to a present day $10^{-18}\mathrm{G}$ in intergalactic scales, we find a lower bound on the PBH mass within the range $[10^{10}- 10^{16}]M_\odot$ depending on the radius of the PBH disk.

astro-ph.CO↗

arXiv2023

Dynamical Friction due to fuzzy dark matter on satellites described by axisymmetric logarithmic potentials

A plausible dark matter candidate is an ultralight bosonic particle referred to as fuzzy dark matter. The equivalent mass-energy of the fuzzy dark matter boson is $\sim 10^{-22}$eV and has a corresponding de Broglie wavelength of kiloparsec scale, thus exhibiting wave behaviour in scales comparable to a galactic core, which could not appear in conventional cold dark matter models. The presence of fuzzy dark matter in galactic clusters will impact the motion of their members through dynamical friction. In this work, we present simulations of the dynamical friction on satellites traversing an initially uniform fuzzy dark matter halo. We focus on the satellites whose shapes are beyond spherical symmetry described by ellipsoidal and logarithmic potentials. We find that the wakes created on the fuzzy dark matter halo due to the passage of such satellites are qualitatively different from those generated by spherically symmetric ones. Furthermore, we quantify the dynamical friction coefficient for such systems, finding that the same satellite may experience a drag differing by a factor of $5$ depending on its ellipticity and the direction of motion. Finally, we find that the dynamical friction time-scale is close to Hubble time, assuming a satellite of $10^{11}$M$_{\odot}$ traversing at $10^{3}$km/s a FDM halo whose mean density is $\sim 10^6$M$_{\odot}$kpc$^{-3}$.

astro-ph.CO↗

arXiv2023

Orbital analysis of the Pluto-Charon moon system's mutual interactions and forced frequencies

Context. The orbits of the four small moons in the Pluto-Charon system, Styx, Nix, Kerberos and Hydra, are circumbinary, as the former form a binary dwarf planet. Consequently, the orbit of each one of them is characterized by a number of frequencies, arising by the central binary and the mutual gravitational interactions. Aims. In this work, we identify the most prominent of these forced frequencies using Fast Fourier Transformations. Methods. Two methods are implemented, a semi-analytic and a numerical one, and comparisons are being made. Results. The results indicate that as a first approximation, moon orbits may well be modelled as the superposition of a series of inevitable oscillations, induced by Pluto and Charon, deviating from circular ones, even if the eccentricity is set to zero. Moreover, the mutual gravitational effects are significant in their long term evolution, especially for the lighter moons Styx and Kerberos, activating modes that dominate the low-frequency region of the power spectrum. This becomes evident through the comparison of simulations where only one moon is included along with the binary dwarf planet and simulations of the entire six-body system. These modes become noticeable over long integration times and may affect the orbits of the lighter moons of the system.

astro-ph.EP↗

arXiv2022

Orbits of the TOI-1338 and TIC-172900988 systems

Recent observations by TESS revealed the existence of circumbinary planets in the systems of TOI-1338 and TIC-172900988. The purpose of this work is to model the planetary orbits in these two systems and study them under the perspective of previous theoretical models. Each planet's distance from the barycenter through time is simulated using n-body integrations and is compared with outcomes from a semi-analytic, a geometric and a Keplerian-based approach. Furthermore, we infer the most prominent frequencies of both planets' orbits induced by the central binaries. We confirm that both systems appear to be stable. Lastly, we examine the implications of an additional candidate planet in TOI-1338 system finding that an extra, 48 M$_\oplus$ planet that has been hinted from observations could be located at 0.8 AU without generating any radical changes to the orbits of the other members of the system.

astro-ph.EP↗

arXiv2022

Initial periods and magnetic fields of neutron stars

Initial distributions of pulsar periods and magnetic fields are essential components of multiple modern astrophysical models. Not enough work has been done to properly constrain these distributions using direct measurements. Here we aim to fill this gap by rigorously analysing properties of young neutron stars associated to supernova remnants. In order to perform this task, we compile a catalogue of 56 neutron stars uniquely paired to supernova remnants with known age estimate. Further, we analyse this catalogue using multiple statistical techniques. We found that distribution of magnetic fields and periods for radio pulsars are both well described using the log-normal distribution. The mean magnetic field is $\log_{10} [B/\mathrm{G}] = 12.44$ and standard deviation is $σ_B = 0.44$. Magnetars and central compact objects do not follow the same distribution. The mean initial period is $\log_{10} P_0 [P / \mathrm{s}] = -1.04_{-0.2}^{+0.15}$ and standard deviation is $σ_p = 0.53_{-0.08}^{+0.12}$. We show that the normal distribution does not describe the initial periods of neutron stars sufficiently well. Parameters of the initial period distribution are not sensitive to the exact value of the braking index.

astro-ph.HE↗

arXiv2022

Relativistic magnetic explosions

Many explosive astrophysical events, like magnetars' bursts and flares, are magnetically driven. We consider dynamics of such magnetic explosions - relativistic expansion of highly magnetized and highly magnetically over-pressurized clouds. The corresponding dynamics is qualitatively different from fluid explosions due to the topological constraint of the conservation of the magnetic flux. Using analytical, relativistic MHD as well as force-free calculations, we find that the creation of a relativistically expanding, causally disconnected flow obeys a threshold condition: it requires sufficiently high initial over-pressure and sufficiently quick decrease of the pressure in the external medium (the pre-explosion wind). In the subcritical case the magnetic cloud just "puffs-up" and quietly expands with the pre-flare wind. We also find a compact analytical solution to the Prendergast's problem - expansion of force-free plasma into vacuum.

astro-ph.HE↗

arXiv2022

Force-free and twisted, relativistic neutron star magnetospheres

In this poster we present the structure of an axisymmetric, force-free magnetosphere of a twisted, aligned rotating dipole within a corotating plasma-filled magnetosphere. We explore various profiles for the twist. We find that as the current increases more field lines cross the light cylinder leading to more efficient spin-down. Moreover, we notice that the twist cannot be increased indefinitely and after a finite twist of about $π/2$ the field becomes approximately radial. This could have implications for torque variations of magnetars related to outbursts.

astro-ph.HE↗

arXiv2022

Orbit determination of the moons of the Pluto-Charon system

Four small moons (Styx, Nix, Kerberos and Hydra) are at present known to orbit around the barycenter of Pluto and Charon, which are themselves considered a binary dwarf-planet due to their relatively high mass ratio. The central, non-axisymmetric potential induces moon orbits inconvenient to be described by Keplerian osculating elements. Here, we report that observed orbital variations, may not be the result of orbital eccentricities or observational uncertainties, but may be due to forced oscillations caused by the central binary. We show, using numerical integration and analytical considerations, that the differences reported on their orbital elements, may well arise from this intrinsic behavior rather than limitations on our instruments.

astro-ph.EP↗