arXiv subjects

Włodek Kluźniak

Publications and source records attributed to Włodek Kluźniak.

At least 19 recordsLinked to original sources

arXiv2026

Radiative GRMHD simulations of puffy accretion discs: Numerical versus analytical models of sub-Eddington accretion

A widely accepted picture of an accretion flow in the luminous soft spectral state of X-ray binary systems is a geometrically thin disc structure much like the classic analytic solution of Shakura& Sunyaev. Although the analytic models are troubled by instabilities and miss important aspects of physics, such as magnetic fields, they are successfully used as a framework for interpreting observational data. Here, we compare the results of general relativistic radiative magnetohydrodynamic (GRRMHD) simulations of optically thick, mildly sub-Eddington accretion on a stellar-mass black hole (the puffy disc) with established analytic and semi-analytic accretion models in the same regime. From the simulations, we find that the accretion flow is stabilised by the magnetic field, with a puffed-up, optically thick region resembling a warm corona surrounding a denser and cooler disc core. However, the stratified vertical structure of the disc significantly influences the observational picture of such a system. We analyse the inner disc structure, flow properties, effective viscosity, and inner edge position, and compare them to the predictions of standard models. We find that the simulated discs share some similarities with the models; however, they differ in several important aspects, most notably: the photosphere is geometrically thick, the inner edge is located closer to the central black hole than the analytic models assume, the surface density is significantly lower than analytically predicted, and the effective viscosity parameter is not constant but rises steeply in the innermost region.

astro-ph.HE↗

arXiv2026

Scattering of massive particles from black holes and naked singularities

We performed a numerical study of the dynamics of massive particles orbiting black holes and naked singularities in the Reissner-Nordström geometry. We modeled a stream of particles with a constant angular momentum and with a range of energies. We then solved the geodesic equation of motion and compared the trajectories around black holes and naked singularities by tuning the charge parameter of the metric. The setup {allows us to explore the orbital dynamics relevant for} astrophysical scenarios such as tidal disruption events{, particularly for deep encounters}. We discussed differences and similarities in the orbital dynamics and deflection angles. We found that particles reflected by a black hole follow a stream-like family of orbits within a narrow range of deflection angles, whereas in the case of naked singularities particles are scattered in all directions on the plane of motion. We explained this behavior as an interplay between the presence of a centrifugal barrier at the location of the unstable circular orbit and an absorbing event horizon in the case of a black hole or a {repulsive core} in the case of a naked singularity. These qualitative differences are expected to impact the observable signatures of tidal disruption events.

gr-qc↗

arXiv2025

Accretion onto Reissner-Nordström naked singularities

Nearly every galactic core contains a supermassive compact object, hypothesized to be a Kerr black hole. It was only with the advent of Event Horizon Telescope observations that the predictions of this hypothesis could be observationally tested for our own Galaxy, and the nearby elliptical M87, on spatial scales comparable to the gravitational radius. At the same time it became possible to test whether alternatives such as naked singularities in general relativity, or similar objects in alternative theories of gravity, are excluded by the data. These and other observational developments renewed interest in non-Kerr spacetime metrics, also in the context of active galactic nuclei at cosmological distances. Recently, we have shown that accreting naked singularities in the Reissner-Nordström metric of general relativity tend to produce strong outflows. The geometry and origin of these winds is studied here, and their parameter dependence is investigated. To this end we performed numerical GR hydrodynamical simulations of accretion of electrically neutral matter in the Reissner-Nordström metric and discussed the results in the context of analytic predictions of fluid motion in this spacetime.

astro-ph.HE↗

arXiv2025

GRMHD Study of Accretion onto time-like Naked Singularities

Naked singularities (NkS) are solutions to the Einstein field equations that violate the cosmic censorship conjecture. Recent studies indicate that these objects may serve as compelling mimickers of black hole shadows. In this work, we investigate the accretion dynamics of selected time-like naked singularities using general relativistic magnetohydrodynamic (GRMHD) simulations. Our objective is to determine whether naked singularities exhibit distinct signatures compared to black holes. We find that, unlike black holes, naked singularities exhibit a centrifugal barrier that prevents direct accretion of rotating matter onto the NkS. Despite reduced magnetization in the funnel region, these objects are capable of generating jet powers comparable to those observed in black holes. Additionally, we observe that accreting matter releases gravitational energy as it is driven towards the NkS, powering the strong outflow via local fluid pressure gradient or magnetic pressure forces.

astro-ph.HE↗

arXiv2025

Energy flow and radiation efficiency in radiative GRMHD simulations of neutron star ultraluminous X-ray sources

We investigate numerically the energy flow and radiation efficiency of accreting neutron stars as potential ultraluminous X-ray sources (ULXs). We perform ten simulations {in radiative general relativistic magnetohydrodynamics (GRRMHD)}, exploring six different magnetic dipole strengths ranging from 10 to 100 GigaGauss, along with three accretion rates, 100, 300, and 1000 Eddington luminosity units. Our results show that the energy efficiency in simulations with a strong magnetic dipole of 100 GigaGauss is approximately half that of simulations with a magnetic dipole an order of magnitude weaker. Consequently, radiation efficiency is lower in simulations with stronger magnetic dipoles. We also demonstrate that outflow power increases as the magnetic dipole weakens, resulting in stronger beaming in simulations with weaker magnetic dipoles. As a result of beaming, simulations with magnetic dipole strengths below 30 GigaGauss exhibit apparent luminosities consistent with those observed in ULXs. As for the accretion rates, we find that higher accretion rates lead to more powerful outflows, higher kinetic efficiency, and lower radiation efficiency compared to those of lower accretion rate simulations.

astro-ph.HE↗

arXiv2025

Magnetic precession as a model for QPOs in PULXs: flat-top noise ULXs are different

Context. Several instances of low frequency Quasi-Periodic Oscillations (QPOs) have been reported in ultraluminous X-ray sources (ULXs), including three in pulsating ones (PULXs) to date. The nature of many ULXs is still unclear, as are the detailed properties of accretion in PULXs. Aims. We seek an answer to questions such as: Is there a QPO model that fits the data? Can mHz QPOs be used to constrain the magnetic field and accretion rate of the neutron stars in PULXs? Are all the low frequency QPOs in ULXs a manifestation of the same phenomenon? Methods. We apply Dong Lai's precession model to the PULX data, with the magnetic threading of the accretion disk constrained by recent simulations. Results. Based on the magnetic precession model, and on recent progress in understanding the inner structure of accretion disks, we predict an inverse scaling of QPO frequency with the neutron star period in PULXs. The theoretical curve is largely independent of the stellar magnetic field or mass accretion rate and agrees with the data for the known QPOs in PULXs. The flat-top QPOs detected in ULXs have observational properties that seem to be very different from the QPOs detected in PULXs, indicating they might have a different origin.

astro-ph.HE↗

arXiv2025

Launching of asymmetric outflows from the star-disk magnetosphere

In resistive and viscous magnetohydrodynamical simulations, we obtain axial outflows launched from the innermost magnetosphere of a star-disk system. The launched outflows are found to be asymmetric. We find the part of the parameter space corresponding to quasi-stationary axial outflows and compute the mass load and angular momentum flux in such outflows. We display the obtained geometry of the solutions and measure the speed of propagation and rotation of the obtained axial outflows.

astro-ph.SR↗

arXiv2025

ULX collimation by outflows in moderately magnetized neutron stars

We perform radiative magnetohydrodynamics simulations in general relativity (GRRMHD) of super-Eddington disk accretion onto neutron stars endowed with a magnetic dipole corresponding to surface strengths not exceeding 100 GigaGauss. Accretion is found to power strong outflows which collimate the emergent radiation of the accretion columns, leading to apparent radiative luminosities of $\sim 100$ Eddington, when the true luminosity is a few Eddington units. Surprisingly, the collimation cone/angle widens with increasing magnetic field. Thus, in our simulations the apparent luminosity of the neutron star is substantially larger for the weaker magnetic fields ($10^{10}\,$G) than for the stronger ones ($10^{11}\,$G). We conclude that a super-Eddington accreting neutron star with the dipole magnetic field $10^{10}\,$G is the most likely source of ultraluminous X-rays.

astro-ph.HE↗

arXiv2025

Pseudo-Newtonian simulation of a thin accretion disk around a Reissner-Nordström naked singularity

We present the first numerical simulations of a thin accretion disk around a Reissner-Nordström (RN) naked singularity (a charged point mass). The gravity of the RN naked singularity is modeled with a pseudo-Newtonian potential that reproduces exactly the radial dependence of the RN Keplerian orbital frequency; in particular, orbital angular velocity vanishes at the zero gravity radius and has a maximum at 4/3 of that radius. Angular momentum is transported outwards by viscous stresses only outside the location of this maximum. Nonetheless, even at that radius, accretion proceeds at higher latitudes, the disk having thickened there owing to excess pressure. The accretion stops at a certain distance away from the singularity, with the material accumulating in a toroidal structure close to the zero-gravity sphere. The shape of the structure obtained in our simulations is reminiscent of fluid figures of equilibrium analytically derived in full general relativity for the RN singularity. The presence of a rotating ring, such as the one found in our simulations, could be an observational signature of a naked singularity. For charge to mass ratios close to but larger than unity, the inner edge of the quasi-toroidal inner accretion structure would be located well within the Schwarzschild marginally stable orbit (ISCO), and the maximum orbital frequency in thin accretion disks would be much higher than the Schwarzschild ISCO frequency.

astro-ph.HE↗

arXiv2024

Conservative limits on the electric charge of Sgr A* in the Reissner-Nordstrom metric

In general relativity, the gravitational field of an electrically charged, non-rotating, spherically symmetric body is described by the Reissner-Nordström (RN) metric. Depending on the charge to mass ratio, the solution describes a black hole or a naked singularity. In the naked-singularity regime, a general property of this metric is the existence of a radius, known as the zero-gravity radius, where a test particle would remain at rest. As a consequence of repulsive gravity there is no circular orbit inside this radius, and at least a part of any quasi-stable structure must necessarily lie outside of it. Assuming the iconic torus in the compact source Sgr A* at the Galactic center observed by the Event Horizon Telescope (EHT) to be the image of a quasi-stationary fluid structure, we provide rigorous constraints in the RN metric on the electric charge-to-mass ratio Q/M of Sgr A*. A comparison between the EHT observations and the space-time zero-gravity radius provides the most conservative limit on the charge of Sgr A* to be |Q/M| < 2.32 in geometrized units. A charged naked singularity respecting this charge-to-mass constraint is consistent with the current EHT observations, if the image is not interpreted as a photon ring.

gr-qc↗

arXiv2024

Outflows from naked singularities, infall through the black hole horizon: hydrodynamic simulations of accretion in the Reissner-Nordström space-time

We performed the first simulations of accretion onto the compact objects in the Reissner-Nordström (RN) spacetime. The results obtained in general relativity are representative of those for spherically symmetric naked singularities and black holes in a number of modified gravity theories. A possible application of these calculations is to the active galactic nuclei (AGNs) with their powerful jets. It is now possible to compare the results of such simulations with the accreting supermassive objects in our own Milky Way and the nearest spiral galaxy: observations of the core regions of galactic nuclei (Sgr A* and M87) performed with unprecedented resolution by the Event Horizon Telescope (EHT) collaboration allow fairly direct tests of the spacetime-metric of the central compact object. In this context we present general-relativistic hydrodynamical simulation results of accretion from an orbiting accretion torus (with a cusp) onto a RN black hole and a RN naked singularity. The results could not be more different for the two cases. For a black hole, just as in the familiar Kerr/Schwarzschild case, matter overflowing the cusp plunges into the black hole horizon. For the naked singularity, the accreting matter forms an inner structure of toroidal topology and leaves the system via powerful outflows. It is an open question whether this inner structure can give rise to an image quantitatively similar to the ones reported by EHT for M87 and Sgr A*.

astro-ph.HE↗

arXiv2024

General Relativistic Magnetohydrodynamic Simulations of Accreting Tori: Resolution Study

We present two-dimensional general relativistic radiative magnetohydrodynamical simulations of accretion disks around non-rotating stellar-mass black hole. We study the evolution of an equilibrium accreting torus in different grid resolutions to determine an adequate resolution to produce a stable turbulent disk driven by magneto-rotational instability. We evaluate the quality parameter, $Q_θ$, from the ratio of MRI wavelength to the grid zone size and examine the effect of resolution in various quantitative values such as the accretion rate, magnetisation, fluxes of physical quantities and disk scale-height. We also analyse how the resolution affects the formation of plasmoids produced in the magnetic reconnection events.

astro-ph.HE↗

arXiv2024

Equilibrium tori orbiting Reissner-Nordström naked singularities

In general relativity, the asymptotically flat space-time of a charged, spherically symmetric (non-rotating) body is described by the Reissner-Nordström metric. This metric corresponds to a naked singularity when the absolute value of charge, $Q$, exceeds the mass, $M$. For all Reissner-Nordström naked singularities, there exists a zero gravity sphere where a test particle can remain at rest. Outside that sphere gravity is attractive, inside it gravity is repulsive. For values of $Q/M>\sqrt{9/8}$ the angular frequency of circular test-particle orbits has a maximum at radius $r=(4/3)\,Q^2/M$. We construct polytropic tori with uniform values of specific angular momentum in the naked singularity regime of the Reissner-Nordström metric, $(Q/M>1)$.

gr-qc↗

arXiv2023

Truncated, Tilted Discs as a Possible Source of Quasi-Periodic Oscillations

Many accreting black holes and neutron stars exhibit rapid variability in their X-ray light curves, termed quasi-periodic oscillations (QPOs). The most commonly observed type is the low-frequency ($\lesssim 10$ Hz), type-C QPO, while only a handful of sources exhibit high-frequency QPOs ($\gtrsim 60$ Hz). The leading model for the type-C QPO is Lense-Thirring precession of a hot, geometrically thick accretion flow that is misaligned with the black hole's spin axis. However, existing versions of this model have not taken into account the effects of a surrounding, geometrically thin disc on the precessing, inner, geometrically thick flow. In Bollimpalli et. al 2023, using a set of GRMHD simulations of tilted, truncated accretion discs, we confirmed that the outer thin disc slows down the precession rate of the precessing torus, which has direct observational implications for type-C QPOs. In this paper, we provide a detailed analysis of those simulations and compare them with an aligned truncated disc simulation. We find that the misalignment of the disc excites additional variability in the inner hot flow, which is absent in the comparable aligned-disc simulations. This suggests that the misalignment may be a crucial requirement for producing QPOs. We attribute this variability to global vertical oscillations of the inner torus at epicyclic frequencies corresponding to the transition radius. This explanation is consistent with current observations of higher frequency QPOs in black hole X-ray binary systems.

astro-ph.HE↗

arXiv2023

Magnetically threaded accretion disks in resistive magnetohydrodynamic simulations and asymptotic expansion

Aims. A realistic model of magnetic linkage between a central object and its accretion disk is a prerequisite for understanding the spin history of stars and stellar remnants. To this end, we aim to provide an analytic model in agreement with magnetohydrodynamic (MHD) simulations. Methods. For the first time, we wrote a full set of stationary asymptotic expansion equations of a thin magnetic accretion disk, including the induction and energy equations. We also performed a resistive MHD simulation of an accretion disk around a star endowed with a magnetic dipole, using the publicly available code PLUTO. We compared the analytical results with the numerical solutions, and discussed the results in the context of previous solutions of the induction equation describing the star-disk magnetospheric interaction. Results. We found that the magnetic field threading the disk is suppressed by orders of magnitude inside thin disks, so the presence of the stellar magnetic field does not strongly affect the velocity field, nor the density profile inside the disk. Density and velocity fields found in the MHD simulations match the radial and vertical profiles of the analytic solution. Qualitatively, the MHD simulations result in an internal magnetic field similar to the solutions previously obtained by solving the induction equation in the disk alone. However, the magnetic field configuration is quantitatively affected by magnetic field inflation outside the disk; this is reflected in the net torque. The torque on the star is an order of magnitude larger in the magnetic than in the non-magnetic case. Spin-up of the star occurs on a timescale comparable to the accretion timescale in the MHD case, and is an order of magnitude slower in the absence of a stellar magnetic field.

astro-ph.SR↗

arXiv2023

Astrophysical cloaking of a naked singularity

A massive naked singularity would be cloaked by accreted matter, and thus may appear to a distant observer as an opaque \mbox{(quasi-)}spherical surface of a fluid, not unlike that of a star or planet. We present here analytical solutions for levitating atmospheres around a wide class of spherically symmetric naked singularities. Such an atmosphere can be constructed in every spacetime which possesses a zero-gravity radius and which is a solution of a (modified-)gravity theory possessing the usual conservation laws for matter. Its density peaks at the zero-gravity radius and the atmospheric fluid is supported against infall onto the singularity by gravity alone. In an astrophysical context, an opaque atmosphere would be formed in a very short time by accretion of ambient matter onto the singularity -- in a millisecond for an X-ray binary, in a thousand seconds for a singularity traversing interstellar space, and a thousand years for a singularity that is the central engine of an AGN.

astro-ph.HE↗

arXiv2021

Levitating atmospheres around naked singularities

For a wide class of spherically symmetric naked singularities there is a sphere within which gravity is effectively repulsive. In such spacetimes accreting matter cannot reach the singularity and will instead form a levitating atmosphere, which is kept suspended by gravity alone. The density of the atmosphere has a maximum at a definite radius. In its qualitative properties the atmosphere is analogous to the recently discussed atmospheres that are supported by radiation pressure above luminous neutron stars, however for the levitating atmospheres around a naked singularity no radiation need be present.

gr-qc↗

arXiv2021

Beamed emission from a neutron-star ULX in a GRRMHD simulation

We perform a global 2.5D general-relativistic radiation magnetohydrodynamic simulation of super-critical accretion onto a neutron star with a $2\times 10^{10}$ G dipolar magnetic field, as a model of a neutron-star-powered ultraluminous X-ray source (ULX). We compute a lower limit on the total luminosity of $\sim 2.5\,L_\mathrm{Edd}$, and find the radiation to be highly beamed by the accretion disk outflows. The apparent isotropic luminosity, which is a function of the viewing angle, reaches a maximum above $100\,L_\mathrm{Edd}$, consistent with the luminosities observed in ULXs.

astro-ph.HE↗