arXiv subjects

Vladimír Karas

Publications and source records attributed to Vladimír Karas.

At least 19 recordsLinked to original sources

arXiv2026

Radiation-hydrodynamics of star-disc collisions: From system parameters to outflows and lightcurves

Quasi-periodic eruptions (QPEs) are nuclear transients producing bright, repeating soft X-ray flares superimposed on quiescent emission. A promising interpretation is that they are powered by star-disc collisions, in which a star crosses an accretion disc around a supermassive black hole, drives shocks, and launches dense outflows from which radiation emerges. We present a systematic study of star-disc collisions, linking the physical parameters of the collision to the resulting outflows and emerging bolometric luminosities. We perform three-dimensional local radiation-hydrodynamics simulations, varying the disc surface density and vertical density profiles, stellar velocity and radius, and local collision angle. We focus on the regime where the star remains unperturbed by the collision. We find that variations in stellar velocity and accretion disc surface density leave the bow shock and outflow morphology largely unchanged. Faster stars produce brighter flares, while denser discs mainly increase the flare duration. Increasing the stellar radius increases the momentum of the forward outflow and produces brighter and longer flares. More centrally concentrated discs yield brighter and shorter flares because radiation escapes more efficiently through outer low-density layers. More oblique crossings reduce the momentum and luminosity asymmetry of two outflows, and lengthen the flares. We provide empirical scalings of the peak luminosity and flare duration with the individual system parameters and apply them to GSN 069. The best candidate solutions favour a star with a radius $\sim R_\odot$ on a retrograde orbit, colliding with a dense post-TDE disc with a vertically concentrated density profile. Our findings suggest that specific combinations of system parameters can reproduce characteristic flare amplitudes, durations, duty cycles, and strong-weak flare patterns observed in QPE sources.

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arXiv2026

Does IRS13 require an intermediate-mass black hole?

We investigate whether the Galactic-centre association IRS13 requires an intermediate-mass black hole (IMBH) to remain bound. Using high-precision $N$-body calculations of IRS13-like systems orbiting the Milky Way supermassive black hole (SMBH), as well as simulations of more massive infalling clusters, we find that an IRS13-sized cluster dissolves on a very short timescale even when an IMBH of $4\times10^4\,M_\odot$ is present. The observed velocity dispersion can arise naturally from the tidal field of Sgr A* and the infall event itself; therefore, it need not imply a central IMBH. In the infalling-cluster runs, tidal stripping produces transient disks, spiral-like structures, and ring-like overdensities. These features suggest a broader interpretation: IRS13 is best viewed as a temporary phase-space overdensity, analogous in mechanism (though not in scale) to phase-wrapped shells and streams in disrupted galaxies.

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arXiv2026

Evolution of Quasi-Periodic Eruptions in the post-TDE Accretion Disk Perturbed by an Orbiting Star

Quasi-periodic eruptions (QPEs) are a recently discovered class of highly variable X-ray bursts originating in galactic nuclei. These high-amplitude bursts exhibit periodicity ranging from tens of minutes to several days. QPEs are also characterized by variable peak amplitudes that can vary by a factor of few. While multiple physical models have been proposed to explain QPE light curves, none can fully account for all the observed features. A possible connection between QPEs and tidal disruption events (TDEs) has been suggested, particularly due to the past optical/UV outbursts that can be traced back for several sources, the long-term decay in the continuum luminosity, and the soft, thermal-dominated X-ray spectrum. Our primary goal is to verify whether the long-term decrease in eruption amplitudes detected for some QPE sources is consistent with the accretion disk being formed following a TDE. In this work, we adopt a simplified extreme mass ratio inspiral (EMRI) scenario, where a Solar-type star orbits a supermassive black hole (SMBH) and collides with an accretion disk twice per orbit, generating eruptions. We assume a post-TDE disk that follows a temporal power-law decline in mass accretion ($\propto t^{-p}$, $p>0$). As our aim is to develop a toy-model scenario, we have used purely analytical methods without considering all intervening processes in their full generality. Indications are that (i) the observed long-term decline in QPE amplitudes can be reproduced if the first monitored epoch occurs years to a few decades after the tidal disruption, (ii) stellar mass loss caused by ablation can play an important role in the evolution of QPE amplitudes in systems with heavy main-sequence (MS) stars.

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arXiv2026

Tidal disruption of a low-mass star in an active galactic nucleus as the origin of the PS16dtm outburst

The event PS16dtm, which occured in the center of the Narrow Line Seyfert 1 (NLS1) galaxy SDSS J015804.75-005221.8 (z = 0.080440), is one of the few candidates for a tidal disruption event in an already-acretting active galactic nucleus (AGN). We aim to shed light on the character of the tidal disruption event in this source since it exhibits unusual peculiarities, such as the double-peak optical/UV light curve and a low blackbody temperature with a lack of X-ray emission. We perform spectral analysis of the source before and during the event. We model the time evolution of the luminosity profile using a numerical code that describes the viscous evolution of the flow. From the combined spectral and timing studies, we interpret the event as the disruption of a $\sim 0.3 M_{\odot}$ main-sequence star, or gradual partial disruption of the low-mass giant star. The star is likely on a circular orbit, embedded in the accretion disc. The discussion of the evolution of the star rather suggests that the orbit is counter-rotating. We observe the system at a sufficiently large viewing angle that the actual disruption process is not directly observed. The disrupted star and inner disc are shielded from the observer by a gaseous envelope. Further observations of the system returning to the previous NLS1 state, particularly in the X-ray band, are needed to confirm the proposed scenario and to put constraints on the return to a regular NLS1 state.

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arXiv2026

Formation of stable exoplanetary systems around pulsars by capture: An exercise in computational classical mechanics

The study of our Solar System -- its formation, evolution, and long-term stability -- has been ongoing for centuries and is now a standard part of scientific education. While the formation of other Solar-like exoplanetary systems is generally explained using the same mechanisms that describe our own, the discovery of exoplanets around pulsars in 1990s has raised new questions about their origin. Several scenarios were proposed, including formation by capture during a close encounter of a compact stellar-mass remnant and a pre-existing planetary system. It was, however, also conjectured that captured planets should exhibit high eccentricities and -- if more planets are captured -- their evolution would lead to chaos We revisit classical mechanics as applied to planetary systems. As an example and follow-up to previous works, we use an open-source high-precision $N$-body code to investigate dynamical interactions between planetary systems and stellar remnants, the orbital properties of captured planets, and their long-term stability over gigayears. We corroborate that the captured planets often exhibit high eccentricities (unlike some observed pulsar planetary systems), but we also present a student's simulation where a Jupiter-like planet undergoes a series of planet-planet encounters and planetary ejections, eventually stabilising at a low eccentricity of ~0.146. This shows that a chaotic post-capture evolution may eventually lead to long-term stability, making the dynamical formation channel viable for producing low-eccentricity systems. These results warrant more detailed investigation in future work. Beyond their astrophysical significance, they also illustrate general principles of non-linear dynamics and computation, where aspects of the analysis can even be carried out at the high-school or undergraduate level, making this type of research accessible to students at an early stage.

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arXiv2025

High-cadence observations of galactic nuclei by the future two-band UV-photometry mission QUVIK

The Quick Ultra-VIolet Kilonova surveyor (QUVIK), a two-band UV space telescope approved for funding as a Czech national science and technology mission, will focus on detecting early UV light of kilonovae (Werner et al., 2024). In addition, it will study the UV emission of stars and stellar systems (Krtička et al., 2024) as well as the intense and variable emission of active galactic nuclei (AGN) or galactic nuclei activated by tidal disruption events (Zajaček et al., 2024). In this contribution, we describe the role of this small ($\sim 30$-cm diameter) UV telescope for studying bright, nearby AGN. With its NUV and FUV bands, the telescope will perform high-cadence ($\sim 0.1$-$1$ day) two-band photometric monitoring of nearby AGN ($z<1$), which will allow us to probe accretion disk sizes/temperature profiles via photometric reverberation mapping. Thanks to its versatility, QUVIK will be able to perform a moderately fast repointing ($<20$ min) to target candidates for tidal disruption events (TDEs). Early detection of the UV emission following a TDE optical flare, in combination with the subsequent two-band UV monitoring performed simultaneously with other observatories, will enable us to infer the time delay (or its lack of) between the optical, UV, and X-ray emission. In combination with theoretical models, it will be possible to shed more light on the origin of the UV/optical emission of TDEs. Furthermore, the two-band monitoring of nuclear transients will be beneficial in distinguishing between TDEs (nearly constant blue colour) and supernovae (progressive reddening) in the era of intensive wide-field surveys.

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arXiv2025

Revealing EMRI/IMRI candidates with quasiperiodic ultrafast outflows

The first detection of the quasiperiodic ultrafast outflow in the ASASSN-20qc system was reported by Pasham et al. (2024). The outflow is revealed in the soft X-ray spectra as an absorption feature, which is enhanced periodically every $\sim 8.3$ days. The repetitive nature of the ultrafast outflow is tentatively explained by an orbiting massive perturber, possibly an intermediate-mass black hole (IMBH), trajectory of which is inclined with respect to the accretion flow around the primary supermassive black hole (SMBH). In this scenario, the orbiting body pushes the disc gas into the outflow funnel, where it is accelerated by the ordered magnetic field (Suková et al. 2021). Quasiperiodic ultrafast outflows (a.k.a. QPOuts) are thus a novel phenomenon that can help reveal new extreme-/intermediate-mass ratio inspiral (EMRI/IMRI) candidates. These then would be prime candidate sources for a simultaneous detection and monitoring in electromagnetic as well as gravitational wave domains.

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arXiv2025

Modulation of X-ray flux by obscuration of neutron star boundary layer

The quasi-periodic oscillations (QPOs) observed in the X-ray variability of both black hole (BH) and neutron star (NS) systems provide a tool for probing strong gravity and dense matter equations of state. Nevertheless, the mechanism of QPO modulation in NS systems, where the amplitudes of QPOs with frequencies approaching kHz range are very high in comparison to BH high-frequency QPOs, remains an unsolved puzzle. Relativistic ray tracing of photons emitted from the immediate vicinity of compact objects has, to date, been used to investigate various mechanisms that explain the observed weak BH QPOs. However, it has not been applied to model the NS QPO signal, which requires incorporating the NS surface and a bright boundary layer (BL) on it. Here, we explore the QPO modulation mechanisms based on the BL obscuration. Using simplified models of axisymmetric oscillations of thick accretion discs (tori), we demonstrate that the disc oscillations drive the high NS QPO amplitudes through BL obscuration, which is relevant especially for vertical oscillations. We also demonstrate that obscuration effects enable the observability of the Keplerian frequency in the case of discs that decay due to instabilities.

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arXiv2024

Discovery of a Dense Association of Stars in the Vicinity of the Supermassive Black Hole Sgr A*

We focus on a sample of 42 sources in the vicinity of the bow-shock source IRS 1W (N-sources), located at the distance of $6.05''$ north-east of the supermassive black hole (SMBH) Sagittarius A* (Sgr A*), within the radius of $1.35''$. We present the first proper motion measurements of N-sources and find that a larger subset of N-sources (28 sources) exhibit a north-westward flying angle. These sources can be bound by an intermediate mass black hole (IMBH) or the concentration that we observe is due to a disk-like distribution projection along the line of sight. We detect the N-sources in $H$, $K_s$, and $L$' bands. The north-westward flying sources could be a bound collection of stars. We discuss a tentative existence of an IMBH or an inclined disk distribution to explain a significant overdensity of stars. The first scenario of having an IMBH implies the lower limit of $\sim 10^4~M_\odot$ for the putative IMBH. Our measurements for the first time reveal that the dense association of stars containing IRS 1W is a co-moving group of massive, young stars. This stellar association might be the remnant core of a massive stellar cluster that is currently being tidally stripped as it inspirals towards Sgr A*. The second scenario suggests that the appearance of the N-sources might be influenced by the projection of a disk-like distribution of younger He-stars and/or dust-enshrouded stars.

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arXiv2024

Investigating the consistency of the shape and flux of X-ray reflection spectra in the hard state with an accretion disk reaching close to the black hole

The observed spectra from black hole (BH) X-ray binaries (XRBs) typically consist of two primary components: multitemperature blackbody (BB) originating from the accretion disk in soft X-ray, and a power-law like component in hard X-ray due to Comptonization of soft photons by the hot corona. Illumination of the disk by the corona gives rise to another key component known as reflection. A fraction of the incident hard X-ray radiation is naturally absorbed and re-emitted as a BB at lower energies, referred to as reprocessed BB. For densities relevant to XRBs and typical ionization values, the reprocessed BB may become significant in the soft X-ray region and should be noticeable in the observed spectra as a consequence of reflection. The absence of any BB component in the low/hard state of BH XRB may not be consistent with reflection of high irradiating flux observed as power-law from appropriately dense disk of XRB. We focus on the low/hard state of the BH XRB MAXI J1820+070. We simultaneously fit the shape and flux of the reflection spectra, allowing us to estimate the correct density and ionization of the slab and, correspondingly, the reprocessed BB. Our fitting suggests that the disk in principle may extend close to the BH and still the reprocessed BB due to disk illumination remains small enough to be consistent with the data as opposed to earlier study. The inner reflection component is highly ionized and its fit is primarily driven by its contribution to the continuum. The reprocessed BB cannot resolve whether the disk is extended close to the BH or not in the hard state. For this specific observation, the flux in inner reflection component turns out to be quite low with respect to outer reflection or power-law. Outflowing slab corona covering the inner region of the disk could be the possible geometry of the source with the underlying disk reaching close to the BH. (shortened)

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arXiv2024

Dynamics of star associations in an SMBH-IMBH system: The case of IRS13 in the Galactic centre

Context: The existence of intermediate-mass black holes (IMBHs) still poses challenges to theoretical and observational astronomers. Several candidates have been proposed, including the one in the IRS13 cluster in the Galactic centre, where the evidence is based on the velocity dispersion of its members, however, none have been confirmed to date. Aims: We aim to gain insights into the presence of an IMBH in the Galactic centre by a numerical study of the dynamical interplay between an IMBH and star clusters (SCs) in the vicinity of a supermassive black hole (SMBH). Methods: We use high-precision N-body models of IRS13-like SCs in the Galactic centre, and of more massive SCs that fall into the centre of the Galaxy from larger distances. Results: We find that at IRS13's physical distance of 0.4 pc, an IRS13-size SC cannot remain gravitationally bound even if it contains an IMBH of thousands $M_\odot$. Thus, IRS13 appears to be an incidental present-day clumping of stars. Furthermore, we show that the velocity dispersion of tidally disrupted SCs (the likely origin of IRS13) can be fully accounted for by the tidal forces of the central SMBH; the IMBH's influence is not essential.

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arXiv2024

Accretion tori around rotating neutron stars II: Oscillations and precessions

The four characteristic oscillation frequencies of accretion flows are, in addition to the Keplerian orbital frequency, often discussed in the context of the time variability of the black hole and neutron star (NS) low-mass X-ray binaries (LMXBs). These are namely the frequencies of the axisymmetric radial and vertical epicyclic oscillations, and the frequencies of non-axisymmetric oscillations corresponding to the periastron (radial) and Lense-Thirring (vertical) precessions. In this context, we investigate the effect of the quadrupole moment of a slowly rotating NS and provide complete formulae for calculating these oscillation and precession frequencies, as well as their convenient approximations. Simple formulae corresponding to the geodesic limit of a slender torus (and test particle motion) and the limit of a marginally overflowing torus (torus exhibiting a critical cusp) are presented, and furthermore, more general approximate formulae are included to allow calculations for arbitrarily thick tori. We provide the Wolfram Mathematica code used for our calculations together with C++ and PYTHON codes for calculations of the frequencies. Our formulae can be used for various calculations describing the astrophysical signatures of the NSs' superdense matter equation of state. For instance, we demonstrate that, even for a given fixed number of free parameters, a model accounting for fluid flow precession better matches the frequencies of twin-peak quasiperiodic oscillations observed in NS LMXBs than a model using geodesic precession.

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arXiv2024

Accretion tori around rotating neutron stars I: Structure, shape and size

We present a full general relativistic analytic solution for a radiation-pressure supported equilibrium fluid torus orbiting a rotating neutron star (NS). Previously developed analytical methods are thoroughly applied in the Hartle-Thorne geometry, including the effects of both the NS's angular momentum and quadrupole moment. The structure, size and shape of the torus are explored, focusing especially on the critically thick solution - the cusp tori. For the astrophysically relevant range of NS parameters, we examine how our findings differ from those obtained for the Schwarzschild spacetime. The solutions for rotating stars display signatures of the interplay between relativistic and Newtonian effects where the impact of NS angular momentum and quadrupole moment are almost counterbalanced at the given radius. Nevertheless, the spacetime parameters still strongly influence the size of tori, which can be shown in a coordinate-independent way. Finally, we discuss the importance of the size of the central NS, determining whether or not the surrounding torus may exist. We provide a set of tools in a Wolfram Mathematica code, which poses a basis allowing for a further investigation of the impact of the NSs' superdense matter equation of state on the spectral and temporal behaviour of accretion tori.

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arXiv2024

Inferring the iron K emissivity profiles of accretion discs irradiated by extended coronae

One of the most promising methods to measure the spin of an accreting black hole is fitting the broad iron K$α$ line in the X-ray spectrum. The line profile also depends on the geometry of the hard X-ray emitting corona. To put constraints on the black hole spin and corona geometry, it is essential to understand how do they affect the iron K$α$ line emissivity profile. In this work, we present calculations of the illumination and the iron K$α$ emissivity profiles performed with the Monte-Carlo GR radiative transfer code Monk. We focus on distinction between the illumination and emissivity profiles, which is in most previous studies neglected. We show that especially for the case of black hole X-ray binaries (BHXRBs), the difference is very large. For active galactic nuclei (AGNs), the emissivity profile has a more similar shape as the illumination profile, but it is notably steeper in the innermost region within a few gravitational radii. We find out that the different behavior between AGN and black hole X-ray binary discs is due to the different energy spectra of the illuminating radiation. This suggests that the emissivity profile of the iron K$α$ line cannot be determined by black hole spin and corona geometry alone and the energy spectrum of the illuminating radiation has to be taken into account. We also examined the effect of including the self-irradiation, and find it to be more important than the corona emission in BHXRBs.

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arXiv2024

On Innermost Stable Spherical Orbits near a rotating black hole: A numerical study of the particle motion near the plunging region

According to General Relativity, astrophysical black holes are described by a small number of parameters. Apart from the mass of the black hole (M), among the most interesting characteristics is the spin (a), which determines the degree of rotation, i.e., the angular momentum of the black hole. The latter is observationally constrained by the spectral and timing properties of the radiation signal emerging from an accretion disk of matter orbiting near the event horizon. In the case of planar (standard, equatorial) accretion disk, it is the location of the Innermost Stable Circular Orbit (ISCO) that determines the observable radiation characteristics, and this way allows us to measure the spin. In this paper, we discuss a more general case of the Innermost Stable Spherical Orbits (ISSO) extending above and below the equatorial plane. To this end, we study the non-equatorial geodesic motion of particles following inclined, spherical, relativistically precessing trajectories with the aim of exploring the boundary between the regions of stable (energetically bound) and escaping (energetically unbound) motion. The concept of the ISSO radius should play a role in determining the inner rim of a tilted or geometrically thick accretion flow. We demonstrate that the region of inclined bound orbits has a complicated structure due to enhanced precession near the inner rim. We also explore the fate of particles launched below the radius of the Marginally Bound Spherical Orbit (MBSO): these may either plunge into the event horizon or escape to radial infinity.

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arXiv2023

MAD UFOs: Magnetically Arrested Discs with persistent Ultra-Fast Outflows

General-relativistic magneto-hydrodynamical (GRMHD) simulations of accreting black holes suggest that the accretion flows form toroidal structures embedded in a large scale component of magnetic field, which becomes organized on length-scales exceeding the gravitational radius of the central black hole. Magnetic field grows gradually until a Magnetically Arrested Disc (MAD) develops that diminishes or inhibits further accretion. We study an outflow that develops in the MAD state in 3D GRMHD simulations. We show that the outflow can be accelerated to relativistic velocities and persist over the course of our simulation. We compare the properties of the outflow from MAD discs with those launched by orbiting secondary at close orbit. The main difference is that the orbiting body launches a more coherent, quasiperiodic ultrafast outflow at lower velocities ($v<0.5c$) while the outflow launched in the MAD state (without the body) has a stochastic behaviour and has an approximately flat velocity distribution between lower and higher outflow velocities, $0.2c 0.5c$.

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arXiv2023

Spectral and polarization properties of reflected X-ray emission from black-hole accretion discs for a distant observer: the lamp-post model

Rebirth of X-ray polarimetric instruments will have a significant impact on our knowledge of compact accreting sources. The properties of inner-accreting regions of active galactic nuclei (AGNs) or X-ray binary systems (XRBs), such as black-hole spin, their disc inclination and orientation, shape and size of their corona, can be polarimetrically studied, parallelly to the well-known X-ray spectroscopic and timing techniques. In this work, we provide a new spectropolarimetric numerical estimate of X-rays in the lamp-post coronal model for a distant observer, including a polarized reflected radiation from the accretion disc. The local disc reflection was simulated using the codes TITAN and STOKES and includes variable disc ionization as well as Monte Carlo treatment of Compton multiple scatterings. We introduce a relativistic code KYNSTOKES based on our well-tested KY package that accounts for all relativistic effects on radiation near a black hole, apart from the returning radiation, and adds a possibility of polarized coronal emission. We study the spectrum, polarization degree and polarization angle at spatial infinity for various global system parameters and we demonstrate the difference at infinity, if analytical local reflection computations are used. We newly predict that in the hard X-rays the reflected component can be 25% polarized and the total emission can be 9% polarized in the most favourable, yet realistic configurations of radio-quiet AGNs. Thus, the relativistic disc reflection remains important for the interpretation of X-ray polarimetric observations.

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arXiv2023

The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a rich population of dusty objects in the IRS 13 cluster

A detailed analysis of the Nuclear Stellar Cluster (NSC) concedes not only the existence of the Scluster with its fast-moving stars and the supermassive black hole (SMBH) Sgr A*. It also reveals an embedded region of gas and dust with an exceptionally high stellar density called IRS 13. The IRS 13 cluster can be divided into the northern and the eastern counterparts, called IRS 13N and IRS 13E, respectively. This work will focus on both regions and study their most prominent members using rich infrared and radio/submm data baselines. Applying a multiwavelength analysis enables us to determine a comprehensive photometric footprint of the investigated cluster sample. Using the raytracing-based radiative transfer model HYPERION, the spectral energy distribution of the IRS 13 members suggests a stellar nature of the dusty sources. These putative Young Stellar Objects (YSOs) have a comparable spectroscopic identification to the D and G sources in or near the S cluster. Furthermore, we report the existence of a population of dusty sources in IRS 13 that can be mostly identified in the H-, K-, and Lband. Together with the objects reported in literature, we propose that this population is the outcome of a recent star formation process. Furthermore, we report that these presumably young objects are arranged in a disk structure. Although it cannot be excluded that the intrinsic arrangement of IRS 13 does show a disk structure, we find indications that the investigated cluster sample might be related to the counterclockwise disk.

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