arXiv subjects

D. Borka

Publications and source records attributed to D. Borka.

34 records · Page 2Linked to original sources

arXiv2016

Line shifts in accretion disks - the case of Fe K$α$

Here we present a short overview and main results of our investigations of several effects which can induce shifts in the broad Fe K$α$ line emitted from relativistic accretion disks around single and binary supermassive black holes. We used numerical simulations based on ray-tracing method in the Kerr metric to study the role of classical Doppler shift, special relativistic transverse Doppler shift and Doppler beaming, general relativistic gravitational redshift, and perturbations of the disk emissivity in the formation of the observed Fe K$α$ line profiles. Besides, we also investigated whether the observed line profiles from the binary systems of supermassive black holes could be affected by the Doppler shifts due to dynamics of such systems. The presented results demonstrate that all these effects could have a significant influence on the observed profiles of the broad Fe K$α$ line emitted from relativistic accretion disks around single and binary supermassive black holes.

astro-ph.GA↗

arXiv2014

Masses of constituent quarks confined in open bottom hadrons

We apply color-spin and flavor-spin quark-quark interactions to the meson and baryon constituent quarks, and calculate constituent quark masses, as well as the coupling constants of these interactions. The main goal of this paper was to determine constituent quark masses from light and open bottom hadron masses, using the fitting method we have developed and clustering of hadron groups. We use color-spin Fermi-Breit (FB) and flavor-spin Glozman-Riska (GR) hyperfine interaction (HFI) to determine constituent quark masses (especially $b$ quark mass). Another aim was to discern between the FB and GR HFI because our previous findings had indicated that both interactions were satisfactory. Our improved fitting procedure of constituent quark masses showed that on average color-spin (Fermi-Breit) hyperfine interaction yields better fits. The method also shows the way how the constituent quark masses and the strength of the interaction constants appear in different hadron environments.

hep-ph↗

arXiv2014

Constraining Extended Gravity Models by S2 star orbits around the Galactic Centre

We investigate the possibility to explain theoretically the observed deviations of S2 star orbit around the Galactic Centre using gravitational potentials derived from modified gravity models in absence of dark matter. To this aim, an analytic fourth-order theory of gravity, non-minimally coupled with a massive scalar field is considered. Specifically, the interaction term is given by analytic functions $f(R)$ and $f(R,ϕ)$ where $R$ is the Ricci scalar and $ϕ$ is a scalar field whose meaning can be related to further gravitational degrees of freedom. We simulate the orbit of S2 star around the Galactic Centre in $f(R)$ (Yukawa-like) and $f(R,ϕ)$ (Sanders-like) gravity potentials and compare it with NTT/VLT observations. Our simulations result in strong constraints on the range of gravity interaction. In the case of analytic functions $f(R)$, we are not able to obtain reliable constraints on the derivative constants $f_1$ and $f_2$, because the current observations of S2 star indicated that they may be highly mutually correlated. In the case of analytic functions $f(R,ϕ)$, we are able to obtain reliable constraints on the derivative constants $f_0$, $f_R$, $f_{RR}$, $f_ϕ$, $f_{ϕϕ}$ and $f_{ϕR}$. The approach we are proposing seems to be sufficiently reliable to constrain the modified gravity models from stellar orbits around Galactic Centre.

astro-ph.GA↗

arXiv2013

Constraining the range of Yukawa gravity interaction from S2 star orbits

We consider possible signatures for Yukawa gravity within the Galactic Central Parsec, based on our analysis of the S2 star orbital precession around the massive compact dark object at the Galactic Centre, and on the comparisons between the simulated orbits in Yukawa gravity and two independent sets of observations. Our simulations resulted in strong constraints on the range of Yukawa interaction $Λ$ and showed that its most probable value in the case of S2 star is around 5000 - 7000 AU. At the same time, we were not able to obtain reliable constrains on the universal constant $δ$ of Yukawa gravity, because the current observations of S2 star indicated that it may be highly correlated with parameter $Λ$ in the range $(0 <δ< 1)$. For $δ> 2$ they are not correlated. However, the same universal constant which was successfully applied to clusters of galaxies and rotation curves of spiral galaxies ($δ=1/3$) also gives a satisfactory agreement with the observed orbital precession of the S2 star, and in that case the most probable value for the scale parameter is $Λ\approx 3000 \pm 1500$ AU. Also, the Yukawa gravity potential induces precession of S2 star orbit in the same direction as General Relativity for $δ> 0$ and for $δ< -1$, and in the opposite direction for $-1 <δ< 0$. The future observations with advanced facilities, such as GRAVITY or/and European Extremely Large Telescope, are needed in order to verify these claims.

astro-ph.GA↗

arXiv2013

Composite profile of the Fe K$α$ spectral line emitted from a binary system of supermassive black holes

We used a model of a relativistic accretion disk around a supermassive black hole (SMBH), based on ray-tracing method in the Kerr metric, to study the variations of the composite Fe K$α$ line emitted from two accretion disks around SMBHs in a binary system. We assumed that the orbit of such a binary is approximately Keplerian, and simulated the composite line shapes for different orbital elements, accretion disk parameters and mass ratios of the components. The obtained results show that, if observed in the spectra of some SMBH binaries during their different orbital phases, such composite Fe K$α$ line profiles could be used to constrain the orbits and several properties of such SMBH binaries.

astro-ph.HE↗

arXiv2012

The First Spectroscopically Resolved Sub-parsec Orbit of a Supermassive Binary Black Hole

One of the most intriguing scenarios proposed to explain how active galactic nuclei are triggered involves the existence of a supermassive binary black hole system in their cores. Here we present an observational evidence for the first spectroscopically resolved sub-parsec orbit of a such system in the core of Seyfert galaxy NGC 4151. Using a method similar to those typically applied for spectroscopic binary stars we obtained radial velocity curves of the supermassive binary system, from which we calculated orbital elements and made estimates about the masses of components. Our analysis shows that periodic variations in the light and radial velocity curves can be accounted for an eccentric, sub-parsec Keplerian orbit of a 15.9-year period. The flux maximum in the lightcurve correspond to the approaching phase of a secondary component towards the observer. According to the obtained results we speculate that the periodic variations in the observed Hα line shape and flux are due to shock waves generated by the supersonic motion of the components through the surrounding medium. Given the large observational effort needed to reveal this spectroscopically resolved binary orbital motion we suggest that many such systems may exist in similar objects even if they are hard to find. Detecting more of them will provide us with insight into black hole mass growth process.

astro-ph.HE↗

arXiv2012

Constraints on $R^n$ gravity from precession of orbits of S2-like stars

We study some possible observational signatures of $R^n$ gravity at Galactic scales and how these signatures could be used for constraining this type of $f(R)$ gravity. For that purpose, we performed two-body simulations in $R^n$ gravity potential and analyzed the obtained trajectories of S2-like stars around Galactic center, as well as resulting parameter space of $R^n$ gravity potential. Here, we discuss the constraints on the $R^n$ gravity which can be obtained from the observations of orbits of S2-like stars with the present and next generations of large telescopes. We make comparison between the theoretical results and observations. Our results show that the most probable value for the parameter $r_c$ in $R^n$ gravity potential in the case of S2-like stars is $\sim$100 AU, while the universal parameter $β$ is close to 0.01. Also, the $R^n$ gravity potential induces the precession of S2-like stars orbit in opposite direction with respect to General Relativity, therefore, such a behavior of orbits qualitatively is similar to a behavior of Newtonian orbits with a bulk distribution of matter (including a stellar cluster and dark matter distributions).

astro-ph.CO↗

arXiv2012

Mass formulas for single-charm tetraquarks with Fermi-Breit hyperfine interaction

In this paper we present the main results of our investigation of the $cq \bar{q} \bar{q}$ single-charm scalar tetraquarks and their SU(3)$_\mathrm{F}$ representations: $\bar{15}_S$, $\bar{3}_S$, $6_A$ and $\bar{3}_A$. We use the Fermi-Breit interaction Hamiltonian with SU(3) flavor symmetry breaking to determine the masses of the single-charm tetraquarks. We also discuss mass spectra obtained from meson and baryon mass fits. The mass spectra are very similar to those obtained with Glozman-Riska hyperfine interaction, and they indicate that some of the experimentally detected states may have tetraquark nature.

hep-ph↗

arXiv2011

Spectra of HB 21 supernova remnant: Evidence of spectra flattening at the low frequencies

We use observations of the continuum radio emission at 1420, 820, 408, 34.5 and 22 MHz to estimate the mean brightness temperatures of the HB 21 supernova remnant (SNR) at five frequencies. We also presented mean spectral index of HB 21. The spectra of HB 21 are estimated for mean temperatures versus frequency for 1420, 820, 408, 34.5 and 22 MHz. We also presented $T-T$ plots of three frequency pairs: between 1420--34.5, 1420--22, 34.5--22 MHz. We noticed flatter spectral indices at frequencies below 408 MHz. Probably this is due to the absorption by thermal plasma at low frequencies.

astro-ph.GA↗

arXiv2011

High-energy plasmon spectroscopy of freestanding multilayer graphene

We present several applications of the layered electron gas model to electron energy loss spectroscopy of free-standing films consisting of $N$ graphene layers in a scanning transmission electron microscope. Using a two-fluid model for the single-layer polarizability, we discuss the evolution of high-energy plasmon spectra with $N$, and find good agreement with the recent experimental data for both multi-layer graphene with $N<10$, and thick slabs of graphite. Such applications of these analytical models help shed light on several features observed in the plasmon spectra of those structures, including the role of plasmon dispersion, dynamic interference in excitations of various plasmon eigenmodes, as well as the relevance of the bulk plasmon bands, rather than surface plasmons, in classifying the plasmon peaks.

cond-mat.mes-hall↗

arXiv2010

Constituent quark masses obtained from hadron masses with contributions of Fermi-Breit and Glozman-Riska hyperfine interactions

We use the color-spin and flavor-spin interaction Hamiltonians with SU(3) flavor symmetry breaking to obtain meson and baryon mass formulas. Adjusting these masses with experimental masses we determine the constituent quark masses. We discuss the constituent quark masses obtained from meson and baryon mass fits. The results for constituent quark masses are very similar in case of two different phenomenological models: Fermi-Breit and Glozman-Riska hyperfine interactions.

hep-ph↗

arXiv2010

Donut and dynamic polarization effects in proton channeling through carbon nanotubes

We investigate the angular and spatial distributions of protons of the energy of 0.223 MeV after channeling through an (11,~9) single-wall carbon nanotube of the length of 0.2 $μ$m. The proton incident angle is varied between 0 and 10 mrad, being close to the critical angle for channeling. We show that, as the proton incident angle increases and approaches the critical angle for channeling, a ring-like structure is developed in the angular distribution - donut effect. We demonstrate that it is the rainbow effect. When the proton incident angle is between zero and a half of the critical angle for channeling, the image force affects considerably the number and positions of the maxima of the angular and spatial distributions. However, when the proton incident angle is close to the critical angle for channeling, its influence on the angular and spatial distributions is reduced strongly. We demonstrate that the increase of the proton incident angle can lead to a significant rearrangement of the propagating protons within the nanotube. This effect may be used to locate atomic impurities in nanotubes as well as for creating nanosized proton beams to be used in materials science, biology and medicine.

cond-mat.mes-hall↗

arXiv2009

Dynamic polarization of graphene by moving external charges: random phase approximation

We evaluate the stopping and image forces on a charged particle moving parallel to a doped sheet of graphene by using the dielectric response formalism for graphene's $π$-electron bands in the random phase approximation (RPA). The forces are presented as functions of the particle speed and the particle distance for a broad range of charge-carrier densities in graphene. A detailed comparison with the results from a kinetic equation model reveal the importance of inter-band single-particle excitations in the RPA model for high particle speeds. We also consider the effects of a finite gap between graphene and a supporting substrate, as well as the effects of a finite damping rate that is included through the use of Mermin's procedure. The damping rate is estimated from a tentative comparison of the Mermin loss function with a HREELS experiment. In the limit of low particle speeds, several analytical results are obtained for the friction coefficient that show an intricate relationship between the charge-carrier density, the damping rate, and the particle distance, which may be relevant to surface processes and electrochemistry involving graphene.

cond-mat.mtrl-sci↗

arXiv2008

Proton channeling through long chiral carbon nanotubes: the rainbow route to equilibration

In this work we investigate the rainbows appearing in channeling of 1 GeV protons through the long (11, 9) single-wall carbon nanotubes. The nanotube length is varied from 10 to 500 micrometers. The angular distributions of channeled protons are computed using the numerical solution of the proton equations of motion in the transverse plane and the Monte Carlo method. Each rainbow is characterized by a sharp decrease of the proton yield on its large angle side. As the nanotube length increases, the number of rainbows increases and the average distance between them decreases in an easily predictable way. When the average distance between the rainbows becomes smaller than the resolution of the angular distribution, one cannot distinguish between the adjacent rainbows, and the angular distribution becomes equilibrated. We call this route to equilibration the rainbow route to equilibration. This work is a demonstration of how a simple one-dimensional bound dynamic system can exhibit a complex collective behavior.

cond-mat.mtrl-sci↗

arXiv2008

Channeling of Protons Through Carbon Nanotubes Embedded in Dielectric Media

We investigate how the dynamic polarization of the carbon atoms valence electrons affects the spatial distributions of protons channeled in the (11, 9) single-wall carbon nanotubes placed in vacuum and embedded in various dielectric media. The initial proton speed is varied between 3 and 8 a.u., corresponding to the energies between 0.223 and 1.59 MeV, respectively, while the nanotube length is varied between 0.1 and 0.8 $μ$m. The spatial distributions of channeled protons are generated using a computer simulation method, which includes the numerical solving of the proton equations of motion in the transverse plane. We show that the dynamic polarization effect can strongly affect the rainbow maxima in the spatial distributions, so as to increase the proton flux at the distances from the nanotube wall of the order of a few tenths of a nanometer at the expense of the flux at the nanotube center. While our findings are connected to the possible applications of nanosized ion beams created with the nanotubes embedded in various dielectric media for biomedical research and in materials modification, they also open the prospects of applying ion channeling for detecting and locating the atoms and molecules intercalated inside the nanotubes.

physics.atom-ph↗

arXiv2008

Dynamic polarization effects on the angular distributions of protons channeled through carbon nanotubes in dielectric media

The best level of ordering and straightening of carbon nanotube arrays is often achieved when they are grown in a dielectric matrix, so such structures present the most suitable candidates for future channeling experiments with carbon nanotubes. Consequently, we investigate here how the dynamic polarization of carbon valence electrons in the presence of various surrounding dielectric media affects the angular distributions of protons channeled through (11,~9) single-wall carbon nanotubes. Proton speeds between 3 and 10 a.u., corresponding to energies of 0.223 and 2.49 MeV, are chosen with the nanotube's length varied between 0.1 and 1 $μ$m. We describe the repulsive interaction between a proton and the nanotube's atoms in a continuum-potential approximation based on the Doyle-Turner potential, whereas the attractive image force on a proton is calculated using a two-dimensional hydrodynamic model for the dynamic response of the nanotube valence electrons, while assigning to the surrounding medium an appropriate (frequency dependent) dielectric function. The angular distributions of channeled protons are generated using a computer simulation method which solves the proton equations of motion in the transverse plane numerically. Our analysis shows that the presence of a dielectric medium can strongly affect both the appearance and positions of maxima in the angular distributions of channeled protons.

physics.atom-ph↗