arXiv subjects

Anuar Idrissov

Publications and source records attributed to Anuar Idrissov.

8 recordsLinked to original sources

arXiv2026

Thin accretion disk and gravitational capture cross sections of a quantum Oppenheimer--Snyder black hole immersed in an external magnetic field

We study a geometrically thin, optically thick Novikov--Thorne accretion disk and the gravitational capture cross sections of a quantum Oppenheimer--Snyder black hole immersed in an external, asymptotically uniform magnetic field. The exterior geometry carries a single quantum parameter, and we work over its entire admissible range, which contains a two-horizon black hole, an extremal configuration and a horizonless compact object, and which ends where the photon sphere disappears. Since the spacetime is static, the Wald potential is purely axial, so the field acts on the disk only through the Lorentz force on weakly charged accreting matter and all of its effects are controlled by a single magnetic coupling. We compute the charged circular orbits, the innermost stable circular orbit, the radiative flux, the effective temperature, the redshift factor, the differential and spectral luminosity and the radiative efficiency, together with the marginally bound orbit and the capture cross sections of massless, massive and charged particles. The construction is checked against the exact Schwarzschild limit and against the identity that relates the bolometric luminosity to the efficiency. The quantum parameter changes the disk observables only weakly, whereas the magnetic coupling raises the efficiency, the peak flux and the height of the spectral peak by large factors and shifts that peak to higher frequency. The two parameters act with opposite signs on absorption. The quantum parameter shrinks every capture cross section, while the magnetic coupling enlarges the one for massive particles and leaves the photon cross section unchanged, so that within this test-field model shadow observables respond to the quantum parameter alone. For charged particles the uniform field confines the motion, and no particle reaches the hole from beyond a magnetic shielding radius.

gr-qc↗

arXiv2026

Tidal forces in the quantum Oppenheimer--Snyder black hole

We investigate the tidal forces experienced by massive particles in radial free fall in the quantum Oppenheimer--Snyder black hole, a loop quantum gravity correction to the Schwarzschild geometry that describes the exterior of a bouncing dust ball. Using an orthonormal tetrad adapted to a freely falling observer, we show that the radial and angular components of the tidal tensor reproduce their Schwarzschild counterparts at large distances but reverse sign in the interior. The angular zero remains confined between the Cauchy and event horizons and coincides with the latter only in the extremal limit. A particle released from rest does not reach the center, it stops at a turning point inside the Cauchy horizon, where the ratio of the tidal components is independent of the parameters of the solution and is fixed by the transverse equation of state of the effective source. Solving the geodesic deviation equations for two sets of initial conditions, we find that the radial deviation vector attains its maximum at the minimum of the metric function, exactly for one set of initial conditions and asymptotically for the other, and remains finite throughout, in contrast with the Schwarzschild case, in which it diverges at the singularity. This regularity originates in the bounce rather than in a regular core, and therefore protects timelike radial infall while leaving radial null geodesics unaffected. Examining the full range of the quantum parameter, we find that its sign determines whether the tidal sector possesses any of this structure, while its magnitude determines only whether that structure is hidden: beyond the extremal value the geometry becomes horizonless, and the entire deformation history, bounce included, is exposed to distant observers.

gr-qc↗

arXiv2026

Newtonian Shirokov Effect: Epicyclic Frequency Splitting from Mass Multipoles

We analyze small oscillations of nearly circular orbits in an axisymmetric Newtonian potential expanded in mass multipoles, as the classical counterpart of the relativistic Shirokov effect. Computing the full Hessian of the effective potential at the true (possibly tilted) equilibrium and solving the coupled two-mode oscillator exactly, we obtain a complete picture. (i) A quadrupole splits the radial and vertical epicyclic frequencies, $\Omega_\theta^2-\Omega_r^2=-3GQ/r_0^5=6GMJ_2R^2/r_0^5$, at first order in $J_2$; the Newtonian analogue of the Shirokov splitting, equivalent to the classical statement that an oblate body's apsidal and nodal rates differ. (ii) A gravitational dipole produces no splitting: it equals $M r_{\rm CM}$, is removable by re-centering at the center of mass, and cannot appear in any coordinate independent frequency; the apparent first order coupling cancels at the true tilted equilibrium, any residual absorbed by the induced quadrupole of the shifted source, confirmed by direct orbit integration. (iii) A genuine octupole does split the frequencies, $\omega_+^2-\omega_-^2\approx6G|O|/r_0^6$. The selection rule is thus not even/odd parity: every multipole splits the frequencies except the dipole. These yield two complementary probes of an axisymmetric source: the frequency splitting measures the oblateness $J_2$, while the orbital plane tilt, $\delta\theta_0\simeq-r_{\rm CM}/r_0$, measures the center of mass offset $r_{\rm CM}$, an orbital geometry observable rather than a frequency one. We give solar system estimates for both. Carried through to Shirokov's original observable -- the secular transverse drift after $n$ orbits -- the quadrupole effect gives $\xi^\theta=\xi_0^{\theta}\,\pi n\,(6J_2R^2/r_0^2)$, of order $10^{-8}$ cm at $1$ au and $\sim10^{-6}$ cm near $0.1$ au, comparable to Shirokov's Schwarzschild estimate.

gr-qc↗

arXiv2026

Shirokov and Shapiro Effects in the Hartle-Thorne Spacetime

We investigate the influence of rotation and quadrupole deformations of astrophysical compact objects on the Shirokov and Shapiro effects within the Hartle-Thorne spacetime, which describes the exterior gravitational field of slowly rotating, slightly deformed celestial objects. Using geodesic deviation equations, we analyze the oscillatory motion of neighboring test particle trajectories and show how the combined impact of angular momentum $J$ and quadrupole moment $Q$ affects the Shirokov effect. The results are compared with our previous analysis for the Lense-Thirring and Zipoy-Voorhees metrics, revealing consistent trends in the coupling between radial and azimuthal oscillations. Importantly, by evaluating the period splitting in the weak-field regime we show that the dominant contribution to the Shirokov effect is the Newtonian quadrupole moment of the source rather than the relativistic mass term originally identified by Shirokov. For the Shapiro time delay, we examine two limiting configurations: (i) the Lense-Thirring frame -- dragging case with $J^2=0$, $Q=0$ and $J\neq0$, where the effect persists for both positive and negative values of the angular momentum; and (ii) the static quadrupolar case with $J=0$ and $Q\neq0$, where more oblate sources produce a stronger gravitational time delay with increasing distance. We also study these effects in the Hartle-Thorne spacetime without employing the weak-field approximation, performing a full numerical analysis. In particular, we examine the mimicking effects produced by the quadrupole deformation and the angular momentum of the compact object. These results illustrate how the deformation and rotation of compact objects influence the relativistic observables in the surrounding spacetime.

gr-qc↗

arXiv2025

Constraining quadrupole deformations with relativistic effects

We investigate two general relativistic effects - namely, the Shirokov and Shapiro effects - within the framework of the Zipoy-Voorhees spacetime ($q$-metric), which generalizes the Schwarzschild solution by incorporating a quadrupole moment. By analyzing the geodesic deviation equations, we explore the oscillatory motion of test particles and demonstrate how the source's quadrupole parameter influences the Shirokov effect. Furthermore, we derive an expression for the Shapiro time delay in this deformed spacetime and examine the quadrupole moment's impact on the gravitational time delay experienced by radio waves propagating near a massive object. The first-order approximation reveals a pronounced effect of the quadrupole parameter on the time delay, in contrast to similar recent analyses. These findings deepen our understanding of how deviations from spherical symmetry influence gravitational phenomena, with potential implications for the study of compact astrophysical objects such as neutron stars and naked singularities or ''black hole mimickers'' that exhibit significant multipolar structures.

gr-qc↗

arXiv2025

Geodesic deviation in the $q$-metric

We consider the tidal forces between test particles falling along geodesics in the exterior spacetime generated by a static and axially symmetric compact matter source with non-vanishing mass quadrupole. Specifically, we analyze the radial and angular geodesic deviation, compare it with that of the Schwarzschild spacetime, and investigate the impact of the deformation parameter $q$, at different polar angles $\theta$ with respect to the vertical symmetry axis. Furthermore, we examine the geodesic deviation for the case of non-constant $\theta$ during the radial fall. It is shown that the presence of the deformation parameter affects the behavior of the geodesic deviation vectors, depending on its value. In particular, we observe that for arbitrary values of $q$ and $\theta$ the behavior of the deviation vector differs as it approaches the singularity at $r = 2m$. Above all, we can witness either stretching or compressing of the deviation vector for various combinations of $q$ and $\theta$. These findings provide insight into the effects of quadrupole deformation on the motion of test particles in the vicinity of the central object.

gr-qc↗

arXiv2023

Quasi-periodic oscillations for spherically symmetric regular black holes

We consider the recent data sets of quasi-periodic oscillations from eight different low mass X-ray binaries. We here interpret their physical features in the context of given regular black hole solutions and verify their applicability to neutron star configurations. We evaluate the numerical constraints over the free parameters of Bardeen, Hayward and Dymnikova regular solutions by performing a set of Markov chain Monte Carlo analyses, based on the Metropolis algorithm. For each source, we evaluate the best-fit parameters, among which mass and magnetic charge, and compare and contrast them with the current literature. We also infer the corresponding innermost stable circular orbit radii and the radial extents of the accretion disks. Focusing on how to identify discrepancies among theoretical models and observations, our results show that, in most of the cases, regular black holes, in particular the Bardeen and Hayward spacetimes are slightly more suitable to describe neutron stars than Schwarzschild geometry, whereas the Dymnikova metric is ruled out.

astro-ph.HE↗

arXiv2020

Accretion disk luminosity for black holes surrounded by dark matter

We consider the observational properties of a static black hole space-time immersed in a dark matter envelope. We thus investigate how the modifications to geometry, induced by the presence of dark matter affect the luminosity of the black hole's accretion disk. We show that the same disk's luminosity produced by a black hole in vacuum may be produced by a smaller black hole if surrounded by dark matter under certain conditions. In particular, we demonstrate that the luminosity of the disk is markedly altered by dark matter's presence, suggesting that mass estimation of distant super-massive black holes may be changed if they are immersed in dark matter. We argue that a similar effect holds in more realistic scenarios and we discuss about the refractive index related to dark matter lensing. Hence we show how this may help explain the observed luminosity of super-massive black holes in the early universe.

astro-ph.HE↗