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Z. Yasakov

Publications and source records attributed to Z. Yasakov.

5 recordsLinked to original sources
arXiv2026

Shadow signatures and near-horizon energy accumulation in coherent quantum sine-integral black holes

Quantum modifications of black hole spacetimes can lead to observable departures from the predictions of classical general relativity, particularly in the strong-gravity regime near the event horizon. In this context, the coherent quantum black hole model introduces a sine-integral correction to the gravitational potential, characterized by a coherence scale \(R_s\), which softens the central singularity and alters the near-horizon geometry. Motivated by the growing precision of black hole imaging and strong-field observations, we explore how these quantum-induced effects influence photon motion and the resulting shadow structure. Considering representative values within the interval \(R_s \in [0.5M, 1.2M]\), we show that the photon sphere shifts inward while the critical impact parameter decreases, reaching values such as \(b_{\rm crit}\approx 4.764M\) for \(R_s=0.5M\). Consequently, the shadow radius becomes systematically smaller and develops a more intricate structure than in the Schwarzschild case (\(b_{\rm crit}=3\sqrt{3}M \approx 5.196M\)). The analysis of the corresponding intensity distributions also reveals a pronounced enhancement of radiation near the shadow boundary, especially for intermediate coherence scales. This feature can be attributed to photons remaining longer within the modified near-horizon region before escaping toward distant observers. Such an increase in brightness may therefore provide a possible observational signature of quantum corrections at horizon scales. Furthermore, although the geometry allows radiation and energy to accumulate around an effective horizon surface, its response to perturbations differs from that expected for compact objects with stable light rings. Instead of contracting under backreaction effects, the accumulation surface shifts outward.

physics.gen-ph↗
arXiv2026

Coherent quantum state corrections in RN--black holes: Photon dynamics, shadow casts, and energy backreaction

We revisit the Reissner-Nordström (RN) geometry within the coherent quantum black hole framework, introducing a version that remains well-behaved at short distances while preserving the expected classical behavior at large scales. The key ingredient is a smooth Gaussian regularization, which replaces the usual sharp cutoff and {suppresses} the unphysical oscillations. This construction introduces a characteristic core radius $R_{\rm s}$, constrained relative to the mass $M$ and charge $Q$ to ensure either a {Ricci-regular} center or a softened integrable singularity. Near the origin, the spacetime exhibits distinct behaviors depending on the choice of parameters. When $R_{\rm s}$ satisfies the critical relation $R_{\rm s}^* = \sqrtπ\,R_{\rm Q}^2 / R_{\rm M}$, the geometry is {Ricci-regulFar}, while deviations from this value produce a milder, integrable singularity. {We clarify that for generic parameters, the Kretschmann scalar retains a $1/r$ divergence even at $R_{\rm s}^*$, so the center is not fully regular in the sense of finite curvature invariants.} The effective stress-energy tensor can be interpreted as an anisotropic fluid, whose density and pressure smoothly interpolate between the quantum-dominated core and the classical RN region at large radii. The Misner-Sharp mass accumulates gradually from the center, recovering the ADM mass asymptotically. The quantum scale also modifies the horizon structure, giving rise to standard, extremal, or horizonless configurations depending on the ratios $α=R_{\rm Q}/R_{\rm M}$ and $β=R_{\rm s}/R_{\rm M}$. Photon motion is correspondingly affected: the photon sphere moves inward, light deflection and Shapiro delay are slightly reduced, and the resulting shadow appears more compact and smoother, reflecting the quantum-corrected geometry.

gr-qc↗
arXiv2026

Joint Constraints on Quantum Oppenheimer--Snyder Black Holes from High-Frequency Quasi-Periodic Oscillations in X-ray Binary Systems and the S2 Star Orbit

We study a quantum-corrected version of the Oppenheimer--Snyder (OS) spacetime in which classical collapse is modified by a single effective-parameter \(α\), representing small-quantum-gravitational effects in a phenomenologically rescaled-form. Although the correction enters through a term proportional to \(αM^2/r^4\), it produces noticeable changes in the strong-field region while leaving weak-field physics almost unchanged. One of the main results is the appearance of a minimum-mass for horizon-formation, \(M_{\min} \sim(0.5-1.2)\,M_\odot\) for the effective-parameter \(α\approx0.22\). We emphasize that this \(M_{\min}\) is expressed in terms of the effective-parameter \(α\); in terms of the bare loop-quantum-gravity (LQG) parameter, \(M_{\min}\sim M_{\text{Pl}}\sim10^{-5}\)~g, consistent with the theoretical-expectation. Orbital properties are also slightly modified, with the innermost-stable-circular-orbit (ISCO) shifting to \(r_{\rm ISCO}=6M-α/(12M)\), giving corrections-of-order \( \sim 10^{-4} \) for stellar-black-holes and \(\sim 10^{-10}\) for Sgr A$^\ast$. In the same region, radial epicyclic frequencies change by about \(5\%-10\%\) near \(r\sim5M\), while vertical modes are less affected, leading to small but structured shifts in quasi-periodic oscillation (QPO) behavior. Using four X-ray binaries (GRS 1915+105, XTE J1550-564, XTE J1859+226, GRO J1655-40), we find a consistent range \(α=0.22\pm0.10\), with masses between \(5.4-12.4\,M_\odot\) and emission radii \(r/M \sim 5.5 - 8.6\). The observed QPO frequencies, lying in the range \(100-450\) Hz, are well reproduced within this framework. Weak-field tests such as S2 and Mercury still allow \(f_{\rm sp}=1.10 \pm 0.19\), leaving room for these small strong-gravity corrections.

gr-qc↗
arXiv2026

Observational Constraints on Kazakov-Solodukhin Quantum-Deformed Black Holes from M87$^*$ and Sgr A$^*$ Shadows

We explore the Kazakov-Solodukhin quantum-deformed black hole spacetime, characterized by a single deformation parameter \( η\) that encodes quantum corrections to the classical Schwarzschild solution. The model preserves the correct general-relativistic limit as \( η\to 0 \), while introducing significant and physically meaningful deviations in the strong-field regime. A central and remarkable feature of the geometry is the regularization of the classical singularity: curvature invariants remain finite near the minimal radius \( r = η\), effectively replacing the divergent core with a smooth and well-behaved region. This behavior naturally introduces a minimal length scale into the spacetime structure, offering a geometrically motivated resolution of the singularity problem. The deformation modifies the horizon structure, shifts the event horizon location, and alters the mass-radius relation. It also reduces the surface gravity, leading to a lower Hawking temperature and a slower evaporation process, thereby enhancing the thermodynamic stability of the black hole. Photon dynamics are correspondingly affected, resulting in a displaced photon sphere and modified strong-lensing characteristics. While the shadow remains perfectly circular due to spherical symmetry, its size depends sensitively on \( η\). Observational constraints can be expressed through \( \left| R_{sh}(η) - R_{obs} \right| \leq ΔR_{obs}, \) which places an upper bound on the deformation parameter. In the weak-field limit, the deflection angle acquires a quadratic correction proportional to \( η^2 \), ensuring consistency with precision tests while allowing potentially detectable deviations in strong-gravity observations. These features make the model both theoretically appealing and observationally testable.

gr-qc↗
arXiv2026

Slowly rotating traversable wormholes supported by radially varying string-fluid matter: From regular geometries to photon trajectories

This work investigates slowly rotating traversable wormholes supported by string fluids whose properties vary with distance from the throat. This radial variation allows the matter to transition smoothly from a de Sitter-like core near the center to a string-dominated environment further out, producing a regular, horizon-free, and asymptotically flat spacetime. By letting the transverse pressure depend on radius, the fluid naturally adapts to the surrounding geometry, resulting in a well-behaved energy density and shape function. Even modest rotation introduces frame-dragging effects that gently twist photon paths, creating subtle differences between co-rotating and counter-rotating trajectories. These effects are strongest near the throat, while at larger distances the spacetime is largely governed by the static gravitational potentials. Circular photon orbits reveal that the interplay of the redshift function, wormhole shape, and rotation shapes the photon-sphere structure. Different radial profiles of the string fluid generate distinctive photon-ring patterns, offering potential observational signatures of both the rotation and the internal matter distribution. Overall, radially varying string fluids provide a flexible and physically consistent source for traversable wormholes, bridging smoothly between vacuum-like and string-dominated regions while maintaining regularity and supporting slow rotation. This study highlights how anisotropic matter can influence both curvature and light propagation, providing a realistic framework for horizonless exotic spacetimes and suggesting new avenues to explore subtle observational effects around traversable wormholes.

gr-qc↗