arXiv subjects

Sushant G. Ghosh

Publications and source records attributed to Sushant G. Ghosh.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics, Ringdown, and Accretion-Driven Multiple Quasi-Periodic Oscillations of Kerr-Bertotti-Robinson Black Holes

We study the motion of test particles around the Kerr--Bertotti--Robinson (KBR) black hole (BH) and explore how the three defining parameters, the mass $M$, rotation parameter $a$, and magnetic parameter $B$ influence their dynamics. We derive analytical expressions for the energy and angular momentum of stable equatorial circular orbits, along with the corresponding radial and latitudinal oscillation frequencies, as functions of $M$, $a$, and $B$. We also examine the key features of the quasi-periodic oscillations (QPOs) of test particles near stable circular orbits, including the precession effects such as periastron precession and the Lense-Thirring effect. We compare our results with those corresponding to the Kerr BH. We find that the particle motion is strongly shaped by the BH parameters. Using a WKB approach, we also study scalar quasinormal modes of rotating KBR BH in an external magnetic field and show that the magnetic field increases damping, while rotation and angular momentum mainly set the oscillation frequencies. Alternatively, general relativistic modeling of Bondi-Hoyle-Lyttleton (BHL) accretion onto rapidly rotating KBR BH shows that two distinct physical structures emerge and cyclically transform into one another over time. These processes produce either a strongly oscillating flip-flop shock cone or a nearly stationary toroidal structure, with their formation governed by the BH spin and magnetic curvature. Power spectral analysis shows that these configurations give rise to low- and high-frequency QPO, providing a unified theoretical framework to understand how multiple QPO-like features can arise in rapidly spinning accreting systems.

gr-qc↗

arXiv2026

Periodic orbits as probes of charged loop quantum gravity black holes through gravitational waves

Gravitational waves from extreme-mass-ratio inspirals (EMRI) provide a direct probe of the strong-field geometry of black holes. Motivated by this, we study the motion of test particles and the resulting gravitational wave emission in the spacetime of a charged black hole inspired by loop quantum gravity (LQG), where the classical singularity is replaced by a smooth transition surface arising from the LQG polymerization, in which its radius is set by the LQG area gap condition. As a result, the polymerization parameter $δ_b$ is uniquely determined by the mass $M$ and charge parameter $Q$, so that all cases examined in this work contain LQG correction. By constructing the effective potential, the innermost stable circular orbit (ISCO) and the marginally bound orbit (MBO) are determined. Periodic orbits are classified using the Levin-Perez-Giz zoom-whirl taxonomy, showing how the orbit topology shapes the waveform, so that each closed trajectory is labeled by the triple integer $(z, w, v)$ and located through the rational frequency ratio $q = ω_ϕ/ω_r - 1$. Within the quadrupole approximation, the gravitational waveforms for an EMRIs are estimated, and the resulting polarizations are obtained in the time-domain and frequency-domain. The resulting polarizations in the time-domain exhibit a zoom-whirl morphology, with the waveform amplitude and phase dependent on the LQG parameter. The characteristic strain peaks in the millihertz band for all values of the charge parameter $Q$, and they exceed the projected sensitivities of LISA, Taiji, and TianQin, suggesting that future observations could place meaningful constraints on the LQG polymerization parameter in the strong-field regime.

gr-qc↗

arXiv2026

Testing loop quantum gravity through EHT observations of M87* and Sgr A* using rotating holonomy-corrected black holes

The Event Horizon Telescope (EHT) has provided a new tool for testing the strong-field regime of gravity by imaging the shadows of M87* and Sgr A*. These observations provide the first real opportunity to test whether quantum gravity--specifically loop quantum gravity--leaves observable imprints on spacetime. We use the EHT observations of M87* and Sgr A* to examine the observational signs of rotating holonomy-corrected black holes (RHCBHs). We discover that, in comparison to the typical Kerr black hole, the quantum correction parameter $b$ increases the size of the black hole shadow. As the deviation parameter $b$ increases in RHCBH, the prograde photon orbits shift outward, indicating a weaker effective gravitational field near the central region. Unlike Kerr naked singularities, which produce open arc-like shadows, the RHCBH spacetime can still produce closed shadow rings even in the absence of an event horizon. We find that photon rings continue to exist in the parameter range $b_E \leq b \leq b_p$, due to the presence of unstable circular photon orbits.We apply the Kumar--Ghosh method based on the shadow observables: the shadow area $A$ and the oblateness $D$ that together allow a unique determination of the spin parameter $a$ and the quantum correction parameter $b$. At $θ_o=17$\textdegree~, the angular diameter bound of M87$^{*}$ yields $b \leq 0.1319\,M$ at $a = 0\,$ and $b \leq 0.421\,M$ at $a = 0.784\,M$, while at $θ_o=50$\textdegree~, the angular diameter bound of Sgr A$^{*}$ yields $b \leq 0.5764\,M$ at $a = 0\,$ and $b \leq 0.7482\,M$ at $a = 0.6253\,M$ the Sgr~A$^{*}$. Our results show that nonzero values of the holonomy correction parameter are consistent with current EHT data, indicating that RHCBHs provide viable alternatives to the classical Kerr geometry in the strong-gravity regime and are strong astrophysical black hole candidates.

gr-qc↗

arXiv2026

Probing Gravitational Wave Signatures from Periodic Orbits of Regular Black Holes in Asymptotically Safe Gravity

We investigate bound and periodic timelike geodesics and their associated gravitational-wave (GW) signatures in the spacetime of a regular black hole arising in asymptotically safe gravity (ASG). The geometry incorporates quantum corrections via a running gravitational coupling, encoded in a dimensional scaling parameter $ξ$, that modifies the near-horizon structure while preserving asymptotic flatness. We derive the effective potential for massive test particles and determine the conditions for stable circular and bound motion as functions of $ξ$, including the shift in the innermost stable circular orbit (ISCO). The three topological integers $(z,w,v)$, which represent the number of zooms, whirls, and vertices per radial cycle, are used to categorize the test particles' periodic orbits using Levin's zoom -- whirl taxonomy. Moreover, we employ the rational frequency ratio $q = \frac{ω_ϕ}{ω_r} - 1$ to find closed orbits, where $ω_ϕ$ and $ω_r$ stand for the azimuthal and radial frequencies, respectively. We examine how the orbital frequency spectrum is altered, whirl behaviour is enhanced, and deviations from the Schwarzschild limit are produced by the quantum parameter $ξ$. The GW forms for extreme mass-ratio inspirals (EMRIs) are calculated within the quadrupole approximation. We find that as $ξ$ increases, the signals that are released exhibit detectable amplitude modulations and phase shifts. The corresponding typical strain spectra fall within the anticipated sensitivity limits of space-based detectors such as LISA, Taiji, and TianQin, as they peak in the millihertz frequency band. Peak strain increases monotonically with $ξ$, indicating that observational restrictions on quantum-gravity-induced deviations from classical general relativity in the strong-field domain can be obtained from precise measurements of zoom -- whirl dynamics in EMRIs.

gr-qc↗

arXiv2026

Probing Kalb-Ramond gravity with charged rotating black holes: constraints from EHT observations

The Event Horizon Telescope (EHT) has guided strong-field gravitational physics by providing the first direct images of the supermassive black holes M87* and Sagittarius A*. The EHT observations offer unprecedented opportunities to test modified gravity theories against general relativity (GR). Motivated by this, we investigate charged rotating black holes in KR gravity, a framework motivated by string theory that incorporates spontaneous Lorentz symmetry breaking. The spacetime geometry is characterized by a Lorentz--violating parameter $\ell$ and electric charge $Q$, which modify the Kerr--Newman metric through a radial-dependent mass function. We compute black hole shadows and derive constraints on $\ell$ and $Q$ using EHT observations of M87* and Sgr A*. For angular shadow diameter $θ_{\rm sh}$ of M87* at inclination $θ_o=17^\circ$ and fixed $Q=0.2$, the EHT-allowed range $θ_{\rm sh}\in(35.1,\,40.5)\,μ\mathrm{as}$ constrains the Lorentz--violating parameter to approximately $-0.019\lesssim\ell\lesssim0.075$ and $-0.076\lesssim\ell\lesssim0.029$ across the admissible spin interval. For angular shadow diameter $θ_{\rm sh}$ of Sgr A* at inclination $θ_o=50^\circ$ and fixed $Q=0.2$, the corresponding EHT-allowed range $θ_{\rm sh}\in(41.7,\,55.7)\,μ\mathrm{as}$ permits approximately $-0.075\lesssim\ell\lesssim0.110$ and $-0.124\lesssim\ell\lesssim0.076$ across the admissible spin interval. Our analysis reveals that the Lorentz-violating parameter suppresses the shadow radius by a factor $\sqrt{1-\ell}$, while charge introduces additional distortions. Using the angular shadow diameter measured by EHT, we obtain an upper bound $\ell \lesssim 0.19$ from Sgr A* data with the stellar dynamics mass prior.

gr-qc↗

arXiv2026

Thermodynamics and phase transitions of charged-AdS black holes in dRGT massive gravity with nonlinear electrodynamics

Investigating black holes in modified theories of gravity offers fertile ground for exploring phenomena beyond the scope of general relativity. We investigate a novel class of charged anti-de Sitter (AdS) black holes within the ghost-free de Rham-Gabadadze-Tolley (dRGT) massive gravity, minimally coupled to an exponential form of nonlinear electrodynamics (NED). The NED sector is modelled by an exponential electrodynamics Lagrangian, which leads to singular black hole geometries in contrast to many regular configurations known in other NED models. In turn, we systematically investigate the thermodynamic properties and phase structure of the obtained black holes. The results show that the system has a rich thermodynamic structure. For different values of the magnetic charge $q$, the black hole can exhibit several types of phase transitions. These include van der Waals-like first-order phase transitions, second-order critical behavior, and a reentrant phase transition between small and large black holes without extending the phase space ($Λ=$constant). Our study enhances the understanding of AdS black holes in ghost-free massive gravity, providing further insights into the interplay between graviton mass and NED. The results highlight how the combined effects of graviton mass and electromagnetic nonlinearity can yield a rich and complex thermodynamic phase space, offering further insights relevant to the gauge/gravity duality and the ongoing search for observational signatures of modified gravity.

gr-qc↗

arXiv2026

Topological signatures in Kerr-Sen AdS black hole thermodynamics

Black hole thermodynamics and topology have emerged as a strong foundation for a coordinate-independent understanding of phase transitions. Using both Duan's topological current theory and a novel complex residue method, we perform a topological study of the Kerr-Sen AdS black hole arising in heterotic string theory. In turn, we find the zero points corresponding to on-shell black hole states and calculate their winding numbers to find the global topological charge by building the generalized off-shell free energy and examining the corresponding vector field in a parametric space. Our analysis reveals that the Kerr-Sen AdS black hole exhibits three distinct thermodynamic phases -- small, intermediate, and large black hole branches -- characterized by critical points with winding numbers $+1$, $-1$, and $+1$ respectively, culminating in a total topological charge $W = +1$. Significantly, this topological number remains invariant under variations of the dilaton charge parameter, indicating that the dilaton field does not alter the fundamental topological class established for Kerr-AdS and RN-AdS black holes. However, the rotation parameter proves crucial in determining the phase structure and the emergence of multiple critical points. We systematically examine three limiting configurations: the full Kerr-Sen AdS spacetime, the GMGHS AdS limit ($a = 0$), and the asymptotically flat Kerr-Sen case ($Λ= 0$). In addition, we propose a novel approach that analytically continues the thermodynamic characterisation into the complex plane. The characterized complex function, derived from the off-shell Gibbs free energy, possesses isolated singular points whose residues directly encode the winding numbers. Our results indicate that topology offers deep insights into black hole phase transitions, with potential implications to holographic dualities.

gr-qc↗

arXiv2026

Black hole solutions surrounded by an anisotropic fluid in a Kalb--Ramond two--form background

We investigate static, spherically symmetric black hole spacetimes induced by the spontaneous Lorentz--symmetry breaking of a Kalb--Ramond (KR) two--form field, non--minimally coupled to gravity, coexisting with an anisotropic fluid. By adopting a general equation of state where the radial pressure relates to the energy density via $w_1 = -1$ and the tangential pressure via an arbitrary parameter $w_2$, we derive exact analytical solutions representing black holes surrounded by diverse matter fields, including dust ($w_2=0$), radiation ($w_2=1/3$), and dark energy--like distributions ($w_2=-1/2$). A rigorous analysis of curvature invariants confirms a genuine core singularity, while the global geometry and adherence to standard energy conditions are shown to be highly sensitive to the interplay between the KR coupling ($\ell$), the fluid density parameter ($K$), and $w_2$. Furthermore, we analyze null geodesics in detail to determine the photon sphere and shadow radii. Using the Gibbons--Werner geometrical approach and the Gauss-Bonnet theorem applied to the optical metric, we compute the weak deflection angle of light and demonstrate that both the KR field and the anisotropic fluid significantly enhance light bending, particularly in dark--energy--like backgrounds. In the strong deflection limit (SDL), we calculate the lensing observables--$θ_\infty$, $s$, and $r_{\mathrm{mag}}$--for the supermassive black holes Sgr A* and M87*. Using EHT observations, we obtain constraints on the model parameters: for dust ($w_2=0$), the data of Sgr A* restricts $0\le \ell \le 0.065$ and $0\le K \le 0.04$, while for radiation ($w_2=1/3$), $K$ lies in $0.65\le K \le 0.85$ with $\ell$ unconstrained. We also derive similar bounds from M87*.

gr-qc↗

arXiv2026

Accretion flow around Kerr metric in the infra-red limit of asymptotically safe gravity

We investigate accretion disk dynamics and the formation of quasi-periodic oscillations (QPOs) in the infrared limit around Kerr-like black holes in asymptotically safe gravity. Relativistic hydrodynamic solutions of Bondi-Hoyle-Lyttleton (BHL) accretion reveal that quantum corrections significantly modify the structure of the shock cone formed around the black hole. The black hole spin controls the azimuthal asymmetry of the shock cone through frame-dragging effects, whereas the quantum correction parameter effectively reduces the strength of gravitational focusing by modifying the metric coefficients in the strong-field region, resulting in a wider shock opening angle, weaker post-shock compression, and reduced density concentration within the cone. Time-dependent mass accretion rates reveal oscillation modes trapped within the shock cone. The power spectral density (PSD) investigations suggest that these modes naturally generate low-frequency QPOs, whose amplitudes, coherence, and harmonic structure depend on both the spin and the quantum correction parameter. The PSD analyses performed at different radial locations reveal that identical QPO frequencies are obtained in all cases. The numerically detected frequencies result from the excitation of global oscillation modes trapped within the post-shock region. The resulting global modes are found to consist of fundamental frequencies, their associated harmonic overtones, and near-commensurate frequency ratios such as 2:1 and 3:2. Coherent oscillations are enhanced and near-commensurate frequency ratios are produced when moderate rotation and moderate quantum corrections are coupled. Large quantum correction parameters, on the other hand, wash out unique spectral peaks and suppress oscillation amplitudes.

astro-ph.HE↗

arXiv2026

Scale-Invariant Bounce Cosmology in Weyl f(Q) Gravity with Quintom Signature

We investigate a bouncing cosmological model within the Weyl-type $f(Q)$ gravity framework, employing a power-law form of the non-metricity scalar $Q$. The model successfully resolves the initial singularity problem by demonstrating a nonsingular bounce, where the universe transitions from a contracting phase $ \dot{a}(t)<0 $ to an expanding phase ($ \dot{a}(t)>0 $) at the bouncing point $t \approx 0.$ Key features include the violation of the null energy condition (NEC) near the bounce and the crossing of the phantom divide line ($ω=-1$) by the equation of state (EoS) parameter, indicating quintom-like behavior. The model exhibits accelerated expansion post-bounce, suggesting an inflationary phase. Stability analysis via the adiabatic index reveals instability near the bouncing point, while energy conditions highlight the dominance of dark energy. Additionally, the study explores scalar fields, showing that quintessence-like kinetic energy becomes negative and phantom-like kinetic energy peaks positively near the bounce, aligning with dark energy dynamics. The Hubble parameter, deceleration parameter, and Hubble radius further validate the bouncing scenario, with the latter displaying symmetric behaviour around the bounce. These results underscore the viability of Weyl-type $f(Q)$ gravity as a framework for nonsingular bouncing cosmologies, offering insights into early universe dynamics and dark energy behaviour.

gr-qc↗

arXiv2026

The Influence of Uniform Magnetic Fields on Strong Field Gravitational Lensing by Kerr Black Holes

We investigate strong gravitational lensing using magnetized Kerr black holes (MKBHs), which are accurate Kerr-Bertotti-Robinson solutions for Kerr black holes in a uniform magnetic field with additional magnetic field strength $B$ apart from mass $M$ and spin $a$. Unlike previous magnetized spacetimes, the MKBH geometry is Petrov type D, devoid of conical singularities, allowing photons to reach asymptotic infinity and making the concept astrophysically feasible. We use the strong deflection limit formalism to calculate the photon sphere radius, critical impact parameter, deflection angle, and lensing observables including the image position $θ_\infty$, angular separation $s$ and relative magnification $r_{\text{mag}}$, as well as their relationships with the parameters $a$ and $B$. Our results reveal that the relativistic image's photon sphere and angular size increase with $B$, whereas lensing observables deviate significantly from the Kerr scenario. For M87*, with $a=0.9$, the angular position of relativistic images increases from $10.8~μ$as (Kerr) to $12.02~μ$as, and the time delay between the first two images increases from $158.5$ h to $176$ h at $B=0.4$. Similarly, for Sgr A*, the image position increases from $14.4~μ$as to $16~μ$as, with time delays enhanced by approximately $0.7$ minutes. The relative magnification $r_{\text{mag}}$ grows with $B$ and deviates by $0.53$ from Kerr black holes at $B=0.4$. Our findings highlight strong gravitational lensing as a powerful tool to probe the presence of magnetic fields around astrophysical black holes, and in particular, we demonstrate that the MKBH spacetime enables constraints on the parameters $a$ and $B$.

gr-qc↗

arXiv2026

Parameter estimation of Kerr-Bertotti-Robinson black holes using their shadows

We investigate the shadow of Kerr-Bertotti-Robinson black holes (KBRBHs), which have a deviation parameter $B$ that captures the effect of an external magnetic field on the spacetime geometry. These spacetimes of Petrov type $D$ are asymptotically non-flat. We utilise the separability of the Hamilton-Jacobi equation to generate null geodesics and examine the crucial impact parameters for unstable photon orbits that define the black hole shadow. We carefully investigate how the magnetic field strength $B$ and spin parameter $a$ influence black hole shadows, discovering that increasing $B$ increases the shadow size while also introducing additional distortions, especially at high spins. We calculate the shadow observables, viz., area $A$ and oblateness $D$ and create contour plots in the parameter space $(a, B)$ to facilitate parameter estimation. We also investigate the dependence of the shadow on the observer's position, specifically by altering the radial coordinate $r_O$ and the inclination angle $θ$. For far viewers, the shadow approaches its asymptotic shape, but finite-distance observers perceive substantial deviations. The energy emission rate analysis reveals that the magnetic field parameter $B$ modifies the Hawking radiation spectrum, with increasing $B$ suppressing emission via backreaction, which lowers the Hawking temperature. Our findings confirm that KBRBH shadows encode imprints of magnetic deviations, thereby offering a potential avenue to distinguish Kerr from non-Kerr spacetimes and to probe magnetic effects in the strong-gravity regime.

gr-qc↗

arXiv2026

The extended phase space thermodynamics and Ehrenfest scheme for the Kerr-Sen AdS black holes

In the present work, we numerically investigate the horizon structure of the Kerr-Sen black holes in anti-de Sitter (AdS) spacetime. Further, we investigate the phase transitions and critical phenomena in Kerr-Sen-AdS black holes at the critical points. Such black holes are characterized by its mass ($M$), the dilaton charge ($Q$), and the negative cosmological constant, $Λ(<0)$. We define a dimensionless parameter $ε=\bar{J}/{\bar{Q}^2}$ and express the mass, temperature, volume, and Gibbs free energy in terms of $ε$ and its polynomials. Moreover, we numerically fit the data for the critical points and find that in the appropriate limit, the expressions for critical points would correspond to the respective critical points of the Kerr-AdS black hole thermodynamics. Such a study involves a systematic analysis of temperature, Gibbs free energy, and volume in the extended phase space. We provide an analytical verification of the nature of the phase transitions at the critical points by introducing the Ehrenfest equations. We also check that all three quantities, e.g., the specific heat at constant pressure, $C_P$, the volume expansion coefficient, $α$, and the isothermal compressibility, $κ_T$, diverge at the critical points. We find the $Prigogine$-$Defay$ ratio using the expressions of $C_P$, $α$, and $κ_T$, and find that it identically equals unity. Hence, the phase transition behavior of the Kerr-Sen-AdS black holes at their critical points is of second order. In addition, we propose investigating the energy extraction process via the Penrose process. Later, we calculate the speed of sound and adiabatic compressibility for the rotating Kerr-Sen-AdS black holes. Finally, on a specific note, we calculate the thermodynamic quantities of the boundary conformal field theory (CFT) dual to the extended phase space.

gr-qc↗

arXiv2025

Probing Loop Quantum Gravity black holes through gravitational lensing

We investigate strong gravitational lensing by a charged loop quantum gravity (LQG) black hole obtained through the polymerisation scheme of Borges \textit{et al.} \cite{Borges:2023fog}. These effective geometries replace the Reissner--Nordström singularity with a symmetric transition surface and admit an extremal, cold remnant determined by the minimal area gap in LQG. In turn, we derive the null geodesic equations, investigate the photon effective potential, and obtain expressions for the photon-sphere radius and critical impact parameter. We compute the weak-field deflection angle and Einstein ring size, highlighting the deviations induced by the polymerisation parameter and the Barbero--Immirzi parameter. In the strong-field regime, we compute the strong deflection coefficients $(\bar{a},\bar{b})$ and evaluate the lensing observables $θ_\infty$, $s$, and $r_{\rm mag}$. Unlike the Reissner--Nordström case, the LQG corrections enhance the deflection angle and increase the angular separation of relativistic images, with deviations growing as the geometry approaches the LQG remnant limit. We further compute the corresponding observables for Sgr~A* and M87*, finding that the quantum-gravity modifications lie within the potential sensitivity of next-generation VLBI facilities. For M87*, the angular separation $s\in(0.05712,0.19123)\,μ\text{as}$, while it is $s\in(0.07595,0.25426)\,μ\text{as}$ for Sgr A*. The relative flux ratio is found to lie in the range, $r_{\rm mag}\in(4.49272,5.96397)$. Our analysis demonstrates that LQG-induced corrections leave characteristic strong and weak-lensing imprints, offering a promising observational pathway to probe quantum gravity using near-future high-resolution observations.

gr-qc↗

arXiv2025

Probing Lorentz Symmetry Violation through Lensing Observables of Rotating Black Holes

We find a Kerr-like black hole solution-a rotating Bumblebee black hole (RBBH) with a Lorentz-violating parameter $\ell$ and examine the strong lensing by it. The parameter $\ell$ changes the event horizon radius and photon sphere, resulting in a different lensing signature compared to the Kerr black hole of general relativity. Using the strong deflection limit formalism, we compute key observables such as the angular positions of relativistic images, their separation, magnification, and time delays for supermassive black holes Sgr A* and M87*. Our results show that the parameter $\ell$ has a profound influence on these observables, with $\ell > 0$ suppressing and $\ell < 0$ increasing the deflection angle compared to the Kerr case. We compare RBBH observables with those of Kerr black holes, using Sgr A* and M87* as lenses to observe the effect of the Lorentz symmetry-breaking parameter $\ell$. For Sgr A*, the angular position $θ_\infty$ in $\in~(18.25-33.3)~μas$, while for M87* $\in~(13.71-25.02)~μas$. The angular separation $s$, for supermassive black holes (SMBHs) Sgr A* and M87*, differs significantly, with values ranging $\in~(0.005-0.81)~μas$ for Sgr A* and $\in~(0.003-0.6)~μas$ for M87*. The relative magnitude $r_{\text{mag}}$ $\in~(3.04-8.15)~μas$. We also compared the time delays between the relativistic images in the SMBHs and found that RBBH can be quantitatively distinguished from Kerr black holes. Our analysis concludes that, within the 1$σ$ region, a significant portion of the parameter space agrees with the EHT results of M87* and Sgr A*. This demonstrates the feasibility of utilizing strong gravitational lensing to identify Lorentz symmetry violations in extreme gravity regimes. Weak lensing analysis and Einstein ring observations provide further constraints, producing an upper bound of $\ell \lesssim \mathcal{O}(10^{-6})$.

gr-qc↗

arXiv2025

Lyapunov Exponent Approach to Phase Structure of Schwarzschild AdS Black Holes Surrounded by a Cloud of Strings

We investigate Schwarzschild black holes in anti-de Sitter (AdS) spacetimes surrounded by a cloud of strings (BH-AdS-CoS), incorporating both electric- and magnetic-like components of the string bi-vector. Thermodynamically, these systems exhibit small/intermediate/large black hole phases with first- and second-order transitions governed by the string parameter $c_0$. Dynamically, we probe the phase structure using Lyapunov exponents $λ$ from unstable circular geodesics. For massless particles ($δ= 0$), analytical expressions $λ$ reveal multivalued behavior in first-order transition regimes ($c_0 < c_{\text{cri}}$), with branches mapping to thermodynamic phases ($λ_{\text{SBH}}, λ_{\text{IBH}}, λ_{\text{LBH}}$). The discontinuity $Δλ= λ_{\text{SBH}} - λ_{\text{LBH}}$ at $T_p$ follows mean-field scaling: $Δλ/ λ_{\text{cri}} \propto (T_\text{cri} - T)^{1/2} \quad (β= 1/2)$. For massive particles ($δ= 1$), numerical computation of timelike geodesics confirms $λ$ as an order parameter, with critical exponent $β= 1/2$ universally. Key distinctions emerge: $λ\to 1$ asymptotically for photons, while $λ\to 0$ in the significant black hole phase for massive particles due to vanishing unstable orbits. The transition of $λ$ from multivalued to single-valued at $c_0 = c_{\text{cri}}$ establishes it as a universal dynamical probe of black hole criticality. The universal critical exponent of 1/2 for $Δλ$ further reinforces the analogy with conventional thermodynamic systems. Our results confirm a direct connection between the thermodynamic phase structure of BH-AdS-CoS and the dynamics of test particles, with the Lyapunov exponent emerging as a sensitive diagnostic of black hole criticality.

gr-qc↗

arXiv2025

Testing Strong Gravitational Lensing Effects of Supermassive Black Holes with String-Inspired Metric: Observational Signatures and EHT Constraints

We analyze gravitational lensing in the strong field limit for spherically symmetric string-inspired Euler-Heisenberg black holes, characterized by magnetic charge ($q$) and Einstein-Maxwell-dilaton coupling constants ($α, β$) from the low-energy limit of heterotic string theory. Our results show that the string coupling has a weak impact on the positions of relativistic images, deflection angles, photon orbit radii, and shadow sizes, making these black holes indistinguishable from the Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) black holes with the same mass and charge. Compared to Schwarzschild black holes, the string-inspired Euler-Heisenberg black holes exhibit smaller deflection angles, decreasing with increasing charge. Moreover, the time delay for Sgr A * and M87 * can reach $~11.477$ and $~17349.8$ minutes, respectively, at $q=0.1$ and $η=-1$, deviating from Schwarzschild black holes by $~0.0198$ and $~28.9$ minutes, which are not very significant. For Sgr A* and M87*, we determine $θ_\infty$ range within $(11.52, 26.33)~μas$, and $(9.17, 19.78)~μas$ respectively, with angular separations $s$ ranging from $(3.29-6.85)~nas$ for Sgr A* and $(2.47-5.15)~nas$ for M87*. EHT bounds on the $θ_{sh}$ of Sgr A* and M87* within the $1σ$ interval bound the $q$ as: for Sgr A* $0.54109\le q \le 0.7796 $ and for M87* $0< q \le 0.29107$, while in both the cases, we did not find any bound on the parameter $η$. We show that string-inspired Euler-Heisenberg black holes and EHT observations agree in the finite parameter space. A discussion on the effective metric has been included.

gr-qc↗

arXiv2025

Testing Quantum-Corrected Black Holes with QPOs Observations: A Study of Particle Dynamics and Accretion Flow

We study the epicyclic oscillations of test particles around rotating quantum-corrected black holes (QCBHs), characterized by mass $M$, spin $a$, and quantum deformation parameter $b$. By deriving the radial ($Ω_r$) and vertical ($Ω_θ$) oscillation frequencies, we explore their dependence on spacetime parameters and show that quantum corrections ($b \neq 0$) significantly modify the dynamics compared to the classical Kerr case. Through numerical modelling of accretion around QCBHs, we further examine how $b$ influences strong-field phenomena, comparing the results with test-particle dynamics and observational data. Our analysis reveals: 1. Quantum corrections shift the ISCOs outward, with $b$ altering the effective potential and conditions for stable circular motion. 2. The curvature of the potential and thus the epicyclic frequencies change $Ω_r$ shows up to 25% deviation for typical $b$ values, underscoring sensitivity to quantum effects. 3. Precession behavior is modified: while Lense-Thirring precession ($Ω_{LT}$) remains primarily governed by $a$, periastron precession ($Ω_P$) is notably affected by $b$, especially near the black hole. 4. Accretion disk simulations confirm the physical effects of $b$, aligning well with the test particle analysis. Moreover, quasi-periodic oscillation (QPO) frequencies obtained via both approaches agree with observed low-frequency QPOs from sources like GRS $1915+105$, GRO $J1655{-}40$, XTE $J1550{-}564$, and $H1743{-}322$. The distinct frequency profiles and altered ratios offer observational signatures that may distinguish QCBHs from classical black holes. Our findings present testable predictions for X-ray timing and a new avenue to constrain quantum gravity parameters.

gr-qc↗