arXiv subjects

G. Mustafa

Publications and source records attributed to G. Mustafa.

At least 37 records · Page 2Linked to original sources

arXiv2026

Unified Mass-Scaled QPO Signatures of Kerr Sen Black Holes from Stellar Mass to Supermassive Sources

In this study, we numerically investigate Bondi-Hoyle-Lyttleton (BHL) accretion around Kerr-Sen black holes and examine how the charge-related deformation of the spacetime affects the shock-cone morphology, the variation of the mass accretion rate, and the quasi-periodic oscillation (QPO)-like temporal behavior. The relativistic BHL flow is solved numerically in the equatorial plane for two different black hole spin parameters, a = 0.9 M and a = 0.5 M. From the numerically computed mass accretion rate signal, we calculate the power spectral density (PSD) and perform multi-component Lorentzian fits to identify the dominant QPO-like modes excited around the black hole. The results show that the Kerr-Sen deformation shifts the characteristic frequencies, changes the coherence properties of the oscillation modes, and produces near-resonant harmonic structures close to 3:2 and 2:1. By using inverse mass scaling, the numerically computed frequencies are compared with observed QPOs from stellar-mass, intermediate-mass, and supermassive black hole systems. In particular, reasonable agreement between the numerical simulation results and observations is found for the sources GRS 1915+105, IGR J17091-3624, M82 X-1, NGC 5408 X-1, RE J1034+396, 1H 0707-495, and ESO 113-G010. This comparative analysis indicates that Kerr-Sen black hole shock-cone oscillations may provide a unified framework for interpreting timing features over a broad range of black hole masses and may additionally contribute to constraining the mass and spin parameters of sources whose properties are not yet fully established observationally. These findings further imply that combined hydrodynamical and timing diagnostics constitute a promising approach for assessing the extent to which deviations associated with the Kerr-Sen geometry can be empirically distinguished from those of the Kerr spacetime.

astro-ph.HE↗

arXiv2026

QPO-like Signatures and Hydrodynamical Variability in Accretion around a JNW-type Compact Spacetime in Freund-Nambu Scalar-Tensor Gravity

Scalar tensor theories of gravity provide a broad as well as physically rich extension of general theory of relativity by allowing the gravitational interaction to be mediated not only by the spacetime metric but also by scalar degrees of freedom. In this manuscript, we present a new exact solution in the Freund-Nambu scalar-tensor (FNST) gravity scenario, representing a nontrivial scalar-tensor generalization of the Janis-Newman-Winicour naked-singularity geometry, characterized by an additional coupling parameter q in the scalar sector. We also numerically solve the general relativistic hydrodynamic equations in order to investigate the shock-cone mechanism formed by Bondi-Hoyle-Lyttleton accretion around this compact spacetime on the equatorial plane. We show that stronger scalar-tensor deviations modify the shock-cone morphology, significantly increase the amount of matter accumulated near the central compact object, and enhance the oscillatory behavior of the shock cone. The Lorentzian-like peaks obtained from the numerically computed power spectral density are interpreted as hydrodynamically generated QPO-like modes. These modes are driven by shock cone oscillations and by the compression and rarefaction of the plasma trapped inside the cone. Finally, for a compact object with mass parameter M = 10M_sun, the numerically extracted frequencies are found mainly in the range from a few Hz up to approximately 100 Hz. These frequencies overlap with the QPO ranges reported in stellar-mass black-hole-candidate systems. In particular, the frequencies obtained for the FNST2-FNST4 models fall within the range of timing features reported for the source GRS 1915+105. These results suggest that the exterior hydrodynamical variability of FNST compact spacetimes may provide phenomenological diagnostics of scalar-field-induced deviations from the Schwarzschild reference case.

astro-ph.HE↗

arXiv2026

Dilaton-Induced Resonant Production of Ultralight Vector Dark Matter

A dilatonic half-mass resonance can produce ultralight vector dark matter only if the Floquet instability becomes efficient before the oscillating spectator scalar dominates the cosmic expansion. We formulate this requirement in terms of the microscopic modulation parameter $ε_i=Φ_i/M$ and the gravitational onset fraction $r_i=Φ_i^2/(6\Mpl^2)$. For a background with constant equation-of-state parameter $w_b$, the narrow-band Floquet exponent obeys $μ/H\propto a^{3w_b/2}$; during radiation domination this ratio grows as $a^{1/2}$, while it remains constant for matter-like expansion. Imposing that the delayed instability occurs before spectator domination yields the amplitude-independent bound $M/\Mpl\lesssim\sqrt6,c_1\simeq0.31$, with $c_1\simeq1/8$ determined by the linear half-mass branch. An explicit expanding-background analysis confirms that $a_\star<a_{\rm dom}$ for sub-Planckian $M$, whereas $M\simeq\Mpl$ postpones efficient growth until after domination. Combining this embedding condition with the efficient-transfer normalization gives $m_{γ'}\propto r_i^{-2}$, implying that the ultralight range $m_{γ'}\sim10^{-20}$--$10^{-18},{\rm eV}$ corresponds to $r_i\sim10^{-5}$--$10^{-4}$ rather than to early spectator domination. The polarization-resolved canonical analysis shows that longitudinal production is more strongly concentrated in the infrared than transverse production, while derivative terms from canonical normalization modify the leading Floquet exponents at order unity. Stückelberg and Higgsed completions impose distinct ultraviolet consistency conditions, including radial decoupling and symmetry-restoration constraints. The viable branch is therefore radiation-era, perturbative, infrared-dominated, and associated with a sub-Planckian kinetic scale.

hep-ph↗

arXiv2026

Disformal Kerr Imprints on BHL Accretion: Shock Morphology, PSD Signatures, and Observational QPO Counterparts

We reveal the effect of the spacetime parameters on the accretion morphology formed through the BHL mechanism around a slowly rotating disformal Kerr black hole. Thus, we investigate the measurable signatures of these parameters on the hydrodynamical morphology and the timing behavior of the accreting flow. It is shown that even weak disformal deviations from the Kerr solution modify the shock-cone structure, enhance the density in the post-shock region, and produce coherent oscillations in the accretion rate. The Kerr model produces coherent peaks at 42.99 Hz and 68.13 Hz, and these frequencies are consistent with the high-frequency QPOs observed from the source GRS 1915+105. In the models where the deviations from the Kerr solution are weak, low-frequency QPOs are produced and found to be coherent. These frequencies also fall within the frequency range observed in Galactic black-hole binaries. On the other hand, the models with large deviations from Kerr can be used to explain observational results that are more irregular, broad-band, and contain multiple peaks. In addition, by using inverse-mass scaling in this work, the numerically calculated frequencies are also compared with observations of intermediate-mass and supermassive black holes. In particular, the disformal black-hole models are found to be consistent with the observational results obtained from the sources M82 X-1, NGC 5408 X-1, and RE J1034+396. This comparison also allows the possible black-hole mass range of observed sources to be inferred from the relation between simulated and observed frequencies. This makes BHL accretion in disformal Kerr geometry a powerful framework for connecting modified-gravity black-hole spacetimes with observable QPO phenomenology.

astro-ph.HE↗

arXiv2026

Evidence of dynamical dark energy found via the DESI DR2 Lyman$α$ forest

We present a comprehensive analysis of the cosmological implications of the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2) Lyman-$α$ forest baryon acoustic oscillation (BAO) measurements, combined with DESI DR2 galaxy BAO, Type Ia supernova samples (Pantheon$^+$, DES-Dovekie, and Union3), and the cosmic microwave background CamSpec likelihood. We consider several dark-energy parameterizations, including Chevallier-Polarski-Linder, logarithmic, exponential, Jassal-Bagla-Padmanabhan, Barboza-Alcaniz, and generalized emergent dark energy, as well as the $w$CDM model and non-flat extensions of $Λ$CDM and $w$CDM. Using the Metropolis-Hastings MCMC algorithm, we constrain cosmological parameters and compute Bayesian evidence with \texttt{MCEvidence}. We find that non-flat extensions remain consistent with spatial flatness, with $Ω_k \approx 0$. All parameterizations favor a dynamical dark-energy scenario with $w_0 > -1$, $w_a < 0$, and $w_0 + w_a < -1$, consistent with a Quintom-B behavior. A moderate preference for dynamical dark-energy models is found relative to $Λ$CDM, reaching up to $\sim3.10σ$ for Ly$α$ + CMB + galaxy BAO. When combined with SNe~Ia datasets, the deviations decrease to $\lesssim2σ$, corresponding to inconclusive preference. The Bayes factor ($\ln B_{ij}$) shows that model preference depends strongly on the dataset combination: $w$CDM and o$w$CDM exhibit moderate evidence for Ly$α$ + CMB + galaxy BAO, while most other models show weak or inconclusive evidence. With Pantheon$^{+}$ or DES-Dovekie, o$w$CDM shows strong evidence, whereas other models remain moderately favored.

astro-ph.CO↗

arXiv2026

Cosmological Tests of $f(R,G,T)$ Dark Energy Model in FRW Universe

This research article presents a new cosmological model formulated within the $f(R,G,T)$ framework, focusing on the observational signatures and parameter constraints of the model. The Markov Chain Monte Carlo (MCMC) technique is employed to effectively explore the parameter space using data from 36 Cosmic Chronometers and 1701 Pantheon Plus data points. A comparative analysis is conducted between the proposed $f(R,G,T)$ model and the widely accepted $Λ$CDM model, considering various cosmological parameters, such as Deceleration, Snap, and Jerk. By evaluating these parameters, valuable insights into the dynamics and evolution of the universe within the context of the new model are obtained. Diagnostic tests including Statefinder and Om Diagnostic are performed to further investigate the behavior and consistency of the $f(R,G,T)$ model. These tests provide deeper insights into the properties of the model and its compatibility with observational data. The model is subjected to statistical analysis using Information Criteria to rigorously assess its goodness of fit to the data. This analysis helps determine the level of agreement between the $f(R,G,T)$ model and the observational data, establishing the viability and reliability of the proposed cosmological framework. The results highlight the potential of the $f(R,G,T)$ framework in understanding the fundamental aspects of the universe's evolution and dynamics. The comparative analysis with the $Λ$CDM model, along with the comprehensive diagnostic tests performed, demonstrates the efficacy and validity of the $f(R,G,T)$ model in explaining observed cosmological phenomena. These findings contribute to the ongoing pursuit of accurate and comprehensive models that provide a deeper understanding of the nature of our universe.

gr-qc↗

arXiv2026

Evidence for evolving dark energy from DESI DR2 BAO and Pantheon$^+$, DES-Dovekie, and Union3

Evidences for evolving dark energy are shown using baryon acoustic oscillation measurements from the recent Dark Energy Spectroscopic Instrument Data Release 2 , combined with different Type Ia supernova datasets (Pantheon$^+$, DES-Dovekie, and Union3) and the CMB compressed likelihood. We examine several dark energy parameterizations, including the Logarithmic, Exponential, CPL, BA, JBP, Thawing, Mirage, and GEDE models. Analyzing the DESI DR2 measurements alone, we find that evidence for evolving dark energy is primarily driven by the LRG1-2 tracers, as their inclusion yields a preferred value of $w_0 > -1$. However, as each tracer provides only limited observables, this preference can result in an underconstrained and potentially unstable inference. Further, we find that each dark energy model predicts values in the $w_0 > -1$, $w_a < 0$ quadrant, a region characterized by the Quintom-B type dark energy scenario. The logarithmic bayes factor shows that, among all models, the Mirage model shows the inconclusive-to-moderate evidence across all dataset combinations. Consistently, the statistical significance remains modest, with $Nσ\sim 1.1$-$2.3$, and no model showing a robust preference for dynamical dark energy using late-time datasets alone. The evolution of $w(z)$ shows a phantom crossing around $z \sim 0.5$ in most dynamical dark energy models, and the evolution of $f_{\mathrm{DE}}(z)$ converges to $f_{\mathrm{DE}}(0) = 1$ in all dark energy models.

astro-ph.CO↗

arXiv2026

Long Inflation Screens Euclidean-Wormhole Initial States

Euclidean wormholes can prepare inflation in non--Bunch--Davies initial states, but long Lorentzian expansion screens this memory from the CMB. We derive a visibility bound for Euclidean-matched Bogoliubov data: the pivot excitation satisfies $|β_*| \lesssim e^{-2N_{\rm pre}}$, and smooth Euclidean filters confine residual signatures to a comoving edge $k_w=a_iM$. Only near-minimal inflation, or an edge inside the observable window, leaves detectable scalar, tensor, and higher-point imprints. For longer inflation, wormhole-prepared perturbations are driven to the Bunch--Davies prediction. Euclidean memory therefore, becomes a quantitative bound on inflationary duration, with direct targets in CMB polarization and large-scale structure: the longer inflation lasts, the less of the wormhole remains on the sky.

hep-ph↗

arXiv2026

Probing departures from $Λ$CDM by late-time datasets

Observational data play a pivotal role in identifying cosmological models that are both theoretically consistent and empirically viable. In this work, we investigate the level of preference for dynamical dark energy over a cosmological constant using current late-time observational datasets, including Cosmic Chronometers , Baryon Acoustic Oscillations from DESI DR2, and different Type Ia supernova catalogs (Pantheon$^+$, DES-Dovekie, Union3). We analyze various dynamical dark energy models, including $ω$CDM, o$ω$CDM, $ω_0ω_a$CDM, Logarithmic, Exponential, JBP, BA, and GEDE. In most cases, the o$Λ$CDM and o$ω$CDM models favor an open Universe. For the o$ω$CDM, the inclusion of DES-Dovekie or Union3 data together with CC and DESI DR2 favors a nearly flat geometry. Using the CC + DESI DR2 dataset, the preference for dynamical dark energy lies between the $1$-$2σ$ level. When different supernova catalogs (DES-Dovekie or Union3) are included, the deviation from $Λ$CDM in the $ω$CDM, $ω_0ω_a$CDM, Logarithmic, JBP, BA, and GEDE models increases to the $2$-$2.74σ$ level, while the Pantheon$^{+}$ sample yields deviations below the $2σ$ level. We find consistent evidence for $ω_0 > -1$ and $ω_a < 0$ across all dark energy models, indicating a preference for dynamical dark energy characterized by a Quintom-B type scenario. The $Λ$CDM paradigm has long served as the standard framework of modern cosmology; however recent DESI DR2 results have exposed emerging tensions with the cosmological constant $Λ$, hinting at possible new physics in the dark energy sector. Even so, the currently available data are still not strong enough to definitively rule out the $Λ$CDM model.

astro-ph.CO↗

arXiv2026

Localization and Confidence Region Estimation of Short GRBs with the COSI BGO Shield Using a HEALPix-Based Deep Learning Approach

The Compton Spectrometer and Imager is a NASA satellite mission under development that will survey the entire sky in the 0.2-5 MeV range using a wide-field germanium detector array, surrounded on the sides and bottom by active shields (the Anticoincidence Subsystem, ACS). The ACS aims to suppress and monitor background events, as well as detect transient sources, such as Gamma-Ray Bursts (GRBs), through its onboard triggering algorithm. The data related to GRBs are sent to the ground and analyzed by an automated pipeline to localize the GRBs and share their positions with the community. In this work, we present a brief GRB localization method based on ACS data, utilizing deep learning (DL) techniques, which can estimate the 90\% confidence region, including cases where it is split into multiple areas. To address this, we developed a neural network classifier that predicts the GRB location as a probability distribution across the sky map following the HEALPix framework. The distribution can be used to compute the 90\% confidence regions. Future work will compare this DL-based localization approach with classical methods such as $χ^2$ fitting and Maximum Likelihood Estimation.

astro-ph.HE↗

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↗

arXiv2025

Relativistic accretion process onto rotating black holes in Einstein-Euler-Heisenberg nonlinear electrodynamic gravity

In this study, we uncover the accretion dynamics and oscillatory behavior around rotating black holes within the EEH nonlinear electrodynamic framework by analyzing both the motion of test particles and numerically solving the general relativistic hydrodynamic equations. Using EEH geometry, we compute the structure of circular motion, the effective potential and force, and we evaluate the orbital, radial, and vertical epicyclic frequencies together with the Lense-Thirring and periastron precession rates. Our calculations show that, compared to the Kerr model, the charge parameter $Q$ and the spin parameter $a$ significantly modify the strong gravitational field and shift the characteristic frequencies. We then model the dynamical structure formed by matter accreting toward the EEH black hole through the BHL mechanism, finding that the parameter $Q$ increases the amount of infalling matter and strengthens shock-cone instabilities near the horizon, while farther from the black hole it suppresses accretion and reduces turbulence. Time-series analysis of the accretion rate reveals robust QPOs, whose low-frequency components arise from the precession of the shock cone, while high-frequency components appear as a consequence of strong-field instabilities modified by $Q$ and $a$. A systematic parameter-space exploration identifies the regions where EEH corrections maximize QPO activity, indicating that nonlinear electrodynamics can leave observable imprints on accretion flows and may be testable with QPO and horizon-scale observations.

gr-qc↗

arXiv2025

Confining nonlinear electrodynamics black holes: from thermodynamic phases to high-frequency phenomena with accretion process

We investigate a static, spherically symmetric black hole solution arising from Einstein gravity coupled to a confining nonlinear electrodynamics model that reproduces Maxwell theory in the strong-field regime while introducing confinement-like corrections at large distances. The resulting metric function is asymptotically Schwarzschild but carries a characteristic Q^3/(9ξ^2 r^4) correction, where $Q$ is the magnetic charge and $ξ$ is the nonlinear electrodynamics parameter, with the conventional Reissner-Nordström term Q^2/r^2 absent. We analyze the horizon structure and construct three-dimensional embedding diagrams to visualize spatial geometry. Using the Gauss-Bonnet theorem, we compute the weak-field deflection angle in vacuum, cold plasma, and axion-plasmon media, finding that the nonlinear electromagnetic corrections reduce the total bending compared to Schwarzschild at fixed Arnowitt-Deser-Misner mass. The gravitational redshift, Joule-Thomson expansion coefficient, and heat capacity are derived, revealing phase transitions and inversion curves that depend on the model parameters. We obtain closed-form expressions for the photon sphere radius, Lyapunov exponent, and shadow size, demonstrating their sensitivity to Q and $ξ$ along observable Intensities. Fully relativistic hydrodynamical simulations of Bondi-Hoyle-Lyttleton accretion show that the confining geometry produces a $\sim 40\%$ enhancement in mass accretion rate relative to Schwarzschild and generates quasi-periodic oscillations with stable 3:2 and 2:1 frequency ratios matching observations from black hole X-ray binaries. These results establish the confining nonlinear electrodynamics black hole as a testable model that can reproduce high-frequency quasi-periodic oscillation pairs without invoking black hole spin.

astro-ph.HE↗

arXiv2025

Origin of Quasi-Periodic Oscillations and Accretion Process in X-Ray Binaries around Quantum Lee-Wick Black Hole

In this study, we investigate the accretion dynamics and test particle motion around a non-rotating, spherically symmetric Lee-Wick black hole (BH) to reveal how the model parameters affect orbital stability and the quasi-periodic oscillations (QPOs) observed in X-ray binary systems. The spacetime geometry, characterized by the BH mass and the coupling parameters $S_1$ and $S_2$, includes exponential and oscillatory corrections arising from the Lee-Wick terms. Using the effective potential approach, we derive specific energy, angular momentum, epicyclic frequencies, and the locations of the innermost stable circular orbits (ISCOs) of test particles. In addition to the analytical analysis, we explore the effects of the Lee-Wick spacetime parameters on the shock-cone morphology produced by Bondi-Hoyle-Lyttleton (BHL) accretion. To this end, we perform general relativistic hydrodynamic simulations in two characteristic regimes: Block-1 (weak Lee-Wick regime) and Block-2 (strong Lee-Wick regime). The results show that Block-1 solutions closely resemble the Schwarzschild case, while Block-2 models develop denser and asymmetric shock cones accompanied by stronger QPOs activity, shifting from low-frequency to high-frequency QPOs. These variations yield distinct observational signatures that may be detectable in high-resolution X-ray timing data. Our analytical and numerical findings demonstrate that the Lee-Wick parameters $S_1$ and $S_2$ cause measurable changes in the morphology of the accretion flow and in the frequency ratios near the BH. This suggests that future multi-wavelength observations could provide an important avenue to test higher-derivative gravity theories.

gr-qc↗

arXiv2025

Compact Objects in 4D Einstein Gauss Bonnet Gravity A Data Based Perspective

Cosmic evolution is the most sensational topic among researchers of modern cosmology. This article explores cosmic evolution in 4D Einstein Gauss Bonnet gravity, focusing on mass accretion of compact objects (black holes and wormholes) by dark energy. Three DE models CPL, JBP, and BA parameterizations are studied within 4D EGB gravity, with their Hubble parameters derived and compared against observational data (Cosmic Chronometers, Type Ia Supernovae, and Baryon Acoustic Oscillations). Bayesian analysis favors the CPL and BA models, with CPL providing the best fit. For black holes, mass accretion of CPL and JBP DE shows transitions between quintessence and phantom eras, while BA and $Λ$CDM strictly exhibit quintessence-like behavior, driving cosmic acceleration. In contrast, wormholes exhibit the opposite trend, favoring a phantom-dominated era for the BA and $Λ$CDM models. The study highlights the dynamic nature of DE in 4D EGB gravity and its role in cosmic expansion.

gr-qc↗

arXiv2025

Structure and Mass-Radius Stability of Charged Compact Objects in Symmetric Teleparallel Euler-Heisenberg Gravity

In this work, we develop a new relativistic model for a charged anisotropic compact star in the framework of modified symmetric teleparallel gravity, namely $f(Q)$-Euler-Heisenberg gravity. By employing the MIT bag model equation of state, we establish a relation between the metric potentials, leading to an exact solution of the field equations for an anisotropic fluid configuration coupled with a non-linear electromagnetic source. The interior spacetime is smoothly matched with the exterior geometry calculated from the theoretical setup of $f(Q)$-Euler-Heisenberg gravity using the Darmois-Israel junction conditions, ensuring the continuity of the metric functions and their derivatives at the stellar boundary. The physical viability of the model is examined through regularity, energy, and causality conditions, all of which are satisfied throughout the stellar interior. The study highlights how the pressure anisotropy, the propagation speeds of sound, and the Tolman-Oppenheimer-Volkoff balance condition are interconnected, showing that the star remains in mechanical equilibrium only when the gravitational, hydrostatic, electric, and anisotropic contributions counterbalance one another appropriately. The dynamical stability of the configuration is further supported by the requirement $Γ> \tfrac{4}{3}$ for the adiabatic index, indicating resilience against small radial perturbations. The plots of compactness, surface redshift, and the mass--radius profiles confirm that all physical quantities behave regularly and vary smoothly throughout the stellar interior. We graphically plotted the mass-radius curves.

gr-qc↗

arXiv2025

Magnetized particle motion and accretion process with shock cone morphology around a decoupled hairy black holes

Relativistic accretion onto compact objects such as black holes and neutron stars is one of the most efficient known mechanisms for converting gravitational potential energy into radiation. In the case of rapidly spinning black holes, up to $40\%$ of the rest-mass energy of accreting matter can be released, far exceeding the efficiency of nuclear fusion. In this work, we investigate magnetized particle motion and relativistic accretion processes around a decoupled hairy black hole via extended geometric deformation. The developed geometry involves two hairy parameters that preserve the horizon structure with the additional feature of the fulfillment of weak energy conditions outside the event horizon. We provide the foundation with necessary formalism for magnetized particle motion around a decoupled black hole. The effective potential and innermost stable circular orbits are then derived, which demonstrate a significant reduction of the radius of the latter quantity under the hairy parameters for the magnetized particle. Afterwards, we obtain exact analytical expressions for radial velocity profiles, mass accretion rates, and a few others which reveal improved energy efficiency and emissivity as compared to the standard black hole. Furthermore, the decoupling parameter shows strong influence on oscillations, accretion presenting fantastic agreement between analytical predictions and numerical simulations, and thus offering noticeable observational signatures for future gravitational wave and X-ray astronomy.

astro-ph.HE↗

arXiv2025

Analytical and numerical study of accretion processes around charged spherically symmetric black holes in scalar-tensor Gauss-Bonnet gravity

We investigate the physical phenomena occurring around a spherically symmetric, non-rotating charged black hole (BH) to explore the effects of scalar-tensor Gauss-Bonnet gravity on circular motion, accretion disk properties, and Bondi-Hoyle-Lyttleton (BHL) accretion flow. By analytically and numerically examining the influence of the Gauss-Bonnet coupling constant $c_1$ and the cosmological parameter $Λ$, we reveal how these modified gravity parameters alter the underlying physical processes. Using geodesic analysis, we compute the specific energy, angular momentum, innermost stable circular orbit (ISCO) radius, and radiation flux of test particles, providing insight into how the modified gravity framework affects orbital stability and the organization of the accretion flow. Subsequently, through numerical solutions of the general relativistic hydrodynamic (GRHD) equations, we describe the morphology of the shock cone formed via the BHL accretion mechanism around the BH. The numerical results demonstrate that increasing the values of $c_1$ and negative $Λ$ reduce gravitational focusing. Consequently, depending on the parameter choices, the opening angle of the shock cone either widens or narrows compared to the Schwarzschild case. However, because of weakened gravitational focusing, both the amount of accreted matter and the density of material trapped inside the cone decrease significantly. These results indicate that scalar-tensor Gauss-Bonnet corrections act as an effective gravitational damping term, transferring turbulence and transforming shock-dominated accretion into more stable configurations. The consistency between theoretical and numerical results suggests that the observable properties of accretion disks and quasi-periodic oscillations (QPOs) can serve as probes to constrain the parameters of scalar-tensor Gauss-Bonnet gravity in strong-field regimes.

gr-qc↗