arXiv subjects

Abdul Malik Sultan

Publications and source records attributed to Abdul Malik Sultan.

10 recordsLinked to original sources

arXiv2026

Impact of a Cold Dark Matter Halo on Magnetic Reconnection and Energy Extraction from Kerr-like Black Holes

Recently, Comisso and Asenjo introduced a new energy extraction mechanism based on magnetic reconnection. In this paper, we investigate the power and efficiency of magnetic reconnection energy extraction in a rotating black hole surrounded by a cold dark matter halo. We first examine the properties of the underlying spacetime and its physical quantities, including the event horizon, photon sphere, and ergosphere. We then analyze the allowed energy extraction region and the corresponding energy extraction efficiency for circular orbits. Our results show that energy extraction is feasible for black holes with a spin parameter.

gr-qc↗

arXiv2026

Magnetic Reconnection and Energy Extraction from a Rotating Black Hole in the Einstein-AdS SU(N)-Nonlinear Sigma Model

In the present study, we analyze the power and efficiency of energy extraction via magnetic reconnection in the rotating Einstein-AdS-SU($N$)-NLSM black hole, both in the circular-orbit regime and in the plunging region. Initially, we define the background properties of this spacetime, and then analyze the physical quantities such as the size of the ergoregion, the event horizon, and the boundaries of the ergosphere. We analyze the magnetic reconnection process within circular orbits. We plot energy-extraction parameter diagrams and analyze the power and efficiency of energy extraction. Our results indicate that energy extraction remains feasible even at a spin parameter as low as $0.7$, significantly below previously reported thresholds, and the extracted power can exceed that of the Blandford-Znajek mechanism with specific constraints. The coupling constant $K$, AdS radius $l$ and the flavors number $N$, collectively participate in lowering the spin threshold for energy extraction. Consequently, we further investigate the permissible energy extraction region for the energy extraction mechanism in the plunging region, as well as the corresponding power output and efficiency. We observe that the energy extraction is possible even at a spin as low as $0.2$. Importantly, the parameters $N,~K$ and $l$ mainly contribute to lowering the energy extraction spin threshold. This behavior is similar to that in circular orbits. Finally, comparing the plunging region with the circular orbits, we observe that the energy extraction power in the plunging region is higher than in the circular orbits.

gr-qc↗

arXiv2026

Magnetic Reconnection and Energy Extraction from a Rotating Black Hole in a Four-dimensional Einstein-Gauss-Bonnet Gravity

Recently, Comisso and Asenjo proposed a new mechanism for energy extraction based on magnetic reconnection of plasma within the ergosphere. In this paper, we analyze the power and efficiency of energy extraction through magnetic reconnection in a rotating four-dimensional Einstein-Gauss-Bonnet gravity black hole. Firstly, we analyze the background properties of this spacetime and its physical quantities, including the event horizon, the boundary of the ergosphere, and the size of the ergosphere. We analyze the magnetic reconnection in circular orbits and investigate the energy extraction region, as well as the power and efficiency of energy extraction. Our results indicate that energy extraction remains feasible even for a low spin parameter of $0.4$, which is well below the previously reported threshold. We also find that the energy extraction power exceeds that of the Blandford-Znajek mechanism. The Gauss-Bonnet coupling parameters $α$ lower the spin threshold for energy extraction. Similarly, we analyze the energy extraction region in the plunging regime, together with the corresponding power and efficiency. We find that energy extraction is possible even for a low spin parameter as $0.22$. We also observe that the Gauss-Bonnet coupling parameter $α$ further lowers the spin threshold. This behavior is consistent with that observed in the circular orbit case. Finally, we compare the energy extraction power in the plunging and circular orbit regimes and find that the plunging regime yields a higher energy extraction power than the circular orbit case.

gr-qc↗

arXiv2026

Optical Appearance of a Rotating Black Hole in Nonlinear Electrodynamics Surrounded by Thin Accretion Disks

This work investigates the optical appearance of a rotating black hole (BH) in nonlinear electrodynamics (NED) using two illumination models, namely a celestial sphere and a thin accretion disk. The BH images are constructed using a backward ray-tracing method together with a fisheye camera model. We examine the effects of the electric charge $Q$ and the NED parameter $β$ on the event horizon, shadow, photon ring, and optical appearance for both prograde and retrograde accretion flows. The results indicate that an increase in $β$ leads to a larger shadow radius with reduced distortion, whereas increasing $Q$ decreases the shadow size and enhances its deformation. These features are further quantified through the shadow radius and distortion parameter. We also analyze the direct and lensed images of the thin accretion disk, together with their corresponding redshift distributions and emission bands. The redshifted emission is found to dominate the observed images, while the blueshifted region is confined to the vicinity of the photon ring. Our results demonstrate that both $Q$ and $β$ leave distinct signatures on the optical appearance of rotating NED BHs, providing useful insights for future high-resolution observations.

gr-qc↗

arXiv2026

Repetitive Penrose Process in Rastall Rotating Black Holes Immersed in Quintessence Dark Energy

We investigate the repetitive Penrose process in the spacetime of a Rastall rotating black hole surrounded by a quintessence dark energy field. After reviewing the fundamental properties of the black hole geometry, we formulate the repetitive Penrose process by deriving the conservation equations governing particle splitting within the ergoregion, along with the corresponding iterative evolution equations. The physical conditions required for terminating the energy extraction iterations are established, and the minimum spin thresholds of the decay particles are analyzed to identify the critical stopping criterion. Our analysis reveals that the termination of the repetitive Penrose process is consistently governed by Particle~$0$, which possesses the highest minimum spin threshold among all decay products. Numerical results further demonstrate that the dimensionless Rastall structure parameter $\hat{N}_s$ and the Rastall coupling parameter $α$ significantly influence the evolution of the energy extraction process. At the same decay radii increasing initial values of both parameters boosts the energy utilization efficiency and energy return on investment. Specifically, smaller values of $\hat{N}_s$ enhances the energy utilization efficiency at lower decay radii, shifts the maximum extracted energy toward lower decay radii, and accelerates the depletion of the remaining extractable energy reservoir. This indicates that the repetitive Penrose process is highly favored at lower decay radii. Smaller initial values of $\hat{N}_s$ yield a larger maximum energy return on investment. Similarly, increasing $α$ enhances the energy utilization efficiency, alters the location of the peak extracted energy, and reduces the total extractable energy. But the effects of $α$ on these energetics are very small as compared to $\hat{N}_s$.

gr-qc↗

arXiv2026

Probing Primordial Cosmology Through BBN Observational Constraints Under Extended Gravitational Dynamics

In this article, We investigate the cosmological consequences of a recently developed $f(R,G,\mathcal{T})$ gravitational framework, in which the action is formulated as a general function of the Ricci scalar $R$, the Gauss-Bonnet invariant $G$, and the trace of the energy-momentum tensor $\mathcal{T}$. As one of the most reliable probes of the physical conditions in the early universe, Big Bang nucleosynthesis offers a stringent framework for testing deviations from standard cosmology. We consider four representative models that are analyzed and constrained using observational limits on $\left|ΔT_f/T_f\right|$ and the primordial helium mass fraction $Y_p$. The bounds obtained identify the allowed parameter regions for each model and demonstrate that significant departures from standard cosmology are compatible with nucleosynthesis observations. Our analysis shows that broad regions of the parameter space satisfy existing nucleosynthesis constraints, indicating the consistency of $f(R,G,\mathcal{T})$ gravity with the observed primordial light-element abundances and the established picture of the early universe preserving the observed abundances of light nuclei.

gr-qc↗

arXiv2026

Testing the Nature of Rotating Black Hole Shadows Surrounded by a Thin Accretion Disk within Rastall Gravity

We investigate the observational appearance of a rotating black hole (BH) in Rastall gravity by analyzing its shadow and accretion signatures under different illumination environments. The spacetime geometry is characterized by the Rastall parameter $μ$, the structure parameter $γ$, and the rotation parameter $a$. To visualize the BH environment, we employ a ray-tracing algorithm that follows photon trajectories from the observer's screen to the emission region. We analyze how the shadow radius, distortion, and photon ring morphology respond to changes in the spacetime parameters. For a fixed value of $a$, the shadow observables exhibit a pronounced dependence on the Rastall gravity parameters. In particular, increasing the structure parameter $γ$ leads to a gradual enlargement of the shadow radius, indicating an expansion of the photon capture region surrounding the BH. At the same time, the distortion parameter decreases, implying that the shadow boundary becomes progressively more circular and less deformed. These results suggest that larger values of $γ$ tend to suppress the asymmetry induced by rotation and enhance the apparent size of the shadow. Similar modifications are observed for different values of the Rastall parameter $μ$, demonstrating that the combined effects of $μ$ and $γ$ leave distinct signatures on the shadow morphology. Consequently, shadow observations may provide an effective tool for constraining the parameter space of rotating BHs in Rastall gravity.

gr-qc↗

arXiv2026

Visual Characteristics of a Rotating Black Hole in $4$D Einstein-Gauss-Bonnet Gravity with Thin Accretion Disk Under EHT Constraints

This study investigates the visual characteristics of a rotating black hole (BH) within the fabric of $4$D Einstein-Gauss-Bonnet gravity illuminated with two illumination models, such as a celestial light sphere and a thin accretion disk. To visualize the BH shadow images, we use a recent fisheye camera model and ray-tracing method. And then, we focus on investigating the impact of the coupling parameter $α$ and the spin parameter $a$ on the shadow images. The results exhibit that the shadow radius decreases, while the shadow deviation increases with the aid of $α$. However, with respect to $a$, the shadow radius is slightly increased compared to the corresponding shadow deviation. For a celestial light sphere, the increasing values of $α$, lead to a decrease in the corresponding photon ring, while the space-dragging effect becomes more prominent with increasing $a$. For a thin accretion disk, we enhance its inner edge to the BH event horizon, and the particle motion is different in the regions inside and outside the innermost stable circular orbit. The result demonstrates that the shadow becomes progressively asymmetric with $a$, while the overall size of the inner shadow gradually decreases with the variations of $α$. Subsequently, we also investigated the distinct features of red-shift configurations on the disk for both direct and lensed images. Additionally, we used the latest observational data from M87* and Sgr A* to impose certain parameter constraints on $α$; the results depict the consistency of our considering the BH model.

astro-ph.HE↗

arXiv2025

Gravitational Baryogenesis in Extended Proca-Nuevo Gravity

In this article, we investigate the gravitational baryogenesis mechanism in the framework of Extended Proca-Nuevo (EPN) gravity, a theory where a massive vector field is non-minimally coupled to the curvature. This analysis is carried out for an early universe, encompassing three separate cosmological scenarios defined by power-law, exponential, and modified exponential scale factors. By deriving the modified field equations from the EPN action, we obtain precise solutions for each scale factor, including the influence of the vector field. We compute the baryon-to-entropy ratio using the gravitational baryogenesis formalism, where the baryon asymmetry arises from a dynamical coupling of the baryon current with the derivative of the Ricci scalar. Our findings demonstrate that the baryon-to-entropy ratio is consistent with observational constraints in all scenarios, highlighting the potential of EPN gravity as a viable theory to explain the matter-antimatter asymmetry in the early universe. The study further stresses the contribution of anisotropy and vector field dynamics to the cosmological evolution within modified gravity models.

gr-qc↗

arXiv2024

Stellar structure in $f(R,T)$ gravity: some exact solutions

We find some exact solutions for constant-density and quark matter equations of state in stellar structure models framed within the $f(R,T)=R+λκ^2 T$ theory of gravity, where $R$ is the curvature scalar, $T$ the trace of the stress-energy tensor, and $λ$ some constant. These solutions correspond to specific values of the constant $λ$, and represent different compactness states of the corresponding stars, though only those made of quark matter can be regarded as physical. The latter modify the compactness (Buchdahl) limit of neutron stars upwards for $λ>0$ until saturating the one of black holes. Our results show that it is possible to find useful insights on stellar structure in this class of theories, a fact that could be used for obtaining constraints on limiting masses such as the minimum hydrogen burning mass.

gr-qc↗