arXiv subjects

A. Sulaksono

Publications and source records attributed to A. Sulaksono.

At least 19 recordsLinked to original sources

arXiv2026

Photon propagation and black hole imaging in Kruglov nonlinear electrodynamics

Nonlinear electrodynamics (NLED) modifies photon propagation through an effective optical geometry, allowing optical phenomena to deviate from those predicted solely by the underlying spacetime metric. In this work, we revisit the Kruglov model, which generalizes Born--Infeld electrodynamics, and investigate the corresponding null geodesics in theeffective geometry through direct numerical integration, without relying on strong-field analytical approximations. We analyze the resulting photon spheres, gravitational lensing, black-hole shadows, and thin accretion-disk images, together with their dependence on the parameter $q$. For sufficiently small positive $q$, the effective angular metric develops a pole that generates stable photon orbits outside the event horizon and locally reverses the azimuthal evolution of photon trajectories, producing a distinctive bending pattern that resembles, although does not originate from, frame dragging. Conversely, negative $q$ introduces a zero of the effective correction factor, which acts as a turning point for photon trajectories and prevents them from entering the region where the background metric becomes complex. It produces measurable changes in light deflection, shadow radius, photon-ring morphology, and relativistic images. In particular, smaller positive values of $q$ produce thinner photon rings and enlarged shadow radii, whereas negative values generally lead to broader photon rings and smaller shadows. Comparison with the current Event Horizon Telescope constraints on Sgr~A* shows that a substantial region of the Kruglov parameter space remains compatible with present observations.

gr-qc↗

arXiv2026

Axial Perturbations of Extended Tolman III in the New $f(R,T)$ Theory

We investigate compact objects using the extended Tolman III equation of state in the framework of $f(R,T)=R+2χT$ gravity. In the matter sector of the Lagrangian density, we employ a new formalism based on the evaluation of the second-order derivative of the matter Lagrangian with respect to the metric \cite{Jayawiguna:2026das}. We analyze the corresponding observational signatures through axial gravitational perturbations including stellar stability and gravitational echo. We find that the effective potential in $f(R,T)$ gravity is wider and deeper than its counterpart in general relativity (GR). The quasinormal-mode (QNM) frequencies are calculated using the Bohr--Sommerfeld (BS) fitting and both the lower- and higher-order Wentzel--Kramers--Brillouin (WKB) approximations. For the ultracompact configurations considered, trapped modes can tunnel through the potential barrier, giving rise to gravitational-wave echoes. The imaginary parts of the QNM frequencies are found to be negative, $\Im(ω_n M)<0$, indicating that the perturbations decay exponentially with time. Our results show that the echo train in GR contains more echoes than that in $f(R,T)$ gravity. Finally, assuming that the stellar object is a neutron star and adopting a stellar radius consistent with measurements by the Neutron Star Interior Composition Explorer (NICER), we find that the $f(R,T)$ gravity model yields a characteristic frequency consistent with the estimated frequency associated with GW170817 for $β= 4.663\times10^{-1}$ and $χ=0.96π$, whereas the corresponding GR model does not exhibit comparable agreement.

gr-qc↗

arXiv2026

Parameter Constraints on $f(R^{1+ε},T_{μν}T^{μν})$ Gravity from Exotic Compact Objects

According to General Relativity (GR), gravity is described as the curvature of spacetime caused by mass and energy. Although GR has been highly successful in describing gravitational phenomena, exotic compact objects with high mass and strong gravitational fields, as well as low-mass objects, have motivated the study of modified theories of gravity. One of the modified gravity frameworks is $f(R,T^2) = R^{1 + ε} + ακ^2 T_{μν} T^{μν}$, which provides an extension of GR by introducing modifications to both the geometric and matter sectors. In this study, we investigate the mass-radius structure, physical viability, and stability of quark stars in $f(R,T^2)$ gravity using the MIT Bag Model equation of state. We compare results from the exact $f(R,T^2)$ model, its first-order Taylor expansion, and GR. The values of the gravity parameter $ε$ and $α$ are constrained by evaluating the ability of the model to reproduce the observed mass-radius configurations of compact objects, including HESS J1731-347 with a mass of $M = 0.77_{-0.17}^{+0.20} \, M_{\odot}$ and a radius of $R = 10.4_{-0.78}^{+0.86}$ km, GW190814 with a mass of $M = 2.6 \, M_{\odot}$, PSR J1614-2230, with a mass of $M = 1.928 \pm 0.017 \, M_{\odot}$ and a radius of $R > 10$ km, and PSR J0437-4715, with a mass of $M = 1.418 \pm 0.037 \, M_{\odot}$ and a radius of $R = 11.36_{-0.63}^{+0.95}$ km. We assess the physical viability and stability of the stellar configurations using the energy conditions (strong, weak, null, and dominant) and the speed of sound. Overall, the $f(R,T^2)$ gravity model can accommodate quark star configurations and reproduce the observed properties of massive compact objects, particularly those associated with GW190814 and the lightest compact object observed, HESS J1731-347, respectively.

gr-qc↗

arXiv2026

Non-extensive entropy signatures in compact star

The non-extensive entropy models applied to the black-hole horizon are connected to the generalized Einstein--Hilbert action, $f(R)$, via the Wald entropy formalism. We demonstrate that quark star configurations characterized by the MIT bag model under this modified gravity approach conform to the observational lower and upper bounds established for compact objects linked to HESS J1731$-$347 and GW190814. Furthermore, we assess the effective energy conditions and corresponding speed of sound to evaluate the physical plausibility and stability of the stellar configurations. All non-extensive entropy models studied satisfied these conditions. Our findings show that the exotic compact objects offer a compelling and credible framework for exploring aspects of non-extensive entropy models, such as the Barrow, Tsallis-Cirto, and Rényi formalisms, and vice versa.

gr-qc↗

arXiv2026

A Quantum-Gravity-Motivated GUP Effective Metric

Recent critiques have addressed certain aspects of the generalized uncertainty principle (GUP) effective metric (Ong 2023). This study presents a scale-dependent quadratic GUP effective metric, constructed through analyses of the gravity-induced phase shift (COW experiment) and the Einstein-Bohr photon box Gedanken experiment. In contrast to the procedure outlined in (Xiang et al. 2018), the momentum-dependent metric is improved by introducing an interpolating function $Δp (r)$, which employs the effective distance concept to accurately capture the distinct behavior of $Δp (r)$ in both short and long distance regimes. The resulting effective metric exhibits the same structure as that derived from the Renormalization Group (RG) theory. However, the RG parameter $\hatγ$ can now be related to the dimensionless GUP parameter $β_0$, thereby distinguishing this metric from the RG-based approach. The corresponding effective metric prediction demonstrates internal consistency of the model, and phenomenologically the predictions are in agreement with some quantum black hole models in some limits. The effective metric improved black hole thermodynamics and shadow predictions compared to the heuristic approach and other proposed GUP effective metrics. Furthermore, the relationship between GUP and $f(R)$ gravity (D'Agostino et al. 2026) may provide a possible future route toward a more fundamental description of GUP.

gr-qc↗

arXiv2026

Ambiguity in matter sector for modified gravity involving $δ^2 \mathcal{L}_{m}/δg^{μν}δg^{αβ}$ and its implications to astrophysics and cosmology

Matter density ($ρ$) and radial pressure ($p$) are often used as the matter Lagrangian density ($\mathcal{L}_{m}$) because both are thermodynamically consistent and produce the same Einstein field equation (EFE) in general relativity (GR). New gravity models with explicit links between matter and geometry instead involve second-order derivatives of $\mathcal{L}_{m}$ relative to the metric tensor. So, picking either $p$ or $-ρ$ for $\mathcal{L}_{m}$ gives different effective EFEs. This confusion appears because one usually treats the four-velocity ($u_μ$) and the metric tensor ($g_{μν}$) as independent. Here, we revisit the basics and offer a consistent framework by relaxing that assumption, thereby making the modified gravity theory independent of the choice of $\mathcal{L}_{m}$. Finally, we test this approach on neutron and quark stars (ultraviolet region) and on cosmological situations with radiation-dominated ($p=ρ/3$) equations of state (infrared region), showing how it clarifies the ambiguity in picking $\mathcal{L}_{m}$ for gravity models.

gr-qc↗

arXiv2026

Astrophysical environment around a black hole in the braneworld and its optical signatures

We investigate the impact of braneworld theory on the astrophysical environment surrounding a black hole. The black hole is sourced by localized matter from the bulk, which could describe both regular and singular (Schwarzschild) black hole. Employing an Einstein cluster description for the environment, we find that the anisotropic nature of the cluster, coupled with finite brane tension, leads to a weakening of gravity due to the quadratic and nonlocal corrections to the effective four-dimensional field equations. Consequently, this effect prevents horizon formation within the environment. Applying current constraints on the brane tension derived from neutron star observations, we demonstrate that this effect is particularly relevant for sub-stellar mass black holes embedded in compact environments. Furthermore, we investigate the optical signatures of finite brane tension in this scenario, specifically focusing on the black hole shadow and Einstein ring radii. We show that the Einstein ring radius decreases with a smaller brane tension, whereas the black hole shadow radius increases--somewhat contradicts the weakening gravity effects. Ultimately, these two observables may jointly serve to constrain the value of the brane tension in a very specific astrophysical scenarios.

hep-th↗

arXiv2026

Two descriptions of dark matter around a black hole: photon sphere, shadow, and lensing

We examine the observational discrepancies of two widely used models describing anisotropic (dark) matter distributions around a black hole, focusing on their photon spheres, shadow radii, and lensing observables. The models considered are the vacuum and Einstein cluster dark matter models, characterized by negative and zero radial pressure, respectively. The analysis reveals that these models display contrasting photon sphere behaviors. In particular, the Einstein cluster results in a more pronounced deviation in the shadow radius relative to the standard Schwarzschild black hole. Additionally, a distinctive lensing phenomenon associated with the matter halo is identified in both models.

gr-qc↗

arXiv2025

$I-$Love$-$Curvature: Exploring compact stars' quasi-universal relation with curvature scalars

We investigate quasi-universal relations in neutron stars linking standard observables, such as tidal deformability ($Λ$) and normalized moment of inertia ($\bar{I}$), with normalized curvature scalars in general relativity. These curvature scalars include the Ricci scalar ($\mathcal{R}$), the Ricci tensor contraction ($\mathcal{J}$), the Weyl scalar ($\mathcal{W}$), and the Kretschmann scalar ($\mathcal{K}$). We systematically examine both piecewise polytropic and color-flavor-locked equations of state, finding: (1) significant correlations between both local (central and surface) and global (volume-averaged) curvature scalars with $\bar{I}$ and $Λ$; (2) especially strong correlations between surface and volume-averaged curvature scalars and both $\bar{I}$ and $Λ$; (3) a near equation-of-state-independent maximum for the normalized Ricci scalar, suggesting a link to the trace anomaly; and (4) new universal relations involving normalized central and volume-averaged pressure and energy density, which also correlate strongly with $\bar{I}$ and $Λ$. Using constraints from GW170817 and low-mass X-ray binaries, we demonstrate that $Λ$ measurements directly constrain both scalar curvature quantities and the interior properties of canonical-mass neutron stars. These findings agree with the literature on equation-of-state-dependent Bayesian inference estimates. Our identified relations thus provide an equation-of-state-insensitive connection between stellar observables, spacetime geometry, and the microphysics of compact stars.

gr-qc↗

arXiv2025

Imaging the destruction of a rotating regular black hole

A regular black hole, unconstrained by the weak cosmic censorship conjecture, can exceed its critical spin limit and transition into a superspinar. In this paper, we investigate the observational appearance of a rotating regular black hole, specifically the Ghosh black hole and its superspinar counterpart, when surrounded by a thin accretion disk. The resulting images reveal distinct features: the black hole closely resembles its Kerr counterpart with slight deviations, while the superspinar configuration exhibits an inner photon ring structure. Furthermore, we investigate the image transition of the Ghosh black hole that has recently been destroyed by a collapsing null shell carrying a specific angular momentum. The results indicate that, apart from a possible sudden burst of light, the inner photon ring undergoes gradual transitions over time, with the transition times depending on the additional angular momentum gained by the black hole. Our findings also suggest that the transition timescale becomes significant for supermassive black holes, with masses at least less than about twice that of M87*.

gr-qc↗

arXiv2025

Horizonless star based on regular black hole with finite radius and its observational signatures

The horizonless configuration of regular black holes has recently attracted attention as a model for ultracompact stars. In this paper, we propose a new class of regular black hole models sourced by a de Sitter vacuum with a finite radius. We focus on studying its horizonless configuration, which is modified into an anisotropic gravastar by proposing an ansatz of equation of states. We confirm that an anisotropic gravastar approaching horizon formation must violate the dominant energy condition. We also found that the proposed object has an effectively similar structure as a frozen star on the time geometry at the extremal configuration. From the proposed model, we investigate the photon geodesics inside the object and predict the optical appearance of the object surrounded by a thin accretion disk. Our imaging results indicate that, assuming light does not interact with the object's interior, its optical appearance differs from that of a thin-shell gravastar. ``Chaotic" photon ring merges for $x>x_{m}$, where $x_{m}$ represents the minimum value required for the photon sphere to exist. In addition to its optical appearance, we investigate the axial gravitational perturbations emitted by this horizonless star. Notably, echo trains are found to exist for $x>x_{m}$, as determined by numerically solving the time-dependent Regge-Wheeler equation. By comparing the echo time with the GW170817 observation, we find that a frequency of 72 Hz can be achieved, albeit at the cost of requiring a relatively high value of $\ell$.

gr-qc↗

arXiv2024

Shadow images of regular black hole with finite boundary

Regular black hole is one of the bottom-up solutions designed to eliminate the singularity at the center of black holes. Its horizonless solution has gained interest recently to model ultracompact star. Despite interesting, this proposal is problematic due to the absence of a well-defined boundary. In this work, we introduce a novel regular black hole model inspired by the Hayward black hole, incorporating additional terms to define a clear and well-defined `surface' radius $R$. We analyze the null geodesics around the object, both horizonful and horizonless configurations, by studying the photon effective potential. We further simulate the shadow images of the object surrounded by a thin accretion disk. Our results indicate that for $R > 3M$ the horizonfull shadow differs slightly from that of a Schwarzschild black hole. In the horizonless configuration, we identify distinct inner light ring structures near the central region of the shadow image, which differ from those observed in horizonless Hayward black holes.

gr-qc↗

arXiv2024

Anisotropic gravastar as horizonless regular black hole spacetime and its images illuminated by thin accretion disk

A connection between regular black holes and horizonless ultracompact objects was proposed in~\cite{Carballo-Rubio:2022nuj}. In this paper, we construct a model of a horizonless compact object, specifically an anisotropic gravastar with continuous pressure, that corresponds to regular black hole spacetime in the appropriate limit. The construction begins by modeling an equation of state that satisfies the anisotropic gravastar conditions and transitions to the de Sitter ($p=-ε$) upon horizon formation. The spacetime structure is similar to the {\it Quantum Horizonless Compact Object} (QHCO) described in~\cite{Chen:2024ibc}. Within this model, we also generate images of the corresponding objects surrounded by a thin accretion disk. The resulting images reveal that assuming that the emitting matter exists only outside the object, the inner light ring structure closely resembles that of the horizonless configuration of a regular black hole and the QHCO, yet it exhibits a distinct light ring structure compared to the thin-shell gravastar model. However, the opposite occurs when emitting matter is taken into account inside the object.

gr-qc↗

arXiv2024

Observational Constraints on the Maximum Masses of White Dwarfs, Neutron Stars, and Exotic Stars in Non-Minimal Derivative Coupling Gravity

The advancement of astronomical observations opens the possibility of testing our current understanding of gravitational theory in the strong-field regime and probing any deviation from general relativity. We explore to what extent compact stars predicted by non-minimal derivative coupling (NMDC) gravity theory agree with observed data. We investigate white dwarfs (WDs), neutron stars (NSs), and quark stars (QSs) mass and radius in various values of constant scalar $|Q_{\infty}|$ at coupling strength of $η=\pm1$. This study focuses on the astrophysical impacts of altering maximum masses by values of $|Q_{\infty}|$ and $η$. From an observational point of view, we found that WD stars are consistent with ultra-cold WD data at $|Q_{\infty}| \lesssim 0.2$. We also found that QS has a similar impact of mass-radius to NS, where the modification is more significant at higher (central) density. For NS and QS EoSs, the value $|Q_{\infty}|$ strongly alters the critical mass and might eliminate the $M-ρ_c$ turning point in the negative $η$ case. In that case, the sufficiently large $|Q_{\infty}|$ could predict $M>2.6 M_\odot$ NS and QS, i.e., larger than GW190814 secondary counterpart. We suggest that the lower mass gap in the gravitational wave and x-ray binary mass population data might restrict the theory's $|Q_{\infty}|$.

gr-qc↗

arXiv2024

Impact of modified gravity theory on neutron star and nuclear matter properties

New observational data, measured with a high degree of accuracy, of compact isolated neutron stars and binary stars in gravitational wave remnants have the potential to explore the strong field gravity. Within the framework of energy-momentum squared gravity (EMSG) theory we study its impact on several properties of neutron stars and plausible modifications from the predictions of general relativity. Based on a representative set of relativistic nuclear mean field models, non-relativistic Skyrme-Hartree-Fock models and microscopic calculations, we show deviations of neutron star mass-radius sequence in EMSG theory as compared to general relativity. The variation in the effective nuclear equation of state in EMSG, results in distinct magnitudes in the reduced pressure, speed of sound, and maximum compactness at the center of neutron stars. We perform extensive correlation analysis of the nuclear model parameters with the neutron star observables in light of the new observational bounds. Perceptible modifications in the correlations are found in the models of gravity that provide different estimates of the slope and curvature of nuclear matter symmetry energy. The available neutron star data however do not impose stringent enough constraints for clear evidence of deviations from general relativity.

gr-qc↗

arXiv2022

Minimal Length, Nuclear Matter, and Neutron Stars

In this paper, we employ one variant of the Generalized Uncertainty Principle (GUP) model, i.e., the Kempf-Mangano-Mann (KMM) model, and discuss the impact of GUP on the EoS of nuclear and neutron star matter based on the Relativistic Mean Field (RMF) model. We input the result in the Serrano-Liška (SL) gravity theory to discuss the corresponding Neutron Star (NS) properties. We have shown that the upper bound for the GUP parameter from nuclear matter properties is $β\leq 2\times10^{-7}$ MeV$^{-2}$. If we used this $β$ upper bound to calculate NS matter, and considering SL parameter $\tilde{c}$ as an independent parameter, we have found that the upper bound for the SL parameter, which modifies the Einstein field equation, is $\tilde{c} \leq 10^7$ m$^2$. This beta upper bound is determined by considering the anisotropy magnitude smaller than the pressure magnitude. By employing $β=2\times10^{-7}$ MeV$^{-2}$ and $\tilde{c} = 10^7$ m$^2$, we obtain the mass-radius relation that satisfies NICER data for both PSR J0740+6620 (whose mass is $\sim 2.1M_\odot$) and PSR J0030+0451 ($M\sim 1.4M_\odot$). Our GUP parameter upper bound perfectly matches the constraint from $^{87}$Rb cold-atom-recoil experiment. If we consider that the same strength from the additional logarithmic term in the entropy from both GUP and SL model are dependent, for $β< 2\times10^{-7}$ MeV$^{-2}$, it is clear that SL parameter lower bound is $\tilde{c} > -16\times 10^{-34}$ m$^2$. The magnitude of this bound is $10^{-40}$ smaller than the upper bound magnitude of SL parameter considering as independent parameter i.e., $\tilde{c} \leq 10^7$ m$^2$.

nucl-th↗

arXiv2021

Effects of neutrino magnetic moment and charge radius constraints and medium modifications of the nucleon form factors on the neutrino mean free path in dense matter

Effects of neutrino charge radius and magnetic moment constraints obtained from the astrophysical observations and reactor experiments, as well as in-medium modifications of the weak and electromagnetic nucleon form factors of the matter on the neutrino electroweak interaction with dense matter, are estimated. We use a relativistic mean-field model for the in-medium effective nucleon mass and quark-meson coupling model for nucleon form factors. We analyze the neutrino scattering cross section and mean free path in cold nuclear matter. We find that the increase of the cross section relative to that without neutrino form factors results in the decrease of the neutrino mean free path when neutrino form factors and the in-medium modifications of the nucleon weak and electromagnetic form factors are simultaneously considered. The quenching of the neutrino mean free path is evaluated to be about 12-58\% for the values of $μ_ν= 3 \times 10^{-12} μ_B$ and $R_ν= 3.5 \times 10^{-5}~\textrm{MeV}^{-1}$ compared with that obtained for the $μ_ν=0$ and $R_ν=0$. The decrease of the neutrino mean free path is expected to decelerate the cooling of neutron stars. Each contribution of the neutrino form factors to the neutrino mean free path is discussed.

hep-ph↗

arXiv2021

$2.6 M_\odot$ Compact Object and Neutron Stars within Eddington-Inspired Born-Infeld Theory of Gravity

In the context of whether a massive compact object recently observed in the GW190814 event is a neutron star (NS) or not, we have studied the role of the parameters $κ$ and $Λ_c$ of the Eddington-inspired Born-Infeld (EiBI) gravity theory on the NS mass-radius relation, moment of inertia, and tidal deformability. The results are compared to recent observation constraints extracted from the analysis of NS observation data. The NS core equation of state (EoS) is calculated using the relativistic mean-field model with the G3 parameter set. In the hyperon sector, the SU(3) and hyperon potential depths are used to determine the hyperon coupling constants. For the inner and outer crusts, we use the crust EoS from Miyatsu et al. (2013). We also maintain the sound speed to not exceed $c$/$\sqrt{3}$ at high densities. We have found that, in general, the NS mass significantly depends on the value of $κ$, and the radius $R$ is sensitive to the value of $Λ_c$. Moreover, as $Λ_c$ is equal to zero or less than the accepted bound of the cosmological constant, the NS within the EiBI theory is compatible with observation constraints, including $2.0 M_\odot$ mass, canonical radius $R_{1.4 M_{\odot}}$, moment of inertia, and tidal deformation. Our investigation also reveals that the $2.6 M_\odot$ mass compact object and current observational constraint of canonical radius $R_{1.4 M_{\odot}}$ can simultaneously be satisfied only when the $Λ_c$ value is unphysically too large and negative. Therefore, within the spesific EoS employed in this work, we conclude that the secondary object with $2.6 M_\odot$ observed in the GW190814 event is not likely a static (or a slow-rotating) NS within the EiBI gravity theory.

gr-qc↗